The Beekeeper’s Companion Since 1861
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Notes from the Lab

Honey bees practice social distancing when infested with varroa

- December 1, 2022 - Scott McArt - (excerpt)

I imagine most readers of this article are tired of social distancing. Our human world did plenty of social distancing in 2020 and 2021, and at least some people still practice some amount of social distancing in 2022. But most people are tired of it.

But as sobering as it is to admit, social distancing for us humans does indeed work in the face of disease. I have two small children who were admirably diligent about social distancing between April 2020 and December 2021; I never got a single sickness from them during that 1.5-year span. What’s happened since December 2021 as we and nearly everyone else has started to relax our social distancing? I’ve been sick about once every two months. Yes, social distancing works for us humans.

What about honey bees? Do they practice social distancing in the face of parasites or disease? If they do, how do they do it given the incredibly dense bee populations within colonies? Are there particular behaviors or ways they organize interactions within hives that change in the face of parasites or disease? These are the topics for the sixtieth Notes from the Lab, where I summarize “Honey bees increase social distancing when facing the ectoparasite Varroa destructor,” written by Michelina Pusceddu and colleagues and published in Science Advances [2021].

For their study, Pusceddu and colleagues conducted two types of experiments: whole-colony observations of varroa-infested vs. varroa-free colonies (Photo 1, Figure 1) and high-resolution observations in metal hoarding cages using small groups of individually tagged bees assembled from varroa-infested vs. varroa-free colonies (Photos 2 and 3).

For the whole-colony observations, three varroa-free colonies (average infestation level 0.1%) were obtained by treating colonies with oxalic acid every week for three consecutive weeks, starting two months before the observations. These varroa-free colonies were compared to three varroa-infested colonies (average infestation level 6.2%) obtained by allowing varroa levels to grow naturally without treatment. Colony strength was balanced in both experimental groups by removing brood frames from the strongest colonies.

Behavioral observation videos were made on varroa-free and varroa-infested hives using four high-definition cameras, two for each of the two observation hives being compared simultaneously (Photo 1). For three consecutive days, each colony was recorded for three 15-min sessions (morning, afternoon, and evening) between 10:30 a.m. and 6:30 p.m. Observations of dancing and allogrooming were made on two types of comb: one taken from the central part of the nest (central frame) and the other taken from the sides of the hive (lateral frame). One hour before starting the video recording, selected combs were placed behind the glass windows of each hive. Moving the central combs to a lateral position for the time necessary to do video shooting should not alter their status of “central combs,” as numerous previous studies have shown that foragers mark the positions of combs where they perform dances after the first foraging flights and return to the same positions to repeat them during the rest of the day.

For the high-resolution behavioral observations in metal hoarding cages, bees were obtained from nine varroa-free colonies and mites were obtained from nine varroa-infested colonies that were treated with oxalic acid (or not) in an identical manner to the colonies used for whole-colony observations. Newly emerged adult workers and mites were harvested from capped brood frames, individually tagged, and placed in groups of 12 bees in each hoarding cage (Photos 2 and 3). For each bioassay, groups of 12 varroa-free bees were compared to groups of varroa-infested bees where six of the 12 bees were parasitized and the other six bees were not parasitized. Frequency of antennation, trophallaxis, and allogrooming was monitored for 20 minutes in each experimental cage.

So, what did they find? Do foraging bees alter behaviors when infested with varroa? Yes. As seen in Figure 2, foraging bees altered the location where they performed dances (round and waggle) when infested with varroa. When colonies were free of varroa, foraging bees danced throughout the  ….