Caddisfly egg mass morphology mediates egg predation: potential costs to individuals and populations

Abstract

Summary Egg predation is seldom considered in life‐history studies of freshwater insects, but could be an important source of mortality with potential to limit population numbers. Costs of egg predation to prey can be considered at two levels: (i) fitness costs to individuals via reduced reproductive output; and (ii) population costs via reduced recruitment of benthic larvae. Larvae of Orthotrichia armata (Trichoptera: Hydroptilidae) feed on the egg masses of caddisflies in the Little River, central Victoria, Australia, but the small size of O. armata suggests that they may be unable to penetrate egg masses that are covered with a thick layer of spumaline jelly. We predicted that predation by O. armata would be more severe for egg masses with thinner spumaline. With field surveys throughout the peak caddisfly oviposition period (summer), we compared predation by O. armata on the egg masses of nine caddisfly taxa. Egg masses were categorised, according to the thicknesses of spumaline, into bulbous (thickly jellied), thinly jellied and jelly‐free morphotypes, with three species per morphotype. Bulbous egg masses were rarely consumed. Thinly jellied egg masses comprised just 4–13% of all egg masses in each survey, but attracted 52–93% of all egg predators. Jelly‐free egg masses were readily consumed, but O. armata were disproportionately scarce on them. In a second survey, we compared the proportions of eggs that were consumed from egg masses with small and large clutch sizes, by monitoring the thinly jellied egg masses of Taschorema sp. (598 ± 95 eggs per mass) and Ethochorema turbidum (1195 ± 192 eggs per mass) to eclosion. More than 25% of Taschorema sp. egg masses were entirely consumed, causing complete reproductive failure of parent females (fitness costs to individuals). Consumption rates were < 25% for most egg masses of E. turbidum. Orthotrichia armata consumed 48% of all Taschorema sp. eggs and 20% of E. turbidum eggs, suggesting that egg predation may substantially curb larval recruitment (costs to populations), particularly for species with small clutch sizes. Our results demonstrate that egg mortality varies for aquatic insect species that package their eggs in different ways. Combined with recent data from the same system, our results suggest egg mass morphology–mortality trade‐offs, with bulbous egg masses particularly vulnerable to flow forces (shear) and thinly jellied/jelly‐free egg masses particularly vulnerable to predation. Given the wide array of aquatic invertebrate species that lay single egg masses on hard substrata, it is feasible that this is a common trade‐off that warrants more research on the impacts of egg predation on aquatic species.