The chemical composition of plant surfaces plays a role in selection of host plants by herbivorous insects. Once the insect reaches the plant, these cues determine host acceptance. Laboratory studies have shown that the stem borer Busseola fusca (Lepidoptera: Noctuidae), an important pest of sorghum and maize in sub-Saharan Africa, is able to differentiate between host and non-host plant species. However, no information is available on the cues used by this insect to seek and accept the host plant. Thus, the role of surface phytochemical stimuli on host selection and oviposition by B. fusca was studied in the laboratory using two host plants, sorghum, Sorghum bicolor, and maize, Zea mays, and one non-host plant, Napier grass, Pennisetum purpureum. The numbers of eggs and egg masses deposited on the three plant species were compared first under no-choice and choice conditions. In both cases, more eggs and egg masses were laid on maize and sorghum than on the non-host. Artificial surrogate stems treated with a water or chloroform surface extract of each plant were then compared with surrogate stems treated with, respectively, water or chloroform as controls, under similar conditions. Surrogate stems treated with plant water extracts did not show an increase in oviposition when compared to controls, indicating that the major compounds in these extracts, i.e., simple sugars and free amino acids, are not significantly responsible for the oviposition preference. By contrast, a chloroform extract of sorghum enhanced oviposition on the surrogate stems compared to the control, while those of maize and Napier grass showed no significant effects. Analysis of the chloroform extract of sorghum showed higher amounts of α-amyrin, ß-amyrin, and n-nonacosane compared to those of maize and Napier grass. A blend of the three chemicals significantly increased oviposition compared to the chloroform-treated control, indicating that these compounds are part of the surface chemical signature of the plant responsible for host recognition and oviposition by B. fusca.
Keywords: Africa; Epicuticular chemistry; Lepidoptera; Maize; Napier grass; Noctuidae; Nonacosane; Sorghum; Triterpenoids; n-alkanes; ß-amyrin; α-amyrin.