Comb irradiation has limited, interactive effects on colony performance or pathogens in bees, Varroa destructor and wax based on two honey bee stocks

Authors: De Guzman, Lilia; Simone-Finstrom, Michael; Frake, Amanda; Tokarz, Philip

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2018
Publication Date: 1/8/2019
Citation: De Guzman, L.I., Simone-Finstrom, M., Frake, A.M., Tokarz, P.G. 2019. Comb irradiation has limited, interactive effects on colony performance or pathogens in bees, Varroa destructor and wax based on two honey bee stocks. Insects. 10(1):1-20. https://doi.org/10.3390/insects10010015.
DOI: https://doi.org/10.3390/insects10010015

Interpretive Summary: Gamma irradiation is currently used to inactivate bee-infectious pathogens, such as American foulbrood, that linger in combs. This study was conducted to determine if this benefit of comb irradiation can be maximized by examining its effects on colony performance, varroa mite population, and viral load in bees, varroa mites and wax in Russian and Italian honey colonies. We found that the use of mite-resistant stock was more helpful in regulating mite population than comb irradiation. Also, bee and brood production of both stocks was similar, and was not substantially improved by comb irradiation. Deformed wing virus (DWV), Varroa destructor virus (VDV-1), Chronic bee paralysis virus (CBPV), and Black queen cell virus (BQCV) were commonly detected in wax, newly emerged bees, and pupae and their infesting varroa mites. Both irradiated and non-irradiated combs had about 4 viruses or pathogens including European foulbrood (EFB). Because VDV-1 levels increased through time in wax and the bees that emerged from the non-irradiated combs of Italian colonies, this observation suggests that wax could be a transmission route of viral infection. In addition, Russian bees and their infesting mites had lower levels of DWV than the Italian bees indicating potential DWV resistance by Russian honey bees. CBPV was also reduced in mites from Russian colonies in general and in mites, mite-infested pupae, and newly emerged bees that were collected from irradiated combs. However, BQCV was not reduced by comb irradiation. Overall, the contribution of irradiating comb in improving honey bee health and colony survival appears to be subtle, but may be useful as part of an integrated pest management strategy with the addition of using mite-resistant stocks.

Technical Abstract: Parasitic mites and pathogens compromise honey bee health. Development of sustainable methods of managing these problems will minimize their detrimental impact on honey bees. Here, we compared the overall health of Italian and Russian honey bee colonies with or without gamma-irradiated combs in 2015 and 2016. The major finding concerned honey bee genotype confirming that Russian honey bees are more resistant to Varroa destructor than Italian honey bees. The effect of comb irradiation was inconsistent showing significant increase in adult bee population, Varroa infestation on adult bees, and amount of stored pollen in 2015, but not apparent in 2016. Increased infestation on adult bees in the irradiated group in 2015 was largely influenced by high infestations of the Italian bees regardless of comb treatment. The increased amount of stored pollen was probably associated with larger adult population in colonies with irradiated combs in September 2015 regardless of honey bee stock. The contrasting results between the two years of observations may be due to other factors such as weather and nectar flow that may have contributed to the buildup of both mites and colony population. Nevertheless, the ability of bees to collect and store more pollen in the irradiated group cannot compensate the negative impact of mite parasitism on honey bees especially in the Italian bees, which consistently suffered significant colony losses during both years. Results of viral analyses of wax, newly emerged bees, and Varroa and their pupal hosts showed common detections of Deformed wing virus (DWV), Varroa destructor virus (VDV-1), Chronic bee paralysis virus (CBPV), and Black queen cell virus (BQCV). Wax samples had on average ~4 viruses or pathogens detected in both irradiated and non-irradiated combs. Although pathogen levels varied by month, some interesting effects of honey bee stock and irradiation treatment were notable indicating how traits of mite resistance and alternative treatments may have additive effects. Further, this study indicates that wax may be a transmission route of viral infection. For example, VDV-1 levels increased through the experimental period in wax and the bees that emerged from Italian colonies with non-irradiated combs. In addition, pupae and their infesting mites from Italian colonies exhibited higher levels of DWV than those from Russian colonies suggesting potential DWV resistance by Russian honey bees. CBPV was also reduced in mites from Russian colonies in general and in mites, mite-infested pupae, and newly emerged bees that were collected from irradiated combs. However, BQCV was not reduced by comb irradiation. Overall, the contribution of irradiating comb in improving honey bee health and colony survival appears to be subtle, but may be useful as part of an integrated pest management strategy with the addition of using mite-resistant stocks.