Directed chemical spray of the peppermint stick insect (Megacrania batesii) is induced when predation risk is at its highest

The risk of mortality during a predator–prey sequence is at its greatest when a predator makes contact with the prey. Theoretically then, if prey have an active secondary defence, it makes strategic sense that prey should attempt to deter or dissuade a predator prior to contact. Here, we investigated the triggers for the dynamic spraying behaviour of the peppermint stick insects (Megacrania batesii) relative to predatory threat status. A variety of non-contact and contact cues representing an escalation in predation risk were applied to the insects. None of the non-contact treatments elicited a spraying response, but instead contact was required. Contrary to our expectations, the likelihood of a spraying event depended on the type of contact and even the particular body regions contacted. The strongest response came from contact that prevented escape, while contact with body regions of higher survival significance (i.e. thorax, abdomen) elicited significantly higher response to body parts less vital to survival (such as tarsi). Staged ‘attacks’ revealed that contact with the initially less ‘vital’ body regions was more likely to trigger a spray response when ‘vital’ regions were subsequently contacted. In light of our findings, we argue that waiting to spray upon contact potentially increases the likelihood of successively deterring predators and we highlight the need to establish a predation risk paradigm for the strategic deployment of chemical defences.