Stage-specific mortality of Helicoverpa armigera in three smallholder crops in Kenya.
Abstract
Partial life tables of Helicoverpa armigera were constructed for 3 crops, sunflower, maize and sorghum, commonly grown on smallholder farms in Kenya. Oviposition coincided with early flowering of the crops, and was due to the preference of ovipositing moths for flowering plants rather than to narrow periods of oviposition activity. Consequently, single cohorts of H. armigera developed on each crop. The partial life tables show that mortality during development was generally high (82-99.3%) on sunflower, maize and sorghum, but stage-specific mortality varied greatly from season to season. Mortality was highest on maize, particularly during the young stages. Mortality was generally higher in the short rainy seasons than in the long rainy seasons. The contribution of parasitism and pathogens was rather small; only a tachinid fly attacking old larvae was common. Most mortality was due to unknown causes including predation. The potential role of predators was partially evaluated by relating their temporal association with the prey to stage-specific mortality, but was obscured by the variety of predators that feed on H. armigera, and thus requires further study. Mortality of young stages of H. armigera was most strongly associated with the incidence of anthocorid bugs. Ants (Myrmicaria sp.) ants are likely to be more important on sunflower than on maize and sorghum. Implications of the findings for improving biological control through habitat manipulation and importation of exotic natural enemies are discussed.