Hydrocarbons of pest aphids and their detection by a braconid parasitoid

The research here reported describes the presence and semiochemical functions of cuticular hydrocarbons of cotton aphids {Aphis gossypii Glover), corn leaf aphids (Rhopalosiphum maidis Fitch), and greenbugs (Schizaphis graminum Rondani) Potential ecological pressures that may influence the production of hydrocarbons in geographically separated cotton aphid populations were considered. Both the alarm pheromone, (E)-^- farnesene, and a total of 14 n-alkane hydrocarbons from hexane extracts of cotton aphids collected from Lubbock and Brazos Counties, Texas, and Kern County, California, were identified by GC/MS analysis. The alkanes were common to both color varieties, but the relative proportions of the compounds differed significantly within each population. Among populations, however, hydrocarbons differed significantly not only in relative concentrations, but also in their types.

Next, the potential of using the hydrocarbon profiles of different aphid species in systematics was investigated. GC/MS analysis revealed 4, 5 and 14 «-alkane hydrocarbons, respectively, from the hexane extracts of corn leaf aphids, greenbugs, and both green and yellow varieties of cotton aphids collected from Lubbock and Hale Counties in Texas. Unlike the shorter chain compounds, the longer chain compounds between C25 and C29 were common to all three species, and were the most abundant hydrocarbons of each species. There were clear differences in the hydrocarbon profiles of apterous females among species and between color varieties of the cotton aphid, demonstrating that although the pests each produce many of the same compounds, when viewed as a whole, their hydrocarbon profiles are species-specific.

Lastly, the attractiveness of naive female parasitic wasps (Lysiphlebus teastaceipes) to various host-associated chemical cues was measured via a series of Y-tube olfactometer bioassays to investigate the foraging strategies of Z. testaceipes as influenced by the host-associated chemical cues. Semiochemicals of cotton aphids elicited stronger responses from Z. testaceipes than semiochemicals produced by the cotton plant. All the bioassays resulting in significant preferences by the wasps were aphid-related. Finally, separating the aphid extract into smaller fractions did not elicit a response from Z. testaceipes, indicating that this wasp only responded to the entire extract profile of the aphid.