Yield response of wheat infested with Chnootriba similis Thunberg (Coleoptera: Coccinellidae)

Chnootriba similis Thunberg (Coleoptera: Coccinellidae) is a sporadic pest that often causes extensive defoliation of wheat and other crops during outbreaks in many parts of Africa. Both the larvae and the adults cause characteristic perforations of foliage and can be present in large numbers over a wide area (Graham 1953). Where infestation is severe, growth and ear formation in wheat or barley are completely arrested. The beetles are widely distributed in Africa south of the Sahara and parts of Yemen (Anon. 1994). In West Africa, the beetles have specialized in defoliation of rice crops (Taylor & Kamara 1974), while in Ethiopia the beetles are sometimes called tef beetles and are found on wheat and barley crops in wet land and dry land habitats. These beetles are also considered major pests of pearl millet, Pennisetum t yphoides Burm., i n t he S udan (Schmutter 1969). In East Africa the beetles can attack a range of graminaceous plants including pastures, maize, rice, barley and wheat (Le Pelley 1952). The pest occurs in Kenya mainly in two endemic habitats: Naru-Moru marshland and along Malewa river towards Lake Naivasha. These areas are adjacent to the low rainfall zones that are currently developed by the government for expansive wheat production. At the moment the scenario presents a great risk for the wheat crop. Information on yield losses associated with these beetles is yet to be developed, especially on wheat. The beetles affect wheat grain yield indirectly by feeding on plant leaves especially the flag leaf (last leaf) (growth stages according to Zadoks, et al. 1974) prior to ear formation. The beetle density that can cause economic injury to the wheat crop has not been studied. The aims of this study were to determine the critical infestation level that can result in a significant yield reduction and the most susceptible growth stage in wheat. The beetles used in the test were laboratoryreared on a grass host, Digitaria scalarum (Schweinf.) Chiov., from the first instar to pupation. Emergent