Behavioural and physical adaptations of three insects that feed on the phototoxic plant Hypericum perforatum

Chrysolina hyperici, C. quadrigemina (Coleoptera: Chrysomelidae), and Anaitis plagiata (Lepidoptera: Geometridae) all feed on St. John's-wort (Hypericum perforatum), in spite of the presence of the phototoxin hypericin, which has been shown to be toxic to other insects. Fourth instar Chrysolina spp. larvae fed at dawn, then hid in the soil for the remainder of the day. When larvae were forced to remain in the light, mortality was positively correlated with light intensity. This susceptibility to light may be related to the highly transparent larval cuticle which transmitted over 60% of the photosensitizing wavelengths (540–610 nm). Chrysolina adults differed from the larvae in that they were sun loving, fed on St. John's-wort during the day, and basked at the tips of the branches. Adult cuticles only transmitted 0.1–0.2% of the photosensitizing wavelengths. Third instar A. plagiata larvae remained on the plant at all times, feeding both during the day and night. In the 5th instar, larvae burrowed into the soil during the day and fed mostly at night, much like late instar Chrysolina larvae. However, A. plagiata larvae suffered no ill effects if forced to remain in the light. Part of their resistance to photosensitization may be explained by the low transmittance of their cuticle (20%).