Treating for Mites in High Temperatures

Most of us have 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. Bees collect water from outside the hive and spread it on cell caps and comb surfaces, and then fan their wings to evaporate the water and blow heat out of the hive. When there are too many warm bee bodies in the hive, the colony will grow a beard - workers without any work to do will hang out on the front porch of the hive (and on each other) to help the inside of the hive cool down. Since the bees actively cool their hive, insulated hives or insulated covers can help a colony manage the heat. Just like a well insulated house, insulated hives won’t absorb as much heat from the sun, and can keep its interior cooler through the heat of the day. As long as your bees have access to water, they can generally tolerate high temperatures that can threaten humans and other animals. 

Even if most heatwaves are unlikely to harm your bees directly, it may make it impossible to treat your hives’ varroa mites at the right time with the miticide you wanted. Therefore, it's important to be able to weigh your warm-season mite-control options and make an informed decision.

First, do you need a “knock-down” or a “hold-down” treatment?

The first question to answer is whether you need to knock the mite population down, or if you just need to keep it from getting any bigger with what some call a “hold-down” treatment. If your mite levels are high, you’ll need to knock them back hard to protect your colonies. However, if the mite populations are relatively low, you may be able to treat with a hold-down treatment to buy time for the weather to cool down, at which point you can use a stronger knock-down treatment. What kinds of products are hold-down treatments? Well, VarroxSan strips are one product that has proven itself capable of holding mite populations relatively steady. Another product is the new genetic-technology-based product Norroa. Both of these products can be used in hot weather, with supers on the hives, to limit mite population growth. (However, Norroa may not fare as well during the nectar dearths that can accompany hot weather, since the bees will eat the miticide-laced syrup instead of putting it where the mites will encounter it.) If your mite levels are only moderately high, perhaps holding them steady through hot weather will be enough. But these hold-down tools aren’t the first products you’d reach for when your mite infestation is high, and you want to bring it down quickly. For that, you’ll need a knock-down treatment that can handle summer temperatures.

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

It's no secret that many beekeepers rely heavily on formic acid 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 the 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 might lead to more brood and bees being killed, unreliable mite control, and, have been associated with more queen issues. (For more on how heat impacts formic acid treatments, check out our interview with Dr. Heather Broccard-Bell, the staff scientist at the company that makes Formic Pro.) 

Careful readers will notice that the manufacturer's FAQ mentions the danger of hot weather in the first three 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°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, or find one or two mites in an alcohol wash or sugar shake, you may not need to do anything for a while. If our goal is to always keep our mite level below 6 mites per 300 bees all summer, and we find 7 mites on our 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 something like formic acid until your local weather is cooler in a week or so. The big risk is that sometimes the local weather doesn't cool down in a week, or two, or three, and now your mite levels are higher and it's still too hot for your preferred treatment! If you've got to treat and the temperatures won't allow your favorite miticide, there are summer alternatives that you can use.

Oxalic acid

Powdered oxalic acid (such as 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 be used with honey supers on the hive. Both treatment methods can be used to kill mites on the adult bees, but neither method penetrates the brood caps. Because it kills mites on the bees but not in the brood, oxalic acid will provide only partial control in summer colonies, since the more capped brood in a hive, the more places the mites will have to hide from the treatment. Oxalic acid treatments can certainly help knock the mite levels down in a summer hive, but this miticide really shines when the colony is broodless or nearly broodless. Repeated treatments will be needed wen the colony has brood to really significantly lower the mite population in the hive. 

On very very hot days, it may feel cruel to plug up your colony and blast them with super-heated oxalic acid vapor. Hot weather vaporization also means you’ll be putting on your heavy nitrile gloves and full-face respirator to stay safe during the treatment, which is definitely no fun on 100°F Though the bees are likely to handle the few minutes of confinement and treatment without much issue, it can all be avoided by using an OA dribble treatment instead. A relatively new product, Api-Bioxal RTU (“ready-to-use”), offers a pre-mixed oxalic acid dribble treatment that can be repeatedly applied to hives with minimal stress to the bees. It contains no sugar, so the bees aren’t tempted to eat it. Weekly treatments with Api-Bioxal RTU for 4 weeks can help control mite even in colonies with capped brood, and don’t have temperature restrictions for hot temperatures.

Apivar 2.0 (amitraz)

Apivar 2.0 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 2.0. 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 2.0 in the first summer to control mites as your (non-food-producing) colony grows.

Apivar 2.0 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 makes it a poor choice to treat hives that you're hoping to harvest summer or 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 currently 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). Api-Life Var is limited to temperatures below 95°F, since the multiple plant-derived compounds in the miticide become a mite-killing vapor at lower temperatures.

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 recommended 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!