Studies on Species Diversity, Host Preference, Host Suitability, Longevity and Fecundity of Native Egg Parasitoids of Stemborers in Semi-Arid Kenya

Abstract

Infestation and damage by lepidopteran stemborers is one of the major causes of low maize yields in Kenya. Among the biological management options of stemborers, egg parasitoids are some of the potential agents that can be included in the integrated management strategy for the pests. Information on the species diversity and the role of egg parasitoids in stemborer control is lacking in the semi-arid eastern and central regions of Kenya. The aim of this study was to determine the egg parasitoid species diversity and to assess their contribution in enhancing lepidopteran stemborer eggs' mortality in semi-arid eastern and central regions of Kenya. This was first investigated through field surveys by collecting stemborer eggs from farmers' maize fields in Kitui and Thika districts. No egg batches were recovered from the plants in Kitui in eastern province and Thika in central province during the long rains, of 2003 unlike the short rains, 2003, where 9 egg batches of Chilo partellus and 19 batches of Sesamia calamistis were recovered from Thika district. Five out of the 19 egg batches of S. calamistis yielded Telenomus busseolae (Gahan). On-station studies were also conducted where maize plants were artificially infested with Chilo partellus and Sesamia calamistis eggs in order to determine egg parasitism in the eastern region. Egg parasitism ranged from 0.34 -12.54% in both Katumani and Kiboko. However, Kiboko had significantly higher parasitism levels than Katumani during both the long (t =5.04; df= 2209; P < 0.0001) and short rainy seasons of 2003 (t= 11.73; df=202l; P<0.0001 The parasitoid species recovered from the stemborer' eggs in Kiboko were T_ busseolae, T. lSlS, Telenomus spp., Trichogramma spp. and richogrammatoidae spp., while in Katumani, it was Trichogramma spp. The abundance of the parasitoids in descending order was T. busseolae, T. isis (Polaszek), Trichogramma spp., Telenomus spp., and Trichogrammatoidae (58.9, 17.81, 13.01, 9.59 and 0.68 % respectively) at Kiboko. If confirmed, this would be the first record of T. isis in East Africa. The suitability of different ages of stemborer eggs was determined using the dominant egg parasitoid, T. busseolae. Ages 1-3 days of both B. fusca and S. calamistis were found to be successfully parasitized and suitable for the development of T. busseolae. However, T. busseolae took a longer time to develop in older eggs of S. calamistis. A Y- tube experiment was used to compare the preference by T. busseolae for the pheromones produced by B. fusca and S. calamistis females, but no significant difference in choice for either of the stemborer species was observed (X2 = 1.667; df 1,60; P = 0.1967). Similarly, in the field, T. busseolae females had non-preference for either B. fusca or S. calamistis (X2=0.72; df=1; P>0.05)eggs for parasitization. In a study to determine the longevity and fecundity of T. busseolae, the females begun ovipositing on the first day of their emergence, and average daily oviposition rate decreased with the age of the 'female and was considerably higher on the first day than any subsequent day for both B. fusca and S. calamistis. The mean longevity and the mean fecundity of T. busseolae females on B. fusca and S. calamistis did not vary significantly. This study has generated baseline information on egg parasitoids, which can be used in the development of a biological management strategy for stemborers in the semi-arid Kenya