Bees

The mite problem

Wisconsin winters are hard on bees, but the problem every beekeeper here has to manage is varroa mites and the viruses the mites carry. Varroa feed on bees’ body fluid (hemolymph) and spread viruses such as deformed wing virus, which kills bees ¹. The mites breed inside capped brood cells, alongside the developing bees, so they multiply right along with the colony ¹.

The threshold to remember: when more than 3 percent of the bees carry mites, a colony declines quickly and dies if it isn’t treated ¹. So the core skill of beekeeping here isn’t building boxes or harvesting honey. It’s counting mites and acting on the count.

Counting

Sample each hive about monthly from April through October ¹. The simplest method, and the one most Wisconsin beekeepers use, is the powdered sugar roll: scoop bees from a brood frame into a jar with a screened lid, add powdered sugar, roll it to coat them, and shake the dislodged mites out onto a white surface to count ¹. The bees go back to the hive unharmed. Alcohol washes and sticky boards are other options ¹.

A mite calendar

The timing of mites in a Wisconsin colony is predictable ¹:

• March: on the first day or two in the 40s, open the hive briefly to check food stores and mites. Overwintered colonies carry mites into spring, often at high levels, so consider treating even at 1 to 2 percent now, because the count only grows through the season.
• May through August: the bee population booms, which hides a rising mite population. A low percentage in July doesn’t mean the problem is solved.
• Late summer and fall: the bee population shrinks and the mite count climbs sharply. This is the dangerous stretch, because the bees that must carry the colony through winter are being raised now.
• Late fall and early winter: when the colony has no brood, all the mites are on the adult bees where treatments can reach them; that’s when oxalic acid works best ¹.

Spring temperatures are often too cool for thymol or formic acid products to work well ¹. Whatever you use, follow the label exactly; misused treatments can kill the queen, contaminate honey, and breed resistant mites ¹. In southern Wisconsin the season runs a week or two longer at each end, and in the north a week or two shorter ¹.

Wisconsin rules and help

• No hive registration is required ².
• Bringing honey bees or used beekeeping equipment into Wisconsin requires a Honey Bee Import Report first ². Used equipment can carry American foulbrood.
• DATCP’s apiary staff offer free hive inspections, May through October, including mite testing and disease diagnosis ². For a first-year beekeeper, that’s an expert second opinion for free.

What to take from this

Varroa mites are the problem to manage. Sample monthly from April to October with a powdered sugar roll and act above 3 percent, or at 1–2 percent in spring. Expect a fall surge hidden by the summer population, get mites down before winter bees are raised, and use the broodless period for oxalic acid. Follow labels. No registration is needed, but used equipment brought into Wisconsin needs an import report, and DATCP inspections are free.