Since the first observation that plants contained protease inhibitors, as identified by their ability to inhibit vertebrate enzymes, it has been postulated that the presence of these substances was related to their phytoprotective abilities. However the following assumptions (1) that phytophagous insects use trypsin, and (2) that ingested inhibitors disrupt digestive proteolysis in insects, have not been adequately tested. Identification of non-tryptic enzymes, cathepsin B, D, and H in phytophagous Coleoptera and unique trypsin-like enzymes in Lepidoptera, indicates insect proteases may differ from their vertebrate counterparts. Putative inhibitor proteins inhibited vertebrate trypsin and chymotrypsin in vitro but had no effect on the trypsin- or chymotrypsin-like activity from the insect midgut. Feeding experiments with the European corn borer, Ostrinia nubilalis (Hübner), indicate that ingestion of inhibitors may not disrupt digestive proteolysis in vivo and the vertebrate trypsin inhibitor in corn may be ineffective as a phytoprotective strategy for this insect. Limitations and implications of ingested inhibitors for future pest control may depend on the origin of the inhibitor, as well as the insect's response.

The 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.

Leaves of the neem tree, Azadirachta indica A. Juss. (Meliaceae), have been used for centuries in India to protect stored grain from insect attack. Recent concern for the effects of synthetic insecticides on the environment and human health has provided the impetus for a reappraisal of the utility of neem derivatives as botanical crop protectants. Neem and its active principle, the limonoid azadirachtin, have several properties that are highly desirable for a natural insecticide. Azadirachtin is the most potent natural insect antifeedant discovered to date, suppressing insect feeding at concentrations of less than 1 part per million. Azadirachtin is also a potent insect growth regulator, which acts by disrupting molting and development, and interfering with reproduction in adult insects. These actions have been observed in over 90% of the more than 200 species of pest insects tested to date. Neem also has systemic action in some plants which could prove extremely valuable against stem- and root-feeding pests that are difficult to control. Finally, neem is essentially non-toxic to vertebrates, and in fact has a long history of medicinal use in southeast Asia.In the present paper we review our research aimed at developing a neem-based insecticide for use against pests of Canadian agriculture. We document the potent antifeedant action of azadirachtin in laboratory bioassays against the European corn borer, Ostrinia nubilalis Hübner, and the variegated cutworm, Peridroma saucia Hübner, and the molt-disrupting action of the compound in the migratory grasshopper, Melanoplus sanguinipes Fab., and the large milkweed bug, Oncopeltus fasciatus Dallas. Both antifeedant and insect growth regulatory activities of various samples of neem seed oil are shown to be correlated to azadirachtin concentrations in the oils. Field trials of an experimental neem insecticide conducted against pests of crucifers, corn, and potato in British Columbia, Ontario, and Prince Edward Island, respectively, indicate that the neem insecticide provides pest control as effective as or better than pyrethrum, the current botanical insecticide of choice for organic growers. The commercial prospectus for neem insecticides in Canada is discussed in light of our results.