An evaluation of potential economic returns from the strategic control approach to the management of African armyworm Spodoptera exempta (Lepidoptera: Noctuidae) populations in eastern Africa

Abstract

The costs of armyworm control in candidate outbreak source areas in Ethiopia, Kenya and Tanzania were compared with estimates of the costs of potential damage if these source outbreaks were not controlled. The latter costs were estimated using a modelling approach, taking account of known moth migration trajectories, their frequency of occurrence, and of the agricultural productivity of potential destinations of the migrant moths. Two parallel studies were conducted: one for the outbreaks in the period January to June, and the other during October to December. Strategic control of armyworm using ground spraying, whereby infestations are controlled even if they are of no immediate economic importance but when their future progeny pose a potential threat, will be worthwhile during the period October to December on the basis of: (a) available estimates of control costs; and (b) a sensitivity analysis of potential losses in relation to moth reproduction. For January to June the potential outbreak sources worthy of control are fewer, but the two most important sources could lead to more devastation than any of those in October to December. The maximum estimated control cost (US$ 33 per hectare, 1987 prices) involved the use of helicopters during an operation in Tanzania. The next highest were for ground-spraying operations in Ethiopia in 1985 (US$ 17) and for an aerial operation in Kenya (US$ 16). However, even if the latter estimates are used, strategic control will still be worthwhile in one fifth of the study area regardless of season, given 20% crop damage and a moth reproductive rate per generation of at least five. Comparison of the results with those based on a hypothetical situation of no migrations showed only minor differences in many cases but the effects of moth movements were more marked for the period October to December than during January to June. This exercise also identified the Meru district in Kenya and the south Morogoro region of Tanzania, in particular, as zones which have unremarkable agricultural value of their own but which can be sources of moth populations capable of causing economically important damage elsewhere. It is proposed that strategic control operations should be restricted to those areas likely to be responsible for the most potential damage. Such target areas have been identified in the study and are mostly in east central and southwestern Kenya and northern and central Tanzania.