Survival and development of the stemborer chilo partellus (swinhoe) (lepidoptera:pyralidae) on selected wild gramineous plants
Mohamud, Mohamed Hassan
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A major constraint to increase the production of maize is competition from phytophagous insects. The Lepidopteran stem borer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), is considered to be one of the most damaging insect in Eastern and Southern Africa. Understanding the role of wild alternate host plants in sustaining stem borer population is important for development of crop protection strategies. A comparative study was therefore undertaken to determine the behaviour and biology of C. partellus on selected wild and cultivated host plants. The host plants included susceptible maize (Inbred A) and five wild host plant species of familily Poaceae (Graminae) - Sorghum versicolor Anderss, Pennisetum purpureum Schumach, Panicum maximum Jacq, Echinochloa pyramidalis (Lam.) and Hyparrhenia rufa (Nees) Stapf. The aspects investigated were ovipositional behaviour, larval arrest and dispersal, larval feeding, food assimilation and larval growth and development. There was no significant (P=0.05) difference between any of the host plants tested in ovipositional response in no- and multiple-choice tests. In two-choice tests, however, there was significant (P=0.05) difference in number of eggs laid on Inbred A (424 eggs) against P. purpureum on which 966 eggs were laid. Larval arrest on leaf cuts obtained from test plants in petri dishes at 4 and 24 hours after infestation was not significantly (P=0.05) different among any of the wild grasses tested in no-choice tests; also no significant (P=0.05) differences were found in the number of larvae arrested on Inbred A (8.66 and 5.0) versus S. versicolor (6 and 4.16) at 1 and 24 hours after infestation in two-choice tests. In multiple-choice tests, the number of first instar larvae arrested on both Inbred A (20.20) and S. versicolor (19.10) was significantly (P.0.001) higher than on the other host plants at 24 hours after infestation. The lowest number of larvae was recorded on H. rufa with only 2.70. When whole plants were used the number of larvae arrested was significantly (P=0.001) higher on Inbred A (10.0) and S. versicolor (8.33) than on the other host plants. Larval feeding on leaf discs of different host plants was significantly (P=0.001) higher on Inbred A (45 percent) than on the wild host plants. There were no significant (P=0.05) more food assimilated of Inbred A (0.45mg) leafcuts than on E. pyramidalis (0.15mg), P. maximum (0.06mg) and H. rufa (0.03 mg). Larval development was significantly (P=0.001) faster on Inbred A and S. versicolor, than on the other test plants. S. versicolor was the most suitable wild host plant for survival and development of C. partellus larvae, while P. purpureum was suitable for oviposition. The wild host plants could be used as a bordering or trap crops for adult oviposition and larval arrest with maize, and cut off after the cultivated crop has passed the susceptible stage. This would have the effect of eliminating high proportion of C. partellus eggs batches as well as first instar larvae.