We often get asked what makes our bees mite resistant..
The answer is simple, varroa sensitive hygiene or VSH for short. VSH is not a breed, race or line of bees. It is simply a trait that all bees possess to a higher or lesser extent. By selecting for this trait over the years, we are able to increase the expression of the VSH trait in our bees. Bees that exhibit a high level of VSH target reproductive varroa mites under the capped brood cells and remove the pupae. This stops the varroa mite’s reproductive cycle. Since the foundress mite can only attempt to reproduce 2-4 times before running out of sperm, the mite eventually becomes non-productive. VSH bees only target reproductive mites and leave the non-reproductive mites alone. By doing this, the mite infestation decreases over time.
The VSH trait was first researched by Dr. Harbo at the USDA bee lab in Baton Rouge during the 1990’s. He found that bees that expressed this trait at a high level were able to survive untreated. To test bees for this behavior, he developed the VSH assay. This assay involves uncapping 100 capped brood cells that have been capped for 7-10 days.
This process is completed under a microscope to carefully inspect the interior of each cell and pupae. We select colonies for the assay that have not been treated for mites. If treated colonies are used, it will skew the results. During the inspection, the number of reproductive varroa mites, non-reproductive varroa mites and cells with no varroa mites are counted. Colonies with no varroa mites or only non-reproductive varroa mites are scored the highest. Any colony that is found to have 3 or more reproductive mites in the 100 cell sample is removed from the breeding program. We adopted Dr. Harbo’s VSH assay and scoring method into our breeding program here at Lloyd St. Bees. By using this assay, we have been selecting for VSH expression at the highest level, then using those colonies to make our queens, produce drones and make our artificially inseminated breeder queens.
We also perform an alcohol wash using a 300 bee sample on every colony in the operation during a one week period in August. These colonies have not received a mite treatment all season. While we don’t use this test as a marker of VSH, we do use it to pre-select colonies we want to perform the VSH assay on the following spring and to gauge how the breeding program is performing. To gauge the performance of the program, we take all our mite counts from this period and average the scores. These are our scores from the past three years:
2020 – 8 mites out of a 300 bee sample
2021 - 4 mites out of a 300 bee sample
2022 – 2 mites out of a 300 bee sample
The fact that mite counts have continued to drop the past three years showcases that we are moving in a positive direction toward high mite resistance. The mite testing also allows us to see what colonies are above the mite threshold that we have determined will need to be treated. For the 2022 season, 79% of our colonies went into winter not requiring treatment all year. We treated the remaining 21% of the colonies because we believe that no colony should suffer, and they can be re-queened or used to make queen mating nucs in the spring.
We want to thank Dr. Harbo for his research into VSH and for being kind enough to patiently answer all our questions while we learned his VSH assay and about the process of the VSH trait. If you want to learn more about the VSH assay Dr. Harbo created, check out this paper!
Thanks for supporting local beekeeping,
Trevor Bawden
Lloyd St. Bees