Fitness costs of cornicle secretions as a defense mechanism for cotton aphid, Aphis gossypii (Hem.: Aphididae)

The cornicle secretion is a defensive mechanism in many aphid species to warn the related individuals of predation by natural enemies. Many researches have been conducted on the benefits of cornicle droplet and alarm pheromone but the cost of this phenomenon is poorly investigated. This study is intended to evaluate the direct fitness cost of cornicle secretion of immature as well as mature cotton aphids، Aphis gossypii Glover. Aphids were artificially forced to produce cornicle droplets at different stages of their development (second، third and fourth instars as well as adults). They were lightly stroked on the anterior portion of the thorax with a fine brush، resulting in the secretion of visible cornicle droplets. After this manipulation، life-history parameters of aphids were recorded until the death of the last adult individual. Experiments were conducted in a growth chamber at 25 ± 1°C، 50 ± 5% RH and a photoperiod of 16 L: 8 D hours. The results showed that the secretion of cornicle droplets by second and third instar nymphs of cotton aphid did not affect their survivorship and the number of offspring produced by their adult stage. In contrast، fourth instar nymphs as well as adults that emitted cornicle droplets had significantly lower survivorship and offspring production than non-secretors. The cornicle secretion has also fitness costs on the late instar and adult cotton aphids.