Impact of Diflubenzuron on Bombus impatiens (Hymenoptera: Apidae) Microcolony Development

Abstract

Reliance on the honey bee as a surrogate organism for risk assessment performed on other bees is widely challenged due to differences in phenology, life history, and sensitivity to pesticides between bee species. Consequently, there is a need to develop validated methods for assessing toxicity in non-Apis bees including bumble bees. The usefulness of small-scale, queenless colonies, termed microcolonies, has not been fully investigated for hazard assessment. Using the insect growth regulator diflubenzuron as a reference toxicant, we monitored microcolony development from egg laying to drone emergence using the Eastern bumble bee Bombus impatiens (C.), a non-Apis species native to North America. Microcolonies were monitored following dietary exposure to diflubenzuron (nominal concentrations: 0.1, 1, 10, 100, and 1,000 µg/liter). Microcolony syrup and pollen consumption was significantly reduced by diflubenzuron exposure. Pupal cell production was also significantly decreased at the highest diflubenzuron concentration assessed. Ultimately, diflubenzuron inhibited drone production in a concentration-dependent manner and a 42-d 50% inhibitory concentration (IC50) was determined. None of the dietary concentrations of diflubenzuron tested affected adult worker survival, or average drone weight. These data strengthen the foundation for use of this methodology, and provide valuable information for B. impatiens; however, more work is required to better understand the utility of the bumble bee microcolony model for pesticide hazard assessment.

Keywords: bumble bee; insect growth regulator; microcolony; pesticide; risk assessment.

Figure 1.. Microcolony configuration and experimental timeline.

(A) microcolony chambers composed of 1/6 size food pan, white light-blocking modified lid, and a perforated stainless-steel insert. Microcolony chambers were provisioned with a nest dish containing a pollen patty and a separate pollen feeding dish. (B) Experimental overview and timeline of key events and monitoring.

Figure 2.. Indicators of nest development in B. impatiens microcolonies exposed to diflubenzuron.

(A) Average honey pots constructed by workers. (B) Average uncapped egg chambers present. (C) Average brood masses present. (D) Average pupal cells present. Microcolony observations were collected once per week for the duration of the experiment. Diflubenzuron concentrations are expressed in μg/L. Data shown as mean ± SD (n = 6). * denotes p<0.05.

Figure 3.. Concentration-dependent inhibition of microcolony development.

Microcolony development from nest initiation on day 0 to experimental termination on day 42 in the presence of diflubenzuron-containing sugar syrup captured photographically. Representative photos are shown from 1 of 6 microcolonies per treatment.

Figure 4.. Microcolony syrup and pollen consumption.

(A) Average syrup consumption by exposure group per week. (B) Average pollen consumption by exposure group per week. (C) Average syrup consumption for the full 6-week study for each exposure group. (D) Average pollen consumption for the full 6-week study for each exposure group. Tracking pollen consumption began during the second week of the experiment (Fig. 1B). All values have been corrected for evaporation. Diflubenzuron concentrations are expressed in μg/L. Data shown as mean ± SD (n = 6). * denotes p<0.05.

Figure 5.. Influence of diflubenzuron on drone production.

Drones were collected from each microcolony and counted as they emerged. (A) Average drone emergence on day 37, 40, and 42 by diflubenzuron concentration (μg/L). (B) Average drone emergence from microcolonies for the entire experiment. (C) Drone inhibition (normalized to vehicle control) due to diflubenzuron exposure. The IC₅₀ was calculated to be 28.61 ± 14.41 μg/L. Diflubenzuron concentrations are expressed in μg/L. Data shown as mean ± SD (n = 6). * denotes p < 0.05.