Studies to identify an alternative aphid host for culturing the predatory syrphid, Ischiodon scutellaris (Fabricius) (Diptera: Syrphidae)

Abstract

Background

Aphidophagous syrphid, Ischiodon scutellaris (Fabricius) (Diptera: Syrphidae) is an effective predator of aphids that infest many agricultural and horticultural crops. Cowpea aphid, Aphis craccivora Koch (Hemiptera: Aphididae), is the natural host for culturing syrphid predator, I. scutellaris. The present study was conducted to evaluate the feeding and development of I. scutellaris on seven species of aphid hosts viz., A. craccivora, Aphis fabae Scopoli, Aphis gossypii Glover (from cotton), Schoutedenia emblica (Patel & Kulkarni), Brevicoryne brassicae (L.), Aphis gossypii (from okra) and Hyperomyzus carduellinus (Theobald) and to identify an alternative aphid host to maintain the culture of I. scutellaris during the situation of non-availability/loss of the culture of A. craccivora.

Results

Aphis fabae was not accepted for feeding by the larvae of I. scutellaris. The larvae of I. scutellaris did not survive on S. emblica. The descending order of feeding potential of aphids by I. scutellaris was A. craccivora > H. carduellinus > B.brassicae > A. gossypii (from cotton) > A. gossypii (from okra) > S. emblica. The larva of I. suctellaris completed its development in combination of the aphid prey (A. craccivora + H. carduellinus). The net reproductive rates (R₀) of I. scutellaris when fed on A. craccivora and H. carduellinus were 308.28 and 302.33, respectively. The intrinsic rate of increase (r) of I. scutellaris was significantly higher when reared on H. carduellinus (0.155 ± 0.003) than on A. craccivora (0.143 ± 0.004).

Conclusion

The results confirmed that H. carduellinus reared on Lactuca virosa could serve as an alternate host for the mass production of I. scutellaris.

Background

Aphids are a major group of sucking pests that cause economic damage by sucking sap from different plant parts like stems, leaves, flower buds, and fruits in several agricultural and horticultural crops (Shylesha et al. 2006; Thakur et al. 2009; Bashir et al. 2013). Syrphids are very important predators of many species of economically important aphids (Chambers et al. 1983). Ischiodon scutellaris (Fabricius) (Syrphidae: Diptera) is an efficient predator of different species of aphids like Aphis craccivora Koch (Joshi et al. 1999), Brevicoryne brassicae (L.) (Sharma and Bhalla 1988), Lipaphis pseudobrassicae (Davis) (Devjani and Singh 2006), Rhopalosiphum maidis (Fitch) (Singh and Mishra 1988) and Toxoptera aurantii (Boyer de Fonscolombe) (Radhakrishnan and Muraleedharan 1993). Hitherto aphid, Aphis craccivora was the only natural host used for the rearing of the larvae of I. scutellaris (Joshi et al. 1998).

To maintain the culture of I. scutellaris, there is a need for year-round maintenance of A. craccivora cultured on cowpea plants. The unavailability of aphid culture to feed the developing syrphid larvae leads to starvation and loss of culture. In the event of the non-availability of A. craccivora, there is a need to identify alternate aphid hosts for maintaining the culture of I. scutellaris. There is a single greater reliance on the cowpea or other pulse crops cultivated in the fields which are infested by A. craccivora to initiate the culture. Many a time it is difficult to ensure the availability of the culture of A. craccivora during the monsoon season. The rainy days negatively impact the population establishment of sucking pests like aphids, whiteflies, and mealybugs (Villanueva 2019). The natural parasitization by parasitoids under field conditions is an additional factor that directly causes the loss of an active culture of A. craccivora (Amin et al. 2009).

Hence, there is a need to identify another suitable aphid host for rearing and maintaining the culture of I. scutellaris rather than depending solely on A. craccivora as a natural host. The present study was conducted to identify another suitable aphid host that could be effectively multiplied on a host plant to serve as a supplement to maintain the culture of I. scutellaris during the non-availability of the natural host, A. craccivora. Although the predatory potential of I. scutellaris on different species of aphids has been well investigated across the globe, there has been no published literature on the identification of any alternate host in addition to A. craccivora for long-term maintenance of the live culture. In the present study, the development of I. scutellaris on different species of aphid hosts was compared and evaluated. The development of larva and adult and population growth parameters of I. scutellaris on A. craccivora were compared with those on other species of aphids so as to identify an alternative host for maintaining and mass rearing of I. scutellaris.

Methods

Study site

The study was carried out at the experimental farm of the National Bureau of Agricultural Insect Resources (NBAIR) of ICAR in Bengaluru, Yelahanka Campus (13.096932N, 77.56759E) which is located in the heart of the rapidly growing capital city of the south Indian state of Karnataka.

Maintenance of stock culture of A. craccivora

Cowpea seeds were sown in earthen pots (20 × 20 cm) at the rate of four seeds per pot and maintained in the greenhouse. The plants after germination were watered regularly. The live culture of A. craccivora was collected from the field-infested cowpea plants. Three to four leaves of cowpea infested with A. craccivora were hand collected and placed gently over the healthy cowpea plants raised in the earthen pots. The settlement and colonization of the aphids was monitored on a daily basis. This served as the stock culture of A. craccivora for the entire set of experiments.

Maintenance of plants for oviposition studies

Another set of cowpea seeds was sown in small-sized plastic pots (10 × 10 cm) at a rate of two seeds per pot to serve as plants for oviposition by the adult flies. The germinated plants after attaining the five-leaf stage were inoculated with aphids. These plants have been monitored for the colonization of inoculated aphids on daily basis. This set of plants has been used in experiments on the oviposition of adult flies.

Culture of the syrphid, Ischiodon scutellaris

Cabbage crop was raised at the experimental farm of the ICAR-NBAIR Yelahanka campus. The crop was regularly monitored for syrphids’ predators. The larvae of I. scutellaris were collected from the aphid-infested leaves using a fine hair brush in a circular plastic rearing container (9 × 4 cm) with fine mesh fitted at the lid for aeration. The collected larvae were brought to the laboratory and supplied with A. craccivora ad libitum till pupation. The pupae were collected separately and kept for the emergence of adult flies.

Rearing of adult syrphid flies

After the emergence of the adult flies, they were released inside a wooden cage (30 × 30 × 30 cm). Honey solution (50%) swabbed on cotton was placed in a Petri plate as a carbohydrate source for the adults. Pollen grains collected from the flowers of castor plants were placed in a dish inside the cage. In addition to the pollen and honey, cowpea plants infested with A. craccivora were placed inside the cage for oviposition by adult flies. Cowpea plants with aphids were checked on a daily basis for the presence of eggs laid by I. scutellaris. Plants were replaced every alternate day for oviposition. The plants taken out of the cage were placed inside a plastic container with a lid fitted with mesh for proper aeration. The hatchlings were monitored regularly. First instar larvae upon hatching were collected using a camel hair brush and released into the circular plastic rearing container (9 × 4 cm) with a fine mesh fitted at the lid for aeration and provided with cowpea leaves infested with A. craccivora on a daily basis for its feeding and development. This served as a stock culture of syrphid predator, I. scutellaris.

Aphid host suitability studies for the rearing of I. scutellaris larvae

Host suitability of seven different aphid species viz., Aphis craccivora from cowpea, A. gossypii (from cotton), A. fabae from S. nigrum, S. emblica from Phyllanthus sp, A. gossypii (from okra) and H. carduellinus from Lactuca virosa (wild lettuce) for the rearing of I. scutellaris was evaluated. From the stock culture of I. scutellaris, the freshly hatched first instar larva was collected gently using a fine camel hair brush and released inside the circular rearing dish (9 × 4 cm) fitted with mesh at the top for aeration. Ten replicates each with one larva per replicate was maintained. The number of aphids consumed (only emptied exoskeleton) was counted on a daily basis in each treatment. A fresh supply of host aphids (100 mixed stages of aphids/day) in each treatment was also supplied on a daily basis. The duration of larva, pupa and pupal weight of I. scutellaris when reared on five aphid host species viz., A. craccivora, A. gossypii from cotton, H. carduellinus, A. gossypii from okra and B. brassicae were recorded. Since the larvae of I. scutellaris failed to accept A. fabae for feeding and did not survive on S. emblica, the development of I. scutellaris was recorded only from the other five aphid prey, which was accepted by the syrphid larvae. The survival percentage of the feeding stage i.e., larvae of I. scutellaris was also recorded in each treatment.

Development of I. scutellaris reared on a combination of aphid species

Out of the five aphid prey species evaluated, the most preferred aphid in terms of the number of aphids consumed per day was H. carduellinus. It was used for further studies. The experiment was conducted with the aim to meet the situations of nonavailability or sudden loss of standard natural host, A. craccivora during the rearing of I. scutellaris. In this experiment, the larvae of I. scutellaris were reared on a combination of aphid preys viz., A. craccivora and H. carduellinus. Five replicates were maintained and the experiment was repeated twice. Newly hatched first instar larvae of I. scutellaris at the rate of one larva per replicate were collected from the stock culture and supplied with A. craccivora for 1–3 days and H. carduellinus for 4–6 days. The number of aphids consumed per day and the duration of the larval and pupal stages were recorded. The survival percentage of the larvae was also recorded when reared on two different aphid preys.

Fecundity of I. scutellaris reared on A. craccivora and H. carduellinus

A mated pair of the newly emerged adult flies of I. scutellaris was released inside a wooden rearing cage (30 × 30 × 30 cm) and supplied with cotton swabbed with a honey solution for feeding. Honey bee pollen (approximately 1 g) was given in a small bowl inside the cage. Two treatments viz., cowpea plant grown in tiny pots at 4–5 leaf stage infested with A. craccivora and Lactuca virosa plant grown in tiny pots at 4–5 leaf stage infested with H. carduellinus were maintained. A uniform density of both aphid species (100 nymphs of each aphid host) was maintained inside the cages. Ten replicates were maintained for this study. Freshly aphid-infested plants were supplied for the adults for oviposition on alternate days. The plants with aphids that were exposed to the flies for oviposition were taken out from the cage and separately maintained outside the cage and observed on daily basis for the hatching of the eggs. The pre-oviposition, the number of ovipositional days and total fecundity were recorded. The rate of hatching of the eggs laid by I. scutellaris was calculated by taking the ratio of the number of hatched eggs with the total number of eggs laid in both treatments. The population growth parameters (net reproductive rate R₀, intrinsic rate of increase r, finite rate of increase λ and mean generation time T_G) (Birch 1948) were worked out using the following formulae:

• 1. Net reproductive rate, R₀ = Ʃlxmx

  where lx indicates the number of individuals surviving to age x and mx indicates the number of female offspring per female of age x.

• 2. Mean generation time, T_G = Ʃxlxmx/Ʃlxmx

• 3. Intrinsic rate of increase, r = log R₀/T_G, where R₀ indicates net reproductive rate and T_G indicates mean generation time.

• 4. Finite rate of increase, λ = e^r

Data analysis

Analysis of variance (GLM in SAS 9.3; SAS Institute, Cary, NC) was used to compare the feeding preference and development of larvae and adults of I. scutellaris in different species of aphids. Where significant difference was detected, treatment means were separated using Tukey’s HSD Test (0.05%).

Results

Aphid host suitability studies for rearing the larvae of I. scutellaris

The influence of seven different species of aphids on the development of larvae of syrphid, I. scutellaris was studied (Fig. 1). The larva of I. scutellaris did not prefer to be fed on A. fabae. A significant difference in the number of aphids consumed by the larva of I. scutellaris per day was recorded across the six different species of aphids (F value = 9.21; df = 5.45; P < 0.0001). Amongst the remaining six different aphids, the mean number of A. craccivora (50.05 aphids/day) consumed by I. scutellaris was the highest, which was statistically on par with the consumption of H. carduellinus (50.20 aphids/day), B. brassicae (43.08 aphids/day), A. gossypii (42.10 aphids/day) and A. gossypii from okra (21.45 aphids/day). The lowest rate of consumption by the larva of I. scutellaris was recorded on S. emblica (3.55 aphids/day).

Fig. 1

Feeding potential of the larva of Ischiodon scutellaris on different aphid hosts. Note: Among all the species of aphids studied, Aphis fabae was not accepted by the syrphid, I. scutellaris hence not included for statistical analysis

There was a significant difference in the larval duration of I. scutellaris reared on different species of aphids (F value = 4.04; df = 5.45; P < 0.0001) (Table 1). The duration of the larval stage of I. scutellaris was the lowest on A. craccivora (4.40 ± 2.50 days), which was statistically on par with that of H. carduellinus (5.60 ± 0.55 days). The larval duration of I. scutellaris fed with A. gossypii was (6.20 ± 0.44 days) which was on par with A. gossypii from okra (7.20 ± 0.84 days) and B. brassicae (7.00 ± 0.71 days). There was a non-significant difference in the duration of the pupal stage of I. scutellaris reared on different species of aphids. The pupal period of I. scutellaris ranged between 4.80 ± 0.81 to 7.00 ± 0.71 days across the different aphid hosts.

Table 1 Development of Ischiodon scutellaris on different aphid hosts

There was a significant difference in the pupal weight of I. scutellaris reared on different species of aphids (F value = 3.53; df = 5.45; P < 0.0001). The highest weight of pupae of I. scutellaris was recorded when the larvae were reared on B. brassicae (27 mg), which was statistically on par with that on H. carduellinus (23 mg), A. craccivora (19 mg) and A. gossypii (19 mg). The lowest weight of pupae of I. scutellaris (9 mg) was when reared on A. gossypii from okra.

The larval survival rate of I. scutellaris differed significantly when reared on the different aphid hosts (F value = 3.51; df = 5.45; P < 0.0001). The larvae of I. scutellaris reared on A. craccivora, A. gossypii, H. carduellinus and B. brassicae recorded a 100% rate of survival. A significantly lower rate of survival of I. scutellaris was observed when fed on A. gossypii from okra (56.40%).

Development of I. scutellaris reared on a combination of aphid species

In the combination of the aphid species feeding experiment, the mean larval and pupal duration of I. scutellaris was 6.80 ± 0.84 and 5.60 ± 0.55 days (Table 2). The average pupal weight was 27.60 ± 0.55 mg. There was 100% survival of the larvae of I. scutellaris when reared on a combination of the two aphid species viz., A. craccivora from cowpea and H. carduellinus.

Table 2 Development of Ischiodon scutellaris on combination of aphid hosts

Fecundity of I. scutellaris reared on A. craccivora and H. carduellinus

Adult fecundity and population growth studies were recorded in cowpea plants infested with A. craccivora and L. virosa infested with H. carduellinus (Table 3). There was a non-significant difference in the pre-ovipositional period and the number of ovipositional days of adult flies of I. scutellaris when fed on both the aphid hosts. The mean pre-ovipositional period of I. scutellaris in A. craccivora and H. carduellinus were 4.40 ± 0.55 and 4.20 ± 0.84 days, respectively. The ovipositional period of I. scutellaris was recorded for a time span of 23.80 ± 2.38 and 26.41 ± 1.81 days in A. craccivora and H. carduellinus hosts, respectively. Fecundity of I. scutellaris differed significantly (F value = 17.91; df = 2.18; P < 0.0001) with respect to the two aphid hosts viz., A. craccivora (353.40 ± 8.56 eggs/female) and H. carduellinus (323.81 ± 13.08 eggs/female). There was a non-significant difference in the hatching rate of eggs laid by adults of I. scutellaris in the colonies of A. craccivora (80.20 ± 3.49%) and H. carduellinus (82.80 ± 4.21%).

Table 3 Comparison of reproductive parameters of Ischiodon scutellaris on the two different aphid hosts

Population growth parameters of I. scutellaris reared on A. craccivora and H. carduellinus

Based on the adult ovipositional studies, the population growth parameters viz., net reproductive rate, intrinsic rate of increase, mean generation time and finite rate of increase of I. scutellaris worked out on two aphid hosts; A. craccivora and H. carduellinus (Table 3). There was a non-significant difference in the net reproduction rate (R₀) between A. craccivora (308.28) and H. carduellinus (302.33). The mean generation time (T_G) of I. scutellaris differed significantly (F value = 20.79; df = 2.18; P < 0.001) between the two aphid hosts; A. craccivora (16.74 ± 0.35 days) and H. carduellinus (16.02 ± 0.04 days). The values of the intrinsic rate of increase (r) of I. scutellaris differed significantly (F value = 22.10; df = 2.18; P < 0.001) between the two aphid hosts A. craccivora (0.143 ± 0.004) and H. carduellinus (0.155 ± 0.003). The finite rate of increase (λ) of I. scutellaris varied significantly (F value = 48.69; df = 2.18; P < 0.001) between the two aphid hosts H. carduellinus (2.02 ± 0.03) and A. craccivora (1.97 ± 0.03).

Discussion

Among the seven different species of aphids evaluated in descending order of preference of hosts for feeding by I. scutellaris were A. craccivora > H. carduellinus > B. brassicae > A. gossypii (from cotton) > A. gossypii (from okra) > S. emblica. Some species of aphids viz., A. fabae were not preferred by I. scutellaris for feeding. A. fabae was seen colonizing the unripe green berries of Solanum nigrum in greater numbers with few nymphs in leaves. The green berries of S. nigrum were reported to contain Solanine, a glycoalkaloid (Kanteh and Norman 2015). Aphids have been reported to sequester host plant alkaloids/allelochemicals for their own defence against their natural enemies (Desneux et al. 2009). A. fabae may have sequestered the alkaloids during the process of feeding on S. nigrum, which later may have resulted in I. scutellaris rejecting A. fabae as a host. The larvae of I. scutellaris consumed a significantly lower number of S. emblica per day. Similarly, most plants belonging to the genus Phyllanthus were reported to contain alkaloids, flavonoids, terpenes and lignans (Seeff et al. 2013). The larvae of I. scutellaris reared on S. emblica were observed to die on the third day after feeding. The dead larvae appeared shrunken and turned pale yellowish brown. The authors also observed the natural oviposition of syrphid predators in the colonies of all studied species of aphids, except in A. fabae and S. emblica. They also attribute the reason for non-preference/underdevelopment of the larvae of I. scutellaris on A. fabae and S. emblica might be due to the sequestration of chemical constituents in the host plants by the respective aphid species.

The developmental duration of larvae and pupae was not significantly influenced by the aphids’ species on which they were fed. However, the larvae of I. scutellaris reared on A. gossypii from okra recorded the least pupal weight. This could be directly correlated with larval feeding activity. Compared to other aphid hosts evaluated, the number of A. gossypii from okra consumed per day by the larvae of I. scutellaris was relatively lower resulting in the decreased pupal weight of I. scutellaris than other species of aphids. The critical weight of the life stage of a fly is directly dependent on the quality and quantity of the larval diet (Davidowitz and Nijhout 2004). The observations on the mean pupal weight of I. scutellaris reared on A. craccivora in the present study (19.0 mg) is in conformity with that recorded by Joshi et al. (1999) when reared on A. craccivora. The survival percentage of I. scutellaris fed on A. gossypii from cotton was observed to be better than that of A. gossypii from okra. The effect of the host plant may be one of the possible reasons for the increased survival rate and pupal weight of I. scutellaris when fed on A. gossypii from cotton compared to A. gossypii from okra. Similar observations were recorded by one of the co-authors where a relatively higher mortality rate of larvae of I. scutellaris was observed when reared on A. craccivora from Glyricidia maculata than A. craccivora from cowpea (Joshi et al., Personal communication).

The results of the combination of aphids feeding experiment concluded that the growth and development of I. scutellaris were not affected by the change in host aphid species during its course of development. This is very crucial in the rearing of I. scutellaris as there could be a sudden loss of the natural host, A. craccivora due to coccinellid predation. Also, sudden loss of colonies of A. craccivora due to the parasitization by Aphidius colemani Viereck. a major parasitoid of A. craccivora as reported by Rakhshani et al. (2005). The results indicated that during the situations of non-availability of A. craccivora, to avoid the disruption in the rearing of I. scutellaris, the larvae could be fed alternately when fed on H. carduellinus.

The egg-hatching rate of I. scutellaris was slightly higher on H. carduellinus in L. virosa than A. craccivora in cowpea. The reason could be that the cowpea plant tends to lose its turgor pressure at a faster rate than L. virosa which might have resulted in a slightly higher egg-hatching rate of the eggs laid in the colonies of H. carduellinus in L. virosa. Cowpea plants at the five-leaf stage showed a tendency to droop even when maintained with a proper supply of moisture in the root zone, unlike L. virosa. The maintenance of the lengthy and sturdy nature of L. virosa plant compared to cowpea might be also a reason for the increased number of ovipositional days. Life table parameters are viable measures to estimate population development, survival and reproduction (Price 1997). The net reproductive rate (R₀) is a factor that indicates the stability of the population (Ning et al. 2017). The non-significant difference in R₀ values in the present study, when I. scutellaris was reared on A. craccivora and H. carduellinus indicated that the syrphid culture could be maintained on both the aphid species. The value of the intrinsic rate of increase (r) indicates the population growth of the organism under study. Insects with shorter pre-ovipositional periods were reported to have a high r-value (Lewontin 1965). In the present study, the r-value of I. scutellaris was significantly higher on H. carduellinus. The pre-ovipositional period of I. scutellaris on both the aphid hosts viz. A. craccivora and H. carduellinus was statistically on par with each other. The mean generation time (T_G) of I. scutellaris was significantly shorter on H. carduellinus than A. craccivora. Lower generation time recorded on H. carduellinus indicated that it is a suitable host for the rearing of I. scutellaris. The authors infer that the host plant turgidity could be one of the probable reasons for higher T_G values in A. craccivora. The leaf branches of cowpea were less sturdy than L. virosa. T_G values are the indicator of the survival rate and fecundity, host plant stature might also be a factor of concern that resulted in lower values of T_G in I. scutellaris reared on H. carduellinus in L. virosa plants. The finite rate of increase and intrinsic rate of increase was higher on H. carduellinus though generation time was lower on H. carduellinus. The reproductive rate of I. scutellaris was comparable on both A. craccivora and H. carduellinus. This showed that I. scutellaris could be a more effective predator while preying on H. carduellinus compared to A. craccivora. However, the finite rate of increase values differed marginally between the two aphid hosts (2.02 in H. carduellinus and 1.97 in A. craccivora), which implies that the predatory syrphid, I. scutellaris could be multiplied on both the aphid hosts.

Conclusions

Active colonization of aphid, H. carduellinus was recorded throughout the year on Lactuca virosa. Natural oviposition by I. scutellaris in the colonies of H. carduellinus was observed in the study area. The host plant propagation through seeds of L. virosa so as to ensure host plant availability for multiplication of H. carduellinus throughout the year was studied.

Considering the various biological parameters like a greater number of aphids consumed per day, shorter larval/pupal duration and higher r- value in H. carduellinus, it was concluded that it is another potential alternative natural aphid host apart from A. craccivora for successful culturing and maintenance of the predatory syrphid, I. scutellaris.