Is ragwort flea beetle (Longitarsus jacobeae) performance reduced by high rainfall on the West Coast, South Island, New Zealand?

摘要

The New Zealand biological control programme against ragwort, Senecio jacobeae L., began in the 1920s, and four biological control insects have been released. The ragwort flea beetle, Longitarsus jacobaeae (Waterhouse) (Coleoptera, Chrysomelidae), wasintroduced from Oregon into the field in 1983. The beetle has established and has had a major impact on ragwort plant populations in many areas of New Zealand, but not on the West Coast of the South Island. Field studies looked at sites where the beetlehad established to determine why it had not reduced ragwort populations. The abundance of ragwort plants (13/m~2) and of flea beetles (three per rosette) was recorded at five sites. The biology of flea beetles and ragwort plants at each site were also recorded and rainfall data were collected. The inability of the ragwort flea beetle to reduce ragwort populations in some areas in New Zealand may be due to the high density and rapid growth of ragwort plants, possibly due to higher rainfall, and this mayalso have a negative effect on flea beetle populations. Numbers of flea beetles per plant was lower on the West Coast (four per rosette) than East Coast sites (ten per rosette). The density of ragwort plants was higher on the West Coast (13/m~2) than the East Coast (47m~2).