Egg parasitoids are able to find their hosts by exploiting their chemical footprints as host location cues. In nature, the
apolar epicuticular wax layer of plants that consists of several classes of hydrocarbons serves as the substrate that retains
these contact kairomones. However, experiments on chemical footprints generally have used filter paper as substrate to study
insect behavior. Here, we explored the ability of
Trissolcus basalis (Scelionidae) females to discriminate between footprint cues left by male and female
Nezara viridula (Pentatomidae) on leaves of their host plant
Brassica oleracea (broccoli). Furthermore, we analyzed the chemical composition of the outermost wax layer of broccoli leaves to evaluate the
degree of overlap in insect and plant cuticular hydrocarbons that could lead to masking effects in the detection of footprint
cues. Our results showed that
B. oleracea epicuticular wax retains the chemical footprints of adult bugs and allows
T. basalis females to differentiate hosts of different sex. Traces of female bugs elicited more extensive searching behavior in egg
parasitoids than traces of males. The application of
n-nonadecane, a compound specific to male
N. viridula, on the tarsi of female bugs prevented parasitoid females from distinguishing between host male and host female footprints.
Analyses of
B. oleracea leaves revealed that epicuticular waxes were mainly composed of linear alkanes, ketones, and secondary alcohols. Alkanes
were dominated by
n-nonacosane (
nC29) and
n-hentriacontane (
nC31), while male-specific
n-nonadecane (
nC19) was absent. The ecological significance of these results for parasitoid host location behavior is discussed.
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