Diversity of alternative hosts of maize stemborers in Trans-Nzoia district of Kenya

Abstract

Genetically-engineered (GE) crops such as those expressing insecticidal Bacillus thuringiensis (Bt) toxin genes have the potential to greatly reduce the use of broad spectrum insecticides and increase crop productivity. However, development of resistance by the target insect species is an important consideration in the deployment of this strategy. In areas where GE crops are deployed on a large scale, current resistance management strategies rely on a 'refuge strategy', comprising the incorporation of a certain proportion of non-GE plants in the agro-ecosystems, to conserve susceptible individuals of the target pests. In the USA, simulation models indicate that at least 20% of the crop should be non-Bt plants. In Africa, the target lepidopteran stemborers attack a wide range of wild grass species as well as cultivated cereal crops. Wild grasses generally occur in the vicinity of maize and other cereal fields, and may provide a refuge if GE crops are in the farming systems. To assess the quality of these grasses as refuges, it is critical to obtain information about their size and spatial distribution. In this study, we have assessed the abundance and diversity of alternative refuge of stemborers, mainly wild grasses occurring in the proximity of maize fields, in Trans-Nzoia district, one of the most important maize growing areas in Kenya. The proportion of wild host plants relative to maize was found to decline from 100% during the non-cropping season to <8% during the maize-growing season. The ShannonWeaver diversity index indicated high variation in the diversity of wild hosts of stemborers between agroecological zones in the district. The results of this study are discussed in light of the possible role that wild host plant species might play in stemborer resistance management following the introduction of Bt maize.

Diversity and abundance of wild host plants of lepidopteran stem borers in two agro-ecological zones of Kenya

“…However, densities of stem borers in wild habitats range between 0.001 and 0.03 per tiller, results that are considerably lower compared to the cultivated habitats (Le Ru et al 2006a, Ndemah et al 2007Matama-Kauma et al 2008). A study carried out in Kitale, Kenya, that assessed the potential of using wild host plants as a refuge for Bt-susceptible pest populations, required in resistance management for Bt maize (Kanya et al 2005), showed that the ratio of wild host plants to maize was below 10%. However, another study carried out in Kakamega and Muhaka, representing the moist transitional and lowland tropical zones, showed that the host range of C. partellus and B. fusca was limited in both number and abundance (Otieno et al 2006), suggesting that Kanya et al (2005) may have been overestimated the populations.…”

Diversity and abundance of wild host plants of lepidopteran stem borers in two different agroecological zones of Kenya

“…However, our study and that done by Kanya et al (2005) showed that the area covered by wild host plants was below 10% and therefore inadequate to sustain susceptible stem borers as it is recommended that 20-50% to be non-transgenic plants (Fitt et al 2004).…”

“…Th e high number of host plants reported earlier was most probably due to misidentifi cation of the stem borers recovered from the wild habitat (Le Rü et al 2006aRü et al , 2006b). However, Kanya et al (2005) reported wild host abundance of < 10% compared to maize 95%. Interaction between vegetation structure and movement patterns of insects ultimately aff ects population dynamics in a heterogeneous landscape (Burel et al 2000).…”

“…In addition, among the wild plant species identifi ed in this study location only three plants (Sorghum arundinaceum, Setaria megaphylla and Pennisetum purpureum) were identifi ed as hosts of B. fusca (Le Rü et al 2006a, 2006b. Coupled with the limited host range of this pest, dense human population is putting more pressure on the remaining (Kanya et al, 2005). Th is was based on earlier report by Polaszek & Khan (1998) that B. fusca and C. partellus are polyphagous.…”

“…Recent studies done on wild host range of stem borers in East Africa by Le Rü et al (2006a, 2006b) indicate that B. fusca and C. partellus are oligophagous, contradicting previous report by Polaszek & Khan (1998) characterising these species as polyphagous. Studies done by Kanya et al (2005) in Kitale in Kenya showed that area covered by wild host plants of these pests was below 10% suggesting that wild host plants might not be adequate as refuge of these pests. Until now, little attention has been given to the role of wild host plants in the invasion of crop fi elds by the stem borer pests.…”