Do parasitoids reduce cereal losses? Estimating the impact of parasitoids on stem borer pest infestation and maize yield

Abstract

Chilo partellus (spotted stem borer) is an economically important stem borer pest in Eastern Africa causing high yield losses in maize and sorghum. The larval endoparasitoid, Cotesia flavipes (parasitic wasp) was released in Kenya to manage C. partellus and post release studies showed it had established and stem borer parasitism was steadily increasing. Despite the rise in parasitism, doubts on whether parasitoids can effectively reduce stem borer populations have been raised.

Objective: This study was carried out to estimate the impact of C. flavipes on stem borer infestation and the resultant effect on maize yield in Kenya. Methodology and results: Using insecticidal check method, selective insecticides were used to exclude stem borer pests and natural enemies in Treatments A and B respectively with controls designated as Treatment C. Results revealed significant differences in stem borer infestation (F2,9=5.835; p<0.05) and parasitism (F2,9 =91.97; p<0.0001) among the three treatments. Significantly higher infestation was recorded in treatment B (2.0±0.2%) compared to A (0.5±0.3%) and C (1.5±0.4%). Parasitism was significantly higher in treatment C (23.5±6.8%) compared to A (0.0±0.0%) and B (0.3±0.1%). However, there was no significant difference in yield among treatments showing that parasitoid action did not translate into changes in yield. Conclusions and application of findings: The parasitoid C. flavipes has made an important contribution towards suppressing stem borer population. This was shown by high infestation recorded in treatment B which was as a result of partial elimination of parasitoids. This deduction was further supported by the results which showed that pest exclusion using chemicals did not have a significant difference from parasitoid action as shown by insignificant difference in infestation between treatment A and C fields. Percentage infestation and parasitism results obtained from this study thus provide quantitative data on the impact of C. flavipes. Release of C. flavipes can therefore be done in other areas both within and beyond the country’s borders where C. partellus is still a problem. However, since reduced infestation and increased parasitism did not translate into yield increase, more suppressive effect may be achieved if the pest’s multiple developmental stages are targeted, most especially before it reaches the destructive larval stage. There is need to carry out further research on C. partellus’ egg parasitoids. Optimum yield may then be realized.

Key words: Classical biological control; Chilo partellus ; Cotesia flavipes; parasitism; insecticidal check technique; Kenya