The factors which influence the passage of flying and crawling insects through a doorway were investigated. It was considered practicable to exclude flying insects from a building or to prevent their successful escape from cages or rearing rooms by means of a downward air current if the speed of this in ft./min. multiplied by the ratio of the length through the doorway to its height is not less than 550. A combination of reflex and illuminated edges, smooth shiny walls without a pattern visible to insects, and a floor grid and underlying screening treated with a long-lasting residual insecticide, will, at all natural population densities, reduce to an insignificant level the number of insects that crawl through and survive. Artificially higher densities, as in rearing rooms, may require that the doorway be fitted with sliding doors in addition. These may be arranged to switch on the air current and lights as soon as they are opened.

A general survey of the species of Anopheles in Zaria Province, Northern Nigeria, carried out between June 1957 and December 1958, was designed primarily to ascertain the bionomics of actual and potential vectors of malaria there. The studies, which were centred upon the four principal domestic Anophelines, A. gambiae Giles, A. funestus Giles, A. nili (Theo.) and A. wellcomei Theo., were carried out by making regular floor-sheet collections and entranceand exit-trap catches in native huts, together with biting catches inside and outside huts. Data on other Anophelines and Culicines, taken at the same time, were also recorded.

The studies show that in this region, where malaria is holoendemic, all four species are endophagous to a greater or lesser extent although both A. gambiae and A. nili prefer to feed outside if a host is available. Only two species have been incriminated as vectors, A. gambiae, which had a sporozoite rate of between 4 and 7 per cent., and A. funestus with between 3 and 8 per cent., according to the season.

Collections by floor-sheets and by catches at human bait showed that A. gambiae, A funestus and A. nili were predominantly wet-season species, although in one village area studied A. funestus also occurred in fairly high numbers throughout the dry season. A. wellcomei, on the other hand, was shown to be a predominantly dry-season species.

By using traps and making collections with human bait, the entrance, exit and biting times of A. gambiae, A. funestus and A. nili were ascertained. The largest numbers of A. gambiae and A. funestus entered huts between 9.0 and 11.0 p.m. and left between 3.0 and 5.0 a.m., the maximum biting activity for A. gambiae being between midnight and 5.0 a.m., and for A. funestus between 11.0 p.m. and dawn. A. nili differed considerably from the other two species, having two peaks of maximum entry, between 9.0 and 11.0 p.m. and 1.0 and 3.0 a.m., the period of maximum exodus being between 1.0 and 5.0 a.m., with a peak of biting activity between 10.0 p.m. and 1.0 a.m. It was found that a very high proportion of the mosquitos caught leaving the huts was unfed; between 1.0 and 5.0 a.m., 64 per cent, of A. gambiae leaving, 63 per cent, of A. funestus and 30 per cent, of A. nili were unfed.

Apart from the four domestic species of Anopheles mentioned above, the only other anthropophilous species which could be described as common in the vicinity of Kaduna were A. coustani Lav., A. theileri Edw., A. flavicosta Edw. and A. rufipes (Gough). A. implexus (Theo.) is recorded from Nigeria for the first time.

The commonest species of Culicines taken at human bait during outside night collections were Mansonia africana (Theo.), M. uniformis (Theo.), M. cristata (Theo.), Aëdes lineatopennis (Ludl.) and Culex poicilipes (Theo.). Of these, M. uniformis was by far the most regular and persistent biter throughout the year. M. africana, on the other hand, was only taken in any numbers during October.

In an appendix, a list of 17 species of Anopheles (including 3 varieties), 65 of the CUlicinae and two of the TOxorhynchitinae known to occur in Zaria Province is given, with notes on their distribution and bionomics.

The name Pseudolitomastix was applied (Eady, 1960) to a genus erected for the new species P. nacoleiae, bred from Nacoleia octasema (Meyr.). This unfortunately overlooked the valid and prior use of the name Pseudolitomastix, by Risbec (1954). The two species for which Pseudolitomastix Risbec was erected are generically quite distinct from P. nacoleiae, so that a new generic name is required for this latter species. The name Pentalitomastix, indicative of the five-segmented funicle of the female antenna, is hereby proposed. I am grateful to Dr. O. Peck, of the Entomology Research Institute of the Canadian Department of Agriculture, for drawing my attention to the homonymy, and to my colleague Mr. G. J. Kerrich for suggesting the new name.

The aerial dispersal of the Pseudococcid vectors of virus diseases of cacao in Ghana and the possible influence on it of wind speed was studied at Tafo. Seasonal variations in wind speed in Ghana are slight, particularly inland in the cacao-growing areas. Observations at Tafo during 1955 showed that air movement was maximal between 12.30 and 3.0 p.m. and minimal during the hours of darkness. Daily wind speeds in the open, from April to November 1955, averaged 1·1 m.p.h., 4 ft. above ground. About 80 per cent, of wind speeds measured in one-minute periods on two days in February 1955 were within the range 1–5 m.p.h. and 11–12 per cent, between 5 and the maximum recorded speed of 7 m.p.h.

In cacao, wind speed is reduced by a factor of 10–20 times; daily speeds averaged 0·09 m.p.h., at 8 ft. above ground during April and May 1955. The highest daily averages under a closed canopy (400–650 ft./hr.) were recorded 2 ft. above ground-level; speeds fell to a minimum (25–125 ft./hr.) in the canopy at 15–25 ft., and rose again above the canopy at 30 ft. to a speed (250–350 ft./hr.) comparable with that just below the branches at 10 ft. At breaks in the canopy caused by the removal of diseased trees, daily averages were lowest (125–150 ft./hr.) 2 ft. above the ground. They rose to a peak (400–600 ft.) on a level with the lower branches of the canopy, dropped markedly (250–450 ft./hr.) at the level of the middle canopy and rose to a maximum (650–1,000 ft./hr.) at 30 ft.

All instars of Pseudococcus njalensis Laing walking on pieces of cacao wood in the laboratory withstood removal at air speeds of 10 m.p.h., but these and corresponding stages of three other species could be dislodged by gently tapping the wood. The late nymphs and adults of Ps. njalensis and Ps. gahani Green were more easily removed than their ‘crawlers’, though this difference was not found between the developmental stages in Planococcus citri (Eisso) and Ferrisiana virgata (Ckll.). Amongst the four species tested, first-instar nymphs of Pl. citri were most easily dislodged, and those of Ps. njalensis or F. virgata more so than those of Ps. gahani.

Airborne mealybugs were caught on adhesive traps, on bait twigs pinned to mature trees, and on cacao seedlings. Eight vector species became established on cacao after dispersal by air currents.

Under a closed canopy, more airborne mealybugs were caught at two and ten feet above ground than at levels in and above the canopy. At breaks in the canopy, catches averaged about 13 per cent, of those obtained under a continuous canopy and were insufficient to show changes in aerial density with height. The distribution of catches over the surface of traps would suggest that mealybugs drop from the branches, are carried by air currents when falling, and become laterally dispersed at levels a few feet above the ground.

In a clearing where cacao had been removed to simulate conditions following the routine cutting out of virus-infected trees, airborne mealybugs became established on seedlings at a distance of 45 ft. from infested cacao trees. The ratios of boxes of seedlings which became infested by aerially dispersed Ps. njalensis at increasing distances from infested standing cacao, in relation to those beneath it (unity), were: 0·86 at 10–20 ft., 0·57 at 30 ft. and 0·14 at 40–100 ft. Under conditions of high insolation, the maximum recorded distance of mealybug aerial dispersal from surrounding vegetation to cacao seedlings was 340 ft. Aerial catches on seedlings 40 to 165 ft. from cacao showed an over-all decrease with distance.

Aerial dispersal is more pronounced during dry conditions, particularly during the main dry season, December–February, and to a lesser extent during the brief dry period experienced in July or August.

The infestation rate of cacao seedlings by windborne mealybugs (predominantly Pl. citri and F. virgata) was increased by 50 per cent, when plants were protected from weather by artificial shade. These traps, insulated from ants, failed to become infested by Ps. njalensis.

Out of 64 young cacao trees, 22 per cent, became infested by airborne vectors during the five-month period May to September 1955; when, for similar trees, normal dispersal was augmented by an initial artificial colonisation with Ps. njalensis, the corresponding figure was 41 per cent. The infestation rate, after either augmented or natural dispersal, was not significantly affected by attempts to establish on the trees nests of the ant, Grematogaster striatula Emery, or by affording protection from the weather in the form of artificial shade. It was evident, however, that the presence of mealybug-attending ants is almost essential for infestations of Ps. njalensis to develop, but there must be other limiting factors, since establishment failed on 58 per cent, of trees on which coccidophilic species were present.

The part played by airborne vectors in extending infection by radial and ‘jump spread’ is discussed, together with the possible use of insecticidal measures to prevent their establishment on the trunks of healthy cacao.

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.