Establishment and Efficacy of Stem Borer Biological Control Agents Released in Kenya

Abstract

Agriculture dominates the economy of most African countries with most farmers in Sub-Saharan Africa growing mainly maize (Zea mays L.) and sorghum (Sorghum bicolor Moench.). However, production of these crops is greatly constrained by lepidopteran stem borers. In Kenya, the four important stem borer pests are Chilo partellus Swinhoe, Chilo orichalcociliellus Strand, Busseola fusca Fuller and Sesamia calamistis Hampson. Due to losses associated with these pests, the International Centre of Insect Physiology and Ecology‟s Biological Control programme spearheaded release of egg parasitoid Telenomus isis Polaszek in Eldoret/Kitale and Wundanyi in 2005. The larval parasitoid Cotesia flavipes Cameron was released in North and South Coast in 1993, Eastern region and Kisumu in 2002. The larval parasitoid Cotesia sesamiae Cameron was redistributed from Kitale to Taita Hills in 2006 and the pupal parasitoid Xanthopimpla stemmator Thunberg was released in Eastern region of Kenya in 2002. However, there is limited information on the establishment status, spread and impact of these agents. Due to this information gap, this study was carried out from March 2014 to September 2016 to determine the establishment status of the above biological control agents and assess their spread and impact on target stem borer pests. Surveys were carried out on maize and sorghum farms at sites where the biological control agents were previously released. In each field, 100 maize or sorghum plants were inspected for stem borer infestation. Ten infested plants were destructively sampled and recovered immature stem borers collected and taken to the laboratory for rearing. Percentage infestation and parasitism computed from this data was then compared to figures recorded before the parasitoids were released in respective localities. Results revealed that T. isis established in Eldoret/Kitale and Wundanyi with mean egg parasitism varying significantly between the two localities (W=57, p=6.8e-10). In comparison to pre-release figures, egg parasitism in Eldoret showed a significant reduction (V=326, p=0.00072) while in Wundanyi, a significant increase was recorded (V=211, p=0.00035). Surveys in moist lowland, dry and moist mid altitude agro-ecological zones (AEZ) where C. flavipes had been released indicated that infestation was 28.9±3.1, 22.5±7.42 and 2.71±0.4% respectively, figures which were significantly lower in comparison to infestation levels documented before parasitoid release (V=3, p=4.26e-10; t=41.63, df=54, p=2.2e-16 and V=0, p=2.24e-08 respectively). Current parasitism levels were 36.1±3.0, 25.3±3.3 and 5.5±2.5% in the moist lowland, dry and moist mid altitude AEZ respectively and were higher compared to parasitoid pre-release figures (V=10.96, p=5.22e-15; V=1213, p=0.0002 and V=1950, p=0.0009 respectively). The larval parasitoid, Cotesia flavipes is steadily suppressing stem borer population in areas in which it was released. Following the highland Cotesia sesamiae redistribution to Taita hills, overall stem borer infestation was computed at 19.8±2.5% and this varied significantly among the three stem borer species, B. fusca, C. partellus and S. calamistis (χ²=16.86, df=2, p=0.00022). The highland C. sesamiae was not recovered from Taita hills and thus parasitism was generally low (10.8±4.3%) and did not show significant variation in comparison to pre-release records (V=3, p>0.05). Xanthopimpla stemmator was not recovered from the Eastern region of Kenya during this survey. This suggests a failure to establish though there is need to sample alternative and wild hosts before this is declared. An insecticide exclusion method was also used to evaluate the effect of parasitoids on maize yield in moist mid altitude and lowland AEZ. In both experiments, infestation varied among treatments (F2,9=5.835; p<0.05 and F2,87(0.05)=6.92; p<0.05 respectively). Mean weight of harvested maize (yield) did not vary among treatments indicating

that though there was a decrease in infestation coupled with increase in parasitism this did not translate into increased yield. It is recommended that for maize and sorghum yields to be increased, there is need to introduce C. flavipes egg parasitoids which can manage the pest before it reaches the most destructive larval stage.