Sylvie DERRIDJ, Ben Rui WU and Philippe COUZI.
INRA, Unité de Phytopharmacie et des Médiateurs Chimiques, Route de St Cyr, 78046 Versailles, FRANCE

In fields of Russia when sorghum and maize are grown simultaneously, Ostrinia nubilalis oviposition preference changes throughout the growth stages. There is no preference at the early stages, then the insect prefers maize to sorghum, and when the maize is flowering and maturing the insect prefers sorghum.

We already demonstrated that on the maize leaf surface are present primary metabolites coming from the leaf tissues. These substances give physiological and plant specific information. Ostrinia nubilalis females have sensorial sensillae on legs and ovipositor which detect soluble carbohydrates and malic acid and we observed that host selection for oviposition is linked to these substances.
To explain the insect shift amongst the two plants along with the development, we analyzed the biochemical composition of the sorghum leaf surfaces in soluble carbohydrates and malic acid throughout growing stages and compared it to maize.
Sorghum (cultivar: kubanskoe krasnoe 1677) coming from Russia and maize (hybrid: Dea from Pioneer) were grown in phytotronic chamber and green-houses. Substances were collected by water spraying at the sunset, derivatized by silylation and quantified by gas chromatography.

Compared to the other plant species already studied, the proportions of soluble carbohydrates found on sorghum leaf surface are very near from those of maize and different from the others. Concentrations varied according to leaf position and growth stage. So the oviposition preference of O. nubilalis between maize and sorghum could be explained by variations of common metabolites related to plant development. Since sorghum takes more time to reach the mature stage than maize, this delay in growth stage produces differences in metabolite concentrations which finally induce host selection shifts for oviposition.