Cashew, Anacardium occidentale L. is a non-traditional crop in Ghana, but it has become a crop of considerable economic importance. It is cultivated mostly in the drier parts of Guinea savanna as well as the forest-savanna transition and coastal savanna zones, where the soils are considered unsuitable for most staple crops including rice and maize. The Government of Ghana, through Ministry of Food and Agriculture (MOFA) and in collaboration with Cocoa Research Institute of Ghana (CRIG), has embarked on massive rehabilitation of the existing plantations, as well as establishing new ones, through the provision of technical support and farming incentives to farmers. The primary objectives are to increase cashew production and to export processed nuts. Government support has resulted in a significant expansion of hectarage under cashew cultivation from 18,000 ha to 60,000 ha and a corresponding rise in nut yield from 3,000 MT to 16,000 MT between 2000 and 2006.
Many insect pests attack the crop but the sap sucking bug, Helopeltis schoutedeni (commonly referred to as cashew mosquito bug) is one of the most important pests. The pest is prevalent all year round and in all the cashew growing areas, infesting transplanted seedlings, establishment phase of the plant and mature trees. Both nymphs and adults feed on growing tips, foliar and floral flush, apples and nuts. Attack on foliar and floral shoots results in dieback and, in severe cases, entire foliar and floral flushes shrivel (Plate 1). Damaged shoots also sometimes exude colourless resin from the point of feeding. Severely damaged developing nuts shrivel, darken and die but the dead nuts remains attached to the apple (Plate 2). Lower portions of the cashew canopy suffer more of its damage than upper portions. The pattern of distribution of the bugs in the plantations is clumped, while both the curves for population distribution and organ damage are mono-modal. H. schoutedeni populations reach peak between November and March and decline from June to September. The population increase coincides with increasing availability of food, low rainfall, low relative humidity and relatively high temperatures.
Plate 1: Curling of cashew Plate 2: Dead nut shrivels, turns
leaves 48h after feeding by black and remain attached to the
H. schoutedeni apple
The pest lays its eggs singly or in batches of 2 to 4 into tender epidermal tissues of shoots, veins and midribs of leaves, leaf petioles and developing nuts of cashew. The egg measures on average 1.0 ± 0.05 mm long and 0.5 ± 0 mm wide and has a mean developmental period of 8.2 ± 0.05 days. There are five nymphal instars and the mean nymphal period is 15.6 days.
Both sexes of the adult are brownish orange in colour, the male (Plate 3a) is relatively smaller and is shorter lived (mean longevity of 19.0 ± 2.5 days) than the female. The female (Plate 3b) has a mean longevity of 22.9 ± 0.9 days with average length and breadth of 12.2 ± 0.1 mm and 2.4 ± 0.1 mm, respectively. The antenna of the adult, as in the nymphs, is four-segmented, with the basal segment stouter and the terminal segment having numerous short hairs. The hemelytron overlaps and covers the entire abdomen, with the distal end showing a triangular brownish black colouration.
Plate 3: Adult H. schoutedeni (a = Male; b = Female)
Currently, two insecticides, Karate (lambda-cyhalothrin) at 4.0 ml litre-1 tree-1 and Cyperdim (cypermethrin+dimethoate) at 3.4 ml litre-1 tree-1 have been recommended for the control of the bug on mature trees. The insecticides are applied as foliar sprays four times in a year at monthly intervals starting November (pre-flowering period) and repeating in December (flowering initiation period), January (flowering period) and February (fruit initiation period), using CRIG recommended motorized mistblower spraying machines