Comparative analysis of 3 pollen sterilization methods for feeding bumble bees

Abstract

Pollen is an essential component of bee diets, and rearing bumble bees (Bombus spp.) for commercial use necessitates feeding pollen in mass quantities. This pollen is collected from honey bee (Apis mellifera L.) colonies because neither an artificial diet nor an economical, large-scale pollen collection process from flowers is available. The provenance of honey bee-collected pollen is often unknown, and in some cases has crossed international borders. Both deformed wing virus (DWV) and the fungal pathogen Ascosphaera apis (Claussen) Olive & Spiltoir (cause of chalkbrood disease); occur in honey bee-collected pollen, and infections have been observed in bumble bees. We used these pathogens as general surrogates for viruses and spore-forming fungal diseases to test the efficacy of 3 sterilization methods, and assessed whether treatment altered pollen quality for the bumble bee. Using honey bee-collected pollen spiked with known doses of DWV and A. apis, we compared gamma irradiation (GI), ozone fumigation (OZ), and ethylene oxide fumigation (EO) against an untreated positive control and a negative control. Following sterilization treatments, we tested A. apis spore viability, detected viral presence with PCR, and tested palatability to the bumble bee Bombus impatiens Cresson. We also measured bacterial growth from pollens treated with EO and GI. GI and EO outperformed OZ treatment in pathogen suppression. EO had the highest sterilizing properties under commercial conditions and retained palatability and supported bee development better than other treatments. These results suggest that EO sterilization reduces pathogen risks while retaining pollen quality as a food source for rearing bumble bees.

Treatment of waxborne honey bee (Hymenoptera: Apidae) viruses using time, temperature, and electron-beam irradiation

“…Gamma and e-beam irradiation do inactivate the honey bee pathogens P. larvae , A. apis , N. ceranae , and viruses ( Hornitzky 1994 , Melathopoulos et al 2004 , Simone-Finstrom et al 2018 , Strange et al 2023 ). As treatment for American foulbrood commonly uses a dose of 10 kGy, there is an opportunity to increase that dose to 35 kGy to achieve control of viruses; however, the potential benefits and costs to colony health should be better studied before recommendations are made.…”

“…As treatment for American foulbrood commonly uses a dose of 10 kGy, there is an opportunity to increase that dose to 35 kGy to achieve control of viruses; however, the potential benefits and costs to colony health should be better studied before recommendations are made. Moreover, the type of sample treated by irradiation could influence the efficacy against viruses, as DWV-contaminated pollen treated with ~30 kGy gamma irradiation did not have lower DWV levels posttreatment compared to untreated pollen ( Strange et al 2023 ).…”

Bee microbiomes in a changing climate: investigating the effects of temperature on solitary bee life history and health

“…Past studies found that larvae acquire fatty and amino acids from microbial prey within provisions, resulting in heavier bees than those reared on sterile pollen [12][13][14]. These opposing results may be explained by differences in pollen sterilization methods: in choice assays, EO exposure does not affect the palatability of pollen to bumblebees [28], but it is unknown whether soaking provisions in ethanol [12][13][14] affects pollen consumption in solitary bees, perhaps leading to differences in biomass, fat content, and trophic positions. Of note, while not statistically significant, bumblebees consumed less pollen per day when fed EO exposed pollen compared to the no treatment control [28].…”

“…These opposing results may be explained by differences in pollen sterilization methods: in choice assays, EO exposure does not affect the palatability of pollen to bumblebees [28], but it is unknown whether soaking provisions in ethanol [12][13][14] affects pollen consumption in solitary bees, perhaps leading to differences in biomass, fat content, and trophic positions. Of note, while not statistically significant, bumblebees consumed less pollen per day when fed EO exposed pollen compared to the no treatment control [28]. This likely contributed to slight differences in biomass and fat content between bees reared on sterile versus microberich provisions, though most pollen consumption occurs while fifth instar larvae in Osmia spp.…”

“…We homogenized provisions allocated to males and females together because there is no evidence of pollen microbiome composition between sexes [27]. To sterilize provision material, pollen was exposed to ethylene oxide (EO) gas overnight with one ampule of 17.5 g Anprolene in a benchtop fumigation chamber (Anderson Sterilizers Inc., Haw River, NC) [28]. Sterilized pollen was then rehydrated with sterile DI water to the initial weight prior to EO exposure.…”