Invasive moth facilitates use of a native food plant by other native and invasive arthropods

Organisms that invade new habitats exploit new resources or niches and influence native species. Here, we examine how an invasive moth, the parsnip webworm (Depressaria radiella, formerly D. pastinacella), facilitates interactions with other arthropods in spatially separated populations of native cow parsnip (Heracleum maximum) in the Rocky Mountains (New Mexico and Colorado). We compare this with results on small hogweed (Heracleum sphondylium) in the Netherlands, where both the plant and herbivore are native. Larvae of D. radiella feed in webs on ripening fruits of their food plants. Mature caterpillars descend the hollow stems into which they chew a hole, enter the stem and pupate. Other arthropods enter the stems through these holes. Plants in all populations of cow parsnip/hogweed contained either moth pupae and/or webworms mummified by their main parasitoid, Copidosoma sosares that also been introduced into parts of the United States. In both countries, earwigs (Forficula auricularia), which are also invasive in the United States, were the dominant arthropod to utilize webworm-perforated stems, although there was more within-site variability in abundance of earwigs in the United States than in the Netherlands. The woodlouse, Porcelio scaber, which is native to Eurasia but also established in the United States, was abundant in stems of Dutch hogweeds but absent in stems of American cow parsnips. Other native herbivores (e.g., mirid bugs), were collected in stems at sites in both continents. Moreover, the number of various arthropods found in perforated stems correlate positively with the number of holes found in these stems.