The sesquiterpene, (E)-b-farnesene, is used by many aphid species as an alarm pheromone. Disturbed cotton aphids, Aphis gossypii Glover, released (E)-b-farnesene into the air as detected by SPME (solid phase microextraction) and identified by gas chromatography mass spectrometry (GCMS) by comparison to a synthetic standard. Solvent extracts of cotton aphids of various life stages and weights also were analyzed by GCMS for sums of ions 69 and 93, which discriminated (E)-b-farnesene from coeluting contaminants. Aphids of all life stages and sizes reared on cotton plants in either an environmental chamber or glasshouse contained (E)-b-farnesene in amounts ranging from 0.1 to 2.5 ng per individual. The quantities of (E)-b-farnesene per aphid increased with fresh weight, and variation in individual weights explained about 66% of the variation in alarm pheromone. However, the concentrations (ng/mg fresh weight) declined logarithmically with weight. These findings suggest that smaller aphids try to compensate by producing relatively more pheromone per weight than adults but still cannot approach an evolutionary optimal load, as assumed in adults, possibly due to the need to balance costs of growth with costs of producing the alarm pheromone.