Role of Wild Host Plants in Population Dynamics of Cereal Stemborers and the Associated Parasitoids in Uganda

Abstract

Lepidopteran stemborers attack several graminaceous and wild host plants. These pests are one of the major constraints to maize production in Africa. However, efforts to control stemborers have aimed at cereal crops and the role of alternative wild hosts has been neglected. Recent reports have indicated that the abundance of wild grass in the maize field surroundings is correlated with lower stemborer incidence. This study was conducted with the aim of elucidating the role of wild grasses in stemborer infestations in maize in Uganda. To achieve this goal, surveys, field and screen house trials were conducted between 2003 and 2005. The study covered four Agroecological zones and these included the Eastern, South eastern, Lake Victoria Crescent and Lake Albert Crescent ecozones. Results showed that mainly four stemborer species occur on maize in Uganda. Busseola fusca Fuller (Noctuidae) and Chilo partellus (Crambidae) were the most important across the four Agroecological zones (AEZs). The distribution of these stemborers varied across AEZs with C. partellus as the major stemborer species in the eastern AEZ while B. fusca was dominant in the Lake Albert Crescent. The major parasitoids of these stemborers were the introduced Cotesia flavipes and the indigenous Cotesia sesamiae Cameron (Braconidae) attacking the larval stage and Dentichasmias busseolae Heinrich (Ichneumonidae) and Procerochasmias nigromaculatus Cameron (Ichneumonidae) that attacked the pupal stage. Surveys of stemborers on the selected grasses revealed that wild sorghum Sorghum arundinaceum (Desv.) Stapf. was the major wild host plant for B. fusca and C. partellus and these stemborers were not common on the wild grasses Panicum maximum Jacq., Pennisetum polystachion (L.) Schult. and Pennisetum purpureum Schumach. However, these grasses were hosts of many stemborer species that are confined to the wild grasses. Host plant preference studies showed that all the four selected grasses studied were not preferred over maize for oviposition by both. C. partellus and B. fusca. These grasses did not support larval development to pupation except for S. arundinaceum, which had high numbers of C. partellus pupating although with reduced weights compared to maize. Therefore, amongst the grasses tested, S. arundinaceum was found to be a suitable host for the development of C. partellus and B. fusca, and may form a reservoir during the off-season for both the pest and parasitoids as several parasitoid species were recovered from this grass species. C. sesamiae was also frequently recovered from P. purpureum indicating that this grass species also forms a reservoir for this parasitoid and C. flavipes to a small extent. Results further showed that wild grass border rows did not have a consistent effect on reducing stemborer infestations and larval parasitism was independent of the grassy borders. The role of grassy border rows in controlling cereal stemborers in crops is not well understood. Leaving wild grasses intact in the vicinity of crop fields might have a greater effect on stem- borer populations in crops than planting grassy border rows around the fields. There is also need to understand better the mechanisms for reduced pest densities when grasses are growing in the vicinity of cereal crops. The role of grasses in maintaining parasitoids need to be further investigated and perhaps grasses that enhance parasitism with out necessarily increasing pest infestation be identified.