HortFACT
Identification
The adult is a shiny, metallic green beetle about 6 mm long. These beetles are conspicuous among the flowers of St John's wort, Hypericum perforatum, on sunny summer days.
The bright orange-red, elongate eggs are laid on prostrate winter foliage, usually on the underside of leaves.
Larvae are greyish in colour with dark heads. They are less conspicuous than adults as they spend much time among debris at the base of their host plants, ascending only to feed. The small first and second instars (growth stages) often feed within the shoot-tips, where they are well protected by developing leaves.
Prepupae and pupae exhibit a bright orange colour derived from hypericin, a quinone compound present in the host plant. Pupae are found in the soil or under stones at the base of plants.
Host plants
Chrysolina hyperici was introduced into New Zealand as a biological control agent for St John's wort, a toxic weed of hill and high-country pastures. The beetle is of European origin, and was imported from Australia in 1943. Although extensive starvation trials in Britain and Australia had shown that St John's wort beetle would not attack other plants, it was known that it could also feed on other Hypericum species and that there might be occasional damage to ornamental plants. There have been no complaints about ornamental plants being attacked, however.
Damage
Very large populations of adult beetles can build up, and may defoliate plants. Feeding by the larvae, although less spectacular, may also be highly damaging. Larvae often feed on new shoots, destroying new spring growth almost as soon as it is produced.
Distribution
St John's wort beetle is found throughout the country, even though the weed is not common in the North Island. It was released at many sites in New Zealand during the 1940s. The beetle is most common in drier parts of the South Island, where it sometimes attains high population levels, causing heavy defoliation of stands of St John's wort.
Life cycle
| Egg |  |
| Grub | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Adults can be found all year-round, but are most numerous in early summer when newly emerged beetles are feeding on the host plants. Later they disappear to aestivation ('over-summering') sites, often at the base of plants or under stones, and sometimes at a considerable distance from the host plants. In autumn these beetles reappear to feed, mate, and lay eggs.
Eggs overwinter on the prostrate growth of the plant, and hatch in the spring. Larvae feed particularly on the developing shoots, and can be very damaging to the plant. There are four larval instars. Pupation occurs in late spring and early summer, and the pupal stage lasts about a month.
P. SYRETT
Revised by R. HILL