Submitted to: Veterinary Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/1/2010
Publication Date: 3/16/2011
Citation: Vojvodic, S., Jensen, A.B., James, R.R., Boomsma, J.J., Eilenberg, J. 2011. Temperature dependent virulence of obligate and facultative fungal pathogens of honeybee brood. Veterinary Microbiology. 149:200-205. Interpretive Summary: Honey bees are susceptible to many diseases, including fungi. Chalkbrood is a common disease of honey bee brood (the larvae) that is caused by an obligate fungal pathogen; occurring only as a pathogen of honey bee brood. Stonebrood is also a fungal disease of honey bee brood, but it is an opportunistic pathogen, which occurs only occasionally in honey bees, but is capable of infecting different hosts. Temperature in the hive is under constant regulation by the bees, since fluctuating temperatures in the brood area can have negative effect on the overall health and survival of the brood. We found that under normal hive temperature conditions, the opportunistic fungus (stonebrood) killed honey bee larvae faster than the obligate fungus (chalkbrood). However, when honey bee larvae were stressed by cooling them for 24 hours, they became more susceptible to the obligate pathogen chalkbrood, but survival was improved in the infections with stonebrood. With these results we demonstrated how honey bee disease resistance depends on the pathogen species, which is tightly coupled to the changes in the environment such as temperature drop.
Technical Abstract: Chalkbrood (Ascosphaera apis) and stonebrood (Aspergillus flavus) are well known fungal brood diseases of honeybees (Apis mellifera), but they have hardly been systematically studied because the difficulty of rearing larvae in vitro has precluded controlled experimentation. Chalkbrood is a chronic honeybee-specific disease that can persist in colonies for years, reducing both brood and honey production, whereas stonebrood is a rare facultative pathogen that also affects hosts other than honeybees and can likely survive outside insect hosts. Hive infection trials have indicated that accidental drops in comb temperature increase the prevalence of chalkbrood, but it has remained unclear whether virulence is directly temperature-dependent. We used a newly established in vitro rearing technique for honeybee larvae to test whether there are systematic temperature effects on mortality induced by controlled infections, and whether such effects differed between the two fungal pathogens. We found that increasing spore dosage at infection had a more dramatic effect on mortality from stonebrood compared to chalkbrood. In addition, a 24 hour cooling period after inoculation increased larval mortality from chalkbrood infection, whereas such a cooling period improved survival after stonebrood infection. These results raise interesting questions about honeybee defenses against obligate and facultative pathogens and 40 about the extent to which stress factors in the host (dis)favor parasites with lesser degrees of specialization.