Host range of sturmiopsis parasitica (currran) (diptera: tachinidae) and enterspecific competition with cotesia sesamiae (cameron) (hymenoptera: braconidae in Kenya

Abstract

Cereal stemborers in Kenya include Busseola fusca, B. phaia, Sesamia calamists, S. botanephaga, Eldana saccharina, Chilo orichalcocifellus and Ch. partellus. The stemborers cause 50% yield losses in cereal crops including maize, sorghum and sugarcane. Damage to the host plants results from the feeding and excavating of tunnels by the larvae. The different stemborer control options include pesticides, early planting, intercropping with non-cereals and host plant resistance. Pesticides are expensive and pose a health and environmental hazard. Moreover, they are not fully effective due to the cryptic feeding behavior of the pest larvae. Therefore there is need to search for more efficient and convenient control methods for these pests. Biological control is one such method where natural enemies are used. These include predators, parasitoids and pathogens. Predators do not keep the stemborers below economic injury levels. Pathogens and nematodes are not of great importance in regulating stemborer numbers. Parasitoids feed on immature stages of other insect host stages and kill them in the process. In Kenya, the parasitoids which have been studied in regard to the control of stemborer pests include Telenomus species, Tricogramma, Cotesia sesamiae, Pediobius furvus and Dentichasmias busseolae. However, the biocontrol status of Sturmiopsis parasitica in Kenya has not been fully investigated. The West African S. parasitica race parasitizes diapausing B. fusca, non diapausing E. saccharina and S. calamistis, while the Zimbabwe race parasitizes B. fusca but is encapsulated in S. calamistis and E. saccharina. The suitability of the West African race of S. parasitica in controlling the cereal stemborers has not been investigated in Kenya. The aim of this study was to determine the suitability of this biotype as an effective natural enemy through assessment of host range and interspecific competition with Cotesia sesamiae for its possible redistribution in Kenya. In this study, the suitability of S. parasifica as an efficient biocontrol agent was determined by inoculating planidia of S. parasitica in fourth instar larvae of different stemborer hosts. Host suitability and acceptability of four graminaceous pests namely, the noctuids Sesamia calamists, Busseola fusca, the crambid Chilo partellus and the Pyralid Eldana saccharina were evaluated for their ability in supporting parasitoid development. The findings of the present study showed that, all the hosts were acceptable for larviposition but suitability varied greatly. Sesamia calamistis was the most suitable and preferred host while B. fusca and E. saccharina showed no significant difference. Ch. partellus was the least suitable host. However the parasitoid had the longest developmental period in B. fusca Interspecific competition was determined by exposing the most suitable stemborer host larvae to the parasitoids in the following sequence: S. parasitica, Co. sesamiae and then Co. sesamiae, S. parasitica at various time intervals. Sturmiopsis parasitica outcompeted Co. sesamiae irrespective of time interval between parasitism and whether it was the first or second species to parasitize. Moreover, S. parasitica had significantly higher rates of emergence in the presence of Co. sesamiae. Similarly Co. sesamiae took longer to develop in the presence of S. parasitica and had a high cocoon-to-adult mortality. The implication of this is that Co sesamiae may be extirpated in the highlands of East and Southern Africa. The present observations indicate the potential usefulness of S. parasitica as a biological control agent