Silent assassins: predation of native New Zealand trichopteran eggs by non-native freshwater gastropods

Aquatic insects that oviposit on rocks may evolve strategies to mitigate loss of eggs to opportunistic predation by common native grazers, but such strategies may be ineffective against non-native grazers. We tested whether the eggs of common New Zealand caddisfly families (Hydrobiosidae and Hydropsychidae) that oviposit on rocks were more susceptible to predation by native or non-native snails. The native snail Potamopyrgus antipodarum (Grey, 1843) and non-native snails Physa acuta Draparnaud, 1805 and Pseudosuccinea columella (Say, 1817) were presented with caddisfly egg masses and the number of individual eggs consumed was recorded after 24 and 48 hours. The larger Pseudosuccinea readily consumed the eggs, especially spumaline-encapsulated Hydrobiosidae eggs. Physa consumed very few Hydrobiosidae eggs and no Hydropsychidae eggs, whereas Potamopyrgus did not consume eggs of either family. The trichopteran egg masses tested did not succumb to predation by the native Potamopyrgus but are highly vulnerable to predation by a larger non-native snail.