Treating for Mites in High Temperatures

Most of us have already noticed that heat waves are more frequent, longer, and hotter. Even "normal" average summer temperatures continue to climb around the globe. Summer heat is sometimes just fine for honey bees, though even bees have their limits. Even if the heat is unlikely to harm your bees directly, it may make it impossible to treat your varroa mites at the right time with the product you choose. Therefore, it's important to be able to weigh your hot-weather mite-control options and make an informed decision.

Formic Pro (formic acid): Only if the weather cooperates

It's no secret that many beekeepers rely heavily on formic acid miticide to control their varroa mite levels throughout the summer. As the only miticide that reliably penetrates the brood cappings to kill reproducing mites, and one of only a handful of miticides that are safe for use with honey supers on the hive, formic acid has a lot of advantages. However, its one big disadvantage is that it can't be used when the outside temperature climbs too high. Higher temperatures will lead to more brood and bees being killed and are also associated with more queen issues.

If you read it carefully, you'll notice that the manufacturer's FAQ mentions the danger of hot weather in the first 3 days of the treatment. That's because the majority of the formic acid is engineered to leave the product and enter the hive in those first three days or so. After those first days pass, warm weather is much less likely to cause any serious problems for your bees. But what are your summer treatment options if you can't even find a 3 day period with temperatures under 85 degrees F?

Should I just wait to treat for varroa until it's cooler outside?

Sometimes, waiting is an acceptable option. As good beekeepers, we hate mites and want to kill them dead as quickly as possible. The problem is, all of our colonies have got mites all the time - the question is just whether they have too many mites and need a treatment. 

If you have three or four mites on your screened bottom board after a day under the hive, you may not need to do anything at all. If our goal is to always keep our mite level below 6 mites per 300 bees all summer, and we do an alcohol wash or a sugar shake and find 7 mites on a sample of 300 bees, we know that we should treat - but how urgently? If you're counting 7 mites now, your bees will probably still be okay if you wait to treat them with formic acid as soon as your local weather cools down in a week or so. The big risk is that sometimes the local weather doesn't cool down in a week, and now your mite levels are higher and it's still too hot for Formic Pro! If you've got to treat and the temperatures won't allow Formic Pro, there are alternatives. 

HopGuard III (hop beta acids)

HopGuard III is safe to use in hot weather, and safe to use with honey supers on. The active ingredient, hop beta acids derived from the same plant used to flavor beer, kill mites riding and feeding on your adult bees. However, HopGuard doesn't penetrate the brood caps to kill mites as they reproduce. HopGuard can knock mite levels down, but it doesn't provide the same intense control that most other miticides do, so it's often best in conjunction with another miticide treatment (with a different active ingredient) later in the year. Because it kills mites on the bees but not in the brood, HopGuard is a great choice for a hive that has little to no brood, but doesn't provide the same level of control in a colony filled with capped brood.

Oxalic acid

Oxalic acid (the only legal source in the United States is the regulated product Api-Bioxal) can be administered to a hive either through the heat-vaporization method, or by dribbling a mixture of oxalic acid and sugar syrup over the bees. Either method can now be used with honey supers on the hive, after recent regulatory changes that declared minor oxalic acid residues in honey as negligible and not harmful. Both treatment methods can be used to kill mites on the adult bees, but neither method penetrates the brood caps. Just like HopGuard III, oxalic acid is useful for partial control in summer colonies, but the more capped brood in a hive the more places the mites have to hide from the treatment. Oxalic acid treatments can easily help knock the mite levels down a bit in a summer hive, but this miticide really shines when the colony is broodless or nearly broodless. 

Apivar (amitraz)

Apivar can be used in hot weather, but CAN NOT be used with honey supers on the hive. The label on the miticide does not allow its use on hives with supers, because the residues from the treatment run the risk of contaminating honey. Thus, a hive full of honey and nectar cannot be treated with Apivar. However, if you have a nucleus colony (or another small colony) that you intend to overwinter and then grow into a honey-producing colony the next spring, you can use Apivar in the first summer to control mites as your (non-food-producing) colony grows.

Apivar does not penetrate the brood cap, but the miticide is in the colony long enough that all mites wind up exposed to it. The long treatment period plus the long mandatory withdrawal period before honey supers can be added back to the hive makes it a poor choice to treat hives that you're hoping to harvest fall honey from, but a perfectly reasonable choice for hives that have finished producing honey crops for the year. 

Apiguard and Apilife-Var (thymol)

Marketed under the brands Apiguard and Apilife-Var, thymol-based miticides smell strongly of the herb thyme, and kill mites as the bees try to chew and drag the smelly medicine out of the hive. The problem is that neither of these miticides penetrate the brood caps, and neither can be used with supers on the hive. A hive that isn't making honey for human consumption can be treated with thymol. If the product lasts long enough in the hive (or if repeated doses are applied) then all mites will be exposed to it. Thymol-based Apiguard is actually more effective the warmer it is (up to 104 degrees F). 

Apistan (tau-fluvalinate)

As we consider alternative miticide options, don't forget about Apistan. Apistan (active ingredient tau-fluvalinate) was a highly effective miticide for years (99% mite kill against non-resistant mites). However, overuse of Apistan contributed to the evolution of Apistan-resistant mites, so it can only be used now on mites that are not resistant. However, if you haven't used Apistan on your hives for a while, you may be able to use it for a year and get good mite control. It can leave residues in wax, especially if used heavily and repeatedly, but it's still a useful weapon to have in your mite-killing arsenal. It has similar restrictions to Apivar, like not using it while you have honey supers on the hive, but it is another miticide that you may consider during high summer temperatures for hives that aren't making honey that you intend to eat. 

Beat the heat and beat the mites

So, does hot summer weather mean that you can't treat for mites? Of course not! Formic Pro might not be suitable today, but perhaps another miticide would suit your bees just as well. Or maybe the weather will be cooler next week and the bees will be able to tolerate the mites just a few days longer.

High mites are deadly to bees, but they can survive beyond the treatment threshold for a short while if the right miticide isn't available. Know the weapons that you can use against mites, and know which weapon to reach for in different weather and different circumstances. 

It doesn't hurt to have a few different options on hand, in case the weather forces you to change treatment plans. Just be mindful of the expiration dates on your miticides, and don't buy so much that everything expires before you get a chance to use it!