Do invasive terrestrial invertebrates subsidize north-temperate fish populations? The case of the spongy moth (Lymantria dispar dispar)

Invasive species can achieve incredibly high densities in invaded ecosystems, introducing a novel resource base for willing consumers. Hyperabundant invaders in one ecosystem that spillover into adjacent ecosystems (e.g. terrestrial to aquatic) create new opportunities for multichannel omnivory, whereby generalist consumers feed on prey from different trophic levels and ecosystems. However, our understanding of how invasive organisms originating in one ecosystem are utilized by consumers in adjacent ecosystems remains poorly studied. The spongy moth (Lymantria dispar dispar; LDD) is an invasive invertebrate that exhibits cyclical hyperabundance, with larvae defoliating millions of hectares of deciduous forest during regular outbreaks in eastern North America. We sought to determine if larval LDD could represent an impactful spring and early summer resource for native fish species during years of high larval abundance. Here, we quantified the diets of pond-dwelling largemouth bass (Micropterus nigricans) and pumpkinseed sunfish (Lepomis gibbosus) during 2020 and 2021 respectively, two historic outbreak years for LDD in Ontario, Canada. Both pumpkinseed and largemouth bass failed to exhibit meaningful exploitation of LDD larvae, regardless of their overwhelming abundance. Of 315 pumpkinseed sampled across four pond populations from April to August of 2021, only two contained individual LDD larvae. Of the 82 largemouth bass sampled in June 2020, only 1 individual contained a single LDD larvae. Serendipitously, we discovered one pumpkinseed population relied heavily on a different invasive terrestrial invertebrate, the earthworm (60% of all pumpkinseed stomach contents by mass in early spring). Though hyperabundant LDD larvae appeared to be largely avoided by fish predators, a less well-defended invasive invertebrate (earthworms) acted as a terrestrial subsidy for a native fish. Despite LDD larvae not being consumed by fish, the replacement of leaf litter with the carcasses and frass of LDD larvae could represent an important modification to detrital food webs in ponds and lakes. Thus, understanding how invasive species impact both resident consumers and nutrient cycling will be critical for the appropriate management of invasive species and their resident food webs moving forward.