Pesticides, including neonicotinoids, typically target pest insects by being neurotoxic.
Inadvertent exposure to foraging insect pollinators is usually sub-lethal, but may affect
cognition. One cognitive trait, spatial working memory, may be important in avoiding
previously-visited flowers and other spatial tasks such as navigation. To test this, we
investigated the effect of acute thiamethoxam exposure on spatial working memory in the
bumblebee Bombus terrestris, using an adaptation of the radial-arm maze (RAM). We first
demonstrated that bumblebees use spatial working memory to solve the RAM by showing
that untreated bees performed significantly better than would be expected if choices were
random or governed by stereotyped visitation rules. We then exposed bees to either a high
sub-lethal positive control thiamethoxam dose (2.5ng-1 bee), or one of two low doses (0.377
or 0.091ng-1) based on estimated field-realistic exposure. The high dose caused bees to make
more and earlier spatial memory errors and take longer to complete the task than unexposed
bees. For the low doses, the negative effects were smaller but statistically significant, and
dependent on bee size. The spatial working memory impairment shown here has the potential
to harm bees exposed to thiamethoxam, through possible impacts on foraging efficiency or
homing.