Assessment of host plant resistance in Oryza sativa to the African rice gall midge, Orseolia oryzivora Harris and Gagné (Dipt., Cecidomyiidae), with a description of a new method for screening under artificial infestation

A new method is described for screening rice germplasm for resistance to the African rice gall midge (AfRGM), Orseolia oryzivora, year-round under conditions similar to a severe natural outbreak, using an AfRGM culture to start infestation in a tunnel screenhouse over an irrigated rice field. During 1996 and 1997, 907 test entries of Oryza sativa were screened. Eighteen entries, 17 of which were traditional African lines, had infestation levels at least 50% below those on the partially resistant check variety NHTA 8 at 63-70 days after transplanting, though none showed very strong resistance. When traditional African lines and improved lines were screened together, the latter were on average markedly more susceptible. A field trial using caged plots indicated that even under no-choice conditions the varieties NHTA 8 and Eguazankpa maintained their partial resistance relative to the susceptible variety ITA 306. A comparison of the new screenhouse screening method with one using seedlings in insect cages suggested that the latter was less suitable for detecting partial resistance. An experiment in which NHTA 8 and ITA 306 were infested with one to four neonate AfRGM larvae per seedling showed that the difference in resistance between these varieties decreased with increasing infestation pressure. The potential role of partial resistance in the management of AfRGM, and methods for detecting it, are discussed.