Mechanisms of action of some vegetable oils against Sitophilus zeamais motsch (Coleoptera: Curculionidae) on wheat

The insecticidal activity of some fixed vegetable oils against Sitophilus zeamais on wheat was investigated. Test oils from different sources and levels of purity effectively reduced the development of progeny by similar amounts. When all wheat grains were treated with oil, X-ray studies showed that oviposition by S. zeamais was not affected, although there was considerable subsequent mortality. However, in a two-way choice experiment S. zeamais showed an oviposition preference for grains not treated with oils. Freshly applied oils caused some mortality in adult S. zeamais only at rates above 10.5 ml/kg, but this effect disappeared when treated grains were aged for 14 days before bioassay. Application of oils at up to 17.5 ml/kg to grains pre-infested with late stage larvae/pupae confirmed that these oils had no appreciable effect against such stages. X-ray studies established that the main toxic action of the oils was against the eggs and early stage larvae.