Sesquiterpeno

Naturwissenschaften (2005) 92:69-72

John A. Byers

A cost of alarm pheromone production in cotton aphids, Aphis gossypii
Western Cotton Research Laboratory, USDA-ARS, 4135 East Broadway Road, Phoenix, Arizona 85040-8830, USA

Abstract The sesquiterpene, (E)-β-farnesene, is used by many aphid species as an alarm pheromone to warn related individuals of predation. Disturbed cotton aphids,Aphis gossypii Glover, released (E)-β-farnesene into the air as detected by SPME (solid phase microextraction) and gas chromatography mass spectrometry (GC-MS). Solvent extracts of cotton aphids of various life stages and weights also were analyzed by GC-MS for sums of ions 69 and 93, which discriminated (E)-β-farnesene from coeluting compounds. Aphids of all life stages and sizes reared on cottonplants in both an environmental chamber and glasshouse contained (E)-β-farnesene in amounts ranging from 0.1 to 1.5 ng per individual. The quantities of (E)-β-farnesene in aphids increased in relation to increasing body weight, and variation in individual weights explained about 82% of the variation in alarm pheromone. However, the concentrations (ng/mg fresh weight) declined exponentially withincreasing body weight. These findings indicate that aphid nymphs try to compensate by producing relatively more pheromone per weight than adults but still cannot approach an evolutionary optimal load, as assumed in adults with the greatest total amounts. This suggests that young aphids need to balance costs of growth and maturation with costs of producing the alarm pheromone.__________________________________________ Introduction
The sesquiterpene, (E)-β-farnesene, is the alarm pheromone of aphid species in over 30 genera, including the cotton aphid Aphis gossypii Glover, (Kislow and Edwards 1972; Bowers et al. 1972; Edwards et al. 1973; Wientjens et al. 1973; Pickett and Griffiths 1980; Mondor and Roitberg 2000; El-Sayed 2004). An aphid grabbed by forceps, mimicking a predator, or byan actual predator will release droplets from its cornicles that contain (E)-β-farnesene (Kislow and Edwards 1972;

Nault et al. 1973; Nault and Bowers 1974; Pickett and Griffiths 1980). Studies have shown that (E)-β-farnesene interrupts aphids from feeding and causes them to begin locomotion and often drop off the leaf in attempts to avoid predators that otherwise would devour multiple aphids(Nault and Bowers 1974; Montgomery and Nault 1977; Wohlers 1982; Pickett et al. 1992). Aphid colonies are often comprised of genetically identical individuals since a founding mother deposits clonal daughters that mature and produce additional parthenogenetic daughters (Pickett et al. 1992; Mondor and Roitberg 2000). Therefore, genes for production and detection of an alarm pheromone, while notsaving the aphid preyed upon, would increase its inclusive fitness by warning related individuals nearby to escape the predator. Aphids would maximize their fitness if all individuals in the colony could produce a uniform and optimal quantity of alarm pheromone to warn other clonal members of a predator in the vicinity. It is not known if first instar or juvenile aphids produce alarm pheromone. Anincreasing quantity of alarm pheromone per individual in relation to an increase in size and instar would suggest a cost of pheromone production that smaller nymphs could not afford compared to larger ones. Although producing minute quantities of pheromone requires some energy, the question in chemical ecology has been whether these costs are significant or in fact constrain amounts released. Forexample, bark beetles and female moths typically release ng quantities of aggregation and sexpheromones that represent only 0.0001 percent or less of their weight (Baker 1989; Byers 1989). No studies have reported a cost of pheromone production in insects because the random variation in adult size and pheromone quantities appear to vary independently (Birgersson et al. 1988; Sappington and Taylor...