Side-effects of azadirachtin on the predatory bug Orius laevigatus under controlled conditions

Abstract: In 2023 and 2024, two trials were carried out on potted bell pepper plants to evaluate the side-effects of an azadirachtin-based insecticide on the predatory bug Orius laevigatus. In particular, the studies reported herein aimed at investigating: 1. whether a dose-response effect in the effects of the insecticide on nymphal development of O. laevigatus exists; 2. whether side effects on the offspring of an artificially introduced and not yet established O. laevigatus population vary depending on when the insecticide is applied, i.e. before or after the release of O. laevigatus adults. In both trials, the azadirachtin-based product was tested at half the recommended, the recommended, and twice the recommended field rate, but at different timings of application. While in 2023 the insecticide was applied 2 days before the release of O. laevigatus adults and consequent egg-laying on the plants, in 2024 the insecticide was applied 2 days after the release of the predator, when eggs had already been laid. Untreated plants acted as control. At the expected end of the nymphal development period (approx. 2 weeks after the release of O. laevigatus adults), the number of living nymphs per plant was counted, and percent reduction in nymph numbers compared to an untreated control was determined. When the azadirachtin-based insecticide was applied before the release of O. laevigatus adults, a clear dose-response effect emerged, with the insecticide being harmless at half the recommended rate, slightly harmful at the recommended rate, and moderately harmful at twice the recommended rate. On the contrary, when adults were released before the insecticide spray, the reduction in nymph numbers was high and comparable at all tested rates. These findings show the importance of proper timing of insecticide sprays and release of beneficials.