MSU Sentinel Apiary Update: 2019 Varroa Trends
This article was originally posted on the Michigan Sentinel Apiary Blog, on Dec. 23, 2019
As part of the Bee Informed Partnership (BIP) Sentinel Apiary program, the MSU Honey Bee Program monitors apiaries throughout Michigan. Each month, we took notes on our colonies and management. We also sent a bottle of bees from each colony to the Bee Informed Partnership for the lab to quantify the number of bees and mites in each sample.
We sampled 8 colonies per yard in Lansing and Novi and 4 colonies per yard in Benton Harbor, Escanaba, Hickory Corners (at the Kellogg Bird Sanctuary), and Lake City. After looking at monthly varroa levels, here are some takeaways and trends we’d like to share:
We had to do a lot of work to keep mite populations at safe levels. All of our yards received 3 or 4 mite treatments in total this year. Some colonies may have needed an additional or earlier mite treatment to prevent a spike in mite levels.
Mite level growth can vary among colonies in the same yards. We saw huge increases in mite levels in some colonies, while we didn’t see the same increases in others. In Lansing we saw one colony’s mite levels jump from 0.5% in September to 12.8% in October, while another colony in the same yard increased from 1.1% to 2.4% during the same period. In Lake City, one colony’s mite levels jumped from 0.0% in July to 5.3% in August, while another colony in the same yard remained only jumped from 0.0% to 0.4%.
Skipping early-season mite treatments did not turn out well. In Novi, we skipped a June miticide treatment in 3 colonies that had severe cases of European Foulbrood. By August, the mite levels of these colonies were at 6.6%, 8.1%, and 18.6%, while the other 5 colonies in the yard that received the June treatment were at or below 3.8%.
Beekeepers shouldn’t assume that mite levels are low just because they treated. While the vast majority of mite treatments decreased mite levels, the exact effect was not always easy to predict. For example, we saw one colony’s mite levels decrease from 3.2% to 0.3% after treatment, while the mite levels of another colony in the same yard increased from 0.3% to 2.9% after treatment.
Apiaries close to other bee yards were more vulnerable to huge spikes in mites. We saw the biggest increases in mite levels in Lansing and Novi, and both yards are close to other bee yards. On the other hand, mites levels in 4 colonies at the Kellogg Bird Sanctuary (a yard not close to other bee yards) stayed below 1% through September, and the highest mite load recorded in October was 3.7%. It should be noted that yards far from other bee yards still received at least 3 mite treatments throughout the beekeeping season.
Mite numbers can vary between samples. Since the Bee Informed Partnership calculates the number of bees in each of our samples (instead of assuming each sample has 300 bees) and uses a special machine to dislodge mites, our mite numbers are more accurate than field alcohol washes and powdered sugar rolls. Still, mite levels can vary between samples taken from the same colonies on the same days. For example, 2 samples taken from the same colony resulted in mite counts of 3.4% and 0.0%. In another colony, 2 samples resulted in mite counts of 5.3% and 3.5%.
We expect colonies that experience high mite levels and/or parasitic brood mite syndrome to die, even if we are able to successfully lower their mite levels. Since mites spread viruses within colonies, a high mite load at any point in the year can lead to a high virus load that persists for months after the mite population was knocked down. For example, we had a colony in Novi with a mite infestation of 5.8% in July, 18.6% in August, 0.3% in September, and 0.5% in October. Even though the late summer and fall mite treatments were effective at dramatically dropping the mite levels in this colony, we expect the colony to die because the winter bees that are supposed to survive through the winter were likely exposed to high virus levels.
Mites made a comeback in October. In Lansing, mite levels in one colony were at 0.5% in September and then at 12.8% in October, and another colony was at 0.0% in September and 6.7% in October. We treated all apiaries with an oxalic acid dribble in November. One notable exception to the high October mite levels was the Novi yard in which all 7 of the sampled colonies were at or below 0.8% in October. Full formic acid treatments in both late August and late September may have helped keep Novi mite levels from spiking.
Beekeepers can find varroa-related resources from MSU at keepbeesalive.org.