Persistent Toxicity of Different Insecticides to Trichogramma and Bracon on Guava Leaves

An experiment was conducted at BCKV, Kalyani to study the persistent and residual toxicity of 8 insecticides to the egg parasitoids Trichogramma japohicum Ashmead and larval parasitoid Bracon hebetor Say following standard bioassay techniques and PT values were worked out in each case to identify relatively safer insecticides to formulate a guideline regarding introduction of natural enemies in pesticide treated guava orchard. Endosulfan, quinalphos, monocrotophos and methomyl (@ 0.05%) and cypermethrin, alpha cypermethrin, deltamethrin and fluvalinate (@ 0.005%) were sprayed on guava crop just before flowering. The insecticides differed considerably in their persistence, period of efficacy and index of persistent toxicity (PT) towards all the parasites. Tolerance of T. chilonis was much more than T. japonicum irrespective of the pesticide. Alpha cypermethrin was found to be safer in all cases followed by deltamethrin. Considering the retention period of toxic residues, both the species of Trichogramma could be released in the crop ecosystem after 3–5 days for alpha cypermethrin and fluvalinate, 4–6 days for endosulfan, monocrotophos and deltamethrin spray and 7–8 days of methomyl, quinalphos and cypermethrin spray. In case of Bracon each insecticide persisted for a lesser period than that of Trichogramma.