Toxicity of two neonicotinoid insecticides via the food chain for larvaeof the two spot ladybird Adalia bipunctata


Abstract: The toxicity by ingestion of aphids contaminated by imidacloprid and thiacloprid wasassessed in the laboratory for the larvae of the two spot ladybird Adalia bipunctata. Observedmortality was recorded during the larval and pupal development as well as the time needed toreach adult stage. Adults were then assessed for any adverse effects on reproduction.Aphids used for the first set of experiment were placed in plastic petri dishes and directlytreated with the insecticides at the recommended rate using a Spray-Tower apparatus. Highmortality was rapidly observed and 100% mortality of the larvae fed with contaminated aphidswas obtained after only 3 days of feeding with imidacloprid and 4 days with thiacloprid. For thesecond set of experiments, dwarf bean plants infested with aphids were treated outside with theinsecticides using a knapsack sprayer connected to a sprayer ramp with flat-fan nozzle, as used inopen field. Thiacloprid did not lead to significant mortalities with only 5% corrected mortality.Imidacloprid was more toxic, with 39% corrected mortality. No effect on adult development andfertility was observed.For the last set of experiments, aphids were reared on dwarf bean units treated by irrigationwith the two insecticides. This system was used to avoid direct contact between the aphids andthe insecticides and assess the toxicity by ingestion of aphids only contaminated by feeding onplants treated with systemic insecticides. The doses applied to the units were determined, on basisof preliminary experiments, to limit aphid development of 50%, 75% and 90% after one weekcompared to control. Thiacloprid had no significant effects on larval survival, development timeand adult fertility. Imidacloprid has significant effects at the highest rate tested on larval survivaland development time with a corrected mortality of 25.7% and 20.7 days to reach adult stageinstead of 19.6 days for the control. The two other rates had no significant effects.These results showed that ingestion of contaminated prey could have toxic effects onladybird larvae, but the effects are clearly related on how the food was contaminated.

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