Characterization of the brood cuticular hydrocarbons of the fire ant Solenopsis Invicta and their possible role in brood recognition by workers

Chromatography and mass spectrometry analysis were used to identify and quantify 14 cuticular hydrocarbons in the developmental stages of S. invicta. The following quantitative and qualitative differences in hydrocarbons were observed among brood components. Cuticular hydrocarbons consisted of saturated n-alkanes and methylalkanes, ranging from C23 to C29. The total amount of cuticular hydrocarbons was greatest in prepupae and least in pupal stages. While the total amount of cuticular hydrocarbons was several magnitudes greater in components of sexual brood, as expected since sexual forms are larger than worker forms, the ratio of n-alkanes to methylalkanes in worker brood was generally higher than its counterparts in sexual brood. Discriminant function analysis of the brood subcastes using the five most abundant hydrocarbons within and among colonies showed that the brood subcastes clustered together based on their respective developmental stage, with n-heptacosane being the major component responsible for the observed clustering.

Finally, several bioassays were conducted to determine whether cuticular hydrocarbons play a role in brood recognition. Worker retrieval response toward pupae immersed in hexane for 5 min was significantly lower than that towards a control. Immersing pupae in boiling water for 3 sec. also induced a similar response to that caused by hexane. Several concentrations of cuticular extracts in hexane of sexual prepupae applied to a paraffin dummy failed to induce any retrieval response by workers. In addition, manipulation of the composition of brood cuticular hydrocarbons did not seem to affect worker response toward the brood. Taken together, the results suggest that a chemical signal other than cuticular lipids is involved in brood recognition.