Spittlebug invisibility cloak: experimental tests on the antipredatory effect of the froth of Philaenus spumarius

In Europe, the meadow spittlebug Philaenus spumarius (L.) (Hemiptera Aphrophoridae) is the main vector of the bacterium Xylella fastidiosa Wells et al., the etiological agent of the Olive Quick Decline Syndrome. The froth produced by spittlebug nymphs has a primary function in protecting the insect from dehydration and thermal stress. It is also accepted that the froth protects nymphs from predators, although the underlying mechanism is not completely clear. We investigated such a process using the crab spider Synema globosum (F.) and the ant Crematogaster scutellaris (Olivier) as model species. Nymphs of P. spumarius were divided into two groups, one whose froth was left and one whose froth was removed. The nymphs were then exposed to predators and their survival recorded. The survival of defrothed nymphs was considerably lower than controls with both spiders and ants, though this could be due to increased motility of defrothed nymphs. Moreover, to test the chemical properties of the froth and exclude any physical hindrance effect, P. spumarius nymphs and dead Sarcophaga carnaria (L.) larvae (maggots) under three different conditions (defrothed, water-coated, and centrifuged froth-coated) were offered to workers of C. scutellaris. The survival of the nymphs and ants' bites to both preys were recorded. Again, defrothed nymphs showed a lower survival probability compared to those moistened with water and froth, while no differences were found between these two treatments, suggesting a chemical deterrence or mimicry of the froth. The highest number of ants' bites towards nymphs and maggots was recorded in the defrothed group, while the lowest in the froth-coated nymphs. A significant difference between the water- and froth-coated treatments was only found in nymphs and not in maggots, suggesting the presence of some residual substances on the nymph's integument that could have a deterrent or masking effect. Additionally, our direct observations of ants drinking the froth reinforce chemical mimicry as a more plausible explanation than deterrence. In conclusion, our findings suggest that the froth plays an antipredatory role, at least for predators that use mainly olfactory cues to localise their preys, through a chemical mimicry mechanism.