It has been suggested that nuclear radiation might be used for locust control, but insufficient knowledge has hitherto been available to decide whether such a measure is feasible. In this paper, the results of some laboratory experiments on the effect of γ-radiation on eggs of the desert locust, Schistocerca gregaria (Forsk.), are described.

After a single dose of γ-radiation, the resulting mortality among the eggs was correlated with the size of the dose received and the age of the egg at the time of irradiation. The dose of radiation required to kill the older eggs was 40 times greater than that required to kill young eggs; thus, a dose of 144 rads caused almost complete inviability among eggs deposited one or two days before irradiation, but a dose in excess of 5,500 rads was required to kill eggs deposited eleven days earlier. For comparison, the lethal dose of γ-radiation for humans is estimated to be 400–700 rad.

When the irradiation dose was applied in three small fractions, with intervals between them, the percentage of eggs killed was less than when the same total dose was given in one exposure. This difference was presumably due to tissue recovery during the inter-radiation periods. The temperature to which the eggs were exposed during the inter-radiation periods also affected survival, recovery being greater at higher temperatures.

The possibility of using γ-radiation to control locust egg-fields, the cost of the equipment needed and the hazards involved are discussed. It is concluded that control by this method not only offers no advantage in terms of cost, effectiveness or convenience as compared with conventional insecticide treatment but would be very impractical and dangerous to operators and the inhabitants, human and domestic animal, of the treated area. It is possible that other control techniques utilising radiation, such as sterile-male release or attractant traps treated with a sterilant, may be useful against certain species of locusts after further work on the development of chemical attractants has been carried out.

A Phycitine species bred from larvae boring in stems of black pepper (Piper nigrum) in the Cameroons and Central African Republic is described as new and is named Anonaepestis tamsi sp.n. The species is differentiated from A. bengalella Rag., the only other known representative of the genus, and illustrations are given of the adults and female genitalia of both species and the male genitalia of A. tamsi.

A series of 24-hr catches of Glossina pallidipes Aust., G. palpalis fuscipes Newst. and G. brevipalpis Newst. was carried out on the north-east shore of Lake Victoria over a period of 11 months in 1962–63. The object was to define and compare the daily pattern of activity of the three species as indicated by the numbers caught hourly on three black cattle that were used as bait. Catching during the hours of darkness was done by the light of dimmed hurricane lamps. The period of observation covered successively a dry season, a wet season, a cool dry season and a hot dry season.

At all seasons, males of G. pallidipes were caught in gradually increasing numbers from just before dawn until shortly before sunset, after which a rapid decrease took place. Females of G. pallidipes exhibited a different pattern, with a gradual rise in activity in the morning until about midday, after which activity remained more or less constant until shortly before sunset, when a rapid fall occurred. There was never any indication of a morning peak in activity such as has been described for this species elsewhere. Activity during the night was at an extremely low level.

Activity of both males and females of G. p. fuscipes started about dawn, increased to a peak in the middle hours of the day, and then fell fairly rapidly in the evening; the exact time of the peak was variable. None was caught during the night.

There was some activity of G. brevipalpis throughout the diel, but marked peaks of activity were exhibited by both sexes. In the open, these took place immediately after sunset and immediately before sunrise, the latter being the smaller. In the shade, the morning peak in both sexes was an hour later than in the open, and the evening peak in males alone was an hour earlier.

These results are compared with those of other workers, and the influence of physical factors is discussed. Particular values of temperature and saturation deficit were not closely associated with particular levels of activity of any of the three species, and light intensity is probably the physical factor that is most consistently the same at times of particular levels of activity at all seasons.

It had been noticed in earlier work that there was a lack of close relationship between the intrinsic and contact toxicities of various compounds to adults of Anopheles stephensi List., the intrinsic toxicities being determined by topical application of solutions and the contact toxicities by exposure to dry deposits. Since the reasons probably lay in the relative rates at which compounds penetrated the cuticle from solution and from the solid state, and since these rates are likely to be differently affected by the physical properties of the compounds, the intrinsic and contact toxicities to A. stephensi of some closely related carbamates and of a miscellaneous group of organophosphorus compounds were determined, together with the solubility in n-hexane and the partition coefficient between n-hexane and water of each, and the results were compared.

No definite pattern in the relationship between these physical properties of the compounds and their intrinsic and contact toxicities emerged from consideration of the results. However, there were indications that very low solubility is a limiting factor in the uptake and penetration into mosquitos of a solid insecticide and is associated with low contact toxicity, and that, when solubility is high enough to ensure solution in the wax layer of the cuticle, contact activity is favoured by a low value for the partition coefficient between hexane and water.

Larvae of Pieris brassicae (L.) were reared very successfully on a semi-synthetic diet containing a small amount (1·4% by wt.) of dried powdered green Brassica leaves. At 20°C., development of newly hatched larvae to the time when 50 per cent, had pupated took an average of 23 days, as compared with 21 days for larvae fed on fresh cabbage leaves. Pupal weights and emergence of adults were normal. The cabbage-leaf powder could be omitted if it was replaced by sinigrin and linseed oil, but on this diet development was prolonged by a further one or two days and the average pupal weights were lower. It seems likely that the stocks could be maintained indefinitely on either of these diets. The larvae made satisfactory growth on standard diet which had been stored at – 20°C. for two months.

An economical method of tsetse control aimed at reducing fly numbers to a low level prior to human settlement was tried in northern Tanganyika in 1964–65 in an area containing Glossina morsitans Westw., G. swynnertoni Aust. and small numbers of G. pallidipes Aust. The major central part of the infested area, about 7½ square miles, was sprayed four times at three-weekly intervals with endosulfan from an aircraft, and the remaining peripheral parts were sprayed from the ground. In the aerial applications, the volume of spray averaged 0·0121 gal./acre per application and the amount of endosulfan averaged 0·0242 Ib./acre (a total of about 1 Ib. per 10 acres for the whole operation). The results indicated that the tsetse population was probably eradicated in the part sprayed from the air. Though the ground spraying of the surrounding parts was not completely satisfactory, the tsetse population in the whole area a year later was only about ten per cent, of its original level. The cost of the air-spraying was £88 per sq. mile.

A description is given of the damage caused by the larvae of two weevils, Systates exaptus Mshl. and Mesoleurus dentipes (Mshl.), which are soil pests of germinating maize and other crops in Rhodesia. Information is given on their life-cycles and some features of their biology. The larvae are nearly full grown at the beginning of the dry season, which is passed in an inactive state in the soil. Activity is resumed with the onset of the main rains, and it is at this time that the larvae may cause serious damage by attacking seedlings of maize or other crops growing in infested soil. Factors that encourage local increase of field populations are examined. It is shown that sunn hemp, soya bean and groundnut, all crops with high plant populations, favour the presence of large numbers of weevil larvae in the soil at the beginning of the following season. Growing closely planted maize year after year on the same land has been found to be one of the main causes of serious infestations in farm fields.

Results of soil-insecticide trials are given, and these show that larvae are very difficult to control. In the discussion, attention is drawn to similar weevil larvae indigenous to other parts of Africa, which are tolerant of soil insecticides and are also pests under conditions of intensive crop production.

Ways are suggested for reducing infestation in the following season. It is shown that properly timed applications of DDT to the maize crop will kill adults and prevent oviposition. Otherwise, infested fields may be left fallow, planted to grass or planted very late in the season.

No method of protecting germinating seedlings has been found, but reference is made to a soil-sampling technique that is useful for detecting infested fields before crops are planted.

A description is given of a verandah-trap hut designed to assess the egress and survival of mosquitos escaping through the eaves of the type of window-trap hut used in insecticide testing in East Africa.

Fifty-one per cent. of females of Anopheles gambiae Giles in all gonotrophic stages and 19 per cent. of those recently fed left the hut each night, with 15 and 30 per cent., respectively, of the egress occurring through the eaves.

Ninety per cent of females of Mansonia uniformis (Theo.) in all gonotrophic stages and 91 per cent. of those recently fed left the hut each night, with 69 and 66 per cent., respectively, of the egress occurring through the eaves.

It was concluded that, owing to the high proportion of individuals of M. uniformis that leaves by the eaves, the verandah-trap hut was of a more suitable design for studying the house-frequenting habits of this species, and of other species with similar habits, than the simple window-trap hut.

The results are described of 202 24-hr, catches of Stomoxys and Tabanids on cattle at Lugala on the north-eastern shore of Lake Victoria, and also of six 24-hr, catches at Sukulu, 33 miles from Lugala, all of which were made in 1961–63.

Species of Tabanus and Ancala were more abundant in open grassland than inside the edge of the lake-shore forest. Stomoxys nigra, Macq. and S. Calcitrans (L.) were equally numerous in both situations, whereas S. omega Newst. was most abundant inside the forest edge.

Many species were caught during all months of the year. In both catching areas, there were seasonal rises in numbers of nearly all species between May and August and again in November or December, that is, towards the end of the rains and early in the dry seasons. At one of the two areas there was also a large increase in catch of a few species in April, in the middle of the main wet season.

Diurnal activity of Tabanus and Ancala was greatest near the middle of the day, the exact timing of the peak varying with season. Four species of Haematopota exhibited morning and evening peaks with lower activity at midday, and a rather rare fifth species was apparently more active in the middle of the day. Three species of Stomoxys were also most active in the morning and late evening, but peak activity of a fourth species, S. calcitrans, occurred shortly after midday. Activity during the night was rare.

An account is given of the biology and importance of sorghum midge, Contarinia sorghicola (Coq.), in Ghana. It is distributed throughout the interior savannah areas and because of traffic in grain sorghum (guinea corn) has spread into forest localities and some places in the coastal savannahs. Host-plants comprise all varieties of Sorghum vulgare sens. lat. and the grass Sorghum arundinaceum. Its occurrence in Pennisetum polystachyon has not been confirmed.

The biology of C. sorghicola in Ghana does not differ significantly from that described elsewhere. Adult emergence occurs early in the morning and peak oviposition is from 0800 to 1000 hr., shortly after emergence. The majority of spikelets contain only one larva or pupa, but 64 per cent. of immature stages (larvae + pupae) nevertheless occur two or more per spikelet. Density per spikelet is related to rate of infestation, with maximum density at about 2·5 immature stages per spikelet at infestation rates of about 40 per cent. The mean density over two seasons was l·7 immature stages per spikelet. The cycle from egg to adult requires 17 to 21 days to complete.

The onset of diapause in larvae and the eventual breaking of diapause may be the consequences of threshold reactions to moisture and temperature, with a cumulative process operating in breaking diapause. It is suggested that observed differences between northern and southern localities in dates of appearance of adults and subsequent infestations indicate that moisture is the critical threshold factor in the north and temperature in the south.

Parasites are of little importance. Only Eupelmus popa Gir. has been identified from Ghana, and it exerts no influence on population levels.

A most important relation exists between midge infestation and time of main flowering of the crop. When main flowering is early in the season, midge attack is very low, below 5 per cent. or even non-existent, but as dates of flowering are delayed so the percentage of spikelets infested rises rapidly, an infestation of 81 per cent, having been recorded in a late crop. In general, it seems that infestation is heavier in the less severe climate of the Guinea savannah than in the Sudan savannah areas.

A sampling method is described in which three categories, ‘sound’, ‘midge attacked’ and ‘others’ (which included all undersized or damaged grains not resulting from midge attack) were set up. In respect of midge, it was shown that attack was evenly distributed over the head of sorghum but that in some cases the ‘others’ category gave highest rates at the bottom of the head. This, it is suggested, may be due to differential attack by various species of Hemiptera, which may be responsible for a considerable proportion of the grains included in the ‘others’ class. Surveys carried out in 1952 and 1953 showed that midge was not a very serious problem, infestation rates above 10–15 per cent, being unusual except in late-flowering crops. On the other hand, losses hi the ‘others’ category were rarely below 15 per cent. and were often well over 30 per cent., indicating a major source of loss.

It is confirmed that Nunaba, which typifies the group of varieties in which the glumes remain closed at anthesis, is resistant to midge. It is also confirmed that this resistance breaks down in the absence of a more favoured alternative variety. There is evidence that susceptibility to loss in the ‘others’ category varies between some Nunaba × Belko crosses, one cross, AA226, showing an appreciably smaller loss than two others; the variety Kamolgu may also be less susceptible.

It is suggested oft the basis of field observation in Ghana and references to literature that Hemiptera are the primary cause of loss in the ‘others’ class; in Ghana, Riptortus spp. and Mirperus spp. are important. If this damage is consistently serious, the problem of measures needed to reduce losses is complicated. Midge could, in the long run, be controlled or even eliminated as an economic pest by adoption of practices leading to early flowering in periods before the appearance of midge. In Ghana, these periods are given for the main sorghum areas of the north and north-east, and it is suggested that development of varieties of shorter maturity periods would assist by easing the congestion at normal sowing times in May and June. There is no significant relation between percentages recorded in the ‘others’ category and flowering date, so that other methods of control must be looked for, and it is suggested that search for resistant, or less susceptible, varieties offers a promising avenue of approach. Evidence is presented that losses from both midge and ‘others’ can be reduced by a 1 per cent. DDT spray, but chemical control is unlikely to be of general use.

The most urgent need is for a general investigation of causes of loss in sorghum, with particular attention to the agents other than midge that are responsible for what appear to be large annual losses. Standard trials covering a range of conditions and flowering dates, and aimed at establishing the economic status of the sorghum midge when related to dates of flowering, are proposed. These would also provide much of the information necessary for the formulation of further programmes for the study of the other agents.

In the present study, which represents an extension of earlier work, a method is described for comparing the metabolic status of cacao leaves by measuring the time of survival of excised discs of leaf tissue, and, when used with appropriate sampling technique, is considered reliable for indicating differences in the physiological age of such tissue. Leaf-tissue samples from trees with a history of impeded growth and low yield, and wet-seasonal liability to attack by cacao thrips, Selenothrips rubrocinctus (Giard), were compared with samples from trees of the same clone growing in the same field, with a history of relatively rapid growth, superior yield and freedom from such attack. Their relative survival periods were found to be concordant with their relative physiological ages (as expressed by percentage of dry matter and of total nitrogen and response to foliar injections) and with the subsequent incidence of cacao thrips. A similar comparison, using leaf material from two clones, susceptible and relatively resistant to attack by cacao thrips, respectively, included samples from the susceptible clone collected at a locality where cacao thrips was common in the wet season, and at another where the insect was never common at any time, and samples from the resistant clone taken in the former locality from fully exposed and shaded portions of individual leaves. Leaf samples of the susceptible clone collected at the inferior site and those from the fully exposed parts of the leaf of the resistant clone were found to be similar in length of life, and were both relatively physiologically senescent and demonstrably of subnormal nutrient status, whereas the corresponding samples from leaves of the susceptible clone at the superior site, and from the shaded portions of the leaves of the resistant clone were also similar in length of life, significantly longer lived than the former samples, physiologically younger, and apparently of normal nutrient status. The distribution of cacao thrips that subsequently developed in the field on leaves of the resistant clone was confined to the fully exposed part of the leaf. Leaves of the resistant clone were found to contain more moisture and more solid matter than those from the susceptible clone. A comparison was made of leaf tissue from leaves debilitated by lime-induced chlorosis and of leaf tissue from areas of similar leaves that had recovered as a result of foliar injection with an acidified solution of ferrous sulphate, and the subsequent incidence of cacao thrips in the field on the two types of tissue was recorded. The debilitated tissue was found to be shorter-lived than the restored, and was heavily attacked, whereas the restored tissue remained uninfested. The incidence of cacao thrips on ripening pods was found to be restricted to pods with a history of relatively slow growth prior to infestation. The findings are discussed in the light of information obtained in an earlier investigation, and it is concluded that the results can be explained by assuming that establishment of a cacao-thrips colony on cacao is obligately dependent on the prior occurrence of metabolic derangement in the host leaf leading to a retardation of protein synthesis, and that in Trinidad, W.I., where the investigation was made, such derangement results from the effect on the tree of seasonally adverse factors in its physical environment.

A population of Myzus persicae (Sulz.) that apparently showed resistance to organophosphorus insecticides was found on plants growing in glasshouses at the Plant Breeding Institute, Cambridge, in 1963. Experiments were conducted to compare the resistance to dimethoate and demeton-methyl of this population with that of another population, which had never been treated with insecticides. The resistant aphids were killed much more slowly than were susceptible aphids on plants sprayed with organophosphates, and they were able to transmit beet yellows virus (BYV) and beet mild yellowing virus (BMYV) to the treated plants. Under similar conditions, susceptible aphids were able to transmit BYV, a semi-persistent virus, but less efficiently than the resistant aphids, and were not able to transmit BMYV, a persistent virus, at all.

It is not known how common these organophosphorus-resistant populations of M. persicae are, at present, in the field in Britain. Under normal field conditions, control of such resistant aphids would be difficult, and virus yellows might, in consequence, become widespread even in crops that had been treated with organophosphorus chemicals.

With the aid of reflective patches fixed to flies, nocturnal resting sites of Glossina morsitans Westw. released in teak woodland near the Mashi River in Zambia were detected by torchlight. Within this habitat, sites on the margins of woody vegetation were chosen for the tests, so as to slow up fly dispersal and raise the chances of seeing the marked flies. Over 500 flies were marked and released, and of these 82 (16%) were subsequently seen resting, the maximum height recorded being 20 ft. above ground-level. The distribution of resting heights is shown in tables. Leaves and small twigs comprised 77 per cent, of the nocturnal resting sites. In one series of tests, the proportion of marked flies detected was considerably lower than in other tests, and an environmenal factor is thought to be the cause.

Six species of Cryptolestes that occur in stored products (C. ferrugineus (Steph.), C. capensis (Waltl), G. pusillus (Schönh.), C. turcicus (Grouv.), C. pusilloides (Steel & Howe) and C. ugandae Steel & Howe) showed different susceptibilities to fumigation with methyl bromide at 30°C. and 80 per cent. relative humidity. The susceptibilities were partly correlated with morphological and bionomic differences between the species: C. ferrugineus was the least susceptible.

The influence of population density, temperature, humidity and other environmental factors on the individual behaviour, aggregation in restricted parts of the habitat (accumulation) and spatial structure of the population (dispersion) of five species of beetles found in stored grain in Britain was the subject of laboratory studies by the author in 1959–65. In this paper, the results of these studies are drawn together, compared with field observations, and discussed in relation to behavioural mechanisms, ecology and control. The five species comprised Oryzaephilus surinamensis (L.), Tribolium castaneum (Hbst.).Cryptolestes ferrugineus (Steph.), Rhyzopertha dominica (F.) and Sitophilus granarius (L.). A basic plan of behaviour is considered to underlie the observed dispersion of these pests in grain, which may be summarised as follows: random movement of individuals causes disturbance within the population and leads to dispersal; accumulation occurs where individual movement is most restricted in response to environmental factors and where disturbance is consequently at a minimum.

The dispersion of O. surinamensis is almost entirely determined by kinesis mechanisms, and accumulation takes place in the warmest and dampest parts of a grain bulk. The dispersion of T. castaneum is determined to a lesser extent by kinesis mechanisms; accumulations take place in the drier parts of a bulk and at places where the temperature is about 25°C., but also in regions of damp grain if this is mouldy. The dispersion of C. jerrugineus is mainly determined by behaviour relating to the requirements of oviposition and feeding, which is superimposed on the basic plan. R. dominica accumulates in the driest parts of a grain bulk. The dispersion of S. granarius is largely determined by individual thigmotactic behaviour, so that accumulations are typically at the periphery of the grain bulk.

The ways in which a knowledge of the underlying causes of dispersion and accumulation aid in the detection and control of the pests and point to the optimum storage conditions are described.

Plots on arable land in southern England that had received an annual application of MCPA herbicide for 10 out of 13 years between 1951 and 1963, and similar untreated plots, were sampled to see whether there were any long-term or short-term effects on the arthropod fauna. Samples were taken before and after spring sowing, before and after spray application of the herbicide, and about a month later when weed counts were made. The fauna was divided into orders or suborders initially, and no differences between treated and untreated plots were detected either at the beginning of the season, as a residual effect from previous years, or in the period immediately following spraying. The Acari population was then analysed in more detail for possible qualitative differences between treatments, and the recorded numbers of 12 common species and two genera in treated and control plots were examined. No effect that could be attributed to the use of MCPA was found. The density of the fauna is discussed in relation to other soil arthropod surveys, and the wealth of Acari species is commented on. A list of the species of Acari and Collembola collected is appended.

The deposits found on apples at various times after application of 11 insecticides were determined by chemical analysis of samples from an orchard in south-eastern England sprayed in 1959 and 1960. The results, from samples taken simultaneously, of studies of the effectiveness of these and other chemicals for control of the codling moth, Cydia pomonella (L.), have already been reported.

The period in which 50 per cent, of an initial deposit disappeared in the field (half-life) was calculated from the chemical data and used as a measure of persistence. For comparisons of intrinsic toxicity, the levels of fresh deposits causing 50 per cent, mortality of newly emerged codling-moth larvae (LD50) were determined in the laboratory by chemical and biological tests on apples dipped in a range of concentrations of commercial formulations. The LD50's, in μg./cm.2, were less than 0·025 for carbophenothion, parathion and diazinon, between 0·025 and 0·05 for azinphos-methyl and -ethyl, between 0·05 and 0·10 for malathion, DDT and carbaryl, between 0·1 and 0·5 for DDD and dimethoate, between 0·5 and 1·5 for lead arsenate and above 1·5 for phenkapton.

Chemicals with an LD50 exceeding about 0·1 μg./cm.2 were considered not worth further investigation as codling-moth larvicides. The LD90's of the remainder were found and were taken as the minimum deposits that would have an adequate effect from the standpoint of control (minimum effective levels). From the half-life values, the periods for which the deposits found in the orchard in 1960 should have remained above the minimum effective levels were calculated; these were 24 days for DDT emulsion, 22 for DDT wettable powder, 15 for carbophenothion, 14 for carbaryl, 12 for azinphos-ethyl, 11 for parathion, 7 for malathion and 6 for diazinon. They were in good agreement with the results of the biological tests except in the case of DDT emulsion, the effectiveness of which was unexpectedly short-lived.