The host range, annual cycle and parasitoids of the African rice gall midge Orseolia oryzivora (Diptera : Cecidomyiidae) in central and southeast Nigeria

Host range experiments and field sampling in Nigeria produced no evidence t hat African rice gall midge, Orseolia oryzivora Harris & Gagne, can develop on plants other than Oryza species. Sampling in three outbreak areas durin g 1994 showed that the insect's annual cycle varied according to the agroec ological zone and rice cropping pattern. In the humid forest zone, Orseolia oryzivora persisted through the short dry season on ratoons of cultivated rice Oryza sativa at a rainfed site and on dry season rice crops at an irri gated one. In contrast, at rainfed sites in the moist savannah zone the pes t survived the longer dry season on the perennial wild rice O. longistamina ta, while ratoons and volunteers of O. sativa provided 'bridges' between th e wild host and wet season rice crops. Early in the wet season at rainfed s ites, galls of Orseolia oryzivora were not found at high density on wild ri ce, ratoons or volunteers. The heavy infestations which developed by Octobe r resulted primarily from rapid multiplication on rice crops themselves dur ing the wet season. At all 13 sites sampled, the large majority of galls we re found on fallow or cropped rice fields, rather than in ditches, bunds or uncultivated wetland, irrespective of the time of year or the hosts involv ed. From gall dissections, the parasitoids Aprostocetus procerae (Risbec) a nd Platygaster diplosisae Risbec caused over 30% mortality at some sites by October but generally increased too late to prevent crop damage. Implicati ons of the results for the management of Orseolia oryzivora are discussed.