Population trends of introduced beetles in forests, wetlands, and grasslands across Illinois over 20 years

Introduced beetles can cause widespread damage to crop and natural ecosystems, in part because they can alter populations of native insect species. Two mechanisms for this process are predation and resource competition. The exotic Japanese Beetle (Popillia japonica: Scarabaeidae) is a polyphagous herbivore, and exotic ladybeetles (Coccinellidae) are generalist predators of insects, including aphids (Apidae). There is a need to track the spread and impact of these beetles, particularly in the Midwestern United States where they pose economic threats to agriculture, sustainability threats to natural areas, and biodiversity threats to native species. We used a long-term dataset of plant and insect abundances collected from almost five hundred sites. The data were collected across Illinois by the Critical Trends Assessment Program (CTAP), with repeated visits in five-year intervals. Insects were sampled through sweep netting and are kept in long-term storage in an ethanol solution. We used 20 years of CTAP insect collections to track relative abundance of P. japonica, and native and exotic ladybeetles. We analyzed long-term population trends and geographic patterns of these beetles, and found that native ladybeetles have significantly declined from 2001-2020, while P. japonica has continued to spread west. We explored evidence of interspecific interactions and noted a possible displacement of one exotic ladybeetle by a more recently introduced exotic ladybeetle. In addition, we tested site characteristics associated with invasions, and found an association between beetle populations in agricultural and natural areas.