Effect of intercropping on the parasitoids, Encarsia spp. and Trichogramma spp. in cotton fields, India

Intercropping in cotton is of great significance, which helps to reduce the population of insect pests by attracting natural enemies to the field and helping to get stabilized yield and high profit advantage. The study was conducted with 4 intercrops, viz., sesame, Sesamum indicum L.; pigeonpea, Cajanus cajan L.; pearl millet, Pennisetum glaucum L.; and sorghum, Sorghum bicolor L. in an experimental area during 2016 and 2017 autumn season. The objective was to study the population of parasitoids influenced by the intercrops in cotton. This is a triangular relationship between the host, parasitoids, and environment. As Encarsia spp. (Hymenoptera: Aphelinidae) and Trichogramma spp. (Hymenoptera: Trichogrammatidae) are the main parasitoids of insect pests of cotton, thus the study was undertaken on these parasitoids. The results revealed that the highest parasitization of whitefly, Bemisia tabaci (Gennadius) pupae by Encarsia spp., was recorded in cotton-pigeonpea 1:1 (33.22%), which was at par with cotton-pigeonpea 2:1 (32.16%), whereas the second highest one was in cotton-sesame 1:1 (27.33%), while the lowest parasitization was recorded in the control (16.16%). On the activity of Trichogramma spp., the significant highest percent was recorded in cotton-sesame 1:1 (10.93%), while no activity was recorded in cotton-pigeonpea, cotton-sorghum, cotton-pearl millet, and control treatments.


Background
Cotton, Gossypium arboreum L., is known as the queen of fibers, and it is the most important commercial crop for raw material needs of the textile industry. Although, it is attacked by several insect pests causing drastic reductions in quality and quantity (Dhaliwal et al. 2006). To mitigate the losses caused by insect pests, farmers still rely on chemical pesticides as they considerably control the pests, but injudicious use of pesticides has resulted in harm to non-targeted beneficial organisms and the environment (Patil et al. 2017). So the use of insecticides for the control of the pests has been highly criticized, and therefore, it is forced to switch from insecticides to ecofriendly approaches such as intercropping and cultural control. Intercropping is the agronomic practice of growing two or more crops in the same field at the same time (Andrews and Kassam 1976;Theunissen 1997). Crop diversification or intercrops provide pollen, nectar, and honeydew as alternate non-prey foods to the adult stages of the parasitoids in adverse conditions (Landis et al. 2000;Gurr et al. 2003;Sharma et al. 2009;Lessando 2019). Parasitic wasp belonging to genus Encarsia (Hymenoptera: Aphelinidae) has a number of species and well known parasitoid of the silver leaf whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), with a wide distribution worldwide (Qiu et al. 2003). These are recognized as one of the most important natural enemies of B. tabaci having generated a lot of interest in countries, where B. tabaci is a problem. They oviposit under the host and develop in a vital capsule within the host (Antony et al. 2003). Adult parasitoids preferentially oviposit within the third and the fourth instar nymph of whitefly, and the greatest rate of development occurs when the third instar whitefly nymphs are parasitized (Gerling and Fried 2000). The genus Trichogramma Westwood (Hymenoptera: Trichogrammatidae) has the largest number of species in the family Trichogrammatidae. They are used worldwide and having the ability to parasitize eggs of many economic lepidopterous pests of cultivated plants (Hassan 1993;Smith 1996;Mills 2010). They have been used to treat millions of hectares of agricultural crops and forests against diverse insect pests (Li 1994), and it is reported that increasing the diversity within crops provided a greater number of opportunities for Trichogramma to survive in agricultural systems (Ram et al. 2002). Thus, there is a need to document the effect of habitat manipulation using intercrops along with main crop in sustaining and supporting the parasitoids and subsequently diminish the outbreaks of insect pests.
The present investigation was undertaken to study the effect of intercropping on the abundance of parasitoids, Encarsia spp. and Trichogramma spp. in cotton crop.

Material and methods
The studies were conducted at the experimental area of the Department of Entomology, CCS Haryana Agricultural University (29.14°north latitude and 75.70°east longitude with an altitude of 215 m above mean sea level), Hisar, Haryana, India, during 2016 and 2017.

Treatments
Cotton variety, HD-432, was sown on 14 May and 11 May during 2016 and 2017, respectively. Four intercrops were taken, i.e., sesame (Sesamum indicum L.; variety: HT-1), pigeonpea (Cajanus cajan L.; variety: Paras), pearl millet (Pennisetum glaucum L.; variety: HHB-67i), and sorghum (Sorghum bicolor L.; variety: HC-171). All intercrops were sown in the second fortnight of July in both the years. The plot size was of 22.65 m 2 composed of 6 rows of cotton of 4.8 m length, with a spacing of 67.5 cm between the rows and 30 cm between the plants. There were 9 treatments ( Table 1). The treatments replicated thrice in a randomized block design (Fig. 1, layout plan of experiment). All recommended cultural practices were applied until harvest, without applying chemical control in the experimental areas.

Encarsia spp.
Parasitization of whitefly pupa by Encarsia spp. was recorded in all the treatments which started from the 1st week of July to the end of September at 10 days intervals. Thirty leaves from 10 plants (i.e., 3 leaves per plant) in each plot were plucked randomly to record parasitization of whitefly pupae. These leaves were collected in polythene bags, transferred to the laboratory, and examined under the binocular microscope. Black pupae of whitefly were considered parasitized. Parasitized and healthy pupae of whitefly were counted on the leaves. Percent parasitization was calculated as the parasitized (black) pupae divided by the total number of pupae × 100.

Trichogramma spp.
The activity of Trichogramma spp. was studied by tagging the eggs of rice moth, Corcyra cephalonica (Stainton) (Lepidoptera: Pyralidae) in the field of cotton in all the treatments starting from the 3rd week of July at 15 days intervals. For this study, about 30 fresh and sterilized eggs of rice moth were glued on the paper strip (5.08 × 2.54 cm). In each plot, 5 such strips of rice moth eggs were tagged on 5 randomly selected plants. These strips were collected from the field after 2 days of exposure in the field. The strips were kept in the laboratory under observation, and parasitization of eggs was recorded using a binocular microscope by observing the presence of black eggs. Percent parasitization was calculated by the parasitized (black) eggs divided by the total number of eggs × 100.

Statistical analysis
Data were analyzed, using the analysis of variance (ANOVA), and means were separated, using DMRT (Duncan's multiple range test) with SPSS 19 software.

Results and discussion
Encarsia spp.
The results of intercropping on the parasitization by Trichogramma spp. was studied under the field conditions. The eggs of rice moth, Corcyra cephalonica, were tagged in each plot, and parasitization were recorded after 2 days of exposure in the field. The significantly highest (F = 44, df = 98) parasitization was recorded in cotton + sesame 1:1 (10.93%), followed by cotton + sesame 2:1 (6.13%), and cotton + sesame 3:1 (3.73%), whereas no parasitization was recorded in the rest of the treatments ( Table 2). The present study was in accordance with Ram et al. (2002) who reported that the parasitization of the eggs of til hawk moth, Acherontia styx L. (Lepidoptera: Sphingidae), on sesame crop in large numbers and the eggs of A. styx were quite big from which more than 20 Trichogramma adults emerged from a single egg. Similarly, Lester and Furr (1972) recorded the parasitization by Trichogramma in sesame crop. Amala and Shivalingaswamy (2018) and Zhu et al. (2015) also recorded a higher activity of Trichogramma in the diversified system. However, the present findings showed that there was no parasitization by Trichogramma in cotton-sorghum and pearl millet treatments, but Scholz and Parker (2004); Hegde et al. (2003); Romeis et al. (2003); and Díaz et al. (2012) recorded the activity of Trichogramma in sorghum crop. The difference could be due to the climatic condition and crop cultivation practices of the region where the experiment was conducted.

Conclusion
It is concluded from the studies that the activity of Encarsia spp. and Trichogramma spp. were found to be greater in cotton intercropped with pigeonpea or sesame. Provisioning the parasitoids with intercrops/border crops as pigeonpea and sesame will help in sustaining their population in the agricultural habitats to enhance biological control of insect pests. Habitat manipulation strategies using intercrops should be integrated with the farm landscape in a spatial and temporal way to benefit parasitoids and predators for natural biological control of insect pests.