The Bee-Files

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



V. destructor is an obligate parasite of cavity-dwelling Apis bees. It cannot reproduce on yellow jackets, wasps, bumblebees or any other species. Early reports of this mite on the western honey bee inaccurately identified it as V. jacobsoni Oudemans, which exists in a sustainable association with the eastern honey bee, A. cerana. In 2000, the genus Varroa was reported to consist of at least two species, V. jacobsoni (which infects A. cerana, but not A. mellifera) and V. destructor (which infects both A. cerana and A. mellifera). Consequently, literature reporting on V. jacobsoni and the western honey bee prior to that time actually refers to V. destructor.

The association of V. destructor with the western honey bee reportedly originated in the 1950's when mites transferred to A. mellifera colonies introduced to the home range of A. cerana. Subsequently, V. destructor has established a nearly cosmopolitan distribution with respect to its new host, with Australia being the only mite-free continent. V. destructor was discovered in the US in 1987. Due to the highly mobile nature of both the honey bee and the US beekeeping industry, V. destructor quickly became endemic, and it can now be found in every state in the continental US.

Symptoms and Damage

An adult female V. destructor is elliptical in shape with a width of 1.5 mm, a length of 1.0 mm, and 4 pairs of legs. Mature female mites are brown, dark brown, or cordovan [Fig. 1]. The immature stages are light and translucent, but these are not often found on adult hosts. Usually, there are no obvious symptoms at low levels of infestation. As the mite population increases, a suite of symptoms, collectively designated parasitic mite syndrome, becomes apparent. Initially, adult workers with damaged wings are seen [Fig. 2]. This damage is a result of DWV (deformed wing virus), which is introduced either directly or indirectly to the developing bee by the mite. As the infestation rate increases, more damaged workers are seen and otherwise healthy looking bees may be seen crawling in front of the hive, unable to fly. This condition is also caused by a virus. Finally, the brood begins to deteriorate, appearing to be infected with a variety of pathogens [Fig. 3]. Although these brood symptoms superficially resemble American and European foulbrood, the causative organisms of those diseases have not been identified in the deteriorating brood and treatment with antibiotics does not eliminate the condition. As the syndrome progresses, the worker death rate exceeds the birth rate; and most new worker bees that do emerge are seriously impaired. As a result, the colony’s population begins a rapid decline. From the time that a colony first exhibits brood symptoms until its total collapse can be as little as three weeks.

 
Fig. 2: A worker honey bee with deformed wings.
Fig. 3: Deteriorating brood typical with high levels of V. destructor.

Life Cycle of V. destructor


The life cycle of the mite can be divided into phoretic and reproductive phases. The reproductive phase begins when a mature female leaves her adult host, enters a brood cell containing a worker or drone larva shortly before it is capped, and sequesters herself in the bottom of the cell. Soon, the cell is capped; and shortly thereafter, the immature bee enters the pupal stage. Egg-laying commences about 60 hours after a cell is capped, and both mother and offspring feed on the host’s hemolymph. Mature offspring mate within the cell, but only mature females survive outside the cell.

The number of offspring that reach maturity is positively correlated with the length of the host’s capped stage, which is greatest for drones, intermediate for workers, and shortest for queens. Mites that reproduce on drone brood average 2.2 - 2.6 female offspring per host, while those reproducing on worker brood average 1.3 - 1.4 female offspring per host. Mites cannot reproduce on queen brood due to its short capped period. Not surprisingly, mites are found more often on drone brood than worker brood, with average differences between 5 and 12 fold. Mites are only rarely found on queen brood.

The phoretic phase begins when the host emerges from its cell as an adult bee. The mature female mite may leave the cell with its adult host, or it may walk out of the cell and acquire an adult host. Mites remain on an adult host for a few days or weeks before entering a brood cell for the next round of reproduction. Mites are found twice as often on bees in the brood nest as on bees in the honey supers, and 10 times as often on brood nest bees as on foragers.

Transmission of V. destructor

Robbing by bees is a major source of transmission. As an infected colony become progressively weaker, its defensive capabilities decline, and it becomes susceptible to invasion by workers from nearby colonies (the robbers) seeking its valuable cache of honey. In the process of removing the honey, robbers become infected with mites and transport them back to their own colonies. Swarms from infected colonies also contribute to the local reservoir of mites. These colonies are particularly susceptible to being robbed because they do not receive any treatment for mite control. They weaken and die within a year or two and may be robbed by workers from nearby colonies. Drifting bees, especially in apiaries where colonies are kept close together, also contribute to the spread of mites among colonies.

Beekeepers also play a major role in the transmission of mites. Moving brood among colonies for the purpose of strengthening or equalizing colonies is a common practice that transmits mites. In addition, beekeepers often purchase colonies of bees in the spring to replace winter losses or to increase colony numbers. Some beekeepers purchase small nucleus colonies, usually called ‘nucs’, from local or regional suppliers. These colonies consist of 1-5 combs of bees and brood and usually come with a queen. Others purchase package-bees (2, 3 or 5 pounds of bees, usually with a queen) from a southern location. An estimated one million packages are shipped throughout the country each year. Each of these practices spread mites, including various types of pesticide resistant mites.

Fig. 4: The ‘cappings scratcher’ method for surveying for mites.
Migratory beekeeping also plays a role in transmitting mites. Over a million colonies are moved throughout the country each year as migratory beekeepers fulfill pollination contracts. After the bloom is over, colonies are widely dispersed to other locations for honey production. During the season, some of these colonies may issue mite-infested swarms into local environments, while others may succumb to mites and be robbed by local colonies. Each fall, surviving colonies are returned to a few states in the south where colony numbers are restored. This brings colonies from many different regions of the country into close proximity to one another and provides many opportunities for the transfer of mites among colonies, including various types of pesticide resistant mites. In the spring, these colonies resume their migratory routes throughout the country, and the process is repeated.

Monitoring and Thresholds

Survey methods provide presence/absence information. One such method is the ‘cappings scratcher’ which requires one to impale a number of capped drone cells with a cappings scratcher, and then to pull the immature drones from their cells for examination [Fig. 4]. This method has been found to be highly effective in detecting mites when present at very low levels. A second survey tool is the ‘sticky-board’ which takes advantage of the fact that mites often fall off of bees. Typically, a piece of paper is covered with a sticky substance (petroleum jelly or a vegetable spray) and inserted into the hive where it rests, sticky-side up, on the bottom board. The sticky-board must be protected from the bees. One way to do this is to build a wooden frame, cover one side with 1/8” hardware cloth, and attach the sticky-board to the other side [Fig. 5]. Sticky-boards are also available commercially. The board is removed after 24 or 48 hours and the mites are counted. Strictly speaking, the sticky board does not provide information about pest density; however, it is often used for that purpose. Lastly, mites can sometimes be seen on adult bees or walking on the comb, but this is more common when infestation rates are very high and should not be relied on as a diagnostic method.

Sampling methods provide an estimate of pest density. This is the type of information needed to determine whether or not to apply a pesticide. One method is the ‘ether-roll’, which provides an estimate of pest density in terms of mites per standard volume of bees. Bees are collected from 2 or 3 brood-nest combs and placed in a quart glass jar. If only a few colonies are being sampled, shake bees directly into a dishpan. Scoop up ½ cup of bees and quickly pour them into the quart jar. If larger numbers of colonies are being sampled, a modified ‘Dust Buster’ (DC Insect Vac from BioQuip®) can be used to collect a standard volume of bees, which are then transferred to the quart jar. You will need to experiment with the vacuum collector to determine the exact volume that yields about 300 bees. Spray a 3 second burst of an automotive starting fluid into the jar, replace the lid, shake vigorously for 10 seconds, and then toss and roll the jar 3 times along its long axis. Mites, if present, will be seen adhering to the sides of the jar [Fig. 6]. This method detects 50 - 60% of the mites actually present in the sample. The resulting ether roll count is usually converted to a standardized 300-bee ether roll count (SER) using the formula:

SER = (ER) / (#B/300),

where ER is the ether roll count for the sample and #B is the number of bees in the sample.

An improvement in accuracy can be obtained by calculating the actual mite-to-bee ratio. This is done by collecting the bees as above and then separating the mites from the bees by agitating them for 5 minutes in a container with soapy water or alcohol and straining through a 1/8” hardware cloth screen. The screen catches the bees but allows the mites to pass through. Typically, bees are washed several times to remove all of the mites. Mites and bees are counted and the actual mite-to-bee ratio is calculated. You can convert the mite-to-bee ratio to a standardized 300-bee ether roll count using the formula:

SER = ((R * #B) / 1.783) / (#B / 300),

where R is the mite-to-bee ratio in the sample and #B is the number of bees in the sample.
The conversion factor (1.783) is from Calderone and Turcotte (1998).

 
Fig. 5: The ‘sticky board’ used for surveying for mites.
Fig. 6: Mites adhering to the sides of a jar in the ether-roll test.
Remember! For an estimate of pest density to be meaningful, each step in the sampling method must be standardized. This means monitoring mite levels at the same time each year, and monitoring all colonies exactly the same way. For the ether roll, this means collecting samples from the same place in each colony (2 or 3 brood nest combs), collecting the same number or volume of bees in each sample, applying the same amount of starting fluid, and shaking the jar in the same manner. For the sticky-board, the same size board must be used each time, the sample must be collected at the same time each year, and the board must be left in place for the same length of time.
Fig. 7: A strong population of workers ready for winter.
The decision to use or not to use a pesticide is based on an economic threshold. This is the pest density at which economic damage is expected if a treatment is not applied. Economic thresholds for V. destructor vary widely throughout the country. The values used below are based on studies conducted in the midwest and northwest where blooming patterns, length of winter and winter temperatures are similar to those in the northeast. Significantly different values are used in other parts of the country. Beekeepers should contact their local extension apiculturist for the most current recommendations for their area


Rationale for IPM Program

In order for a colony to survive the winter in good condition, it must have a strong population of healthy worker bees in the fall [Fig. 7]. A colony exhibiting early stages of parasitic mite syndrome in mid-summer can usually be saved by the application of an effective miticide because it has time to produce several more generations of healthy workers in a low-mite environment. However, in the northeastern US, these symptoms often occur during or just prior to the fall nectar flow when chemical treatments are proscribed by label restrictions. By the time the flow is over, mite populations have increased dramatically and colonies have suffered severe damage. The result is a loss of colonies during the fall flow or shortly thereafter. This phenomenon is known as ‘fall collapse’, although it may occur in late summer or winter, or whenever mite populations are allowed to increase to high levels.

Often, infected colonies look strong after the fall flow, and the application of an effective pesticide kills most of the mites present; however, the colony still collapses and dies over the next few weeks or months. Such colonies experienced significant, but less obvious damage while waiting for the fall treatment. The lesson is simple. One cannot assume that a colony will survive the winter if one waits until the end of the fall flow to apply a pesticide. Mite levels must be kept low during the summer in order that colonies can rear healthy workers during late summer and early fall.

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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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