Chemical Ecology for Stored Product Insects

Abstract

Weevils represent a major group of pest insects for stored food products. Three important representatives were selected for our study because of their different host preference. They are: the maize weevil (Sitophilus zeamais), two bean weevils, (Callosobruchus chinensis) and (C. maculatus) and the yellow meal worm (Tenebrio molitor). The maize weevil infests several cereal grains, which are botanically quite different. The host preference is due to the emission of similar volatile ecochemicals which are responsible for the attraction of the weevil to the rice, maize and wheat. The attractants from these grains are C₅-C₇ fatty acids, 2-phenylethanol, 2-nonen-4-olide and nonan-4-olide. In the mating behavior of the yellow meal worm, a sex attractant and a copulation release pheromone are involved, with the former synergizing the latter. The former is (R) (+)-4-methyl-1-nonanol. Different ecochemicals are involved in the behavior of the bean weevil. In the field, the azuki bean weevil prefers only azuki bean and green gram for oviposition. Whether or not such host preference is due to ecochemicals in the plants is not known. The azuki bean weevil deposits eggs not only on the bean but also on any surface with specific curvature. On the other hand, the cow pea weevil oviposits only as a response to a chemical stimulant present in the seed coat. There are two prominant phenomena that are associated with the oviposition behavior of the two bean weevils. Under low population density conditions, the females oviposit on every among the beans, being guided by biological conditioning substance (BCS) which is deposited by the weevil on the surface of beans. Under high density conditions, a large number of eggs are randomly deposited on each bean, but only a few eggs hatch and grow normally inside each bean. The remaining eggs die by the ovicidal action of higher doses of the BCS. Thus, these weevils have developed a strategy to reduce competition among larvae and more fully utilize the host beans by using the same ecochemicals at different levels. Immediately after emerging from the beans, the azuki bean weevils begin their mating activities. A sex attractant released by the female attracts a nearby male. When they are in closer proximity, a second sex pheromone from the female induce male to erect and insert his genital organ and ejuculate. Thus, this copulation release pheromone was named “erectin” and is released from both male and female, but more from the latter. Erectin was found to consist of two synergistically acting fraction, neither having any activity. One is a mixture of several hydrocarbons of BCS; the other is (E)-3, 7-dimethyl-2-octenedioic acid. The bruchid weevils, C. chinensis and C. maculatus grow in many kinds of beans, but not in kidney beans, Phaseolus vulgaris. Extraction from kidney beans, chromatography and electrophoresis of the extract combined with bioassay for larval growth inhibition of C. chinensis using artificial beans resulted in the isolation of a growth inhibitor, a glycoprotein, whose molecular weight was ca. 48, 000 and isoelectric point, 4.46. The inhibitor was present at a level of 0.6% in kidney beans, and showed α-amylase-inhibitory activity, but it was not a trypsin inhibitor.