The most important insect pests on maize in Spain are the pink stem borer S. nonagrioides (Lefèbvre) and the European corn borer O. nubilalis (Hübner). Parasitoid decree of control on stem borers and other maize pests in Spain is not known. The aim of this research was to evaluate the incidence and diversity of parasitoids on maize stem borers in the province of Pontevedra (NW Spain). Samplings of hybrid field maize plants (DMB-1570) were taken in four locations, from March 1999 to November 2000. Plants were dissected and stem borer overwintering and non-overwintering larvae were collected in separated plastic boxes. Larvae of Mythimua spp. and Helicoverpa armigera were also collected. Larvae were reared in the insectary and adult parasitoids obtained from them were determined. Diversity of parasitoids was in general very low. The most frequently found species on non-over-wintering larvae was Lydella thompsoni (Herting) (Diptera: Tachinidae). Differences in percentage of parasitism (PP) between O. nubilatis and S. nonagrioides were significant. Lydella thompsoni was also the dominant parasitoid on overwintering larvae. Its PP was higher on O. nubilalis than on S. nonagrioides. The correlation between number of stem borer per plain and percentage of parasitised larvae was not significant, neither for S. nonagrioides nor for O. nubilalis. Shannon-Wiener index as a measure of parasitoid diversity was similar in the different hosts. In conclusion, a very low parasitoid natural control on maize borers, in hybrid corn crops grown at the province of Pontevedra was found. Low numbers of parasitoids and low percentage of parasitism in the studied area make difficult to draw definitive conclusions. Stem borers seem to be protected from parasitoids by several circumstances. specifically the development of their immature stages within refuges (the stalks), but other factors probably would contribute to the situation.

This research was supported by Xunta de Galicia (project XUGA 40301 B98)

Peer reviewed