Parasitoids use chemical footprints to track down caterpillars.

Parasitoid wasps in search for plant-feeding hosts display typical patterns of behavior. The braconid Cotesia marginiventris, which parasitizes young caterpillars, is guided by herbivore-induced plant volatiles to an infested plant. On the plant, the female wasp searches for further chemical residues (kairomones) originating directly from the host. We showed that caterpillars leave minute amounts of treacherous chemical footprints while walking over a plant surface. Female wasps are able to detect these residues for up to two days after their hosts have left the site. Analyses of the caterpillar footprints revealed that these consisted of linear and monomethyl-branched alkanes as well as few minor unidentified compounds. A reconstructed blend of the major footprint compounds, consisting of linear C(21)-C(32) alkanes, induced characteristic antennation behavior. However, the artificial blend was less attractive than the original one suggesting a role for additional minor compounds in recognizing former caterpillar presence. Previous investigations using wax mutants of barley showed that the physico-chemical traits of the epicuticular leaf wax can modulate the parasitoids' response to host footprints. We hypothesize that long-chain hydrocarbons of insect and plant cuticular origin are important mediators of insect-plant interactions and believe that their role in modulating trophic cascades still awaits full appreciation.