Wild populations of Calandra granaria were allowed to oviposit on clean wheat and the tolerances of the eggs to mercury vapour were estimated. For six such populations the median lethal doses ranged from 55 mg.hr./m.3 to less than 1.0 mg.hr./m.3. A laboratory strain of grain weevils has developed resistance to mercury vapour in the egg stage, probably from contamination of an incubator. Such evidence as is available suggests that this resistance is associated with reduced chorion permeability rather than enhanced metabolic ability. The increase in resistance is such that no attainable concentration × time product will kill more than half the resistant egg population which is thus virtually in equilibrium with air saturated with mercury vapour. Fumigation of gravid females, or of wheat prior to oviposition, does not reduce the viability of any eggs subsequently laid.

Adults of Calandra granaria and C. oryzae have been fumigated with hydrogen cyanide by the methods of sustained pressure reduction and preliminary pressure reduction, the main criterion of response being death. Of the factors studied, starvation and sex of the insects are without influence. With pressure reduction, Haber's Rule relating mortality, concentration and period of exposure is not followed; for a given concentration-time product, the mortality is higher, the shorter the period of exposure.

Preliminary pressure reduction increases mortality much less than sustained pressure reduction. With the latter the mortality of C. oryzae is higher than that of C. granaria. The increases in susceptibility are largely accounted for by increases caused by pressure reduction alone in the absence of fumigant. The number of eggs laid by untreated females and by females surviving treatment was excessively variable and no discrimination between factors could be made.

Experimental huts similar in construction to the dwellings commonly used in East Africa, but with exit window traps, were sprayed with various formulations of the three residual insecticides, DDT, BHC, and dieldrin, and the effect on the A. gambiae and A. funestus entering them was observed.

The almost complete absence of kill recorded by Muirhead Thomson (1950) in experiments in similar huts in Tanganyika treated with DDT Ditreen was not confirmed by these experiments.

A significant proportion of the A. gambiae and A. funestus entering huts treated with DDT did, however, escape unharmed, even immediately after treatment, whereas with the other insecticides, BHC and dieldrin, none of these mosquitos escaped the effect at least in the first month after treatment.

In preliminary experiments in which observations were carried on for nine months after treatments, BHC P.530 still showed some effect after seven months. This was almost certainly due to the fumigant effect of the small amount of insecticide still remaining below the wall surface. The irritant properties of the two DDT formulations, Ditreen and the oil-bound suspension “Supona” D, still existed after nine months.

In a second group of experiments, dosages of less than 80 mg. DDT and less than 60 mg. BHC (8 mg. of the gamma isomer) per sq. ft. gave over 50 per cent. kills of A. gambiae and A. funestus for only one month.

In a third group of experiments, using two formulations of BHC, five of DDT, one of a mixture of DDT and BHC and one of dieldrin:—

(a) Dieldrin was by far the most efficient insecticide and gave very high kills for over seven months.

(b) The DDT formulations, Murphy paste, Murphy wettable powder, suspensions of DDT crystals <30 μ and 30–70 μ in diameter, when applied to the whole internal surface of the huts, produced fairly high kills over the period of the observations (six to seven months), but significant proportions of the mosquitos escaped their action even immediately after treatment.

(c) The BHC formulations, P.520 and the oil-bound suspension “Supona” B, gave high kills for three to four months only.

(d) The mixture of BHC and DDT in oil-bound suspension “Supona” DB gave the high initial kill of BHC and the long-lasting moderately high kill of DDT.

(e) Against C. fatigans all the DDT formulations used in the third group of experiments gave very low kills, the BHC formulations high initial kills and dieldrin high long-lasting kills.

BHC has marked fumigant and particulate properties lasting for three to four months. Dieldrin has a remarkable particulate action, which produces for the whole six-month period of the experiment, very high kills among mosquitos suspended without actual contact with the insecticidal surfaces; DDT only shows this particulate effect to a slight extent.

It is probable that the differences in the toxicities to mosquitos of the insecticides used in these experiments is due partly to differences in the irritant properties of the insecticides. In the case of DDT many of the mosquitos having contact with this insecticide are irritated and escape from the treated surface before acquiring a lethal dose.

The behaviour of mosquitos in contact with deposits from aqueous suspensions of insecticides has been studied by allowing them to alight individually and measuring the time to the first flight.

Anopheles stephensi females flew from all formulations of DDT on various materials after approximately the same time of 2 to 4 minutes, but subsequent kills were high only when the deposit consisted of small (less than 10 microns) particles readily available for pick-up.

Mosquitos were also activated by contact with fresh deposits of BHC wettable powder P.520, but all died very quickly. They were not disturbed, however, by the vapour from BHC deposits, when contact was prevented, until in an advanced stage of intoxication. The time to the first flight from surfaces treated with BHC wettable powder increased as the deposits aged and varied with different materials according to the rate of loss by evaporation and/or sorption.

Mosquitos remained in contact with deposits of chlordane, dieldrin, aldrin and toxaphene for more than 30 minutes.

Experiments with a number of DDT analogues and BHC isomers indicated that lipoid solubility and molecular structure are two factors governing the rate of penetration of a compound.

The reactions of other species tested were similar to those of A. stephensi but kills of Culex p. molestus were significantly different from those of the Anopheline species after contact with deposits from a DDT wettable powder.

C. P. Clausen's and W. R. Thompson's concepts on the time factor in biological control are compared with the writer's experience. All three viewpoints emphasise the influence of environment correlated with time. As Thompson points out, we are almost always forced to ascribe the limitations of the numbers of pests to a combination of factors, biotic and physical, varying from time to time and from point to point. Clausen's views concerning the maximum time (three host generations or three years) necessary for an introduced parasite to demonstrate that it will be fully effective are supported by the observed facts. There is associated with the initial establishment of introduced parasites, however, a time lag that is not evident in the control of subsequent outbreaks. The principal factors are the attainment of an “equilibrium of distribution” and the rate of dispersal in building up a controlling density. When effective biological control can be anticipated, it is characteristic that marked outbreaks are brought rapidly under control, and the pest population then oscillates at a relatively low level. Several examples of fully effective parasites and their limitations are discussed which indicate that the value of a parasite or predator should be regarded not as a constant but rather as a variable.

An equilibrium of distribution is obtained when there is no decrease in the density of a parasite due to outward dispersal into the uncolonised area; the forward and backward surges of dispersal offset each other, producing a minimum uniformity of distribution independently of other influences. Parasites with slow dispersal rates produce an equilibrium of distribution and effective control more rapidly, but in a proportionately smaller area, than those with fast dispersal rates.

When these considerations are applied to the introduced, partially effective parasites of the gipsy moth, the time element is seen to be proportionately extended beyond the three-year period. After establishment, however, the effectiveness of these parasites is conditioned by the normal duration of the heavy outbreaks of the gipsy moth which rarely exceeds three years. Many of these enemies are as effective in the new environment as in the old. The degree of control of the gipsy moth achieved in the north-eastern United States does not differ greatly from that which occurs in central Europe. Examination of this relationship indicates that an enemy that is to become useful, but not completely effective, does not show its value within three years; consequently the adoption of Clausen's conclusions involves a danger of discontinuing programmes prematurely.

A description is given of the hair structures associated with the spiracles of the adult worker honey bee, Apis mellifera L.

The surface hairs around the spiracular orifices vary in size, density and arrangement and with the exception of those of the third, fifth and sixth abdominal spiracles appear to be capable of holding back particles greater than 30 microns.

When living bees were exposed to dust clouds of charcoal and cuprous cyanide, no particles were found in any internal part of the respiratory system beyond the spiracles except in the case of the trachea of the first thoracic spiracle which contained charcoal particles less than 5 microns.

The mechanism of tracheal ventilation was considered and evidence accumulated to suggest that all spiracles could have an inspiratory function.

The colour change and formation of green pigment in Mantis, Acrida, Locusta, Schistocerca and Dixippus are studied.

There is no background reaction to green and brown or yellow colour in these species.Usually the young nymphs of Mantis are green; some ofthe old nymphs and adults have a tendency to lose the green pigment. Phytophagous nymphsbecome green only when fed on fresh or growing grass. Green individuals may be obtained in darkness on this food (Locusta). The green pigmentdisappears on a diet of dry grass.

The colour change from non-green to green, or vice versa, is dependent on the formation or disappearance of the blue component (bile pigment-protein) of green pigment. The blue pigment generally appears first in the blood and is deposited in the integument at the following moult; it does not appear to be a breakdown product of chlorophyll or haematin. It is probably synthesised from a colourless precursor in the peripheral fat body.

Observations made on the pericardial cells of Locusta suggest that the blue pigment may be converted into insectorubin.

Sinoxylon senegalense is essentially the wood borer of felled Acacia seyal Del. in the Sudan, damage due to other beetles being contributory in a very small degree.

The life-history of the Bostrychid is recorded. The period from egg to adult is about eight weeks, but the complete life-cycle is considerably longer. The larval tunnels are superficial and only a few inches in length. The damage is almost entirely due to the adult, which remains boring within the logs for nearly 10 months. A predacious Clerid, Cylidrus fasciatus, and Braconid parasites, a species belonging to genus near Doryctes and Platyspathius pictipennis, were recorded.

Destruction of the timber is very rapid and control methods should be undertaken as soon after felling as possible. Insectaries were prepared containing the timber treated in various ways and the behaviour of the insect noted in each instance. The chemicals used in the experiments were sodium arsenite, boracic acid and pentachlorophenol. The latter, used as a spray on barked logs, was found to be the most effective.

The larval characters of 33 species (largely Palaearctic) of the genus Apanteles have been examined and studied in relation to the division of the species of the genus into four major groups on the characters of the adults. Ingeneral, the larval characters indicate a grouping of the species which is very similar to the grouping of the adults. Groups S and U contrast sharply with groups A and F on the form of the mandible. The form of the mandible also allows group A to be distinguishedfrom group F. The relationships of four aberrant and some other species are discussed.

I wish to thank Mr. J. F. Perkins of the British Museum (Natural History) and Professor G. C. Varley of the Hope Department of Entomology, Oxford, for permission to study material. Mr. G. E. J. Nixon's advice on the relationships indicated by the characters of the adults of the species of the genus Apanteles has been invaluable.

The paper describes the construction, calibration and performance of an electrolytic hygrometer based on the principle of the “Gregory” hygrometer, but considerably smaller. Its dimensions are about 6×6×10 mm.

The element consists of two platinum-clad nickel-iron electrodes, holding a spiral of continuous fibre-glass wool yarn which is impregnated with a solution of calcium chloride. The water content of this hygroscopic salt varies with humidity, so that the resistance between the two electrodes also varies. In order to measure this resistance, alternating current of a known voltage is used, and the current is measured in terms of micro-amperes. A suitable circuit is described. Air temperature is measured by a fine thermocouple permanently attached to each element.

Long exposure to air above acid and water mixtures damages the elements so that calibration is best carried out without acid. An apparatus in which this can be done is described. A stream of air with a high water vapour content is cooled to a precisely known temperature which is below its saturation point, and then raised to the temperature required for calibration, so that its relative humidity is known as it passes over the element being calibrated. Finally the dew-point is again found as a check. Temperature affects the resistance of the elements, and calibration must therefore be carried out at three different constant temperatures.

Curves obtained by means of this apparatus show that one element will give readings from 50 to 98 per cent. R.H. within ±2 per cent. Another element with more hygroscopic salt must be used for humidities between 60 and 10 per cent. R.H.

There is an ageing process whereby the resistance of each element increases for some time after it has been constructed. This period can be shortened by exposing the elements to alternating high and low humidities for a few days. Elements which have received this treatment can be calibrated and used after six weeks, and will remain in calibration for at least a fortnight.

Subsequent calibrations can be made in a simple apparatus (which is described) involving acid-and-water mixtures, but the use of filters and exposure of the elements for no more than 15 minutes avoids damage.

The hygrometer equilibrates in a new humidity within ten minutes.

The instruments possess certain advantages: they are small, they can be read or recorded at a distance, and they do not depend upon evaporation of water, so that disturbance of the natural environment is minimised. The main disadvantages are that they cannot be exposed to saturated air without going out of calibration and that each instrument must be calibrated individually—they cannot easily be standardised for mass production.

The records and descriptions of Eriophyid mites hitherto described from tomato are discussed. A statistical examination was made of morphological measurements of the New Zealand tomato russet mite. The names of gall-mites occurring on tomato have been reduced to two: Acerialycopersici (Wolffenstein) 1879, the tomato erineum mite (causing the production of white, hairy patches on stems and leaf stalks); and Vasates lycopersici (Massee) 1937, the tomatorusset mite (causing stem and leaf bronzing). A key is given to the species of gall-mites occurring on solanaceous plants and several new combinations are recorded.

Methods have been developed for breeding continuous supplies of Lygus vosseleri in the laboratory. Adults readily oviposit in cotton seedlings, but tend to damage the tissues by feeding, so that the eggs fail to hatch. Portions of developing sorghum heads were found more satisfactory; grains in the milk stage stay fresh long enough for eggs laid in them to hatch, if the ratio of adults to grains is kept low.

The method used was to expose sorghum grains to adult Lygus for 3–4 days, afterwards removing the grains to hatching cages to await emergence of nymphs. On emergence, nymphs were transferred to rearing cages and fed immature sorghum grains, changed every two days. Hatching and rearing cages consisted of muslintopped, celluloid cylinders fitting on to shallow tins containing moist vermiculite covered by plaster of paris in which small holes received the sorghum stems.

At insectary temperatures the incubation period averaged 8·4 and the nymphal life 16·7 days; adult females survived up to 57 days and production of nymphs per female per day ranged up to 2·95. The mortalty in the egg stage is not yet known:the growth of fungi on the sorghum grains during incubation causes losses. Mortality in the nymphal stages averaged 44 per cent. and occurred chiefly in the first stage, probably due to handling. Improvements to reduce losses are described.

Difficulties in maintaining continuous supplies of sorghum grains, caused by attacks of Atherigona on sorghum seedlings and of Contarinia on sorghum heads, can be overcome, respectively by the use of insecticides and by bagging the emerging heads.

Contarinia sorghicola was discovered in Uganda in 1951, the only previous African record being from the Sudan. The infestation started in September in early varieties of sorghum grown in observation plots and reached a peak onlate varieties and ratoons in early March, thereafter declining as parasitism, chiefly by Tetrastichus sp. and Aprostocetus sp., increased to reach 100 per cent. in April.

The life-cycle was normally 19–25 days, but between December and April diapause larvae were found; these all produced adults in early August, following a period of high humidity.

Midges morphologically indistinguishable from C. sorghicola have been bred from Eleusine coracana and from wild Sorghum verticilliflorum in Uganda, and the midge may well be endemic wherever wild sorghums grow. Enquiries show that midge damage to sorghum occurs in Kenya, Tanganyika and Nyasaland, and it is likely that all the mainsorghum growing areas of Africa will prove to be infested.

The possible occurrence of resistance is discussed.

An atomizer, with controlling hygrostat, has proved to be a very satisfactory humidifier for raising the humidity of a large breeding room without lowering its temperature.

Simple laboratory methods are described for assessing the potency and persistence of insecticides. Some of the newer synthetic insecticides have been tested by these methods against seven arthropod pests of medical importance.

Initial potency was judged by the mortality of insects exposed to standard films of oil solutions of the compounds, on filter papers. The approximate order of potency of the samples tested was:—

γBHC>dieldrin>aldrin=pyrethrins>chlordane=DDT= “Pyrolan” =allethrin> toxaphene.

Residual action was judged by the mortality of batches of bed bugs exposed to films of the insecticides, at intervals up to six months after preparation. The approximate order of persistence was:—

dieldrin=DDT=toxaphene>“Pyrolan”>γBHC=chlordane=aldrin>allethrin, pyrethrins.

The median lethal concentrations of some persistent insecticides as obtained by a filter-paper-impregnation technique for X. cheopis and P. humanus corporis are given. Their relative toxicity runs in the following order:—

Flea (adults): Gamma BHC > dieldrin > aldrin > chlordane > DDT > pyrethrins > allethrin > toxaphene.

Lice: Dieldrin > gamma BHC > aldrin > chlordane > pyrethrins > allethrin > DDT=toxaphene.

On flea larvae gamma BHC is 36 times as toxic as p,p'DDT.

On flea pupae gamma BHC and DDT powders and wettable suspensions are effective; further tests are required to determine their comparative toxicity.

On mud blocks (London soil) gamma BHC wettable suspension is only 3 times as toxic as DDT compared with 20 times on filter papers impregnated with oil solutions. This is probably due to the more rapid sorption of gamma BHC as well as the difference of surface and formulation.

In the field, at dosages of 2 gr. per m2 of 70 per cent. p,p′ DDT and i43 mg. of gamma BHC from wettable powders, BHC was superior to DDT as it gave a much better control of fleas (P. irritans) over the period of the experiment (24 weeks).

Field trials on body lice by dusting the clothes, while worn, with 40 gr. (per person) of dusts containing, respectively, 10 per cent. (70 per cent. p,p′) DDT dust and 0·4 per cent, gamma BHC, showed that BHC gave more satisfactory results than DDT.

The resistance of lice to DDT shown by Busvine in the local strain of Pediculus and the possibility of its occurrence among fleas may be one of the factors contributing to a better control by BHC.

It was found possible to develop artificially, by selection in the laboratory, a slight degree of resistance in X. cheopis.

Studies are reported on the extent and importance of competition amongst larval and pupal parasites of Oscinella frit.

In Ontario, 30–40 per cent. of frit larvae are parasitised by Hexacola in July–August and superparasitism is thus likely. Subsequent attack by Polyscelis may occur, but probably unsuccessfully.

Competition between the larval parasites and the pupal ecto-parasites Callitula and Spalangia is generally slight, because the former species is not abundant and the latter tends to select only healthy hosts.

Experiments are described to test the inter-relations of the three pupal parasites by exposing puparia of Drosophila melanogaster to two species simultaneously and in sequence.

When Callitula and Spalangia are allowed to oviposit simultaneously, the former is more successful when hosts are relatively few, but otherwise the two species are equally successful. When Callitula parasitises puparia recently exposed to Spalangia, equal numbers of the two parasites emerge, but in the reverse experiment Callitula is superior. Callitula is able to parasitise successfully a proportion of puparia already containing Spalangia whatever the age of the latter; mature Spalangia thus attacked are mostly killed.

Similar comparisons of Callitula and Spalangia with the endoparasitic Loxotropa show that Loxotropa is inferior to both the others, but that all three show marked avoidance of multiple parasitism.

In general, there is little interference between the various species in the parasite complex associated with the frit-fly.

The parasite complex associated with the frit-fly appears to be one that is very well balanced, there being little interference between the various species involved. Moreover, when, for any reason, one species does not attain its accustomed degree of parasitism, this is offset by an increase in the numbers of one or more of the other species.

Two wild strains of Drosophila and one laboratory strain were selected for resistance to γBHC by a contact method. From each of these three strains equally resistant strains developed in about the same time. They did not become more resistant after prolonged selection. The resistance obtained was further investigated in one of the strains.

There was no evidence of any specificity: the susceptibility of the resistant insects to DDT and “Thanite” also appeared to be less than that of the original strains.

Although the strains were selected by a contact method they also showed decreased susceptiblity when the poison was applied to the skin and when it was injected. From the reciprocal crosses F1's were obtained the susceptibilities of which were practically the same and differed little from that of the resistant parent strain. It follows that resistance does not depend on cytoplasmatic heredity and that it is incompletely dominant.