Predicting spillover risk to non-target plants pre-release: Bikasha collaris a potential biological control agent of Chinese tallowtree (Triadica sebifera)

Abstract

Quarantine host range tests accurately predict direct risk of biological control agents to non-target species. However, a well-known direct effect of biological control of weeds releases is spillover damage to non-target species. Spillover damage may occur when the population of agents achieves outbreak densities, depletes the target weed, and some feeding occurs on non-target species. Similar to the assessment of direct risks to non-target species, we assessed the risk of spillover damage to non-target species pre-release. Quarantine experiments were conducted on the flea beetle Bikasha collaris (Coleoptera: Chrysomelidae), a potential biological control agent of Chinese tallowtree, Triadica sebifera, a weed of wetlands, forests, and natural areas in the southeastern U.S.A. Recently emerged, naïve B. collaris adults were fed leaves of T. sebifera (target species), three close relatives that are non-target species (Ditrysinia fruticosa, Gymnanthes lucida, Hippomane mancinella), or water. Adult longevity, egg production, and leaf damage were greatest on the weed and near zero or zero on the non-target species and water control. A spillover event was simulated by transferring adults to non-target species or water after feeding on T. sebifera for 2 or 4 weeks and found similar longevity and egg production as those fed only non-target species or water. These results indicated pre-release that when adults that have fed on T. sebifera are forced to spillover onto the non-target species, there is no risk of damage and that B. collaris will only be able to sustain populations on the target weed.