Aspects of the biology of the St. John’s wort beetle, Chrysolina hyperici (Forst.), relating to its role as a biocontrol agent

Abstract

A questionnaire on St. John's wort was presented to all South Island Noxious Plants Officers. Responses show that St. John's wort biocontrol is perceived as only partially effective. Areas of serious infestation remain, but no alternative to biocontrol is practical.

By means of insecticide interference with biocontrol at an experimental site near Lake Tekapo the findings of Syrett and Hancox (1985) were confirmed, namely that biocontrol by Chlysolina hyperici reduced St. John's wort shoot numbers during the important spring stock grazing period. After spring the beetles had no measurable effect.

Food consumption of laboratory-reared C. hyperici larvae was measured by weighing food sprigs before and after feeding periods. The fourth (last) instar larvae consumed 82% of the total food consumed during the larval period and showed the highest rate of consumption.

Laboratory experiments using controlled daylength cabinets showed that whereas moistening of aestivally diapausing C. hyperici does not break their diapause, a change in day length from sixteen to eight hours of daylight does break diapauses, initiating heavier feeding, reproductive maturation, mating and oviposition. The necessity for "photoperiod matching" of biocontrol agents is emphasized.

Measurements of field-collected insects' flight muscles showed that after peaking in late summer (early in aestivation) the muscles shrank during autumn as the insects became reproductive. It is argued that the benefit of such shrinkage lies in the mobilization of nutrients rather than in vacation of body space or enforcement of winter flightlessness.

Quarantining of the host plant's oil-soluble toxin hypericin in fat or oil globules may have led to a reliance by C. hyperici on muscle tissue as a "negotiable" nutrient store.