The impact of thiamethoxam on bumble bee broods (Bombus terrestris L.) following drip application in covered tomato crops

The side effects of the neonicotinyl compound thiamethoxam on the brood of bumble bees (Bombus terrestris L.) were investigated on tomatoes in plastic tunnels and glasshouses. Preceeding trials had revealed a strong contact and stomach activity of the compound by foliar application. A decisive improvement was obtained by replacing the foliar by drip irrigation of thiamethoxam at rates between 150 and 161 g ai/ha. The mortality figures of all bumble bee stages dropped to the level in the untreated control. No dead bumble bees were found in the thiamethoxam treated tunnels/glasshouses on the ground. Thiamethoxam could be rated as being harmless to bumble bees with a single application through the irrigation system. No spilling of this compound during the application should occur to avoid intoxication of adult bumble bees by oral uptake or contact contamination during the cleaning process. If multiple applications via the irrigation system or under any hydroponic growing conditions is considered, further sequential testing of this mode of use is recommended.