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424 so that a connection can be established. The pheromone produced by the male boll weevil,

Anthonomus grandis, attracts only females in summer; however, in the fall it serves as an aggregation pheromone, attracting both males and females. Male Tenebrio molitor produce both a female attractant and an antiaphrodisiac that inhibits the response of other males to the female's pheromone. The pheromone-producing glands of males are much more varied in their location than those of females. For example, pheromone is secreted from mandibular glands in ants, from glands in the thorax and abdomen of some beetles, from glands at the base of the fore wings in some Lepidoptera, from abdominal glands in other Lepidoptera and cockroaches, and from rectal glands in certain Diptera. As in females, the attractants produced by males are usually long-chain alcohols or their aldehydic derivatives.

Because of their specificity and effectiveness in very low concentrations, the use of sex attractants in pest control has great potential, an aspect that will be more fully discussed in Chapter 24, Section 4.2.

Sexual maturation-accelerating or -inhibiting pheromones are produced by many insects that tend to live in groups, including social species. These substances serve either to synchronize reproductive development so that both sexes mature together or, in social species, to inhibit the reproductive capability of almost all individuals so that their energy can be redirected to other functions. Mature male desert locusts, for example, produce a volatile pheromone that speeds up maturation of younger males and females. The pheromone is a blend of five components, of which phenylacetonitrile appears to be the most critical (Mahamat et al., 2000). Conversely, gregarious juvenile desert locusts secrete a pheromone that retards the maturation of gregarious newly molted adults. This pheromone, which also serves as an aggregation pheromone for the nymphs, is a complex blend of chemicals, some produced by the epidermal cells and others associated with the feces (Assad et al., 1997). The pheromones appear to operate by regulating the activity of the corpora allata in other individuals.

Each colony of social insects has only one or very few reproducing individuals of each sex. Most members of the colony, the workers, devote their effort to maintaining the colony and never mature sexually. Their failure to mature results from the production by reproductives of inhibitory pheromones. In a honey bee colony the queen produces, in the mandibular glands, a material called queen substance, 9-oxo-2-decenoic acid (Figure 13.7B), which is spread over the body during grooming to be later licked off by attendant workers. Mutual feeding among workers results in the dispersal of queen substance through the colony where the pheromone serves to stimulate foraging activity (in older workers) and "household duties" (by young workers). The glands also produce a volatile pheromone, 9-hydroxy-2-decenoic acid (Figure 13.7C), which, together with queen substance, inhibits ovarian development in workers. As the queen ages and/or the number of individuals in a colony increases, the amount of pheromone available to each worker declines, and the latter's behavior changes. The workers construct queen cells in which the larvae are fed a special diet so that they develop into new queens. The first new queen to emerge kills the others and proceeds on a nuptial flight accompanied by drones. Queen substance produced by the new queen now serves as a sex attractant for the drones and to stimulate them to mate with her. When the new queen returns to the hive from the flight, part of the colony swarms; that is, the old queen, accompanied by workers, leaves the hive to found a new colony. Again, it is queen substance, along with 9-hydroxy-2-decenoic acid, that enables workers to locate and congregate around the queen. This is another example of how the function of a pheromone may vary according to the particular environmental circumstances in which the pheromone is released.

In the lower termites, in contrast to the honey bee, there is a pair of equally important primary reproductives, the king and queen. When a colony is small, the development of additional reproductives is inhibited by means of sex-specific pheromones secreted by the royal pair. The pheromones apparently are released in the feces and transferred from termite to termite by trophallaxis. Luscher (1972) suggested that juvenile hormone may be the pheromone that inhibits reproductive development, though how such an arrangement could act in a sex-specific manner has not been satisfactorily explained. In addition, there are indications that the king secretes a pheromone that enhances the development of female supplementary reproductives in the absence of the queen.

In some species of mosquitoes and other Diptera, Orthoptera, Hemiptera, and Lepi-doptera, the male, during mating, transfers to the female via the seminal fluid, chemicals that inhibit receptivity (willingness to mate subsequently) or enhance fecundity (by increasing the rate at which eggs mature and are laid) (Gillott and Friedel, 1977; Gillott, 2003). In contrast to other pheromones, the substances that are produced in the accessory reproductive glands or their analogue are proteinaceous in nature. Both receptivity-inhibiting substances and fecundity-enhancing substances signal that insemination has occurred. The former ensure that a larger number of virgin females will be inseminated; the latter, acting as they do by triggering oviposition, increase the probability of fertilized (viable) eggs being laid (in most species unfertilized eggs are inviable). Both types of pheromones serve, therefore, to increase the reproductive economy of the species.

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