A Closer Look: Varroa Mite Orientation

Studies of Varroa destructor orientation to honey bees were undertaken to isolate discrete chemical compounds that elicit host-finding activity.

by Clarence Collison

Female Varroa mites parasitize both adult bees and bee brood, but only reproduce in capped brood cells. Therefore, the mites have to leave the adult bees and invade the brood cells. Between reproduction periods in capped brood cells, female mites are phoretic (an association between two species in which one transports the other) on adult bees for a variable period lasting a few days to several weeks (Boot et al. 1991). Both worker and drone cells are invaded by the mites, but in drone cells reproductive success is higher than in worker cells (Fuchs 1992; Boot et al. 1995a). More adult offspring are produced in drone cells compared to worker cells. Varroa mites selectively parasitize honey bee larvae within a narrow time window: 15-20 hours before brood cells are capped for pupation for worker larvae, and 40-50 hours for drones (Boot et al. 1992). The mites also prefer to infest drone brood cells, which are eight to 12 times more readily parasitized than worker brood cells (Fuchs 1990; Boot et al. 1995b). In addition, female mites exhibit preferences for adult bees of a specific age as mites readily abandon newly emerged bees and move to nurse-aged workers.

Drone cell preference is partly influenced by the properties of the brood cells. Larger cells contain higher numbers of mites. Cells protruding over the comb surface either naturally (DeJong and Morse 1988) or induced by partly filling them with melted wax (DeRuijter and Calis 1988) were shown to contain increased numbers of mites. Varroa mite infestation levels on worker larvae reared in elevated individual cells was 6-fold higher than in the adjacent six cells surrounding them (Kuenen and Calderone 2000). This differential infestation rate is similar to published values of higher mite infestations of drone cells compared to worker cells. Infestation levels in control cells were the same as in the surrounding cells. In contrast to infestation of these individually raised cells, mites invaded worker larvae in raised cells along the perimeter of a patch of raised cells (10 by 21 rows) 2.5 times more often than surrounding unraised cells, and similarly ca. 2.5 times more often than in the remaining raised cells (interior) of this patch. In similarly prepared frames of drone comb, mites invaded individually raised drone cells 3.3-fold more often than the adjacent surrounding cells and control cells. On the other hand, mites infested drone larvae in the interior of the raised-patch area as often as drones in raised cells along the perimeter of the raised-patch, and this rate was ca. 2.5-fold higher than for drone larvae in unraised cells surrounding the raised-patch and drone larvae in control cells.

Rather than the shape of the cell, the time and construction effort needed for capping might be the relevant factor in determining the degree of mite infestation. In addition, stimuli from the larvae are involved. Drone comb preference was not influenced by the number of infesting mites or the absolute number of available cells (Fuchs 1990).

Invasion of Varroa mites into honey bee brood cells was studied in an observation hive, using combs with cell openings at one side only. The cell bottoms had been replaced by a transparent sheet, through which mites were clearly visible after invasion into a cell. Mites invaded worker cells from 15-20 hours preceding cell capping, whereas they invaded drone cells from 40-50 hours preceding capping (Boot et al. 1992). The larger number of mites generally found in drone cells, when compared to worker cells, may be partly due to the longer period of mite invasion into drone brood.

Invasion of mites into drone cells of honey bees was studied in colonies without worker brood. The probability for a mite to invade was dependent on the brood/bees ratio, which is defined as the number of drone brood cells capped per kg of bees. When compared with invasion in colonies with exclusively worker cells, Varroa mites invaded drone cells 11.6 times more frequently. This suggests that the biased distribution of mites over drone and worker cells in colonies with both types of brood cells results predominantly from the higher rate of invasion into a drone cell per se, when compared to that into a worker cell (Boot et al. 1995b). Since the rate of invasion is high in drone cells, a trapping method using drone combs may be very effective in controlling Varroa mites. When no other brood is present, 462 drone cells are estimated to be sufficient to trap 95% of the mites in a colony of 1 kg of bees.

In colonies of Apis mellifera carnica infested with Varroa jacobsoni, the invasion of worker brood cells and drone brood cells by reproductive female Varroa mites were studied (Fuchs 1990). In 68 choices between brood combs of both cell types, the infestation of mites per cell was, on average, 8.3 times higher in drone brood. This drone cell preference was not affected by the infestation level. It was more marked if drone brood was rare and it decreased toward the end of the drone rearing season.

Studies of Varroa destructor orientation to honey bees were undertaken to isolate discrete chemical compounds that elicit host-finding activity. Petri dish bioassays were used to study cues that evoked invasion behavior into simulated brood cells and a Y-tube olfactometer was used to evaluate Varroa orientation to olfactory volatiles. In Petri dish bioassays, mites were highly attracted to live 5th instar worker larvae and to live and freshly freeze-killed nurse bees.

Olfactometer bioassays indicated olfactory orientation to the same type of hosts, however, mites were not attracted to the odor produced by live pollen foragers. The odor of forager hexane extracts also interfered with the ability of mites to localize and infest a restrained nurse bee host. Varroa mites oriented to the odor produced by newly emerged bees (<16 h old) when choosing against a clean airstream, however in choices between the odors of newly emerged workers and nurses, mites readily oriented to nurses when newly emerged workers were¸Varroa mites may detect relative concentrations of these compounds in order to discriminate between adult bee hosts and preferentially parasitize nurse bees over older workers in honey bee colonies. The volatile profile of newly emerged worker bees also may serve as an initial stimulus for mites to disperse before being guided by allomonal cues produced by older workers to locate nurses. Allomones are chemical substances produced and released by an individual of one species that affect the behavior of another species to the benefit of the originator but not the receiver. Fatty acid esters, previously identified as kairomones for Varroa, proved to be inactive in both types of bioassays. Kairomones are chemical substances emitted by an organism which mediates interspecific interactions in a way that benefits an individual of another species which receives it and harms the emitter.

Several bioassays have been used to test the orientation behavior of the mites to semiochemicals. The activity of contact-chemoreceptive compounds has been examined using simple Petri dish or glass plate assays (Kraus 1993, 1994; Rosenkranz 1993; Zetlmeisl and Rosenkranz 1994; LeDoux et al. 2000; Nazzi et al. 2001; Calderone and Lin 2001; Aumeier et al. 2002; Calderone et al. 2002). These studies have established that mites orient to the stage-specific odor differences of live hosts, and that their movement can be arrested by blends of host cuticular compounds or larval food. Other researchers have employed a semipermeable membrane as a bioassay arena to evaluated the locomotory behavior of Varroa (Rickli et al. 1994; Donze’ et al. 1998), revealing that mites are readily arrested by combinations of straight-chain hydrocarbons or primary aliphatic alcohols and aldehydes derived from extracts of larvae or cocoons.

The detection of airborne host volatiles by the mite also has been examined using several techniques. Le Conte et al. (1989) used a four-arm olfactometer to show that Varroa preferred to orient to the odor of live drone larvae, drone extracts and the fatty acid esters methyl palmitate, ethyl palmitate and methyl linolenate. Rickli et al. (1992) found that mites on a servosphere walked in straight paths confined to airstreams containing the odor of live larvae, adults, larval extracts or palmitic acid, but exhibited only a weak response to methyl palmitate.

Chemcial components of honey bee pheromones also influence the host-finding behavior of Varroa mites. Using wax tube choice tests and a Y-tube wind channel, Kraus (1990) demonstrated that the odor produced by honey bee sting glands, and most of the individual components of alarm pheromone itself, were highly repellent to mites. Hoppe and Ritter (1988) used simultaneous choice tests to show that the preference of Varroa for different ages of adult bees might be explained by the repulsion of mites to Nasonov gland odor or one of its principal components, geraniol.

Varroa mites are attracted to its major prey, drone larvae, by methyl and ethyl esters of straight-chain fatty acids, in particular methyl palmitate. These esters were extracted from drone larvae with n-hexane and were identified by gas chromatography-mass spectrometry. Their behavioral effect was evaluated with the use of a four-arm airflow olfactometer (Le Conte et al. 1989).

Varroa mites exhibit preferences for adult bees of a specific age as mites readily abandon newly emerged bees and transfer to nurse-aged workers, generally three to 12 days old, over older foragers (Kraus et al. 1986; Le Conte and Arnold 1987; Kuenen and Calderone 1997). With no known optical system (Bruce 1997), Varroa must rely on non-visual stimuli for orientation to specific larval and adult hosts. Within the environment of a honey bee colony, semiochemicals appear to be likely candidates for these cues. One alternative for controlling Varroa may be through the use of semiochemicals that either disrupt the normal host-finding behavior of the mite or to attract and trap a portion of the mite population within a colony. The high degree of host specificity exhibited by Varroa suggests that kairomones are used by mites to locate and parasitize larval and adult hosts (Pernal et al. 2005).

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Clarence Collison is an Emeritus Professor of Entomology and Department Head Emeritus of Entomology and Plant Pathology at Mississippi State University, Mississippi State, MS.