Varroa Destructor

Perhaps the most damaging condition affecting honeybees is the presence of Varroa Destructor mites. (Even the name is scary, right?!) Varroa mites have become an international beekeeping problem. Its history is one of the negative ecological consequences that can happen when organisms are moved outside their native ranges. Back in the old days, varroa was a natural parasite of the eastern honeybee, with damage limited to drone brood and overall impact on the hive negligible. However, (as they tend to do), humans made the problem much worse over the last century. Humans moved the western honeybee into eastern Asia giving varroa the opportunity to move into a new host. The presence of varroa on western honeybees became alarmingly obvious in Europe by the 1960s, and today it is found everywhere honeybees are kept except Australia and a few oceanic islands.

Varroa are visible to the naked eye, appearing as shiny and tiny red or brown-colored parasites resembling miniature ticks (see photo below).

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The life cycle of the varroa mite is actually pretty gross. It begins when female varroa seal themselves away in brood (baby bee) cells just before they are capped. Once inside, the female varroa moves into the abdomen of the baby bee and sucks its bee blood (haemolymph). The female varroa also lays several eggs near this feeding site in the baby bee’s abdomen. One son and several daughter mites emerge, mature, and mate. The original female varroa maintains the feeding site on the baby bee so her young can feed.

Varroa parasitizes worker brood as well as drone, and the bees have little behavioral of physiological resistance to this alien parasite. Many larvae simply die in their cells, too crippled by varroa to emerge alive. If the bee survives this trauma, it emerges from its cell and the mated daughter mites emerge with it to disperse and renew the life cycle all over again. This invasive process results in honeybee deformities such as chewed, deformed wings, transmission of viruses, and crippled, sickly bees that are essentially useless to the hive. Here, if you look really close, you can see one of our poor dead honeybees with a varroa mite on her upper left back:

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Varroa are most prevalent in the spring and summer when there is plenty of eggs and larvae to infect. However, their effects are more severe in late season (fall) when brood is naturally declining and a large number of varroa are competing for a shrinking pool of honeybee blood. As the queen bee’s egg-laying tapers off in late autumn and early winter, varroa populations tend to decline, as there are fewer brood cells within which to feed and reproduce. However, the persistent varroa populations survive winter by remaining huddled in the wintertime bee cluster, feeding off of the haemolymph of adult bees.

The first line of treatment for our hives is monitoring. By keeping a watchful eye over varroa populations, varroa can be maintained at a level that will not harm the colony. Yesterday, we conducted one of our weekly varroa counts at the SR apiary. This entails placing a white sheet of paper below a screen at the bottom of the hive. Dead varroa fall through the screen, and we count the number of varroa (adjusted to yield caught per 24 hours). The pictures below show members of our collective diligently counting varroa, every single, tiny speck:

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The treatment threshold is about one varroa per 24-hour period. An infestation of varroa previously meant a death sentence for the colony unless a beekeeper intervenes with treatment. However, in recent years, there is some evidence that a degree of genetic resistance to varroa in the western honeybee is emerging. Varroa has been the focus of a huge body of international research for over the past half century. As a result, there are several well thought-out approaches to management. Unfortunately, there are also a lot of methods which encourage chemical abuse by the beekeeping industry. Our collective has opted for less invasive interventions.

One intervention we do is drone brood trapping. Because varroa tend to prefer drone brood, drone brood trapping is a useful way to remove a large fraction of the varroa population. We insert a special neon green drone frame (see photos below), which has foundation milled to drone size. (Remember, drone calls are larger than worker cells.) The bees build drone cells, the queen fills them with drone eggs, and once the drone brood is capped, the drone frame is removed and frozen to kill the mites (and unfortunately the drone brood). The defrosted brood is then returned to the bees, who will remove and eat the brood, thereby recovering a part of their nutrient investment.

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Another intervention is the use of formic acid, which has been registered for control of varroa mites. An absorbant pad is impregnated with formic acid and placed in the hive where its fumes kill varroa and leave the honeybees relatively unscathed. However, treatment of varroa mites with formic acid and its potential effects on the hive is an ongoing debate within our collective. As a collective, we have found that attempting to fight off varroa infestation if often futile, as almost every beehive on earth will have some level of varroa present. For us, complete removal of the parasite is less the goal than developing a manageable coexistence.

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One thought on “Varroa Destructor

  1. Thank you so much for the detailed entries. I am learning a lot about this little critter for whom I have tremendous affection and fear for their survival. I really appreciate the devotion of you SR apiarists.

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