Biological control of St. John's wort in New Zealand
Pauline Syrett, Manaaki Whenua - Landcare Research, PO Box 69, Lincoln, New Zealand.
Summary
Three insect species introduced for biological control of St. John's wort (Hypericum perforatum) have established in New Zealand. They are two leaf-feeding beetles, Chrysolina hyperici and C. quadrigemina, and a gall-forming fly, Zeuxidiplosis giardi. The earliest introduction, C. hyperici in 1943, is the most common and widespread of the three, occurring on St. John's wort plants throughout their distribution. The second leaf-feeding beetle (first released in 1963) was not rediscovered until 1984, and occurs in mixed populations with C. hyperici. These St. John's wort beetles occasionally outbreak to very large populations, causing complete defoliation of host plants. C. hyperici seems to be the dominant species in New Zealand, and studies of the reproductive diapause of the two species indicate that this is because C. hyperici's reproductive strategy is more successful in areas with colder winters. Z. giardi has a limited distribution in the northern part of the South Island, and where it occurs, plants appear stunted, with fewer flowers. There are no longer reports of areas in New Zealand where St. John's wort is a problem weed, and we conclude that successful biological control with insects is as least partially responsible for this change.
Plant Protection Quarterly (1997) 12 (2) 88-90.