The
sesquiterpene, (E)-
beta-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)-
beta-farnesene into the air as detected by 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)-
beta-farnesene from coeluting compounds. Aphids of all life stages and sizes reared on cotton plants in both an environmental chamber and glasshouse contained (E)-
beta-farnesene in amounts ranging from 0.1 to 1.5 ng per individual. The quantities of (E)-
beta-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 with increasing
body weight. These findings indicate that aphid nymphs try to compensate for their smaller size 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.