Observations, largely based on regular catches along a fly-round, were made over the five years 1959–64 on a population of Glossina morsitans submorsitans Newst. in the Northern Guinea Savannah of Nigeria. The results showed that the largest numbers of flies were caught in the early dry season (November–January) and that, as the climate became progressively drier and more severe, fly numbers declined to reach their annual minimum at the end of the dry season or in the early rains (March–May). These results are tentatively interpreted in terms of the true density of the flies and their activity. Differences occurred between the various years, some of which could be explained by climatic differences.

Of the 7,412 flies caught over the five years, 1,128 (15·2%) were females; the percentage of females was highest in the dry season, rising to a peak of 24·1 per cent, in February, and was below 10 per cent. during the wet season. Many more females were caught on the bodies of the catching team than on vegetation or the ground near the team.

The flies rarely fed on civet cat (Civettictis civetta) or duiker (Cephalophus rufilatus, Sylvicapra grimmia), which were the potential hosts most frequently observed in the experimental area, but fed mainly on wart-hog (Phacochoerus aethiopicus) and man, the next most commonly observed potential hosts.

During the heavy rains, males of G. m. submorsitans were evenly distributed over the fly-round, but at all other seasons they were concentrated to some extent in areas of thicker vegetation. During the dry season, pupae were found in the dry soil of forest islands and riverine vegetation in the savannah; the wet season breeding sites were not discovered. Previously published data on the resting sites and trypanosome infection rate of G. m. submorsitans in the area are summarised.

The results are discussed and compared with the conclusions reached by other workers from earlier more extensive studies on G. m. morsitans Westw. in Tanganyika and on G. m. submorsitans in the Sudan Savannah vegetation zone of Northern Nigeria.

A series of forty 24-hour catches was carried out at seven levels (20-ft. intervals from 0 to 120 ft.) on a steel tower in Zika Forest by means of unbaited and unlighted suction traps that acted simply as air samplers. The object was to study the general flight activity of mosquitos in a tropical forest environment from samples that would not be subject to the selective effects exercised by bait-traps and light-traps.

Altogether, 4,151 mosquitos belonging to thirty-four species or species-groups were taken. Eight species, Aedes (Stegomyia) apicoargenteus (Theo.), Mansonia (Coquillettidia) aurites (Theo.), M. (C.) fuscopennata (Theo.), M. (C.) maculipennis (Theo.), M. (C.) metallica (Theo.), M. (C.) pseudoconopas (Theo.), M. (Mansonioides) africana (Theo.) and M. (M.) uniformis (Theo.), were taken in numbers sufficient for detailed analysis. By far the most abundant of these was M. aurites, comprising 55·7 per cent, of the total sample. The remaining twenty-six species together constituted only 7·6 per cent.

Only 144 or 3·5 per cent, of the mosquitos taken were males; and of the females, only 19 or 0.5 per cent. were blood-fed and only 11 or 0·3 per cent, were gravid. Thus, catches in the suction traps consisted almost entirely of unfed non-gravid females. It is tentatively concluded that male mosquitos in the forest are generally very inactive in the absence of stimuli such as lights or baits. The paucity of engorged and gravid females was expected; few of these would come within range of the unlighted unbaited suction traps.

Dissection of 3,072 females belonging to the eight most numerous species showed all to be inseminated. Age-composition was determined in species of Mansonia only; the majority of females in the subgenus Coquillettidia were nulliparous, whereas in the Mansonioides, nulliparous and parous females occurred in roughly equal proportions.

The unlighted, unbaited suction traps caught far fewer mosquitos than did light- and bait-traps under similar conditions.

In general, the vertical distributions and flight-activity cycles of the various species as indicated by the suction-trap catches differed distinctly from those indicated by bait- and light-trap catches reported by other workers. Very little activity was recorded above the forest. Although, in general, activity was irregular, three main patterns could be distinguished: largely nocturnal activity continuing up to about midday but almost completely suppressed in the afternoon, as in M. aurites, M. fuscopennata, M. maculipennis, M. metallica and M. pseudoconopas; activity spread throughout the entire 24-hour period, as in M. africana and M. uniformis; and mainly diurnal activity, as in A. apicoargenteus.

Leptoconops becquaerti (Kieff.) is one of the most troublesome species of biting midge in Jamaica. The gonotrophic cycle in this insect was studied and is described. In this species a single blood-meal is sufficient for the development of the ovaries and the maturation of eggs.

The stages in follicle development and digestion of the blood-meal adopted for other blood-sucking Diptera have been applied to L. becquaerti with slight modifications. A high degree of gonotrophic harmony is present in this insect.

Fully fed females were kept at five temperatures, 68, 77, 85, 91 and 98°F., to determine the time taken for the ovarian follicles to reach Stage V of development (egg mature, chorion present). The times were approximately 98, 64, 36, 30 and 27 hours, respectively.

It was demonstrated that the number of ovarioles in L. becquaerti was proportional to the size of the individual (as measured by wing length). The average number of ovarioles in an insect of wing length 0.83 mm. (the mean wing length of the population sampled) was about 103.

The number of eggs matured was also proportional to wing length, and was also dependent upon the temperature at which the flies were kept. Thus at the standard wing length of 0·83 mm., the average numbers of eggs matured at the five chosen temperatures (68 to 98°F.) were 78, 85, 65, 61 and 60, respectively. Females that had matured eggs were induced to oviposit by the decapitation technique.

Females provided with a source of carbohydrate (honey) during egg maturation produced slightly more eggs on the average than those fed on blood alone. The comparable figures at 91°F. in the standard individual were 65 and 61, respectively. The probable rôle of supplementary carbohydrate feeding in nature is discussed.

At the five experimental temperatures the greatest number of eggs hatched on days 11, 7, 5, 4 and 4, respectively, after laying. Egg-batches used in these experiments were deposited by decapitated females, and may therefore have been laid somewhat before the usual time in nature. The percentage fertility varied between individual batches at all temperatures, but the over-all fertility was consistently high (70–75%) at the lower four temperatures, and very low (4%) at 98°F. This difference almost certainly resulted from high temperature in the latter instance. Fertility in egg-batches deposited naturally (without decapitation) was of the same order as that in batches deposited by headless females.

The species commonly known in the West Indies as the pepper flower-bud moth has been shown not to be Gnorimoschema gudmannella (Wlsm.), as hitherto supposed, but an undescribed species, for which the name Gnorimoschema capsica is proposed. The species is described and differentiated from G. gudmannella and other species in the Grnorimoschemini.

Three-day-old virgin females of Glossina austeni Newst. that had emerged in the laboratory in Britain from wild puparia collected in Africa or from subsequent laboratory cultures were caged with males over nine days old for 24 hours and dissected after death. Of the 5,474 flies so treated, 38 per cent. died within 14 days after mating day, whereas of the 593 found to be uninseminated, 73 per cent. died during that period, mostly within the first six days. A possible explanation of the discrepancy is that females with a poor expectation of life are unattractive to males, and this could account for the very small proportion of uninseminated flies in field collections, since such flies would tend to die before they could be captured.

The effect of attack by the Coreid bug Pseudotheraptus wayi Brown on the yield of coconut palms was studied in Coast Province, Tanganyika, in 1957–60. Alternate plots in a line of six one-acre plots, each of 50 palms, were sprayed weekly with a mixture containing 0·1 per cent. DDT (to control P. wayi) and 0·1 per cent. malathion (to prevent any increase in scale insects). To facilitate collection of the immature nuts (vidaka) shed by the palms, the plots were kept cleared of surface vegetation. Since this is not a normal practice in local coconut plantations, its effect on yield was checked by leaving the vegetation untouched on six similar, unsprayed, plots immediately adjacent to the first six. All vidaka falling on the latter were collected and recorded daily, and the numbers showing damage by P. wayi were noted. The ripe coconuts on all twelve plots were harvested by climbers, and then counted, at eight-week intervals. Since the nuts take about 12 months to ripen, yields in the first year were in effect those of untreated plots.

The yields from unsprayed plots showed that removing the vegetation was beneficial; the three cleared plots produced 490 and 1,159 more nuts per plot in the second and third years, respectively, than their adjacent, uncleared, partners, whereas in the first year they produced fewer. The mean annual yields from these three cleared but unsprayed plots were successively 817, 2,134 and 3,129 nuts per plot. The corresponding yields from the three cleared plots that were sprayed were 1,102, 3,310 and 2,992. Thus, protection against P. wayi appeared to increase the yield in the second year, but to have no effect in the third, although vidaka damaged by P. wayi were fewer on sprayed plots throughout.

A series of estimates of ‘ total production ’, defined as the sum of the vidaka shed in any period and the ripe nuts subsequently harvested, was obtained by adding the total vidaka collected in successive eight-week periods to the ripe nuts gathered in the harvest taken 11½ months after the middle of each period. These showed that the initially greater yield from sprayed plots came about because a greater proportion of the ‘ total production ’ was converted into ripe nuts, that is, fewer vidaka were shed, doubtless as a result of the diminished attack by P. wayi. The subsequent drop in yield to about the level of the unsprayed plots was due not to any substantial change in this proportion but mainly to a drop in ‘ total production ’.

It is inferred that the number of ripe nuts produced by the palm is subject to an upper limit, which is determined by agronomic factors and greatly exceeded by the number of immature nuts formed. The excess nuts may be shed at an early stage, either for physiological reasons or as a result of biotic factors such as insect attack. Thus there exists a compensating mechanism whereby yields may remain constant under different levels of insect attack and yet show immediate responses to changes in soil or climatic conditions. This could account for the fact that, in the area studied, clearing of plots produced an apparently permanent increase in yield, and insect control only a doubtful and temporary one. It is concluded that, under present conditions, increases in coconut yields are more likely to result from improved methods of cultivation than from pest control with insecticides.

Work at East Malling, Kent, between 1952 and 1963 on the biology of Harpalus rufipes (Deg.) (Carabidae), the strawberry seed beetle, is described in detail, and briefer notes are given on the life-histories of H. aeneus (F.), Feronia melanaria (Ill.) and F. madida (F.), which are less abundant but may also damage strawberries. Soil samples for larvae were washed over suitable sieves, those for pupae and adults were carefully searched in the field. Soil sampling showed that adults of H. rufipes colonised strawberry fields only during the fruiting period, weedy arable land and grassy situations being preferred at other times. By soil sampling, it was found that larvae of H. rufipes hatched from eggs laid in weedy soil in late summer, and evidence from pitfall trapping showed that the first two instars were sometimes active on the soil surface from August to December, before penetrating to a depth of 6–18 in. to overwinter. Development was resumed in April, but the larvae remained and the pupae were subsequently found at the overwinter depth in the soil, the larvae presumably feeding on the seed of fat hen (Chenopodium album) lodged in worm burrows. Third-instar larvae occurred from April to July, and adults emerged soon after. In the laboratory, adults lived for a further two years, hibernating in winter, and becoming active and breeding each summer. Laboratory observations on oviposition periods, fecundity and feeding were also made. Larvae, which in the field were mostly found associated with plants of fat hen, fed on germinating seeds of several plants in the laboratory but growth was most rapid when they were reared on the germinating seed of fat hen.

The life-cycles and habits of the three other species are briefly described and compared with those of H. rufipes, the seasonal occurrence of the different stages in the life-cycles of all species being shown in a table.

Field and laboratory studies of virus disease in the armyworm Spodoptera exempta (Wlk.) were carried out in East Africa in 1962–64. A nuclear polyhedrosis virus was detected microscopically in pupae, prepupae and all larval instars except the first, and was found to be widely distributed in most areas subject to outbreaks of armyworm in East Africa. The signs and course of the disease in larvae of S. exempta are described. Attempts made in the laboratory to infect S. triturata and other species of Spodoptera with virus from S. exempta failed. In unsterile conditions of maintenance, almost all laboratory cultures from wild-caught female moths sustained heavy infections. The appearance of the symptoms of polyhedrosis was accelerated at higher temperatures, and the incidence was higher in more crowded cultures. Transmission took place from contaminated leaves to larvae and probably from one larva to another. There was evidence also of generation-to-generation transmission through the egg.

The effect on polyhedra of immersion in alkali solutions was investigated; it. was found that immersion for 15 minutes in 0·2 per cent. caustic potash effectively dissolved the matrix and inactivated the virus. When all equipment was sterilised in this way (or, later, with 0·4% caustic potash) apparently virus-free cultures were obtained by rearing the larvae from wild-caught females in isolation and breeding from those that survived to become adults.

The possible use of the virus for control of outbreaks of armyworm in the field is discussed; it is concluded that there is at present insufficient evidence to show whether such a measure would be effective.

In laboratory studies of Heliothis armigera (Hb.) in Western Tanganyika, where it may attack cotton severely, pairs of newly emerged adults, derived from sixth-instar larvae collected in the field and allowed to pupate in the laboratory, were confined separately and provided with sucrose solution. The mean number of eggs laid per female fell from 1,226 in March-May to 198 in June—July 1962. Peak egg laying occurred in the fifth to eighth nights after emergence. In April, the mean length of life of adults of either sex was about 11 days and the maximum one month. The mean incubation period of 1,436 eggs laid in March was 2·78 days, and 66 per cent. of them hatched. The mean period from hatching to pupation of 42 larvae reared on cotton squares in April was 21·1 days; each passed through six instars and required three squares in the first 14 days and a further 10 thereafter; only 22 per cent. of the dry weight of the squares was consumed and of this only nine per cent. was converted to pupal dry weight.

Pupation occurs in the soil. The mean pupal period of males increased from 16 days in March to 18 in July, and their pupal weights fell from about 0·27 to 0·23 mg. over the same period, although at any given time pupal period was positively correlated with pupal weight. Female pupae were lighter than male pupae and had a shorter pupal period.

From March to July, a small but progressively increasing proportion of the pupae formed entered diapause; the proportion averaged less than four per cent. in June to July 1962–64. The pupal period ranged from 30 to 171 days, being less in later-formed pupae, so that a general emergence occurred in September–late October.

In Western Tanganyika, Heliothis armigera (Hb.) occurs as a major pest of cotton from February to April, but also feeds on many other host plants. The dry season (August–October) is spanned usually by diapause pupae, the moths emerging from which oviposit on a species of Gleome, on which the first generation develops, giving rise to adults in November–December. Larvae of the second generation feed mainly on early-sown maize, and it is usually those of the third and fourth generations that attack cotton. Two subsequent generations occur on a variety of host plants, including late cotton, maize and legumes. The increasing severity of the attacks of Heliothis on cotton has accompanied the replacement of sorghum by maize in recent years. Maize is not only an intermediate host on which Heliothis builds up before it attacks cotton but also a source of diapause pupae later in the season.

In studies of the natural control of H. armigera, based on regular sampling of eggs and larvae on host plants in the field, the most important parasites were the Tachinids Drino imberbis (Wied.) and Goniophthalmus halli Mesnil, and field observations suggested that a species of Chrysopa (Neuroptera) was the main predator. Increasing use of insecticides may be reducing the effectiveness of natural control. A polyhedral virus was common, mainly found in larvae of Heliothis feeding on maize. The control of Heliothis on cotton by cultural methods was attempted by banning early sowing of maize and by sowing strips of maize in cotton fields as trap crops. Neither method is practicable on a large scale. Early sowing of cotton appears to be the best means of withstanding attack by Heliothis, for the local strains of cotton are able to compensate by later growth even for a very heavy Heliothis attack, provided they are sown early enough to utilise the late rains.

In cotton that was subject to a heavy attack by H. armigera, five applications at 2-week intervals of DDT at 1 lb. per acre gave good control of the larvae and increased the yield from about 200 lb. seed cotton per acre to about 500 lb., the difference being mainly in the early pickings. The addition of toxaphene increased the yield, probably by controlling Earias spp., but neither toxaphene alone nor carbaryl appeared to control Heliothis. The numbers of eggs laid were higher on cotton treated with DDT than on untreated cotton.

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.

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.

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.

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.

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.