Insect pheromones are categorized according to the response they elicit from the receiver, which by nature of the term 'pheromone' is a member of the same species. Some pheromones are released in a defensive context and signalize potential danger – these are called 'alarm pheromones.' Receivers of alarm pheromones respond by either dispersing or attacking the perceived source of danger. In honey bees, two principal alarm pheromones have been identified: the sting alarm pheromone (Boch et al. 1962), and the mandibular gland alarm pheromone (Shearer and Boch 1965), both produced exclusively by worker bees. The role of mandibular alarm pheromone (2-heptanone) in defensive behavior is not well understood, but sting alarm pheromone influences the recruitment, localization and attack behaviors demonstrated in a honey bee colony defense effort.
(Z)-11-eicosenol and isopentyl actetate (IPA, or isoamyl acetate), along with trace amounts of about 40 other compounds, comprise the sting alarm pheromone (Hunt 2007). This pheromone is produced by a mass of excretory cells, called Koschevnikov’s gland, located on the dorsal side of the sting base, and is secreted when the sting is extruded. Alarm pheromone migrates from the Koschevnikov’s gland to setae (hairs) located around the sting base, where it dissipates into the air (Breed et al. 2004). Although (Z)-11-eicosenol is the most abundant component in alarm pheromone, it seems to only serve as a synergist with the more potent isopentyl acetate. This synergistic mixture has been shown to actuate defensive response nearly as effectively as an intact sting (Hunt 2007).
Isopentyl acetate production begins around three to four days of age and peaks at about two to three weeks. It is believed to be mediated by the release of juvenile hormone; however, the onset of IPA production is not related to the shift from nest duty to foraging (Robinson 1985). The amount of IPA released during defensive behavior increases as a worker bee ages, reaching its highest level at about the time when the worker is old enough to perform guarding tasks. The number of guard bees at the hive entrance varies, increasing during times of dearth, when robbing is more likely to occur (Winston 1987). When patrolling guard bees perceive an intruder they raise their abdomens in the air, extruding the sting. Alarm pheromone is released and fanned into the hive by the agitation of wings, and workers come rushing out to pursue the intruder. However, the intruder must be in motion in order for bees to locate and attack it (Breed et al. 2004; Boch and Shearer 1971).
The immediate but rather short-lived defensive response that alarm pheromone produces classifies it as a 'releaser' pheromone. Yet, recent research has indicated that sting alarm pheromone not only provokes a quick defensive response but also influences behavior for a longer period of time by affecting brain gene expression (Alaux and Robinson 2007). This was demonstrated when an initial exposure to IPA affected behavioral responsiveness to subsequent exposures to IPA, inducing the expression of a gene and transcription factor in the antennal lobes. Normally, gene expression is triggered by 'primer' pheromones, so it is probable that alarm pheromone has both releaser and primer qualities.
There is an observable quantitative effect of IPA on honey bee sting behavior that appears to be a function of the number of bees present. The relationship of IPA concentration to oxidative metabolism (indicator of pheromone perception) was measured to determine the sensitivity of worker bees to IPA when the number of bees per group was varied. Small groups (< 100 bees) were the most metabolically responsive to IPA exposure, and large groups (>100) failed to show a dose-dependency. A group of 40 bees was the optimum size for IPA responsiveness. In a group of this size the dose-response correlation was nearly perfect up to a dose of 2.4 ìg/mL, after which IPA responses plateaued; doses greater than 2.4 ìg/mL did not show an increased response (Southwick and Moritz 1985).
Although isopentyl acetate is the only chemical identified directly with sting-releasing activity, an array of other volatile hydrocarbons appears in extracts of the sting apparatus, some of which have an obvious role in defensive behavior (Wager and Breed 2000; Blum et al. 1978). In a combined laboratory and field assay of 11 alarm pheromone components, two of the compounds, 2-nonanol and octyl acetate, gave orientation information, respectively repelling and attracting bees. Isopentyl acetate was the only compound to affect the recruitment and flight behavior of honey bees, but in large concentrations it repelled bees, including drones (Wager and Breed 2000). The latter observation was surprising, considering drones do not participate in colony defense and thus do not need to respond to alarm pheromone. The chemoreception of drones was further investigated in an electroantennogram (EAG) study, in which the responsiveness of worker antennae and drone antennae to IPA was compared. Although drones lack a sting and do not exhibit any behavioral response to IPA, the EAG outputs for drones were nearly as great as those of workers (Vetter and Visscher 1997). The sensitivity of drone antennae to IPA may be due to the greater overall chemosensory potential of drones compared to worker bees.
Honey bee defensive responses are modulated by environmental conditions such as high humidity, heat, and nectar availability (Breed et al. 2004). However, a large portion of the defensive phenotype can be attributed to heritable factors. There is a very strong correlation between lifespan and heightened response to IPA, which merits attention when breeding bees for longevity and vigor (Rinderer et al. 1983). Genetic influences on defensive behavior are also evident from frequent observations that certain lines and races of bees are more 'aggressive' than others. The basis for aggression may be due to differences in the chemical composition of alarm pheromone. For example, at least nine of the components of sting alarm pheromone are produced in greater amounts in Africanized bees (Apis mellifera scutellata) than in the European honey bee (A. m. ligustica) (Hunt et al. 1999). Or, these defensive differences may be attributed to an increased sensitivity of neuroreceptors to alarm pheromones in certain genetic stocks.
Smoke has been used to suppress honey bee defensive behavior in managed hives for thousands of years. Until recently, it has been a mystery as to why smoke interrupts the succession of behaviors following alarm pheromone release. An explanation was offered by the results of an electroantennogram study, which compared the antennal responses of worker bees to isopentyl acetate and 2-heptanone before and after the addition of smoke. In both assays, the addition of smoke significantly decreased antennal responses. This effect was reversible, and the responsiveness of antennae gradually returned to normal within 10-20 minutes of removing the smoke. A similar effect occurred with a floral odor, phenylacetaldehyde, suggesting that smoke interferes with olfaction generally, rather than specifically with honey bee alarm pheromones. A reduction in peripheral sensitivity appears to be one component of the mechanism by which smoke reduces nest defense behavior of honey bees (Visscher et al. 1995).
Alarm pheromones are critically important to the survival of honey bee colonies. However, in relation to the small hive beetle (SHB), Aethina tumida, the honey bee’s alarm pheromones serve a negative function because they are potent attractants for the beetle (Torto et al. 2007). In addition, the beetles vector a strain of yeast, Kodamaea ohmeri, which produces an alarm pheromone mimic when it feeds on stored pollen. The environment of the European honey bee colony provides optimal conditions to promote the unique bee-beetle-yeast-pollen multitrophic interaction that facilitates SHB infestation of hives at the expense of the honey bee. The small hive beetle detects IPA at an even lower threshold than detected by the honey bee, so it is advisable to minimize agitating hives by frequently opening or disturbing them.
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Clarence Collison is a Professor of Entomology and Head of the Department of Entomology and Plant Pathology and Audrey Sheridan is a Research Technician at Mississippi State University, Mississippi State, MS. To comment on this article, or contact the author – firstname.lastname@example.org.