Developing a method to test the repulsive effects of pesticidesfor the parasitic wasp Aphidius rhopalosiphi

Abstract: Methods recommended to test the side effects of plant protection products onbeneficial arthropods are mainly based on the assessment of lethal effects after a prolongedcontact with a treated surface, inert or natural. The effects of this exposure on reproduction,parasitisation ability or feeding capacity of the surviving organisms are usually followed to detectany sublethal effects. For most of the species, these assessments are performed on untreatedunits. There is however an increasing interest to also detect sublethal effects in the presence ofthe pesticide to take into account possible interference of the pesticide residues on the ability ofbeneficial to control pests.The aim of this work was to develop a methodology to detect possible repulsive effects ofplant protection products on the aphid parasitic wasp Aphidius rhopalosiphi and their possibleimpact on aphid parasitisation and aphid parasitoid efficacy. In a first test, adult wasps wereconfined on exposure units with two glass plates, one treated and the other one left untreated, inorder to identify products to be used in a second set of tests: a product with a strong repellenteffect and a lack of direct toxicity to the wasp. a candidate. Five different products were testedand two of them, Stop-Insect and Baïa were considered as repulsive, with significantly morewasps found on the untreated glass plate compared to the treated one.A second test was performed by confining mated female wasps on plants treated with theinsecticidal soap Stop-Insect. The experiments were realized in parallel on plants infested withaphids, where mortality, position of the wasps and parasitisation efficiency were assessed ontreated plants and on plants treated with sucrose, according to the classical method recommendedfor Aphidius, where mortality and position of the wasps were assessed on treated plants andparasitisation efficiency later on untreated plants. Results were indicating the repulsive effects ofthe insecticide with plant infested with aphids, with 30% less wasps observed on plants and 30%less aphid mummies produced. With the plants treated with sucrose, no differences betweencontrol and insecticide were observed. Stop-Insect has also an slight effect on the survival ofA. rhopalosiphi on aphid infested plants with 19.0% corrected mortality, while no effects weredetected on sucrose treated plants.These results are indicating that a plant protection product, because of its possiblerepellence, can reduce the activity of a parasitic wasp and its efficacy in controlling aphids,despite an apparent “harmless” profile.