Evaluating a Heat Therapeutic Control of the Honey Bee Mite Varroa Destructor
Varroa mites first appeared on agricultural honeybees in North America in 1987. Today, virtually all commercially productive honeybee colonies in the US must be treated to control Varroa mite infestations or the hives die within three years. Currently, U.S. beekeepers are relying on fluvalinate (commercial name Apistan), the only approved pesticide available to them. This project evaluated and developed a method of managing varroa mite infestations with heat alone. A simple cabinet-type heating apparatus, similar to that used by beekeepers in Uzbekistan, was constructed, tested and improved.
* Use efficiency studies to determine the percentage of mites removed using bee-safe temperatures and treatment times for sizes ranging up to the entire adult bee population of an established colony.
* Identify and attempt improvements to the heat-therapy apparatus and procedure related to the effectiveness and ease of handling bees during treatment.
* Assess the winter survival of varroa-infested nucleus colonies that were heat-treated during the preceding fall.
* Assess the influence of spring and fall-and-spring heat treatments on the health and honey production of varroa-infested nucleus colonies during the following summer.
* Measure the influence of heat therapy on the population levels of the endoparasite Acarapis woodi.
Heat treatments removed between 82% and 98% of the varroa mites from treated colonies. This rivals the efficiency of experimental organic acid treatments and miticides such as fluvalinate.
Treated colonies showed markedly lower varroa populations through the entire summer and fall following treatment.
Beekeepers who think they have enough labor to treat with heat should begin using this method as part of an integrated pest management approach to honeybee mite control. Heat therapy would likely prove compatible with bee breeding for mite tolerance, the sale of nucleus colonies and package bees, and the production of organic honey and wax.
Effectiveness of most methods of controlling varroa can be improved by treating during broodless periods.
Varroa control requires avoiding post treatment reinfestation from other apiaries. It is important for beekeepers to treat at the same time or to establish sufficient buffer distances between apiaries.
Method and Findings
Varroatosis affects larvael, pupal and adult stages of honeybees with a wide range of symptoms, some of which are associated with known honeybee viruses (parasitic mite syndrome or PMS). Currently only one approved miticide, fluvalinate (trade name Apistan), is available to an estimated 140,000 to 211,600 U.S. beekeepers for controlling varroa. It is generally agreed that the effectiveness of this miticide is unsustainable, and some populations of Varroa already exhibit resistance to fluvalinate.
Although Apistan is a class III pesticide, there exists concern about residues persisting in beeswax comb. Thermotherapy is one of the few effective alternatives available, and as such offers an important tool to beekeepers concerned about chemical residues in their hives. Thermotherapy can replace the use of fluvalinate.
The apparatus used in this project followed the design of a homemade device in use in Uzbekistan, where varroa mites have been an apicultural pest for decades. It consists of a portable wooden cabinet (11" by 16") which accommodates a standard 1500 watt milk-house-type heater that stands on the floor of the cabinet and a wire mesh cylinder for suspending the bees in the upper portion of the cabinet. Plexiglass windows are fitted into the top and sides of the cabinet for ventilation and observation. Bees are shaken, brushed or blown from the hive’s frames into the heat-treatment cage through a large funnel. The actual heating can take less than 15 minutes per hive, while the handling of the bees can take twice as long.
The heat-treatment involves temporarily removing the bees from a hive and exposing them to 46° to 48°C (114.8° to 118.4°F) for several minutes until most of the mites have been detached, but before the bees can be injured by the heat. This project deals exclusively with extrohive thermotherapy of honeybees and should not be confused with a patented device costing several hundred dollars that heats bees and the entire contents of the hive.
Preliminary heat-treatments enabled the project participants to develop bee-safe handling and heating procedures while determining the factors that limit the number of bees per treatment. An efficiency study showed that heat-treatments removed between 82% and 98% of the varroa mites from treated colonies. This rivals the efficiency of experimental organic acid treatments and miticides such as fluvalinate.
The extrohive thermotherapy was shown to be inexpensive, as expected, but heat-treating is labor-intensive; interested beekeepers will need to learn the techniques involved. These techniques are not difficult, but operator inexperience or inattention can result in unnecessary loss of livestock. Findings include labor-conserving techniques such as preparing colonies for treatments and efficiency-improving techniques such as performing treatments nocturnally.
During the entire project an experienced apiculturist regularly inspected all colonies and recorded symptoms of disease, brood levels, varroa-mite counts, and honey production.
The study closely monitored 32 colonies over the course of a year to observe the health and productivity of heat-treated colonies relative to a control group. The project was conducted in areas of mixed woodland and small-scale agriculture in hilly areas of southeastern Vermont and southwestern New Hampshire on sites where varroa infestation pressure from other honeybee colonies was reduced by a two-mile or wider buffer radius.
Participants seeded 32 nucleus colonies of hybrid "Hardy Northern Stock" with varroa mites from donor colonies that had been infested for two years and exhibited symptoms of PMS. In October of 1996 the 32 colonies were randomly assigned to eight sites paired according to expected more or less favorable environmental factors so as to mitigate the effects of site on project outcomes. After normal wintering, 16 colonies at four sites were heat treated during a naturally broodless period in March and April of 1997; and the 16 colonies at the other four sites were maintained as a control group. For the purposes of this efficiency study 4 colonies ranging from 456 to 1133 grams (1 to 2.5 pounds) were heat treated and the bees returned to their hives over a sticky board so that additional mite fall and bee mortality could be measured. The hives were brood-free and the comb reasonably clear of mites. Egress or ingress of bees was prevented by covering the hive entrances with wire mesh. Two plastic strips coated with 10% fluvalinate were inserted, and mite fall was counted over the next nine to fourteen days. This efficient treatment, administered to single-chamber brood-free hives, was expected to remove virtually all the remaining varroa mites. Efficiency of the heat treatments was calculated by dividing the number of mites detached during heat-treatment by total mites, and ranged from 81.9% to 94.6%.
The efficiency of mite removal with two additional colonies was similarly measured except that, instead of a control treatment with Apistan, the bees were sacrificed and all remaining varroa mites counted. The efficiencies of these two treatments were 90.6% and 97.9% for colonies weighing 1420 and 1498 grams (3.13 and 3.30 lbs).
Contrary to expectations that treatment temperatures and durations would need to be minutely adjusted to maintain efficiency in treatment of different sample sizes, the 47° to 50°C range worked for all sample sizes below 1588 grams (3.5 pounds), as long as the cage was not overfilled and other operating procedures observed.
During heat-treatment trials, it became apparent that certain factors grossly affected the efficiency of treatments. For example, if the mixed adult bee population of a large varroa-infested colony was treated in successive batches, it became obvious that while a cage with ample space for air circulation produced an impressive mite fall, a cage overfilled with bees was slow to heat, detached very few mites, and was more likely to result in damage or death to bees. If the treatment cage was oversized for the cabinet, mite fall was also greatly reduced.
If overheating occurs, with the primary thermometer readings above 49° to 50°C, some bees began to regurgitate a clear liquid, became excessively lethargic, and were more likely to clump together in collective helplessness made worse by the cohesive effects of the regurgitated liquid. If the cage was overfilled, or the bees in the cage were in other ways allowed to clump together during heating, far fewer mites fell than with the treatment of loosely distributed bees from the same colony. So, whether clumping is allowed at the beginning of critical heating or results from overheating, bees are undesirably stressed and the efficiency noticeably reduced.
Improvements to the Heat-Treatment Apparatus and Procedure
Experience with heat treating bees enabled the participants to make improvements to the heat-treatment apparatus. It was discovered that bee handling and heat treating were greatly facilitated through preparatory manipulations, inspections, and choices of when to treat. Equipment factors included cage size, cage construction materials, and the funnels used for transferring the bees from the hive to the treatment cage.
The same experience allowed the participants to improve the heat-treatment procedure by avoiding bee clumping and by consolidating bees onto selected frames in advance of treatment to minimize shaking and brushing and the risk of spilling honey or nectar. The timing of treatments, queen management, and treating during cold, broodless periods and after dark were also explored and refined. The need to keep apparatus clean to prevent disease transmission, improve thermotransparancy, and remove accumulated alarm pheromones also became apparent.
The first outcome measure to distinguish treated from control colonies were sticky-board counts begun in July and August. The sticky boards consisted of oil-coated paper, separated from the bees by 1/8" mesh and placed on the floors of the colonies to show the number of varroa mites naturally falling in each colony. Analysis showed that the control group produced, as a mean, 7.2 times more mites per day than the treated group.
In second- or third-year infestations, untreated colonies of honeybees often crash dramatically and die after the annual varroa populations boom. This study was not funded for the two to three years necessary to compare control and heat-treated colonies over the full course of varroa disease.
When using chemotherapy, colonies are treated through the insertion of fluvalinate-coated plastic strips between the brood frames at $3 to $6 per year in materials, plus the labor to open and close each colony two or four times. The cost might be higher when we consider the likely appearance in the U.S. of mite resistance to fluvalinate, a problem already observed in Europe. Although perhaps labor saving, the actual cost of using fluvalinate can compare unfavorably with the more labor-intensive heat method over the course of several years. The coordinator calculates that in the Northeast it costs between $433 and $933 in materials and labor to treat 100 colonies with fluvalinate, not including transportation.
If a beekeeper acquired a heating cabinet, heat source, thermometers, and other needed equipment for a one-time cost of $200, and if it took 45 minutes (labor costs = $7 and $10/hour) with each colony, it would cost the beekeeper between $545 and $770 each year with the initial equipment cost spread out over ten years. An additional argument for using heat therapy arises if we take into account the likely increase in the cost of fluvalinate and the likely increased income from the sale of uncontaminated, premium-priced honey produced using heat therapy.
With organic honey standards in place in Vermont and other states, the number of specialty and certified organic producers is likely to increase dramatically in the next several years. Most standards do not allow use of fluvalinate in the production of honey labeled certified organic, and it is likely that all organic honey standards will similarly exclude or strongly limit the use of other pesticides.
When considering whether to attempt a program of varroa mite control using extrohive thermotherapy, the beekeeper may consider several economic factors. Are the bee yards equipped with 120 volts AC electrical current, can colonies be brought to a treatment site accessible to electrical current on an annual basis, or can bee yards be advantageously relocated with access to electrical current? This is a labor-intensive, materials-cheap varroa control method. Does the beekeeper intend to incorporate sustainable methods into all aspects of colony management and processing, either with the intention of following a sustainable philosophy or achieving organic certification? If so, extrohive thermotherapy would be compatible with most any sustainable apiculture program.
The additional labor required for extrohive thermotherapy can be weighed against the premium price customers are willing to pay for wax, honey and pollen from certified organic or colonies otherwise managed using sustainable methods.
Potential impacts of this project include providing a safe, sustainable alternative to decreasingly effective chemotherapies such as fluvalinate. Current apicultural practices that involve extensive handling of adult bees, such as the formation of nuclei and package bees, could be adapted with relative ease so as to incorporate heat treatments.
Prospects for the production of certified organic honey and other bee products will likely improve with the availability of non-chemotherapeutic varroa-control measures such as heat therapy. Organic honey production in the continental U.S. is currently very low as a direct result of the lack of sustainable alternatives to chemotherapy for the control of varroa infestations.
Reported December, 1997. 1999 Northeast Region SARE/ACE Report.