Overwintering habitat and the survival of the biocontrol beetle,Cassida rubiginosa

摘要

The leaf-feeding beetle, Cassida rubiginosa Muller, was introduced to New Zealand in 2007 as a biological control agent against the pasture weed, Cirsium arvense (L.) Scop. (Californian thistle). The beetle is now established in most regions, but outbreak populations have only been observed in one region (Wairarapa). The overwintering habitat type might affect the survival of the beetle and its ability to achieve outbreak populations. We tested the overwintering survival of C. rubiginosa in three different habitats: ra-diata pine forest, native forest, and pasture. In autumn 2016, adult diapausing C. rubiginosa were placed in overwintering cages established in the three habitats, and in spring their survival was assessed. Temperature and relative humidity was recorded in each habitat, and predatory spiders were also sampled. The total recovery rate of overwintered beetles was greatest in the native forest (42 ± 7.9%), followed by ungrazed pasture (32.5 ± 7.5%), and pine forest (7.5 ± 4.2%). Thetotal percentage recovered was not significantly different between the native forest and pasture habitats, but both of these habitats had greater recovery rates than the pine habitat. The native forest provided the best insulation against temperature andhumidity extremes on a daily basis, and across the winter season, and is the likely explanation for the greater recovery rates in that habitat. This study has provided initial evidence that the particular overwintering habitat may contribute to the success of this biocontrol agent, and suggests that native forest is likely to provide a better habitat than pine forest.