Honey Bee (Hymenoptera: Apidae) with the Trait of Varroa Sensitive Hygiene Remove Brood with All Reproductive Stages of Varroa Mites (Mesostigmata: Varroidae)

Authors: Harris, Jeffrey; Danka, Robert; Villa, Joseph

Submitted to: Annals of the Entomological Society of America
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/16/2009
Publication Date: 2/1/2010
Citation: Harris, J.W., Danka, R.G., Villa, J.D. 2010. Honey Bee (Hymenoptera: Apidae) with the Trait of Varroa Sensitive Hygiene Remove Brood with All Reproductive Stages of Varroa Mites (Mesostigmata: Varroidae). Annals of the Entomological Society of America. 103(2):146-152.

Interpretive Summary: Our laboratory has produced honey bees that strongly resist varroa mites by selectively breeding for a trait called varroa-sensitive hygiene (or VSH). VSH is a form of nest cleaning behavior in which adult worker honey bees somehow smell mite-infested brood, and then they uncap and remove the infested pupae from the broodnest. Although this procedure kills and sacrifices the bee pupa, it also kills the developing offspring of the varroa mite. Over time, this behavior reduces the mite population in a colony because the older varroa mites will eventually die before being able to produce many offspring. Previous experiments with VSH bees suggested that they prefer to uncap and remove pupae that are infested with mites that produce offspring. However, there has never been direct evidence that VSH bees had this preference; there was a simple correlation between hygienic removal of brood and reduced fertility of the mites that remained in brood combs after being exposed to VSH bees for 1 week. This paper describes a couple of experiments in which the behavior of VSH bees was interpreted after mite-infested brood was exposed to the bees for only 2-3 hours. It was found that VSH bees had no preference for removing pupae that were infested by mites that produced offspring. They chewed all types of mite-infested pupae, and the level of fertility in the mites that were chewed was the same as the fertility of mites that were protected from hygiene by a screen. Thus, it appears that VSH bees can smell mite-infested pupae regardless of whether the founding mite has produced offspring. Previous work by other researchers had shown the hygienic bees do not remove mite-infested pupae because of either the smell or movement from the founding mite. Thus, because neither the adult mites nor their offspring trigger hygiene, it seems that the most likely stimulus for the hygienic removal of mite-infested pupae by VSH bees has something to do with changes in the bee pupae that are caused by mites. One author had previously suggested that the chemical profiles on the cuticle of pupae change when mites feed upon them, and this may be the source of the signal to the nest cleaning bees.

Technical Abstract: Varroa Sensitive Hygiene (VSH) is a trait of honey bees, Apis mellifera L., which supports resistance to Varroa destructor mites. VSH is the hygienic removal of mite-infested pupae from capped brood. Bees selectively bred for VSH produce colonies in which the fertility of mites decreases over time. In addition, the fertility of mites decreases after naturally infested combs are exposed to VSH bees for 1 wk. The purpose of this study was to decide if the reduction in mite fertility is caused by the initial selective removal of mites that produce offspring by VSH bees . Initially, we monitored changes in a small patch of capped brood during exposure to VSH bees at 2-h intervals through 60 h, which provided a reference for the subsequent experiment. The time series showed that hygienic manipulations of brood cells by VSH bees occur rapidly, and many pupae are uncapped, recapped and targeted for removal in about 2 h. The approach in the second experiment was to compare the percentage of fertile mites from brood exposed to VSH bees for a 3-h period to the percentage of fertile mites in brood that was protected from hygiene by a screen (on the same comb). Specifically, the fertility of mites found on pupae that were being chewed by VSH bees was measured and compared to mites from protected pupae. There were no significant differences in fertility between the two groups of mites. These results suggest that neither egg-laying by foundress mites nor mite offspring are the stimuli that trigger hygienic removal of mite-infested pupae by VSH bees. It may be that hygienic activities such as the uncapping of brood cells inhibits or disrupts reproduction by varroa mites.