Estimates of the size of populations of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas have been made in Tanganyika from counts of adults caused to fly up by the approach of a slowly moving vehicle. For research purposes it was desirable both to improve and estimate the accuracy of such assessments. For control purposes it was desirable to devise an objective aerial assessment technique which could be used in areas where ground methods were either impracticable or expensive. The errors involved in both ground and air estimates can be divided into those concerned with the ratio of locusts counted in the sample area to those actually present, and those concerned with the distribution of the locusts.

It was found that a Land-Rover moving at five miles an hour, with the wind blowing from a rear quarter, flushed a high and virtually constant proportion of locusts during most of the day and in the most important grasses of the Rukwa outbreak area, throughout the dry season. Variation in locust density and in the state of the grass had little effect, if any, on the value of the equivalent strip (e), i.e., a strip whose width is such that the number of locusts in it is equal to the number flushed by the vehicle in a strip equal in width to its own. The change with time, and thus presumably with locust behaviour, in the value of e, estimated as recommended, was small but significant.

Counts by various observers in one vehicle gave similar results and it seems likely that, even with high numbers, there was no systematic mis-estimation. This is almost certainly true for counts of under a hundred locusts flushed per 0·1 mile. With upwards of one thousand 0·1-mile counts in each assessment, it is believed that the random errors due to mis-counting of the locusts flushed will be negligible.

The locusts are always distributed in an over-dispersed manner, consequently normal techniques for ascertaining the standard error of estimate cannot be used. Three assessments were made by the methods recommended, using a grid of two-mile squares; in one case, where the distribution was very gregarious, the most densely populated sector was more intensively sampled. The standard error of assessments laid out in this way was estimated. The method of analysis overestimates the standard error to some degree. Alternatively, the standard error resulting from the nature of the locust distribution can be computed for assessments where the 0·1-mile counts can be shown to be distributed log-normally.

Provided the population assessment is carried out under the conditions stipulated above, the additional errors will be minor. The effect of such errors will be likely to be so small that the estimated standard error, derived from the lognormal or assessment-grid analysis, may be regarded as a valid estimate of the standard error of the population estimate.

An attempt was made to estimate populations of the red locust in part of an outbreak area from locusts flushed by a ‘ Prospector ’ aircraft. The trials suggested that, given a clear rear view, red locusts which were flushed could be identified as such. Further, the number of locusts flushed could be counted at low densities, and it is thought likely that a reasonably objective technique could be developed for estimation at higher densities.

Flushing appeared to be most effective at an aircraft height of 20 ft. or less and when the direction of flight was with the wind. However, the majority of locusts present failed to take off and the proportion which did rise varied very greatly. The spraying of concentrated ammonia under pressure increased the efficiency of flushing.

A large-scale comparison of air and ground assessments suggested that, under the prevailing conditions, a considerable proportion of the locusts could be flushed only from the dense concentrations. At all densities the proportion of locusts which the aircraft caused to rise was a small fraction of those present. With flushing of such a low level of efficiency, little idea could be obtained of the size of the locust population at a moderate or low density.

The facultative larval diapause of the khapra beetle, Trogoderma granarium Everts, is initiated extrinsically by temperature, crowding, and very low or very high humidity. In crowded populations, in tubes of wheat, the proportion of diapause larvae varies from very low (2 to 3 per cent.) in the upper part of the insect's temperature range to 100 per cent, in the lower part. Diapause larvae become more frequent with increasing population density, the frequency becoming noticeable at a density of about ten larvae per gramme of wheat (about 10 times their minimum food requirements). Accumulation of faecal pellets is a factor that induces diapause in crowded conditions. Many larvae that are kept singly enter diapause when fed on old culture food from which pellets have been removed: possibly in the old culture the pellets impart to the food a substance that initiates diapause. The availability of a suitable refuge, in which larvae can cluster, has no consistent effect on the proportion of diapause larvae. Factors that induce diapause also slightly increase the developmental period of non-diapause larvae.

In malted barley, more larvae enter diapause than in wheat, probably due to the intensification of crowding by the confinement of larvae to a few malt grains.

The relative importance of the factors that induce diapause is discussed. These factors help to explain observations in stores and conflicting results in the literature. The main function of diapause seems to be in assisting the larvae to survive periods of food shortage; diapause may also improve the survival of larvae in a cold winter and synchronise the emergence of adults. Of the factors inducing diapause, accumulation of faecal pellets precedes food shortage and falling temperatures precede the cold of winter, but the role of high and low humidities in this connection is difficult to assess. Diapause larvae play a major part in the dispersal of the species. The diapause of T. granarium is compared with that of other arthropods.

Resistant and normal colonies of the blowfly Chrysomyia putoria (Wied.) were obtained from the Congo and cultured in London. By selection and inbreeding, sub-colonies homogeneous for resistance to (a) the BHC/dieldrin group of chlorinated insecticides and (b) malathion, were obtained. Homogeneous resistant colonies were markedly less fertile and were difficult to perpetuate.

Toxicological investigations of the BHC/dieldrin-resistant colony revealed a characteristic resistance spectrum similar to that of six other insect species. Genetical experiments with the BHC/dieldrin-resistant strain indicated inheritance by a single gene pair, intermediate in dominance, very similar to that reported in nine other insect species.

Toxicological investigations of the malathion-resistant colony, including tests with 19 organophosphorus insecticides, showed a resistance largely specific to malathion and malaoxon. This was rather similar to malathion-resistant strains of two other species, for which data were available. It is known that the toxic action of malathion on mammals can be potentiated by prior treatment with small doses of certain organophosphorus compounds, which inhibit an aliesterase that detoxifies malathion by carboxylic hydrolysis, and furthermore that potentiation has been found in malathion-resistant strains of the house-fly, Musca domestica L. Accordingly, the phenomenon was investigated in C. putoria by adding non-lethal amounts of EPN or tri-o-cresyl phosphate (TOCP) to malathion. It was found that specific malathion-resistance was virtually abolished by this means, a result that suggests that such resistance in this species depends on a carboxyesterase. Genetical experiments with the malathion-resistant strain indicated inheritance by a single gene pair, with nearly complete dominance, similar to that in three other insect species.

It is concluded that both types of resistance in C. putoria have much in common with those occurring in analogous strains of other insect species.

The ecology of the adult mosquitos of the Northern Guinea Savannah was studied from 1960 to 1962 in the Kaduna area mainly in two Hausa villages, Anguwun Rimi and Kangimi, and in nearby Fulani settlements. A description of the area and various types of village huts is given together with an account of the climate and meteorological data. Emphasis was on the Anophelines and their relation to malaria but at the same time information on the Culicines is presented.

Biting catches, in which mosquito scouts acting as human bait collected adults in test-tubes, were performed for over a year in A. Rimi. A total of 38 species was caught in these collections, but many appeared only infrequently. By far the commonest mosquito biting man was Anopheles gambiae Giles which had its peak density towards the end of the rains, in September, and although its population was greatly reduced during the dry season it however remained the principal anthropophilous species. In the area in which the catches were performed A. nili (Theo.) was the second commonest mosquito, and with the exception of March it was more abundant than A. funestus Giles. In other areas it was not so common, and in Kangimi A. funestus was almost as abundant during the rains as A. gambiae, and at the beginning of the dry season it became the main Anopheline. Most of the other Anophelines that were taken at bait were only caught in any numbers during the latter part of the wet season; for the remainder of the year they were virtually absent. Mansonia uniformis (Theo.) was abundant in October, but for the rest of the year it was rarely taken; M. africana (Theo.) was found to be uncommon in the area. Due to the close proximity of Kaduna to A. Rimi, Culex pipiens fatigans Weid., a species now associated with urbanisation and not with villages, was common. Both bait catches and pyrethrum spray-sheet collections in houses showed that it greatly increased in numbers at the beginning of the rains, but that this high level of density was not maintained, and during the greater part of the wet season the species was less common than in the early rains. Aedes aegypti (L.) sens. lat. was rare both in houses and in bait catches; the two principal Aedes species were Ae. vittatus (Big.) and Ae. furcifer (Edw.); the latter species is confined to the Savannah zones. The biting cycles of the three main Anophelines were characterised by maximum feeding occurring after midnight. In both A. gambiae and A. nili there were two main peaks, between 2400 and 0100 and between 0300 and 0400 hr.; in A. funestus, maximum biting occurred between 2400 and 0200 and between 0300 and 0400 hr.

By the use of mosquito nets baited with humans, both inside huts and out of doors, the relative attractiveness, to the anthropophilous Anophelines, of man in the two situations was assessed. It was found that A. nili was equally attracted to man in both situations, and although slightly greater numbers of A. gambiae and A. funestus were found biting out of doors, for practical purposes man is equally attractive in both places. Certain species, notably A. coustani Lav. sens, str. and A. hancocki var. brohieri Edw., were rare or absent in the inside and outside bait-net collections, whereas they were readily taken in both situations in test-tube catches performed in the same locality on the same nights. In fact, more specimens of A. coustani sens. str. were taken in the outside test-tube catches than of either A. funestus or A. nili, and A. hancocki var.brohieri was commoner than A. funestus.

Catches of adults from mud walls and thatch roofing of typical round village huts showed that there was generally no appreciable difference in the attractiveness of these two surfaces to either A. gambiae or A. funestus, but as the thatch only comprised about 34–44 per cent, of the internal surface area of the huts the absolute number resting on the walls will be greater than on the roof.

Spray-sheet collections showed that besides large numbers of A. gambiae and A. funestus resting; during the day-time in normal mud-walled living huts of the Hausa, high densities of these species were found in the Fulani huts, which do not incorporate any mud in their design, and moderate numbers were also caught in Hausa villages in the zaures (huts resembling living ones, but without doors, furniture or fires and not normally slept in at night). Collections from outdoor resting places demonstrated that considerable day-time resting occurred in uninhabited huts, under eaves of huts, both inside and under eaves of goat stables, in kitchens, amongst the tightly woven grass mats termed ‘ zana ’ matting, in rodent holes, tree holes, pots, near wells, amongst grass and vegetation and underneath and under the eaves of grain bins. Surprisingly few examples of A. nili were found in any of the outside resting shelters examined, despite the fact that both exit traps and spray-sheet collections showed that it bit freely in huts but rarely stayed in them. The large number of A. gambiae and A. funestus found out of doors, coupled with the exodus of blood-fed individuals from huts, showed that an appreciable amount of exophily occurred in both species. Entrance traps fitted to village huts caught, in addition to unfed, both blood-fed and gravid (taken to include half-gravid and full-gravid) females of A. gambiae and A. funestus, suggesting that a certain amount of exophagy and re-entry of gravid individuals was occurring. Numerous precipitin tests were made on blood-meals obtained from A. gambiae, A. funestus and A. nili. and a few also from most of the other species. Results showed that the three principal Anophelines were strongly anthropophilous but the exophilous population of A. funestus and A. gambiae showed some deviation to other vertebrates. Most of the other Anophelines, e.g., A. rufipes (Gough) sens, lat.,A. hancocki var. brohieri, A. flavicosta Edw. and A. longipalpis var.domicolus Edw., found resting in goat shelters and other outdoor sites had fed mainly on other vertebrates.

On account of the already available information on the infectivity of both A. gambiae and A. funestus in Northern Nigeria, dissections for estimation of sporozoite rates were only carried out on their exophilous populations, and in both species these were shown to contribute to malaria transmission. Attention was concentrated on dissecting the other Anophelines, special attention being paid to A. nili. Only two species were found with infected salivary glands. A. flavicosta had a crude sporozoite rate of 2·74 per cent, of the 73 dissected, and from 998 examples of A. nili a crude sporozoite rate of 0·80 per cent, was obtained. It was thought that as A. flavicosta was mainly zoophilous it was possible that the sporozoites in the glands were of malaria not of human origin. It appeared that where A. nili was common it could become a secondary vector, and on account of its marked exophilic nature it might prove difficult to interrupt transmisssion by this species.

The relatively recent increase in towns of C. p. fatigans and the almost virtual disappearance of C. nebulosus Theo., formerly the predominant Culicine, and the possibility that this change might have been initiated by the practice in recent years of using DDT larvicidal formulations and not oil, is discussed.