Seasonal population changes in cocoa capsids (Hemiptera, Miridae) in Ghana

Abstract

Pupulations of the West African cocoa capsidsDistantiella theobroma(Dist.) andSahlbergella singularisHagl. were sampled in 1966–67 by several methods: fast knockdown over sheets with pyrethrum, mercury-vapour light-trap, direct counts in a small artificially established area of high population, and observations of presence or absence in randomly selected inspection squares.A population build-up in mature cocoa accompanies and probably directly depends on development of the main crop from July or August to October. When pods become scarce after harvesting there is inS. singilarisa period of dispersal in which the species becomes more widespread as feeding is transferred to vegetative tissues, and inD. theobromaa more local change of feeding sites.Subsequent events vary greatly between areas and between years. It is suggested that the extent to which capsids are able to utilise vegetative tissues varies and depends on nutritional changes in the external parenchymatous tissues from which their food is obtained.Low humidities during spells of harmattan in January and February almost certainly kill larvae in exposed situations, but feeding conditions following such periods may be particularly favourable to capsids as large rapid population increases can occur locally. The factors involved are obscure but may be related to processes of regeneration stimulated by leaf loss and other damage in dry conditions.The densities and seasonal patterns found are discussed in relation to results of previous workers and some implications for control briefly considered.