Contact toxicity of selected insecticides against adult females of Trichogramma chilonis (ISHII) (Hymenoptera: Trichogrammatidae) reared on Corcyra cephalonica (stainton) under laboratory conditions

Introduction

The egg parasitoid Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) occupies a pivotal position in the biological control of economically important lepidopteran pests, including sugarcane borers, cotton bollworms, and vegetable caterpillars. Its amenability to mass production on factitious hosts, particularly the rice moth Corcyra cephalonica (Stainton), has made it the cornerstone of augmentative biological control programs across several countries, including India (Manjunath, 1993; Smith, 1996) [5,13] . The parasitoid's short life cycle, high fecundity, and host-searching efficiency make it an ideal candidate for large-scale inundative releases (Scholler et al., 2005) [11] . Integrated Pest Management (IPM) aims to unify safe and sustainable chemical and biological control methodologies. However, the non-judicious and prophylactic application of broad-spectrum insecticides has precipitated numerous ecological backlashes, including the development of insecticide resistance in target pests, pest resurgence, toxic residues in food commodities, and the destruction of non-target beneficial arthropods such as parasitoids and predators (Armes et al., 1992; Shukla and Jadhav, 2014) [1,12] . Given that approximately 30 per cent of agricultural produce in India is lost to pests, farmers often resort to excessive pesticide applications, inadvertently compromising the very biological control agents that form the backbone of IPM.

For the successful integration of T. chilonis into pest management strategies, it is imperative to ascertain the safety profile of routinely applied insecticides to this parasitoid. The side effects of pesticides on biocontrol agents must be quantified under controlled laboratory conditions before their recommendation in IPM schedules (Finney and Fisher, 1964) [4] . While several new-generation insecticides are marketed as "target-specific," their compatibility with minute hymenopteran parasitoids remains equivocal and demands empirical validation. Against this backdrop, the present investigation was undertaken with the specific objective of evaluating the contact toxicity of selected conventional and new-generation insecticides against adult females of T. chilonis reared on C. cephalonica eggs under laboratory conditions.

Materials and Methods

Experimental site and climatic conditions

The experiments were carried out during 2020-21 in the Bio-control Laboratory, Department of Agricultural Entomology, College of Agriculture, Sardar Vallabhbhai Patel University of Agriculture and Technology, Meerut, Uttar Pradesh, India. The laboratory was maintained at 27 ± 2 °C and 65 ± 5% relative humidity (RH).

Source and rearing of insect cultures

The culture of Trichogramma chilonis was procured from the Bio-control Laboratory, Uttar Pradesh, and reared on UV-irradiated (15 W for 1 hour) eggs of Corcyra cephalonica in glass vials (15 × 7.5 cm) placed in a BOD incubator at 27 ± 2 °C and 65 ± 5% RH. The parasitized eggs turned black 3-4 days after parasitization. Newly emerged adults were provided with a 50 per cent honey solution as food and were subsequently used for the toxicity assays.

Insecticides tested

Six commercially available insecticides and an untreated control were evaluated for their contact toxicity to T. chilonis. The details of the insecticides, their formulations, trade names, and recommended doses are presented in Table -1

Contact toxicity bioassay on adult females

The contact toxicity was evaluated using a residual exposure method. Transparent plastic containers (7.5 cm diameter × 7 cm height) with tight-fitting lids were converted into pesticide-testing units. Small windows were cut on two sides of each container and sealed with fine muslin cloth using waterproof adhesive to prevent fumigant action and to ensure adequate aeration. The inner surfaces of the containers and lids were uniformly sprayed with 2.5 ml of the respective insecticide solution using a manual atomizer. In the control treatment, containers were sprayed with distilled water. All treated containers were air-dried thoroughly for 10 minutes to evaporate the carrier solvent. Twenty newly emerged, mated female wasps were released gently into each treated container. A fine streak of 5 per cent honey solution was provided inside the container as adult food. The containers were then sealed and kept under laboratory conditions. Mortality observations were recorded at 12, 24, 36, and 48 hours after treatment (HAT). The experiment was laid out in a Completely Randomized Design (CRD) with three replications for each treatment. Wasps that remained motionless and did not respond to a gentle probing with a camel hair brush were considered dead.

Per cent adult mortality = (Number of dead adults / Total number of adults exposed) × 100

Statistical analysis

The percentage mortality data were subjected to angular transformation (arcsine √percentage) prior to statistical analysis to homogenize the variance. The transformed data were analyzed using Analysis of Variance (ANOVA) appropriate for a Completely Randomized Design. Critical differences (CD) at the 5 per cent level of significance and standard errors of means (SEm±) were computed to compare the treatment means.

Results

The contact toxicity data revealed significant differences in adult mortality among the six insecticides and the untreated control at all observation intervals (Table 2). The progression of mortality over time indicated that all synthetic insecticides were toxic to T. chilonis females, although the degree of toxicity varied markedly between molecules.

Mortality at 12 HAT

Mortality at 48 HAT and Mean Toxicity

At 48 HAT, cartap hydrochloride (97.69%), fipronil (97.28%), and bifenthrin (96.36%) caused near-complete mortality, leaving virtually no survivors. Imidacloprid (87.21%), chlorantraniliprole (79.27%), and thiamethoxam (79.28%) were relatively less lethal at this terminal observation, though their absolute toxicity had also risen sharply.

Considering the mean mortality across all observation intervals, the overall toxicity ranking from least to most toxic was: thiamethoxam (44.40%) < imidacloprid (52.78%) < chlorantraniliprole (53.78%) < cartap hydrochloride (69.50%) < bifenthrin (74.05%) < fipronil (77.31%). The mean mortality in the untreated control was only 4.56%.

Discussion

The present investigation unequivocally establishes that all six tested insecticides exert significant contact toxicity to adult females of Trichogramma chilonis, albeit with differential intensity. Among the tested molecules, the neonicotinoid thiamethoxam 25 WG emerged as the least toxic, registering the lowest mean mortality of 44.40%. This finding aligns with Renuka and Sawai (2020) [9] , who reported thiamethoxam to be selective towards different life stages of T. japonicum, and with Singh et al. (2018) [7] , who observed comparatively moderate effects of certain neonicotinoids on trichogrammatid emergence. The comparatively lower contact toxicity of thiamethoxam may be attributed to its differential binding affinity to nicotinic acetylcholine receptors in Hymenoptera vis-à-vis its systemic mode of action, which reduces direct contact exposure on treated surfaces. Imidacloprid 17.8% SL, another neonicotinoid, recorded a mean mortality of 52.78%, placing it in the moderately toxic category in the present study. These results corroborate the findings of Saha et al. (2017) [10] and Nirvesh Singh et al.

(2018) [7] , who reported imidacloprid as having low to moderate toxicity to immature stages and adult emergence of T. chilonis. However, its escalating toxicity by 48 HAT (87.21%) suggests that prolonged residual exposure can severely compromise parasitoid survival, necessitating careful timing of its application in the field. Chlorantraniliprole 18.5% SC, an anthranilic diamide, caused a mean mortality of 53.78%. Deshmukh et al. (2018) [3] reported chlorantraniliprole as harmless to Trichogramma spp. when irradiated host eggs were exposed, but slightly harmful on unirradiated eggs under the IOBC classification.

Our direct contact bioassay on adults indicates a moderate level of toxicity, suggesting that while the molecule is relatively safe to immature parasitoids developing inside host eggs, free-living adult wasps are more vulnerable to direct topical or residual contact. This distinction is critical for field integration, as adult wasps foraging on treated foliage are at higher risk. Cartap hydrochloride 50 SP exhibited high mean mortality (69.50%), with mortality exceeding 97 per cent by 48 HAT. This indicates that thiocarbamate insecticides, though often considered less persistent than organophosphates or pyrethroids, possess substantial acute contact toxicity to minute parasitoids. Such high acute toxicity necessitates avoidance of cartap in habitats where T. chilonis releases are planned. Bifenthrin 10% EC, a synthetic pyrethroid, inflicted a mean mortality of 74.05%. Pyrethroids are well-documented broad-spectrum neurotoxins with high toxicity to parasitoids and predators (Bhargavi, 2015; Patra and Samanta, 2017) [2,18] . The rapid knockdown effect and strong residual activity of bifenthrin evidently render it highly detrimental to T. chilonis. Similarly, fipronil 5% SC was the most toxic treatment overall, causing 77.31% mean mortality and exceeding 97 per cent mortality by 48 HAT. These results are in close agreement with Mohapatra and Shinde (2021) [6] , who categorized fipronil as "harmful" to T. japonicum females, causing cent per cent mortality under contact toxicity trials, and with Wang et al. (2012) [15] , who identified fipronil as one of the most toxic insecticides to T. chilonis based on sublethal concentration values. The phenylpyrazole mode of action of fipronil, which disrupts GABA-gated chloride channels, appears to be particularly lethal to parasitic Hymenoptera.

The time-course mortality data revealed that toxicity was generally cumulative, with the sharpest increase observed between 36 and 48 HAT for most treatments. This suggests that even brief residual exposures can lead to delayed mortality, a factor often underestimated in rapid screening assays. The significant disparity between the safest (thiamethoxam) and the most toxic (fipronil) molecules underscores the importance of molecule selection in IPM scheduling. For broad-spectrum insecticides like fipronil and bifenthrin, strict restraint around parasitoid release windows is warranted, whereas molecules such as thiamethoxam and chlorantraniliprole may offer a wider compatibility margin, provided applications are timed to minimize adult wasp exposure.

In the context of the Indian agricultural landscape, where T. chilonis trichocards are widely distributed for management of cotton bollworms and sugarcane borers, the indiscriminate co-application of highly toxic insecticides can nullify the economic investment in biological control. Therefore, the present findings advocate for the incorporation of parasitoid safety data into pesticide recommendation protocols. Future studies should expand this evaluation to include sublethal effects on parasitization efficiency, host-searching behavior, progeny sex ratio, and the safety of these insecticides to immature stages developing within parasitized host eggs to fully characterize their compatibility.

Conclusion

The laboratory screening of six commercially important insecticides against adult females of Trichogramma chilonis revealed significant variation in contact toxicity. Thiamethoxam 25 WG was identified as the comparatively safest molecule among the tested insecticides, with the lowest mean adult mortality (44.40%). Imidacloprid 17.8% SL and chlorantraniliprole 18.5% SC were moderately toxic. Conversely, fipronil 5% SC and bifenthrin 10% EC were highly toxic, causing 77.31% and 74.05% mean mortality, respectively, and should be regarded as incompatible with T. chilonis in integrated management programs. Cartap hydrochloride 50 SP also exhibited high toxicity (69.50%). These findings emphasize that the selection of insecticides in IPM regimes must be predicated not only on efficacy against target pests but also on rigorous safety assessments for key biological control agents. The adoption of comparatively safer molecules, coupled with temporal separation of chemical and biological control interventions, will be pivotal in realizing the synergistic potential of IPM.

References

1. Armes NJ, Jadhav DR, Band GS, King ABS. Insecticide resistance in Helicoverpa armigera (Hub.) in South India. Pesticide Science. 1992;34:355-364.
2. Bhargavi M, Naik KV. Effect of low temperature storage of trichocards parasitised by Trichogramma chilonis (Ishii) and Trichogramma japonicum (Ashmead). Asian Journal of Bio Science. 2015;10(1):43-47.
3. Deshmukh YV, Undirwade DB, Dadmal SM. Effect of some newer insecticides on parasitisation by Trichogramma species under laboratory condition. Journal of Entomology and Zoology Studies. 2018;6(3):228-231.
4. Finney GL, Fisher TW. Culture of entomophagous insects and their hosts. In: DeBach P, editor. Biological Control of Insect Pests and Weeds. New York: Reinhold Publishing Corporation; 1964. p. 84.
5. Manjunath TM. Qualitative and quantitative analysis of techniques leading to economic mass-production of Corcyra cephalonica Stainton (Lepidoptera, Pyralidae). PhD thesis. Bangalore: University of Agricultural Sciences; 1993. 130 p.
6. Mohapatra B, Shinde CU. Contact toxicity of different insecticides against egg parasitoid, Trichogramma japonicum Ashmead under laboratory condition. Journal of Biological Control. 2021;28(3):147-150.
7. Singh N, Agrawal N, Chandra U, Kumar K. Efficacy of novel insecticides and botanicals against parasitization of Trichogramma chilonis. International Journal of Current Microbiology and Applied Sciences. 2018;Special Issue 7:4548-4552.
8. Patra S, Samanta A. Toxicity of some new and conventional insecticides on Trichogramma chilonis Ishii and Bracon brevicornis Wesmael under laboratory condition. Journal of Environmental Biology. 2017;38.
9. Renuka DP, Sawai HR. Evaluation of safety of newer insecticides to immature stages of Trichogramma japonicum. International Journal of Chemical Studies. 2020;8(3):784-789.
10. Saha S, Kumar PS, Bhowmik S, Talukder B. Toxicity of some pesticides to two important parasitoids of lepidopteran tissue borers. International Journal of Current Microbiology and Applied Sciences. 2017;6(7):2415-2421.
11. Scholler M, Flinn PW, Grieshop MJ, Zdarkova E. Biological control of stored product pests. In: Insect Management for Food Storage and Processing. 2nd ed. St. Paul (MN): American Association of Cereal Chemists; 2005.
12. Shukla A, Jadhav DS. Biology of Coccinella transversalis (Fabricius) on different aphid species. The Bioscan. 2014;9(1):17-22.
13. Smith SM. Biological control with Trichogramma: advances, successes and potential of their use. Annual Review of Entomology. 1996;41:375-406.
14. Sudhanan EM, Krishnamoorthy SV, Kuttalam S. Toxicity of flubendiamide 20 WG against egg parasitoid, Trichogramma chilonis (Ishii) (Hymenoptera: Trichogrammatidae) under laboratory conditions. Journal of Biological Control. 2014;28(3):147-150.
15. Wang DS, He YR, Guo XL, Luo YL. Acute toxicities and sublethal effects of some conventional insecticides on Trichogramma chilonis (Hymenoptera: Trichogrammatidae). Journal of Economic Entomology. 2012;105(4):1157-1163.