Asynchronous declines of native and exotic insect predators reduce pest suppression potential in agriculture

Insect declines in agricultural landscapes have been reported as part of the larger biodiversity crisis, but long-term trends are difficult to assess due to natural population fluctuations, interactions with exotic species, and lack of consistent data. Here, we report on population trajectories of a community of predaceous lady beetles (Coleoptera: Coccinellidae) monitored annually over 31 years in a long-term agroecosystem experiment in southwestern Michigan, United States. The total lady beetle community has declined in abundance by 39% during this time, with native species exhibiting larger declines (77%) than exotics (23%). However, these gross trends mask apparent periods of stability, recovery, and, in some cases, very sharp declines lasting many years across individual species and groups. Native and exotic communities exhibit significant nonlinear abundance trends that are often asynchronous in time. Habitat perenniality moderates these patterns, with communities in annual crops exhibiting large changes and those in perennial crops following similar but less pronounced trajectories, while communities in forested habitats remain stable or exhibit gradual, nearly linear declines. Several once-common natives have fallen below detection limits, with six of 10 species not detected on an annual basis in the last 5 years. Over the full length of the study, the pest suppression potential of the entire community has declined 42%, threatening this valuable ecosystem service, which may be further undermined by the effective loss of functionally redundant species. The evidence for varying drivers of these patterns provides unique insights into the broader phenomenon of insect decline.