Samples of aquatic stages, taken from a small lowland stream near Durham, England, showed that the blackfly, Simulium ornatum Mg., emerged in quantity from April to October (7 months) and that oviposition took place between May and October. Oviposition was confined to the period between sunset and dusk.

Flies emerging in the early summer of 1952 were smaller than those emerging in the spring and late summer of that year.

S. ornatum landed on cattle from dawn to dusk, with usually a small peak 2–4 hrs. after dawn, and a large peak between sunset and dusk on warm, sunny days. On cool, cloudy days the number of flies landing showed irregular fluctuations throughout the day.

The number of flies landing on untethered cattle was not markedly dependent on air temperature or saturation deficiency. Winds of over 5 m.p.h. markedly decreased or inhibited landing activity.

On average about 70 per cent, of all blackflies landing spent less than 10 min. on cattle after landing, and this interval sometimes decreased as the total number of flies landing per unit time increased.

Of the flies which landed, some 8 to 25 per cent, were calculated to bite the cow, and the proportion which bit varied significantly from day to day. The number of bites sustained by cattle in the district appeared to cause no ill-effect, apart from the formation of scar tissue in the navel region where most bites were inflicted.

Simuliid larvae and pupae found in phoretic association with the fresh-water crab, Potamonautes niloticus (M.-Edw.) in Kenya rivers far from the haunts of Simulium neavei Roub. gave rise to adults which proved to be S. nyasalandicum De Meillon, a member of the neavei group. Very similar larvae, and pupae that exhibited minor differences from those of S. nyasalandicum, were found in the exhalant passages from the gill chamber of the crab. These yielded adults of S. woodi De Meillon, and no other species was found in this particular situation; nor were examples of S. woodi found on the external surface of the crab. As many as three larvae were found in one passage and, in another crab, a fully grown larva and pupa caused complete obstruction of the passage on one side. Apart from the security enjoyed by the larva in the sheltered position, it is thought probable that the association is essentially a commensal one.

Further observations and trials were conducted with insecticides against larvae of Culicoides impunctatus Goetgh. on Soutra Hill, Midlothian. The analysis of the data was complicated by a large natural reduction (76%) which occurred in the untreated plots. Nevertheless, it appears that the following dosages of insecticidal preparation were still active two years after application:—(a) 50 and 200 mg. p,p′DDT/sq. ft. applied as a dust, wettable powder or water-miscible concentrate (99% control), (b) 50 and 200 mg. γ BHC/sq. ft. as a wettable powder or 200 mg. γ BHC/sq. ft. as a water-miscible concentrate (95% control), (c) dieldrin at 25 mg./sq. ft., which gave 97 per cent, control and (d) chlordane at 50 mg./sq. ft., which gave 84 per cent. control.

The effect of spray volume on insecticidal effect was investigated by applying 25 mg. and 50 mg. p,p′DDT/sq. ft. in the following spray volumes:—5, 10, 15, 25, 50 and 80 gals./acre. The effect of 50 mg./sq. ft. at each volume was less marked than in the previous work, when only one concentration was used. Weather conditions in the present season were unusual, but there were indications that moderate spray volumes (15 and 25 gals./acre) give the best results at this dosage of insecticide. The combined results for 50 mg./sq. ft. at all volumes gave only 43 per cent, residual control, while those for the lower dosage showed no residual control.

The results are discussed with reference to the effect of rainfall on residual control. It is concluded that the volume of spray applied is unimportant provided the droplets of insecticide are closely and evenly distributed on the mossy vegetation that overlies the peat. Subsequent prolonged rain, of the order of 20 inches, is required to attain the most effective distribution of the insecticide, which is at the surface of the peat, where the eggs are laid and the newly hatched larvae will come in contact with the poison.

During 1953 and 1954, experimental plots were set up on two sites in eastern England and regular fortnightly samples were taken in an attempt to evaluate the degree of control of the Cabbage Aphid, Brevicoryne brassicae (L.), exerted by its parasites and predators.

The plot at each site consisted of a block of 5 rows of 40 plants and each main sample consisted of a sample of three leaves (one upper, one middle and one lower leaf) from 25 per cent. of the plants. Two such samples (A & B) were taken at each visit and were so arranged that the plants were sampled in strict rotation. The leaves were taken back to the laboratory and sample A was used to assess the number of Aphids, the other for an assessment of parasitism. The Aphids were removed from sample A and put into 95 per cent. alcohol and counted. The mummies were removed from sample B and put into labelled tubes. The leaves from sample B were then kept in insect-proof containers and examined daily for the next fortnight. All mummies which formed were removed and sorted. The insects which emerged from the mummies were killed, sexed and recorded. Predators taken from this sample were identified and recorded and used for further experiments. The validity of counting Aphids from one sample and parasites and predators from another was investigated by a series of “check-samples” taken from arbitrary plots in the fields. A shorter method for assessing parasitism was tried, using a sub-sample of the main sample, and an explanation is given for the disparity between the two sets of results.

By the method of attacking flying swarms of locusts with insecticides sprayed directly into the swarm by relays of light aircraft, a proportion of the swarm receives a sub-lethal dose from each sortie. For successful operation it is essential that a substantial contribution to the final mortality should be produced by the accumulation of these sub-lethal doses on individual insects over a period of time. The object of the present experiments was to determine whether such doses applied at intervals are wholly additive in their effects. The poison used was dinitro-o-cresol (Mk. IV DNC solution) and to shorten the experimental procedure, the locusts, Schistocerca gregaria (Forsk.) and Locusta migratoria migratorioides (R. & F.), were dosed by means of a single drop of poison applied to the ventral surface of the abdomen by a micro-drop syringe.

When locusts are given regular daily doses of DNC the doses are not wholly cumulative in their effect. After the second or third day the lethal effect of each dose becomes steadily less, and after the fourth or fifth day it tends to zero, representing a steady state in which the rates of application and loss of insecticidal activity in the survivors are equal.

When the dose is applied in two halves with various time intervals between them, the cumulative effect during the first 24 hours is less in S. gregaria than in L. migratoria. In the latter species it is possible that sensitisation occurs. After three days, the first half-dose has fallen to an estimated 20 per cent, of its initial effectiveness in each species.

These two species are equally susceptible to a single dose expressed as μg. DNC/g. body weight. The females are more resistant than the males to daily doses (relative susceptibility in S. gregaria, 1·54) but probably not to single doses (relative susceptibility, 1·09).

Resistance to a daily dose correlates roughly with resistance to a single dose, the total dose required to produce 50 per cent, mortality in four days being about twice the LD50 for a single dose.

If flight activity does not materially alter the present results, it is evident that the non-cumulative effects of sub-lethal doses could cause a serious loss in the efficiency of an air-spray operation if it were unduly prolonged. Thus a quantity of insecticide sufficient to kill over 99 per cent, of the locusts if applied as a single dose would kill less than 40 per cent, if the application were spread over four days.

Since November 1951, trials of residual insecticides have been carried out at Taveta, Kenya, against Anopheles gambiae Giles and A. funestus Giles in native-type huts fitted with exit traps.

DDT wettable powder at over 2·5 g. DDT per sq. metre in a hut with plaster of absorbent mud gave, for eight months, kills of over 50 per cent. of the female Anophelines that entered.

BHC wettable powder in huts with absorbent walls at 0·24 g. γ isomer per sq. metre gave kills of over 50 per cent. for nine months or more. On non-absorbent walls the kill fell rapidly during the third month and was negligible by the fifth. Insecticide persisted in walls of active material after the roof of inactive materials had become relatively innocuous.

Wettable powders combining DDT and BHC were not effective unless the deposit of DDT was at least 2·1 g./m.2. For an equivalent cost, BHC alone was more effective on active walls and dieldrin on inactive ones.

BHC in urea-formaldehyde resin at 2·5 g. γ BHC per sq. metre was persistent but the persistence was not commensurate with the cost of the material and difficulty of application. The insecticide persists longest on mud surfaces, presumably because it is absorbed from the skin of resin and later released as vapour. Absorption would render permanently innocuous a nonvolatile insecticide.

Aldrin wettable powder was found considerably less persistent than BHC on absorbent walls.

It is estimated that in 1953, as a result of attack by Wheat Bulb Fly, Leptohylemyia coarctata (Fall.), 59,000 acres of wheat were re-drilled and 58,000 acres were patched with spring-sown wheat; another 85,000 acres were affected without any action being taken by the farmer. The cost of new seed and the reduction of yield of spring wheat as compared with winter wheat represent a minimum financial loss of about £1,200,000. With the aid of maps it is shown that the distribution of damage is similar to the distribution of wheat and to the area with less than 30 in., rainfall annually. In districts where over 5 per cent. of the combined crops and grass acreage is in potatoes, there is a tendency for the amount of damage by Wheat Bulb Fly to increase with increasing potato acreage.

It is tentatively suggested on circumstantial evidence that high rainfall on the west side of the country is partly responsible for the absence there of damage by Wheat Bulb Fly. Many of the variations in the distribution of damage on the eastern side of the country could be accounted for by differences in cropping.

In Scotland, the distribution of the fly is also confined to the eastern side. There are a few records of the fly in the Republic of Ireland.