Predation on Popillia japonica (Coleoptera: Scarabaeidae) by the Native Robber Fly Machimus rusticus (Diptera: Asilidae): Molecular Detection and Laboratory Experiments

Popillia japonica Newman, 1838, is an invasive scarab that has rapidly spread in northern Italy since 2014, causing damage to crops, turfgrass, and ornamental plants. Native predators may exploit this newly available prey and contribute to its natural regulation. Here, we investigated the predatory role of the native robber fly Machimus rusticus (Meigen, 1820) against P. japonica by combining field observations, molecular detection of prey DNA, and laboratory experiments. Machimus rusticus was repeatedly observed in agricultural and semi-natural habitats within the invaded area, and its larvae were found in soils containing Japanese beetle larvae. Japanese beetle DNA was detected in field-collected M. rusticus larvae and adults, providing molecular evidence of prey consumption under natural conditions. Laboratory experiments provided evidence for predatory capacity of M. rusticus larvae, with P. japonica mortality ranging from 65% to 100% within two weeks. Overall, our results showed that M. rusticus can exploit P. japonica at both larval and adult stages, suggesting a potential role in natural regulation, but further field studies are needed to determine whether predation contributes to population-level regulation and conservation biological control.