Biological control of fall armyworm Spodoptera frugiperda (JE Smith) using egg parasitoids, Trichogramma species (Hymenoptera: Trichogrammatidae): a review

Background Fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is an invasive pest that can cause serious crop loss to various important food crops and risk to global food security. FAW can cause damage to over 350 plant species by feeding on vegetative and reproductive stages of crops. Currently, chemical pesticides and transgenic maize are widely used to safeguard crops from this serious pest. However, biological control is a safer and more sustainable alternative in the long term for FAW management. Of the various natural enemies of FAW, egg parasitoids of genus Trichogramma have been used in augmentative biological control of FAW in the Americas and invaded regions such as Africa and Asia. Main body


Background
Fall armyworm (FAW), Spodoptera frugiperda (J.E.Smith) (Lepidoptera: Noctuidae), is a highly polyphagous pest of maize and other cereals in tropical and subtropical regions of the world (Sparks 1979).FAW is native to the Americas and its recent invasion in Africa, Asia and Oceania (FAO 2023; Fig. 1) has severely impacted yields of several food crops (Overton et al. 2021).This noctuid pest is highly destructive and the caterpillars can feed on over 350 plant species grown commercially or non-commercially across 76 plant families, with a preference for maize (Zea mays L., Poaceae) (Montezano et al. 2018).These caterpillars can feed on different phenological stages of maize and could result in maize yield loss of up to 70% if attacked early growth stage of maize (Hruska 2019).Owing to the increase in international trade and long-distance migration ability, the FAW has rapidly spread in new geographical regions and threatened global maize production (Early et al. 2018).The high adaptability, fecundity and polyphagous nature made them a key pest of maize in the invaded region under suitable environmental conditions (Jing et al. 2021).
Outside its native range, FAW is now recognized as an important pest of maize and several other crops (Rane et al. 2023).Within 5 years of invasion in West and Central Africa, and almost 3 years in India, Asia, this pest has changed the pest status of maize and the existing control strategies become less effective (Prasanna et al. 2022).Nevertheless, several control strategies have been deployed against FAW including chemical methods, biological control agents, and physical methods to reduce the crop yield loss (Kenis et al. 2022).The invasiveness and the destructive nature of FAW have attracted the attention of applied pest management for sustainable crop production (Kumar et al. 2022).In the invaded regions, especially in Africa and Asia, the chemical methods remain a mainstay for FAW control on an emergency basis to reduce crop damage and further spread (Deshmukh et al. 2020).However, several issues remain with chemical control methods due to the heavy application of conventional chemical insecticides such as cypermethrin, lambda-cyhalothrin and chlorpyrifos (Tambo et al. 2019) that cause the development of resistance in FAW populations and redundant use of insecticides kill the nontargets including natural enemies, cause environmental pollution and pose risk to human health (Kebede and Shimalis 2018).The long-term use of chemical measures may be ineffective and non-sustainable (Day et al. 2017).Therefore, the development of efficient control measures is the greatest challenge in the invaded region (Guo et al. 2020).However, in the Americas, the use of transgenic Bt maize is the leading control approach against FAW, besides the use of insecticides and biological control also used by farmers (Burtet et al. 2017).Consequently, there is a need to develop and introduce safer alternatives, such as biological control against FAW in invaded regions.
Egg parasitoids (Hymenoptera: Trichogrammatidae) are considered excellent candidates for the biological control of FAW and are also crucial components during designing the integrated pest management (IPM) of FAW (Navik et al. 2023).In the Americas, several egg parasitoids have been collected, mass-produced and Fig. 1 Global spread of fall armyworm, Spodoptera frugiperda (Source: FAO 2023).Note: Canada (northern provinces) and Alaska with restricted distribution and distribution in all grey countries uncertain or absence (Source: https:// gd.eppo.int/ taxon/ LAPHFR/ distr ibuti on) augmented for FAW management (Bueno et al. 2023).Similarly, efforts were made to collect native egg parasitoids of FAW in the invaded region, so that they could be recruited for biological control or deployed through IPM in sustainable crop production (Sun et al. 2021).It is fascinating that several native egg parasitoids have been found parasitizing the FAW eggs naturally and controlling them at their egg stage before they inflict any damage (Parra and Coelho 2022).This provides an opportunity for the development of augmentative biological control using egg parasitoids against FAW.This manuscript presents a review of the natural occurrence of Trichogramma on FAW, biological attributes, laboratory evaluation, mass-rearing and field application against FAW.

Trichogramma parasitoids of fall armyworm
The egg parasitoids mainly belong to the genus Trichogramma Westwood (Hymenoptera: Trichogrammatidae) considered for FAW control in the Americas and invaded regions such as Africa and Asia.Eight species of Trichogramma have been found naturally parasitizing the FAW egg masses in the maize and other crop environments in its native range as well as in three species in the invaded regions (Table 1).Of these egg parasitoids, various species of trichogrammatids have been mass-reared on suitable factitious hosts such as Sitotroga cerealella (Oliver), Anagasta kuehniella Zeller and Corcyra cephalonica (Stainton) (Table 4) and demonstrated for FAW control at field conditions.Based on reports in the literature, 11 species of trichogrammatid egg parasitoids were found associated with FAW and parasitized egg masses with different rates in various crops (Table 2).In addition, a recent study reported the occurrence of Trichogrammatoidea lutea Girault (Hymenoptera: Trichogrammatidae) from parasitized FAW egg masses based on morphology and molecular analysis (Sun et al. 2021).
Based on the literature survey, 10 species of Trichogramma have been known to parasitize FAW eggs in various crop habitats (Table 1).Of these, 8 species are known to parasitize the eggs of FAW attacking different crops (e.g.Malachra, Sorghum) in the native range, especially in Latin American and Caribbean countries.Amongst the species, Trichogramma pretiosum Riley (Fig. 2A) and T. atopovirilia Oatman and Platner were potential egg parasitoids of FAW and have been used for augmentative biological control.In Africa, Trichogramma chilonis Ishii (Fig. 2B), T. mwanzai Schulten and Feijen (Fig. 2C) and Trichogrammatoidea lutea were found parasitizing the FAW eggs in maize (Sun et al. 2021).In Asia, the natural occurrence of T. chilonis on FAW has been in maize from India (Navik et al. 2021) and China (Jin et al. 2021).However, the natural occurrence and parasitism by Trichogramma spp.parasitizing FAW eggs is largely influenced by egg types, crop growth stages and environmental conditions.

Natural parasitism by Trichogramma
Several species of Trichogramma have been found naturally associated with FAW eggs in the native ranges as well as in invaded regions (Table 2).Parasitism of these species varied between 1 and 52.5% in different crops' ecosystems.In the native range, the parasitism by T. pretiosum and T. atopovirilia was low (2.21%) in maize fields, and T. pretiosum was recorded as the most frequent parasitoid found in 93.7% of parasitized eggs (with an average < 20% parasitism in Brazil maize), followed by T. atopovirilia with 2.07% parasitism (Beserra et al. 2002).Further, Dequech et al. (2013) 2020).In Africa, the natural occurrence of T. chilonis was recorded at the rate of 20.9% in the maize field of Kenya (Sisay et al. 2019).Similarly, in Asia, the parasitism of T. chilonis to FAW eggs ranged from 15.81 to 23.87% in the maize fields of India (Navik et al. 2021).The lower parasitism by Trichogramma species in natural conditions might be due to the distribution of FAW egg masses on plants, eggs in layers and the presence of scales, phenological stages and plant age (Durocher-Granger et al. 2021).In addition, the plant region with the greatest leaf mass preferred for oviposition by the FAW females, which provides protection (Beserra et al. 2002) and the increased scale thickness (Hou et al. 2022) on egg masses reduced the parasitism success of Trichogramma under natural conditions.

Biological characteristics of Trichogramma reared on fall armyworm
For successful biological control programmes, the suitability of Trichogramma species is usually assessed based on biological attributes such as parasitism and emergence rate, adult emergence per egg, female progeny and developmental time on the target pest (Hou et al. 2018).Many species of Trichogramma are specific to host species and some are generalists attack native Lepidoptera and beneficial lepidopteran eggs used for biocontrol projects (e.g.T. chilonis) and their response to the hosts and host age is independent of the egg parasitoid species (Monje et al. 1999).Thus, host age influences the performance of some Trichogramma species by affecting their biological attributes (Hou et al. 2018).In general, the host egg age is known to affect the parasitism potential of Trichogramma parasitoids (Pizzol et al. 2012).Likewise, the different ages of FAW eggs also affected the parasitism rate of Trichogramma species and research indicated that older FAW eggs were less parasitized by parasitoids (Sun et al. 2021).In Zambia, five species of trichogrammatids, viz.T. mwanzai, T. japonicum Ashmead, T. ostriniae (Peng and Chen), T. leucaniae Pang and Chen and T. lutea were exposed to various egg ages of FAW and found that the egg age of FAW had influenced the parasitism rate of the four tested species except T. japonicum preferred to parasitize 1-day-old eggs over 0-and 2-day-old eggs.In addition, T. mwanzai and T. lutea parasitized a  (Li et al. 2023).
Besides, the FAW egg age has not affected the adult emergence of Trichogramma (Sun et al. 2021).The emergence rate of T. mwanzai was 98.3, 94.8 and 93.2% on 0-, 1-and 2-days old FAW eggs, respectively.In the case of T. ostriniae, adult emergence ranged from 93.3 to 97.5% for all egg ages.Similarly, T. leucaniae also produced 96.3-97.1% adult emergence from 0-, 1-and 2-day-old FAW eggs.The emergence rate for T. japonicum and T. lutea varied between 85.7 to 99.1% and 98.2 to 99.4%, respectively, when exposed to various egg ages of FAW (Sun et al. 2021).Further, the emergence rate of T. dendrolimi and T. chilonis was similar on FAW eggs collected on different oviposition days and ranged between 95.4 to 98.7% and 98.3 to 99.5%, respectively (Li et al. 2023).
The developmental time of Trichogramma species is influenced by the various egg ages of lepidopteran eggs (Atashi et al. 2021).In the case of FAW, the age of eggs had a significant effect on the development time of Trichogramma parasitoids.Significant differences were observed in the developmental time of five species of Trichogramma when exposed to various egg ages of FAW (0-, 1-and 2-days old eggs).The developmental time of T. japonicum (10.6-11.7 days) and T. leucaniae (10.3-11.6 days) was increased with the age of the host eggs, while it was similar for T. ostriniae (10.2-10.5 days).In contrast, the developmental time of T. mwanzai (9.5-10.5 days) and T. lutea (10.2-11.4days) was decreased as the age of the host eggs increased (Sun et al. 2021).On FAW, the development period of T. chilonis varied between 8.40 and 10.9 days under different experimental conditions (Yang et al. 2022).Development for T. dendrolimi also varied between 8.17 and 9.5 days on FAW eggs (Li et al. 2023).Although, there was no remarkable difference in the developmental period for T. chilonis, T. dendrolimi and T. pretiosum when exposed to same age of FAW egg masses (24 h) (Yang et al. 2022).However, Li et al. (2023) recorded the shortest development time for T. dendrolimi than that of T. chilonis on FAW eggs.The developmental time for the thelytokous strain of T. pretiosum was 8.93 days (Yang et al. 2022).
Females of Trichogramma can control the sex ratio and allocate the progeny based on their wide range of host sizes (Varshney et al. 2022).During oviposition, female adults control the sex ratio and lay a femalebiased sex ratio which identifies the success of Trichogramma parasitoids in biological control (Zang et al. 2021).The FAW egg age influences the per cent female progeny of Trichogramma species that emerged from parasitized eggs batches.The variable impact of host egg age on the production of female progeny has been reported for Trichogramma species (Atashi et al. 2021).In the case of FAW, the egg age had a significant effect on the Trichogramma spp.sex ratio, although it was female-biased for all species (Sun et al. 2021).The per cent female progeny for T. mwanzai was higher in younger FAW eggs (85.1%) and declined significantly with the age of FAW egg masses (for 3-day-old eggs; 60.1%).Similarly, for T. lutea, the percentage of female progeny was 82.2% on 0-day-old eggs and reduced to 50.8% when offered 2-day-old eggs.The female progeny for T. ostriniae and T. leucaniae varied with the host egg age and ranged between 69.9 to 82.5% and 70.8 to 82.7%, respectively.Conversely, the per cent female progeny for T. japonicum was similar for 0and 2-days old eggs (63.6 and 64.45%, respectively) and was lowest on 1-day-old eggs (49.6%) (Sun et al. 2021).The percentage of females' proportion declined with generations for T. dendrolimi and was 85.13% in F1 and 76.44% in the F2 generations (Yang et al. 2022).However, the per cent female progeny was consistent in F1 and F2 generations for T. chilonis (55.47 and 60.49%) and T. pretiosum (100% in both generations) (Yang et al. 2022).Further, Li et al. (2023) did not find a significant difference in the percentage of female progeny that emerged from FAW egg masses for T. chilonis (81.4-82.1%)and T. dendrolimi (81.6-83.7%)and was comparably higher than other experimental conditions.
Host egg-size influences the oviposition and production of offspring by Trichogramma females (Jin et al. 2021).The optimum number of offspring that emerged from individual hosts adds benefits to applied biological control by avoiding super parasitism (Iqbal et al. 2020).Jin et al. (2021) reported that 0.6 to 1.6 parasitoid progeny emerged per egg from 0 to 24 h-old FAW eggs for seven species of Trichogramma.The number of emerged offspring from an individual parasitized FAW egg was similar for T. chilonis (1.0), T. ostriniae (1.2), T. confusum (1.6), T. embryophagum (1.4) and T. dendrolimi (1.5).However, the number of emerged offspring per eggs by T. pretiosum and T. japonicum at 24-48 h old FAW eggs was 1.0 and 1.1, which was higher than 0-24, 48-72 or 72-96 h of time duration.Further, Jin et al. (2021) also measured the FAW eggs approximately 0.4-0.5 mm in diameter and 0.3-0.4mm in height and the results showed that all seven Trichogramma species produced more than one offspring per FAW egg.This result suggested that an individual FAW egg accepted at least two eggs of Trichogramma and supported the emergence of both adult parasitoids.

Effect of climatic factors on Trichogramma species
Climatic variables influence the performance of parasitoids and therefore must be considered while selecting the potential strain/species for biological control (Grande et al. 2021).Temperature and humidity are the climatic variables that affect the biology and performance of Trichogramma and thereby the overall success of biological control in the fields (Tabebordbar et al. 2022).Developmental time and emergence of Trichogramma parasitoids are greatly affected by temperature.The emergence of T. pretiosum on FAW eggs was lowest at 32 °C (88.9%) and highest at temperatures 18 °C (100%) and 20 °C (99.5%) (Bueno et al. 2010).Temperature is the most critical climatic variable that affects the parasitism rate of Trichogramma (Tang et al. 2023).The parasitism rate by T. dendrolimi and T. lutea was gradually increased with temperature and then decreased with higher temperature.The parasitism rate was higher at the middle temperature at 25 and 29 °C and was lower at 21 and 33 °C at all relative humidity levels for both parasitoids (Tang et al. 2023).Among the two parasitoids, the parasitism rate by T. dendrolimi was significantly higher than that of T. lutea under different combinations of temperature and humidity.This showed that along with temperature, relative humidity was also responsible for changes in parasitism rate by parasitoids.Trichogramma dendrolimi had a better temperature and humidity adaptation than that of T. lutea.Besides, temperature and relative humidity also affected the emergence of T. dendrolimi and T. lutea (Tang et al. 2023).The developmental time of T. dendrolimi and T. lutea was decreased on FAW eggs with increasing temperature (Tang et al. 2023).The higher temperature probably increases the metabolic activity of parasitoids (Bueno et al. 2009).The shorter developmental time for T. dendrolimi was the consequence of higher temperature.Temperature and humidity also influence the sex ratio of Trichogramma species (Tabebordbar et al. 2022).The highest female ratio (94.6%) for T. dendrolimi was recorded at 33 °C and 60% humidity, while the lowest (79.5%) was recorded at 25 °C and 60% humidity.However, there was a non-significant effect of temperature and humidity regimes on the sex ratio of T. lutea (Tang et al. 2023).Further, other climatic variables such as photoperiod and precipitation can also have an impact on the biological characteristics of Trichogramma (Pratissoli & Parra 2000).

Parasitism of Trichogramma in laboratory and field cages
The parasitizing capacity of various Trichogramma spp.has been evaluated in the laboratory before testing in open field conditions (Table 3).Beserra and Parra (2004) evaluated the parasitism potential of T. atopovirilia and T. pretiosum on FAW eggs to select the most suitable species for FAW control in Brazil and found that the females of T. atopovirilia were more aggressive and showed higher affinity towards the FAW eggs with higher parasitism (48.3%) than T. pretiosum (29.7%).Further, Beserra et al. (2005) showed that T. atopovirilia parasitized 100% FAW eggs as compared to T. pretiosum (60%) under laboratory conditions.Three species of Trichogramma namely, T. atopovirilia, T. exigum and T. pretiosum were tested against the FAW and T. pretiosum (34.0%) parasitized a higher percentage of host eggs than the T. atopovirilia (28.81%) and T. exigum (12.8%) (Diaz et al. 2012).In addition, the parasitizing efficacy of field-collected T. atopovirilia and T. pretiosum was tested on FAW eggs in the laboratory and found that T. atopovirilia parasitized 70.14% of FAW eggs as compared to T. pretiosum which could only parasitize 29.23% host eggs under laboratory conditions in Mexico (Jaraleno-Teniente et al. 2020).In China, Junce et al. (2020) evaluated the parasitism potential of five native Trichogramma species on FAW eggs and the results revealed that Trichogramma dendrolimi Matsumura, T. bilingense He and Pang, T. ostriniae Peng and Chen could parasitize 20, 9.6, 1.0 FAW eggs per female wasp, respectively, and T. dendrolimi reported as superior species for FAW control.In another study, Beserra et al. (2003) compared parasitizing efficacy of 20 strains of T. pretiosum on FAW eggs and found that there was the difference in parasitizing capacity between the strains of T. pretiosum and parasitism rate varied between 80.9 and 89.3%.Generally, the parasitism potential of Trichogramma species is assessed in cages/ field cages on a larger to find out potential and suitable species/strain field releases (Chailleux et al. 2012).Beserra and Parra (2005) assessed the parasitism potential of T. atopovirilia in the field cages (1.5 × 1.1 × 1.5 m) and found that T. atopovirilia parasitized 66.24, 45.20 and 40.10% of egg masses with one, two, and three layers, respectively, without taking into account the presence or absence of scales on the egg masses.While, Diaz et al. (2012) tested the efficacy of T. atopovirilia, T. exigum, and T. pretiosum for the control of FAW in Physalis peruviana L. (Solanaceae) in microcosm and results revealed that T. atopovirilia parasitized a higher percentage of FAW eggs with lowest plant damage than other species of Trichogramma.In the field cages, Jaraleno-Teniente et al. ( 2020) found that the parasitism of T. atopovirilia and T. pretiosum did not surpass 8%.T. atopovirilia parasitized a greater number of hosts than T. pretiosum when released using one, two and three parasitoids per pest egg.The parasitism of T. atopovirilia was similar at the dose of two and three T. atopovirilia parasitoids per egg.In another field cage study in Mexico, Jaraleno-Teniente et al. ( 2021) reported that T. pretiosum could parasitize about 7.5% of the FAW egg masses.

Interaction of Trichogramma and Telenomus
Understanding the role of intraguild interactions may contribute to an effective pest management strategy when multiple natural enemies may be essential to control a target pest (Zang and Liu 2007).Before an introduction or release of natural enemies, the intraguild interaction should be well characterized, since several aspects may interfere with the results of pest control as different pest species have different arrays of natural enemies (Ksentini and Herz 2019).FAW eggs are parasitized by several species of egg parasitoids; mainly the species of the genus Trichogramma and Telenomus (Hymenoptera: Platygastridae).The simultaneous occurrence of egg parasitoids of these two genera has been already recorded on FAW eggs from various crop ecosystems (Laminou et al. 2020).In the field conditions in Brazil, Dasilva et al. (2015) reported evidence of exploitative competition among egg parasitoids of FAW in maize.Egg parasitoids, viz.T. pretiosum, T. atopovirilia and T. remus, were engaged in exploitative competition and involved in interference competition towards eggs of FAW.Telenomus remus emerged from every egg mass exposed in the field and reported that it had competitive dominance over both species of Trichogramma.Under laboratory conditions, the intraguild interaction of T. pretiosum and T. remus on FAW eggs when evaluated at different ratios displayed that only two T. remus for each eight T. pretiosum were enough to significantly increase the parasitism rate as compared to other ratios of these two egg parasitoids.However, the higher proportion of T. remus resulted in the most effective parasitism, while the sole release of T. pretiosum was less efficient than when released associated with the parasitoid T. remus (Goulart et al. 2011a).Besides, the interspecific interaction between T. pretiosum and T. remus showed when FAW eggs were earlier exposed to either parasitoid, there was no emergence of the other parasitoid.Interestingly, T. remus showed a greater parasitism rate when both parasitoid females were exposed together with host eggs.Further, both egg parasitoids could recognize host eggs previously parasitized by the other (Carneiro and Fernandes 2012).The comparative parasitism capacity of T. pretiosum and T. remus when exposed together showed that females of T. pretiosum parasitized 121.3 eggs of FAW, while T. remus could parasitize 574 eggs during adulthood with maximum parasitism in the first 24 h of exposure (Pinto and Fernandes 2020).Furthermore, the interaction between Trichogrammatoidea sp. and T. remus resulted in lower parasitism when exposed together to FAW eggs in comparison with T. remus released alone which parasitized 78% FAW eggs under laboratory conditions in Niger, Africa (Laminou et al. 2020).

Effect of egg mass scales and layers on parasitism
Parasitism of Trichogramma is influenced by several factors such as the host egg size, shape, colour and morphological defences such as host scales, hairs and spines on egg batches are the important characteristics for their survival as these insects eggs cannot move and escape from natural enemies (Dong et al. 2021).FAW egg masses often covered with protected scales constitute a physical barrier for egg parasitoids (Floater 1998).Adult female of FAW lays eggs in masses and protect them by depositing scales around and over the eggs at the time of oviposition (Fukuda et al. 2007).These scales often originate from an anal tuft at the tip of the female' abdomen (Hou et al. 2022).The thickness of scales on FAW egg masses varies with egg masses showing no scales (eggs are visible), partially covered and the egg masses being fully covered with scales (egg masses completely invisible) which maximizes the protection of eggs (Hou et al. 2022).These scales modify the oviposition behaviour of egg parasitoids and influence the control efficacy of biocontrol agents, particularly Trichogramma spp.(Hou et al. 2022).Further, the thickness of the scale layer on egg masses of FAW varied with the age of the female (Hou et al. 2022).Furthermore, the poor performance of Trichogramma females on FAW eggs is linked to their smaller body size compared to other egg parasitoids (Dong et al. 2021).
The parasitism rate of FAW eggs by T. dendrolimi varied with the scales thickness and higher parasitism on eggs (31.6%) and egg masses (78.3%) was recorded at level I (71.6 µm thickness).The egg mass parasitism on levels II (169.3 µm) and III (281.6 µm) thickness of scales was 48.2% and 23.1%, respectively.Similarly, egg parasitism was also significantly lower with 12.6% and 1.9% of eggs parasitized by T. dendrolimi on levels II and III of the scale thickness (Hou et al. 2022).Similarly, T. dendrolimi struggled to parasitize the FAW egg masses covered with scales and the average egg parasitism on eggs without scales and with scales was 22.2% and 12.9%, respectively, while T. pretiosum parasitized comparably higher percentage of host eggs and parasitism on without and with scales was 46 and 19.90%, respectively (Dong et al. 2021).However, the parasitism rate by T. pretiosum and T. dendrolimi on egg masses with scales was significantly lower than that on egg masses without scales and parasitized the eggs located on the periphery of the egg mass (Dong et al. 2021).When parasitism of T. pretiosum compared with T. atopovirilia, only 60% of T. pretiosum females were able to parasitize the FAW eggs due to difficulty in parasitizing high-scale-density eggs, while 100% of T. atopovirilia females could parasitize the FAW eggs (Beserra et al. 2005).The egg masses laid first in the first 3 days by FAW females have numerous scales which protect from egg parasitoids like Trichogramma spp.Thereafter, the thickness of scales decreased daily and such eggs with thin layers of scales (< 20.0 μm) were actively parasitized by Trichogramma spp.(Li et al. 2023).Parasitism by T. dendrolimi was 33.4% on such FAW egg masses with decreased scale covers.Besides, the Trichogramma females also parasitize eggs that are half exposed or situated at the periphery of the egg masses (Dong et al. 2021).
The number of egg layers (one to > 3 layers; 89-300 eggs) also influences the biological control efficiency of FAW using Trichogramma (Mohamed et al. 2023).Several studies have reported that some Trichogramma can only reach and parasitize upper-layered eggs, while eggs at the internal layer were difficult to parasitize (Beserra et al. 2005).When T. atopovirilia was exposed to FAW egg masses with different layers, the parasitism on single-layered eggs was higher (66.24%) than those of two (45.2%) and three egg layers (40.1%).It showed that the number of egg layers affects the parasitizing efficiency of T. atopovirilia and the control efficacy may reduce when using this species in control of FAW (Beserra and Parra 2005).Further, the females of T. pretiosum were also found hard to parasitize lower layers of FAW eggs and could parasitize the eggs on the periphery of the egg masses from the side when two layered eggs were offered for parasitism.Further, the emergence rate of T. pretiosum from single and double-layered FAW egg masses ranged from 59.7-73.1% to 36.8-57.2%,respectively (Carneiro and Fernandes 2012).In contrast, T. pretiosum parasitized 98.3% of single-layered egg masses laid on maize in Brazil (De Sa et al. 1994).
The presence of scale layers on FAW egg masses affects the parasitism by Trichogramma species as the females found it difficult to move on the scales of the egg masses, therefore they prefer to parasitize the eggs on the periphery with fewer or with no scales (Carneiro and Fernandes 2012).Besides, several studies indicated that Trichogramma species have the potential to parasitize FAW eggs without scale layers and preferably those are laid in single-layered rather than laid in overlapping layers.The variation in the thickness of scales and layered eggs of FAW explains the obvious difference in parasitism rates by Trichogramma species in the field (Hou et al. 2022).Therefore, the scales on egg masses and layered eggs of FAW should be considered wisely while understanding the interaction between FAW and Trichogramma species and the implications in the biological control of FAW (Dong et al. 2021).Further, considering the mass-rearing cost of Trichogramma and control efficiency, a variety of parasitoids can be combined and released to control FAW to overcome the proportions of egg mass covered with scales and laid in overlapping layers at the field level.

Mass rearing of Trichogramma species for augmentative biocontrol
Mass rearing is a crucial step to achieve the success of biological control or/and to support IPM which involves the production of millions of insects to control the insect pests (Parra 2010).The success of biological control programmes depends on the scale of mass production and the cost efficiency of mass production.Several stages of mass rearing process such as the collection of host eggs, cleaning of egg, drying of host eggs, sterilization of eggs, collection of parasitized host eggs, parasitoid inoculation, preparation of egg cards etc. have been mechanized to reduce the cost of production (Wang et al. 1999).However, the cost efficiency of mass rearing majorly depends on the low-cost and quality factitious hosts for rearing the selected biocontrol agent.Historically, Trichogramma spp.have been mass-reared on several factitious hosts, with the earliest practical attempt for rearing of Trichogramma on Angoumois grain moth, Sitotroga cerealella (Oliver) (Lepidoptera: Gelechiidae) (Flanders 1927).Most successful Trichogramma mass rearing methods are developed using large-size host eggs as large-size eggs are considered suitable for mass rearing.However, small eggs are also used for Trichogramma mass rearing and they are useful for rearing Trichogramma species having a weak ovipositor and mouthparts (Zang et al. 2021).For the control of FAW, Trichogramma spp.are mass-reared on different factitious hosts in the native range and invaded regions (Table 4).Sitotroga cerealella, Ephestia kuehniella (Lepidoptera: Pyralidae) and Corcyra cephalonica (Lepidoptera: Pyralidae) are among the most widely used factitious hosts for mass rearing of Trichogramma spp.across the world (Zang et al. 2021).In Brazil, the eggs of S. cerealella previously used in mass rearing are found nutritionally poorer as well as smaller than eggs of A. kuehniella (Parra et al. 2014).For this reason, eggs of A. kuehniella are the most suitable factitious host for Trichogramma rearing in Brazil, as several species had a lower acceptance and parasitism rate on eggs of S. cerealella (Gomes & Parra 1998).In China, the successful rearing method of Trichogramma was adopted using the host species showing large-size eggs such as Antheraea pernyi which enabled commercial production of T. dendrolimi and T. chilonis (Zang et al. 2021).Besides, the small eggs of C. cephalonica and S. cerealella are highly suitable for mass rearing of T. japonicum and T. ostriniae, in China (Zang et al. 2021).In Brazil, the rearing cost of 100,000 parasitoids on A. kuehniella eggs required for a one-hectare area is US$8-10, adding a cost of US$2-3 for application via drone in the fields (Parra and Coelho 2022).Further, the cost of T. pretiosum wasp and its commercial release was US$15.76 per hectare for a single release to control FAW in Brazil (Figueiredo et al. 2015).The rearing cost of 20,000 parasitoids of T. chilonis on C. cephalonica eggs is US$0.34, in India (Lalitha et al. 2023).In China, the production of 2 billion Trichogramma yearly using C. cephalonica as a factitious host costs about US$0.43 million and using A. pernyi as a host has enabled the production of 400 billion T. dendrolimi per year for a cost of US$11.4 million in production facilities at Jilin province (Zang et al. 2021).Furthermore, the cost of 100,000 Trichogramma adults reared on the Chinese artificial diet was approximately $0.06 (Greenberg et al. 1998).In addition, the mass rearing of factitious host E. kuehniella is costly and the market price is in the range of 600-800 EUR/kg in Europe (Vandekerkhove and De Clercq 2010).In the United States, the production cost of S. cerealella eggs was US$2.43 for 100,000 eggs (Greenberg et al. 1998).The rearing cost of C. cephalonica on a broken sorghumbased diet was US$0.27 for the production of 20,000 eggs in India (Lalitha et al. 2023).Currently, some commercial companies are producing 10-20 kg of A. kuehniella egg per day, with total quantities of up to 30-40 kg eggs per day with a labour cost representing 70-80% of total cost of production in Brazil (Parra and Coelho 2022).Thus, automation is necessary to produce adequate insects under well-controlled temperatures with knowledge of degree-day requirements to maximize the production of factitious hosts and parasitoids (Coelho et al. 2016).

Trichogramma release strategies
Several approaches have been adopted for Trichogramma release worldwide (Parra et al. 2015).Amongst, the inundative release method has been the most adopted method for Trichogramma release programmes (Zang et al. 2021).Besides, the mixed species releases have been also explored to increase the effectiveness of biological control using Trichogramma parasitoids (Zang et al. 2021).In the inundative release, a large number of parasitoids are released for pest control which basically act as a biopesticide (Zang et al. 2021).To release parasitoids in the field, several release techniques have been used in biological control programmes which include-the release of parasitized eggs glued on cardboard just before adult emergence, the release of adult or late pupal stages that are very close to adult emergence (Pinto et al. 2003), or parasitized eggs encased in cardboard capsules (Parra et al. 2015).
In recent times, an inventive method of release has been deployed which uses capsules containing Trichogramma parasitized eggs (Zang et al. 2021).With the advent of new technologies, the use of unmanned aircraft systems (UAS) in the form of drones have been deployed to release the Trichogramma parasitized eggs of E. kuehniella with fine vermiculite for control of maize and forest pests (Martel et al. 2021).
For biological control of FAW, the inundative releases of Trichogramma spp.have been adopted for field release.In Brazil, Figueiredo et al. (2015) released T. pretiosum adults after 24 h of adult emergence using glass containers in the morning hours in maize fields against FAW.These parasitoids were released in the centre of each plot by opening the containers for adult dispersal.Furthermore, Varshney et al. (2021) used T. pretiosum parasitized C. cephalonica eggs glued-on paper cards that were cut into small pieces and stapled on the lower side of the upper part of the maize leaf.These small pieces were uniformly dispersed in the maize field when at least 5% adult emergence (pharate) was observed.In China, eggs of C. cephalonica glued on paper cardboard parasitized by Trichogramma spp.and anticipated to emerge within two days were placed on maize leaves to target egg masses of FAW (Jin et al. 2021).The release of Trichogramma through parasitized egg on cards/cardboard is the most commonly adopted technique at the field level.However, this method has some limitations, as these parasitized eggs prone to predators and exposed to extreme weather conditions that influence the field efficacy of parasitoids.Thus, the protection of these wasps is important to ensure maximum parasitization in the field.Over time field release method has been upgraded (e.g.encased in a capsule) to improve the efficacy of parasitoids and protection from extreme weather conditions.

Integration of Trichogramma with other egg parasitoids
Integration of Trichogramma species with other parasitoids, such as Telenomus remus Nixon could be effective in controlling the FAW.T. remus can penetrate the protective scale covering of FAW egg masses and parasitize the eggs (Fortes et al. 2023).This parasitoid also has the potential to parasitize multi-layered FAW eggs, indicating their ability and effectiveness by creating an efficient avenue for parasitism (Goulart et al. 2011b).Besides, the natural occurrence of multiple parasitoids on FAW is effective in controlling pests (Kenis et al. 2022).Notably, more than one species of Trichogramma can occur and parasitize the same FAW eggs mass in the same field (Jaraleno-teniente et al. 2020).In addition, the parasitism of multiple egg parasitoids, such as T. pretiosum, T. atipovillia and T. remus was also observed from a single FAW egg mass in a maize field (Dasilva et al. 2015).This co-occurrence indicated that T. remus emerge from all parasitized FAW egg masses, while Trichogramma species alone did not emerge from host egg masses.Although, Carneiro and Farnandes (2012) found no evidence of the emergence of T. pretiosum from FAW eggs previously parasitized by T. remus.This suggests that T. remus can recognize the host eggs previously parasitized by Trichogramma spp.Moreover, the larger size, robust body, and ability to penetrate the inner layers of FAW eggs explain its dominance over Trichogramma species (Lacerda et al. 2023).However, the parasitism to egg masses without physical scales and single layered and also partial parasitism to layered eggs (periphery of layered eggs) suggest the implication of Trichogramma species along with T. remus in the biological control of FAW.No doubt, T. remus is the most effective egg parasitoid of FAW, but it is a serious challenge to find a suitable laboratory host for economic mass-rearing (Li et al. 2023).On the contrary, rearing on FAW makes it difficult due to its highly cannibalistic nature and also large-scale rearing is not affordable in terms of time and resources (Li et al. 2023).Further, the rearing of T. remus using C. cephalonica as an alternate host is debatable (Chen et al. 2021), with few reports indicating its feasibility (Queiroz et al. 2017a, b).Thus, large-scale production of T. remus, even on its potential host, Spodoptera litura (Fabricius) is challenging as compared to Trichogramma production.In addition, rearing of Trichogramma on the factitious host is easy and less expensive which reduces the cost of biological control (Zang et al. 2021).Integration of Trichogramma species with T. remus could be effective, by releasing T. remus in the early stage of egg masses with thick scales and Trichogramma subsequently to achieve better control and reduction in overall cast (Li et al. 2023).

Field efficacy of Trichogramma spp. in biological control
Various species of Trichogramma have been released for biological control of FAW in the native as well as in the invaded region worldwide.The inundative release method was followed in most of the cases by releasing these egg parasitoids in large numbers in the various crops for FAW control.Loya (1978) conducted a field experiment in Mexico with mass releases of an unspecified Trichogramma sp. to control FAW and recoded low parasitism level (0.56%) in the parasitoid-released field as compared to the unreleased field (0.18%).In contrast, Montoya (1980) recorded parasitism 45-55% egg parasitism by the release of Trichogramma spp. in the maize fields during different years in Veracruz, Mexico.While, Peralta et al. (1981) reported the average parasitism by Trichogramma sp. on FAW eggs was less than 15% in Tamaulipas, Mexico.Besides, the field release of 30,000 individuals of Trichogramma sp. in maize fields resulted in only 4% egg parasitism.The lower parasitism by Trichogramma sp.owing to a higher number of brachypterous (deformed wings in adult) individuals was responsible for the decrease in the dispersal capacity of the parasitoid and also other factors such as predation by ants and impact of rainfall together affected parasitism (Toonders and Sanchez 1987).In Southern Brazil, two weekly releases of T. pretiosum at 200,000 adults per hectare from a week old plant onwards in maize fields recorded a 63% reduction in plant damage after the second week onwards as compared to non-released fields (Martinazzo et al. 2007).In addition, when T. pretiosum was applied one, two and three times at 100,000 per hectare resulted in 69.8, 79.2 and 68.7% egg mass parasitism, respectively, in Brazil.Three parasitoid releases recorded the lowest maize leaf damage (1.35 leaf damage score on 0-5 score) in the plot as compared to two (1.51 leaf damage) and one (1.53 leaf damage) release.Overall, the inundative field releases of T. pretiosum increased organic maize productivity by 19.4% (Figueiredo et al. 2015).
FAW invasion in Africa and subsequently in Asia also invited the need for biological control, as the pest settled in maize, sugarcane and other crop ecosystems.Field efficacy of four species of Trichogramma, viz.T. chilonis, T. ostriniae, T. confusum and T. pretiosum, were investigated and found that all the species were effectively parasitized FAW egg masses in the field trails and the parasitism rate on the egg masses ranged from 61.5 to 87.5% in China.The field release of T. ostriniae.T. pretiosum, T. confusum and T. chilonis recorded plant damage rates were 36.1, 38.8, 51.9, and 59.7%, respectively, as compared to the non-released field (95.6% plant damage).In addition, parasitoid releases reduced the FAW larval density from 0.83 to 0.43 larvae/plant as compared to the non-released field (1.37 larvae/plant) suggesting these four species are viable biocontrol candidates for FAW in Hainan, China (Jin et al. 2021).In India, four field releases of T. pretiosum at 50,000 per hectare at weekly intervals during the early stage to 60 days old maize crop recorded reduction in FAW egg masses up to 76.25 and 71.64% in winter and rainy seasons, respectively.The reduction in egg masses was evident after the second release of the T. pretiosum in the maize (Varshney et al. 2021).The presence of scales/hairs over the egg masses act as a barrier against parasitism by Trichogramma spp.This difficulty can be overcome by using a more aggressive parasitoid, capable of breaking the physical barrier imposed by scales on the eggs.T. remus has proved very effective in South America and Florida to minimize the egg layers problems progressively superimposed on each other.Thus, a combined release of Trichogramma and T. remus could add a synergistic effect in egg parasitism, which may increase the efficacy of the biological control of FAW.Moreover, Trichogramma spp.parasitizes the upper layer of FAW egg masses, whereas T. remus can parasitize lower layers, thus, there may be a complementary effect towards egg parasitism (Bueno et al. 2023).Although both parasitoids may compete for FAW eggs from the upper layers, yet a complementary effect can still be expected (Bueno et al. 2023).Furthermore, the more frequent releases of wellfed parasitoids at the pupal or adult stages are crucial in achieving increased egg parasitism rates in the fields.In addition, parasitoid releases should be conducted based on moth catches in pheromone traps with intervals of three days, as the average incubation period of FAW eggs is three days (Cruz et al. 1999).This synchronization is crucial for successful parasitism by the Trichogramma species in fields for FAW management (Bueno et al. 2023).
Trichogramma species are generalist egg parasitoids of lepidopterans, concerns exist about possible detrimental effects of inundative releases on non-target native hosts and beneficial lepidopterans used for biological control of weeds (Paraiso et al. 2013).Trichogramma species reported that parasitizing FAW eggs are also known to parasitize the eggs of other lepidopteran pests (Navik et al. 2019).The inundative releases of T. pretiosum are widely adopted to control FAW in corn in Brazil (Parra et al. 2015) and T. chilonis in Asia against noctuid pests (Jin et al. 2021), both parasitoids have a wider host range (Jalali et al. 2017).Nevertheless, the Trichogramma species tend to show a strong preference for certain hosts, plants, and climatic conditions (Hassan and Gou 1991).Given that, Trichogramma are likely to have a limited host range in particular climatic conditions, decreasing the risk for non-target impacts (Orr et al. 2000).Babendreier et al. (2003a) reported the minimal (1% parasitism) impact of inundative releases of Trichogramma brassicae Bezdenko on non-target butterflies including species endangered species in Switzerland and suggested the cautious release of parasitoids (Babendreier et al. 2003b).Similarly, the non-target impacts of inundative release of Trichogramma nubilale Ertle and Davis on endangered and threatened were minimal (Andow et al. 1995).While Trichogramma fuentesi Torre was not advocated for biological control of Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) due to a moderately high level of risk to native non-target cactus moth, Melitara prodenialis Walker (Lepidoptera: Pyralidae) which share the same habitat (Paraiso et al. 2013).Consequently, the specific research on the non-target impact of inundative releases of Trichogramma species used in biological control of FAW is still limited and research on this subject is urgently needed in the near future.

Conclusion and prospects
FAW is a serious pest of cereals and other economic crops in native and invaded regions of the world.None of the single methods is sufficient to control the pest spread in new territory and keep infestations below the economic injury level.Moreover, the egg parasitoids provide an opportunity but not below economic injury level as indicated by the low range of field parasitism to control pests at their egg stage before their larvae hatch out and cause crop damage.Of the various Trichogramma species, T. pretiosum, T. mwanzai and T dendrolimi have been mass-reared and exploited for augmentative biological control of FAW.In addition, several species of Trichogramma have been found naturally parasitized FAW eggs and also tested for laboratory efficacy.Furthermore, the integration of potent species of Trichogramma with other key parasitoids, such as T. remus or/and potential native parasitoids and predators in biological control or IPM strategies to achieve effective FAW control.As Trichogramma parasitoids find difficulty in penetrating egg mass covered with scales or deposited in multi-layered, it becomes essential to combine with other platygasterid parasitoids that can penetrate through scales and parasitize inner layers of FAW egg mass.Further, the integration of Trichogramma with safer and selective pesticides such as green pesticides that are considered environmentally friendly to habitats, and to the ecosystem and conservation of other natural enemies could help in sustainable and environment-friendly FAW control.However, there is a need to investigate more on release time, release rates, number of releases to be made and what stages of Trichogramma to be taken for field releases to maximize the efficacy of biological control or IPM strategies.Further, investigations are needed on release methods, mass rearing protocols and their validation at the field level, determined area-wise so that farmers can release Trichogramma parasitoids effectively to control FAW.

Table 2
Natural parasitism of Trichogramma species on fall armyworm eggs rate was observed for T. ostriniae to the tune of 87.5%.This study reported that the females of T. chilonis, T. ostriniae, and T. confusum prefer to parasitize the FAW eggs within 0-24 h-old, while females of T. chilonis, T. ostriniae, and T. confusum prefer 0-72 h-old FAW eggs.

Table 3
Parasitism of Trichogramma species under laboratory, cage/greenhouse or field conditions used for fall armyworm control

Table 4
Rearing of Trichogramma species for control of fall armyworm in different regions