Parasitoid avoidance behavior is not triggered by airborne cues in a semi-aquatic bug

Until recently, studies on host-parasitoid interactions have rarely reported the detection and resistance of parasitoids by hosts prior to experiencing a direct attack. Several recent studies have indicated that some hosts have an ability to detect parasitoids without being attacked directly. However, the particular cues used for parasitoid detection have received little attention. We investigate the use of airborne cues (such as volatile chemicals or sound) by the water strider Aquarius paludum insularis (Heteroptera: Gerridae) for detecting its egg parasitoid wasp. Since A. paludum females with previous exposure to the parasitoids oviposit at deeper positions relative to unexposed females, we were able to infer whether A. paludum had detected the parasitoid from oviposition depth. We allowed A. paludum to oviposit after one of the three treatments: exposure to the parasitoid directly, airborne cues (the parasitoid was enclosed in the tube of which ends were sealed by mesh), or wasp absent. The oviposition depth was shallower in A. paludum with exposure to airborne cues than in A. paludum with exposure to the parasitoids directly. The depth did not differ between airborne cues and wasp absent treatments. These results indicate that A. paludum were not able to detect the parasitoid from airborne cues alone.