MSc.

The egg parasitoid Trichogramma chilonis Ishii is widely distributed in many crop ecosystems throughout the country. Its inundative field releases have achieved an appreciable success in the suppression of the population density of rice leaf folder. But the potential of T. chilonis can be severely curtailed by the application of insecticides and hence it is essential to identify the insecticides that are compatible with this species. In this context, the present study entitled ‘Impact of novel insecticides on Trichogramma chilonis Ishii (Hymenoptera : Trichogrammatidae)’ was carried out in the Department of Agricultural Entomology, College of Horticulture during 2012-13 to evaluate the lethal effects of insecticides on the egg parasitoid.
Five new generation insecticides viz., buprofezin 25 SC, chlorantraniliprole 18.5 SC, flubendiamide 480 SC, spinosad 2.5 SC and thiamethoxam 25 WG were tested for their toxicity to T. chilonis in the laboratory by adopting the standardised protocols developed by the International Organisation for Biological Control of Noxious Plants and Animals / West Palaeractic Region Section (IOBC/WPRS). The selected newer insecticides are being used in rice ecosystem and have replaced the old conventional insecticides. Hence the acute contact toxicity to adults, dosage-mortality response, detrimental effects to pre-imaginal stages of the parasitoid inside the host eggs and persistent toxicity of the five selected insecticides at field recommended concentrations were investigated.
All the insecticides tested revealed different degrees of acute contact toxicity to the adults of T. chilonis. The recommended concentration of thiamethoxam and spinosad showed highest toxicity by causing 68.42 % mortality at 24 hours after treatment to adult parasitoid. Chlorantraniliprole indicated lowest mortality (50.00%) to the adult wasps.
Median lethal concentration (LC50) of each insecticide was determined. Thiamethoxam indicated highest toxicity to T. chilonis requiring the lowest concentration to cause 50% mortality (0.0011 mg a.i. l-1) to the parasitoid. Chlorantraniliprole was safe to T. chilonis with the highest LC50 value of (1.3860 mg a.i. l-1). Based on risk quotient (the ratio between field recommended concentration and LC50 of the beneficial insects) only chlorantraniliprole was found to be ‘harmless’ to T. chilonis while thiamethoxam was ‘dangerous’ whereas buprofezin, flubendiamide and spinosad were ‘slightly to moderately toxic’ to T. chilonis.
The effect of insecticides on the immature stages egg, larva and pupa along with the impact on the second generation of T. chilonis was also studied. All insecticides significantly reduced parasitisation when T. chilonis eggs were exposed to insecticides. Thiamethoxam caused highest reduction (39.23%) in parasitism while chlorantraniliprole reduced parasitism by 9.50%. Adult parasitoid emergence was also reduced when eggs were exposed to insecticides. Thiamethoxam and spinosad were significantly ‘slightly harmful’ while buprofezin, flubendiamide and chlorantraniliprole were ‘harmless’ to the eggs of T. chilonis.
Spinosad was found to be ‘moderately harmful’ to the larval stage while other insecticides were ‘slightly harmful’ to T. chilonis except chlorantraniliprole which caused 27.89% reduction in adult emergence over control and was thus ‘harmless’.
Exposure of T. chilonis pupae to insecticides caused a significant reduction of adult emergence. Thiamethoxam was most toxic to pupal stage followed by spinosad and buprofezin. Buprofezin, chlorantraniliprole and flubendiamide were ‘harmless’ to pupae of T. chilonis.
Parasitisation efficiency and adult emergence of adult parasitoids emerging from insecticide treated egg, larva and pupa of T. chilonis were studied. The adults emerging from thiamethoxam treated eggs caused 15.00% reduction in parasitism while adult emergence was reduced by 36.13%. Thus thiamethoxam was found to be ‘slightly harmful’ whereas all other insecticide treatments (buprofezin, chlorantraniliprole, flubendiamide and spinosad ) were ‘harmless’. The progenies emerging from chlorantraniliprole treated larvae of T. chilonis resulted in highest rate of parasitisation (62.50%) and were harmless whereas all the other tested insecticides were ‘slightly harmful’. All the insecticide treatments were found to be ‘harmless’ to the second generation when the pupae were exposed to insecticides.
All the tested insecticides were ‘slightly harmful’ for adult emergence when the factitious host eggs were exposed to insecticides.
With respect to persistent toxicity of insecticides to T. chilonis, it was observed that thiamethoxam had the longest persistence up to 21 days and was rated as ‘moderately persistent’according to IOBC ranking whereas buprofezin, chlorantraniliprole and flubendiamide were rated as ‘short lived’.
Among the five insecticides tested, chlorantraniliprole was the safest and thiamethoxam was most toxic to T. chilonis at field recommended concentration. Therefore, potential toxicity of insecticides to parasitoids are be considered before application in rice ecosystem. Let us hope that chemical control and biocontrol will join hands to make Integrated Pest Management a successful venture.