Synergistic negative effects between a fungicide and high temperatures on homing behaviours in honey bees

Interactions between environmental stressors may contribute to ongoing
pollinator declines, but have not been extensively studied. Here, we
examined the interaction between the agricultural fungicide Pristine®
(active ingredients: 25.2% boscalid, 12.8% pyraclostrobin) and high
temperatures on critical honey bee behaviours. We have previously shown
that consumption of field-realistic levels of this fungicide shortens
worker lifespan in the field and impairs associative learning performance
in a laboratory-based assay. We hypothesized that Pristine® would also
impair homing and foraging behaviours in the field, and that an
interaction with hot weather would exacerbate this effect. Both
field-relevant Pristine® exposure and higher air temperatures reduced the
probability of successful return on their own. Together, the two factors
synergistically reduced the probability of return and increased the time
required for bees to return to the hive. Pristine® did not affect the
masses of pollen or volumes of nectar or water brought back to the hive by
foragers, and it did not affect the ratio of forager types in a colony.
However, Pristine®-fed bees brought more concentrated nectar back to the
hive. As both agrochemical usage and heat waves increase, additive and
synergistic negative effects may pose major threats to pollinators and
sustainable agriculture.