How does a carabid predator find aggregations of slugs in the field? Electroantennograms and behavioural assays suggest chemical cues

AbstractField studies suggest that the generalist carabid predator and scavengerPterostichus melanariusIlliger aggregates to patches with a higher density of slug prey. The mechanisms behind such aggregation are unknown. Experiments were conducted to test the hypothesis that they are responding to chemical cues. Electroantennograms (EAGs) showed no response byP. melanariusantennae to live, nematode-infected, injured or freshly killed slugs, but a strong response to dead slugs that had been allowed to decay for up to 48 h. Such a response would enable the beetles to find carrion in the field and may also, as dead prey are likely to be spatially correlated with live ones, provide a mechanism by whichP. melanariusfinds patches with a higher density of live slugs. Subsequent video analyses ofP. melanariusresponses to patches of slug mucus within arenas showed that beetles with intact antennae could detect these patches because they spent more time, and moved greater distances, within them and increased their rate of turning. Thus, at close range,P. melanariusused their antennae to detect slug mucus and hence, by implication, live slugs. The apparent contradiction between EAG data and behavioural experiments is discussed. Together these result confirm thatP. melanariusdoes respond to chemical cues from its slug prey in ways that could lead to aggregation in areas of higher slug density in the field.