Validation of a novel bioassay method for the evaluation of the activity of slow-acting insecticides on tomato borer Phthorimaea absoluta.
TL;DR
The results indicate that the toxic action of insecticides may vary significantly depending on their pharmacological properties and the pest's developmental stage, and incorporating long-term exposure protocols into the validated IRAC methods is strongly recommended for slow-acting compounds.
Abstract
Background
Phthorimaea absoluta is one of the most devastating pests of tomato crops worldwide, with its control relying heavily on chemical insecticides. Assessing the risk of control failure and resistance levels is critical for evaluating insecticide efficacy. The globally adopted bioassay protocol is Insecticide Resistance Action Committee (IRAC) method 022 (IRAC 022), which estimates mortality after a 3-day exposure. However, this short-term method may underestimate the effectiveness of slow-acting insecticides. Recently, the insecticides azadirachtin and spirotetramat were evaluated using both the IRAC 022 method and a modified version of this protocol that allows for prolonged exposure intervals up to pest pupation (12 days after treatment (DAT)) and adult emergence (27 DAT). A susceptible and a wild resistant population from Greece were used in the method evaluation.
Results
For azadirachtin, a more than 100-fold difference in the median lethal concentration (LC50) was observed between 3 DAT and 12 DAT, with minimal changes thereafter up to adult emergence. In contrast, spirotetramat showed at least a ten-fold difference between 3 DAT and 12 DAT, and up to a 17-fold difference between 12 DAT and 27 DAT, demonstrating delayed and stage-specific activity. These results indicate that the toxic action of insecticides may vary significantly depending on their pharmacological properties and the pest's developmental stage.
Conclusion
Incorporating long-term exposure protocols into the validated IRAC methods is strongly recommended for slow-acting compounds. By combining both short- and long-term bioassays, the true efficacy profiles of existing and novel insecticides can be more accurately assessed - an essential factor in developing evidence-based pest management strategies for P. absoluta. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.