Insect infestation of stored raw cocoa in Ghana

Abstract

In Ghana, a considerable part of the main-crop cocoa is stored for six months or more, some up to nine months, and infestation by insects increases during storage. Data are given, obtained chiefly in Ashanti on the 1957–58 main crop.Assessment by sieving the contents of whole bags and counting the numbers of adult insects found (or, in the case of Ephestia cautella (Wlk.), the larvae and pupae also) showed that at the beginning of the main-crop season (September–November), infestation was very light (1–2 insects per bag), but was present in a high proportion of the bags (46–47 per cent, in October). The species present represent those found regularly breeding in cocoa in Ghana, namely, Ephestia cautella (Wlk.), Lasioderma serricorne (F.), Araecerus fasciculatus (Deg.), Tribolium castaneum (Hbst.), Garpophilus dimidiatus (F-), Ahasverus advena (Waltl), Cryptolestes spp., and an unidentified species of Silvanid. After six months' storage, the total count averaged over 100 per bag and comprised chiefly L. serricorne and T. castaneum; larvae of Ephestia were much rarer (less than five per bag), but adults of Ephestia were abundant in the sheds after 2–3 months' storage.Sieving proved a useful technique for obtaining data on infestation, though careful supervision of staff is required to minimise errors. Variation from bag to bag in the same ‘lot’ of cocoa is large, and estimates based on as few as four bags are unreliable.Assessment of insect damage on samples of cocoa beans taken fortnightly by spear-sampling showed an increase from 0·20 per cent, insect-defective beans in November to 1·0–1·9 per cent, in June. Of 21,000 beans examined in April and May, L. serricorne occurred in 0·17 per cent., and 0·6 per cent, contained living insects. Damage by insects affecting more than 2 per cent, of the beans has been very uncommon in Ghana in the past few years. The effect on quality, as assessed by commercial grading and considered financially, is at present slight, as it also is in terms of loss of weight over the whole crop (certainly less than 0·1 per cent.). E. cautella is the most important species economically, because it can infest the premises of cocoa and chocolate manufacturers in importing countries, and spread to finished confectionery. It affects a large portion of the crop in significant numbers.The primary infestation of cocoa before it comes into licensed storage occurs in thousands of small farms and villages, and no one district or type of source is of special importance. Increase of the infestation during storage occurs chiefly as a development of this primary infestation; cross-infestation from outside sources is relatively unimportant.The application of synergised pyrethrin in white oil within storage sheds as a fog, from which a film was deposited on the stacked bags of cocoa, was carried out in many sheds at intervals of 3–4 days, and a comparison made between sprayed and unsprayed sheds, based on data from sieving. Spraying did not substantially reduce the development of infestations of L. serricorne or other beetles, nor was E. cautella controlled to a degree comparable with experience of similar treatment against E. elutella (Hb.) in Britain; this may be due to the fact that E. cautella in Ghana develops in bags throughout the stack, and not chiefly in the peripheral bags, as does E. elutella in Britain.Conditions affecting the film-spraying of bagged cocoa in Ghana differ greatly from those in Britain. Uncontrolled ventilation at the eaves of sheds results in a considerable loss of insecticidal fog, and uneven distribution of deposit; the undersurface of the stacks cannot be sprayed, because they rest on wooden dunnage; it is difficult to maintain an effective oil-film on the topmost horizontal surfaces of the stack, near the roof, of which the temperature frequently exceeds 100°F.; deposits probably do not retain their toxicity for more than a few days.Variations in the materials from which cocoa-storage sheds are constructed, and in their design, cause relatively small differences in internal climatic conditions, and these have even less effect on the temperature of the cocoa in bags, which is generally in the range 82–88°F. The moisture content of the cocoa was about one per cent, less in a hotter type of shed than it was in one of a cooler type, and it is considered that the construction of sheds should aim at producing the hotter, drier conditions that are attainable, in order to keep moisture contents down and restrict the development of moulds, mould-feeding beetles and Araecerus fasciculatus.