The Bee-Files

Integrated Pest Management Varroa destructor in the Northeastern United States
Using Drone Brood Removal and Formic Acid

Nicholas W. Calderone

Key Words: honey bees
Apis mellifera
  
Varroa destructor
IPM
drone brood removal
formic acid
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The methods discussed in this fact sheet were developed and evaluated in the northeastern US. Drone brood removal will benefit beekeepers throughout the US; however, formic acid and other miticides acting as fumigants work best in areas where colonies are broodless or nearly broodless for at least four weeks during the fall or winter. When a colony is rearing brood, most mites are present in brood cells where they are protected from the effects of fumigants. During broodless periods, mites are present on adult hosts and are susceptible to fumigants. Since fumigants have a relatively short treatment period (about three weeks) compared to other pesticides (about six weeks), it is critical that the majority of mites be present on adult hosts for fumigants to be effective.
  • drone and worker comb: wax comb built by bees for storing honey and pollen and for rearing drone (male) and worker (female) honey bees, respectively. The cells that make up drone comb are slightly larger than those that make up worker comb.
  • larva: the feeding stage of an immature insect.
  • pupa: the quiescent stage of an immature insect during which time it undergoes dramatic physiological and morphological changes as undergoes the transition from the larval stage to the adult stage.
  • brood: the immature stages of the honey bee, including the egg, larval and pupal stages. Immature workers and drones develop in worker and drone cells, respectively. Queens are reared in special queen cells, which are seasonal and relatively few in number.
  • capped stage: the period when a cell containing an immature bee is capped with wax. A brood cell is capped from the late larval stage until the bee emerges from the cell as an adult.
  • hemolymph: insect blood.
  • pest density: the number of pests in a sample of known size. Mite density can be measured several ways. Some of these include the number of mites per adult bee, the number of mites per 300 adult bees, or the number of mites in a standard volume of adult bees.
  • economic injury level (EIL): the lowest pest density that causes economic damage.
  • economic threshold level (ETL): the pest density that triggers an action designed to prevent the pest population from reaching the economic injury level. The ETL is always = the EIL.
  • pesticide: includes many kinds of ingredients used in products, such as insecticides, miticides, fungicides, rodenticides, insect repellants, weed killers, antimicrobials, and swimming pool chemicals, which are designed to prevent, destroy, repel, or reduce pests of any sort.
  • pyrethroids: a class of synthetic pesticides with chemical structures similar to pyrethrum, a naturally-occurring substance in chrysanthemums with pesticidal activity. Generally, moderate to high doses of pyrethroids are necessary to cause acute toxicity in mammals. Apistan® (fluvalinate) is a pyrethroid used for controlling V. destructor.
  • organophosphates (OP’s): a class of synthetic pesticides containing phosphorous. Generally, very low doses of OP’s can cause acute toxicity in mammals. OP’s can also cause cumulative, irreversible nerve damage at sub-lethal doses. CheckMite+® (coumaphos) is an OP registered for control of V. destructor in some states.
  • organic acids: a group of carbon-bearing acids, including acetic, formic, lactic and oxalic acids. Organic acids can cause severe burns to the skin, eyes and respiratory system. Mite-Away II™ is a formulation of formic acid registered in the US for control of V. destructor.
  • essential oil: the volatile and aromatic liquid or semi-solid obtained from a single botanical species, primarily through a distillation, expression or extraction process. Essential oils are blends of many compounds, the various compounds being natural products, many of which act as antibiotics and/or pesticides. One such compound, thymol, is derived from thyme oil and is the primary active ingredient in Api-Life VAR™, a product registered for control of V. destructor in some states.
  • tolerance: the maximum residue limit, which is the amount of pesticide residue allowed to remain in or on a treated food commodity. If residues exceed the tolerance level, the commodity is subject to seizure and destruction. Some pesticides are exempt from tolerance (e.g. formic acid), while others have a time-limited exemption that must be periodically renewed (e.g. thymol, menthol and eucalyptus oil).
  • off-label use: the use of any registered pesticide in a manner inconsistent with its label.
  • Integrated Pest Management (IPM): a pest management program based on the coordinated use of multiple tactics (including biological, cultural, genetic, mechanical and chemical) and environmental data (pest densities, economic thresholds) and designed to maintain pest populations below the economic injury level with the least disruption to the environment.

This bulletin focuses on the management of the parasitic honey bee mite Varroa destructor in the northeastern US. It contains information that will allow a beekeeper to: 1) identify V. destructor, 2) recognize the symptoms of mite infestation, 3) determine pest densities, and 4) implement an effective IPM program for keeping mite populations below the economic injury level.

The western honey bee, Apis mellifera, was introduced to the US from Europe in the 1600’s. Today, the honey bee provides essential pollination services for over 45 commercial crops grown throughout the US, adding $14.6 billion to the value of the country’s agricultural production each year. In addition, US beekeepers produce between 170 and 220 million pounds of honey each year, more than 50% of total US consumption. Hence, a sustainable supply of healthy and affordable honey bee colonies is a critical factor affecting farm productivity and the stability of farm incomes and food prices.

The parasitic honey bee mite V. destructor [Fig. 1] is considered to be the most serious global threat to beekeeping and to the sustainable production of crops that rely on A. mellifera for pollination. V. destructor, which kills honey bee colonies of European descent within 1-2 years, has killed millions of managed and wild colonies in the US in the past two decades. Apistan® and CheckMite+® have provided some relief, but control has always been unpredictable due to the fact that mite populations often rise rapidly during the honey producing season when treatment is proscribed by label restrictions. Consequently, colonies often suffer serious damage while the beekeeper waits for a legal treatment window to open.The threat from V. destructor has become a matter of grave concern as resistance to both Apistan® and CheckMite+® has become widespread.

Fig. 1: A mature adult female V. destructor
  To continue as a viable enterprise, the beekeeping industry requires sustainable management practices that will keep mite populations below the economic injury level and maintain the high quality of hive products. The best way to achieve these goals is to use a management program that relies on multiple tactics, rather than solely on chemicals. One such approach is referred to as Integrated Pest Management or IPM. IPM incorporates chemical and non-chemical tactics; however, for several reasons, IPM minimizes the use of chemicals whenever possible. First, chemicals add a recurring cost to a beekeeper’s management program. Second, chemicals inevitably show up as residues in hive products, and that jeopardizes their reputation as pure and natural products. Third, chemicals can be injurious to the applicator and may pose a risk to the consumer. This raises the issue of liability, especially for beekeepers with employees. Fourth, the less a pest population is exposed to a pesticide, the more slowly it develops resistance to that pesticide. So, by minimizing the use of a pesticide, its useful lifetime is extended. 
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© Copyright 2008, All rights reserved, Nicholas W. Calderone, Associate Professor,
Department of Entomology, Cornell University, Ithaca, NY 14853 

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