Sulfoxaflor Exposure Reduces Bumblebee Reproductive Success
Bumblebee colonies (Bombus terrestris) were fed field-realistic levels of sulfoxaflor (5 ppb) in sucrose for two weeks in a lab during colony early growth phase. After lab exposure, colonies were moved to the field (placed around a university campus), allowed to forage freely, and provided with no additional resources for 15 weeks. Colony demographics were monitored weekly, at night, over the study length. Twenty-five colonies were subjected to the treatment and 26 colonies served as controls.
Treated colonies produced significantly fewer workers than unexposed controls and also produced fewer reproductive offspring. The authors concluded that later shortfalls in reproductive individuals in the treated colonies could be attributed to the early drop in worker production.
The bumblebee colony cycle is characterized by an early growth phase in which worker numbers increase rapidly to create a large workforce, followed by a switch to production of reproductive brood later in the season. Between two and three weeks after exposure, detectable differences in worker numbers between treated and control colonies were noted. These persisted until close to the end of the colony cycle.
The authors characterized their study as “conservative” since the dosage of sulfoxaflor used in the study (5 ppb) was less than US Environmental Protection Agency estimates for immediate post-spray concentrations of sulfoxaflor in forager-collected cotton nectar based on labeled rates, and were also less than EPA estimates of residual concentrations for 10 days after spray application. In addition, the nascent colonies while in the lab were fed untreated pollen (under field conditions, post-spray sulfoxaflor residues in pollen have been documented to be more than tenfold higher than those in forager-collected nectar over 10 days post-spray). The regime used in the study most closely mimics spring-flowering crops in temperate environments, allowing comparison with similar neonicotinoid-based studies that also exposed colonies for a short period during the early growth phase.
[Reviewer note: Prior to 2019, sulfoxaflor was prohibited as a foliar spray during bloom to bee-attractive crops in the U.S. In 2019, EPA rejected this prohibition and began to allow bloom-time spray on citrus crops and ornamentals. In addition, spray is now permitted on multiple crops attractive to bees (including pome fruits, stone fruits, tree nuts, berries and small fruits) up to 3 days prior to bloom.]
