AbstractThe thiophenes alpha-terthienyl and methyl-alpha-terthienyl are found in many species of the family Asteraceae and are highly phototoxic to mosquito larvae. These compounds and a synthetic analogue, cyano-alpha-terthienyl, controlled Aedes intrudens Dyar (Diptera: Culicidae) larvae at application rates between 10 and 40 g per hectare in field trials. These concentrations are similar to those currently used with chemical control agents. Piperonyl butoxide, a synergist used with pyrethrin, greatly increased the mortality of mosquito larvae at low application rates of the most potent phototoxin, cyano-alpha-terthienyl.Although we have demonstrated previously that these phototoxic defences are effective against some phytophagous insects, more recently we studied insects that are able to feed on a phototoxic plant, in order to examine modes of resistance to phototoxins. Chrysolina spp. (Coleoptera: Chrysomelidae) larvae are susceptible to phototoxicity but avoid it by feeding on Hypericum perforatum L. (Hypericaceae) at dawn and by hiding during the day. Chrysolina adults avoid phototoxicity by the presence of opaque cuticles that block the sunlight. First-instar larvae of Anaitis plagiata (L.) (Lepidoptera: Geometridae) avoid feeding on the glands that contain the phototoxin. Later-instar larvae feed on the entire leaf, yet are not susceptible to phototoxicity, indicating they have biochemical defenses against photo-induced damage.