Trials are described with window-trap huts to test residual insecticides against vector mosquitos in Malaya. DDT, BHC and dieldrin were tested as wettable powders against Anopheles maculatus Theo., A. sundaicus (Ednw.) and Culex pipiens fatigans Wied. Some results were also obtained with Anopheles barbirostris Wulp, A. letifer Sandosham and A. umbrosus (Theo.), with species of Culex other than C. p. fatigans, with species of Mansonia, and with Aëdes albopictus (Skuse) and Aë. butleri Theo. All these mosquitos, except C. p. fatigans, are essentially outdoor species which enter houses only to bite, and which feed freely on animals as well as on man.
The validity of the window-trap hut method is discussed. It is concluded that although some mosquitos escape through the entrance louvres, and a treated hut is not quite the same as a treated and occupied room, the kills recorded are a good guide to the kills that may be expected in treated houses.
It is shown that there is usually some reduction, seldom sufficient to be important, in the number of mosquitos entering the huts in the first few weeks after treatment. This is probably due to air-borne particles of insecticide drifting through the louvres and acting on mosquitos waiting to enter.
The effect of treatment on the biting and resting behaviour of the mosquitos varied widely, being due to a combination of the properties of the particular insecticide and the habits and susceptibilities of the different species. Broadly speaking, all three insecticides tended to reduce the proportion of mosquitos obtaining a blood-meal, and the proportion remaining in the hut in the morning. But these tendencies might be modified or quite obscured by particular characteristics of the mosquito species or of the insecticide. Thus, for example, the percentage of A. maculatus obtaining a blood-meal was only reduced by 11 per cent, in the first month after treatment with DDT, compared with a 60 per cent, reduction in C. p. fatigans. It is suggested that perhaps A. maculatus did not touch the treated walls before biting and C. p. fatigans did. This difference applies to other species of Anopheles and Culex, and the average reductions in biting rate for the two genera were 19 and 62 per cent.
As another example, before treatment rather more than half of the C. p. fatigans, an indoor-resting species, remained in the hut; after treatment with DDT, which does not easily kill C. p. fatigans, over 80 per cent, were found in the window trap. By contrast, in the outdoor-resting species, A. maculatus, which is easily killed by DDT, over 90 per cent, were in the window trap before treatment, and fewer after treatment. The effect of DDT (and BHC) was to kill some of the A. maculatus before they could leave the hut, thereby reducing the proportion reaching the window trap to about 70 per cent. There was no reduction in the percentage of A. maculatus reaching the window trap with the slow-acting dieldrin.
Judged chiefly by the effect upon C. p. fatigans, DDT influenced mosquito behaviour mainly by its irritant effect, which persisted after the insecticide was no longer killing. The effect of BHC upon behaviour was due to a combination of irritance and rapid lethal action, but this did not last as long as the effect of DDT. Dieldrin has no irritant effect and it influenced behaviour only by lethal action for a short time while it was fresh; it continued to kill after it was no longer doing so rapidly enough to affect behaviour.
The most important finding was the wide range of susceptibility to the insecticides among the ten species or groups of species tested, and the fact that only two (Anopheles maculatus and A. umbrosus) seemed susceptible enough to be effectively controlled in practice by the fairly heavy doses used (200 mg. DDT, 40 mg. γ BHC or 40 mg. dieldrin) per sq. ft. With A. maculatus (and probably A. umbrosus) the 24-hour kill remained above 50 per cent, for about six months or more. With A. sundaicus (and probably A. barbirostris, A. letifer, Mansonia and Aë. butleri) the 24-hour kill fell below 50 per cent, in from one to four months. With C. p. fatigans (and probably Culex spp. and Aë. albopictus) the kill never reached 50 per cent, with DDT, and was only above 50 per cent, for one to two months with BHC and dieldrin.
A. maculatus was the most susceptible species to all three insecticides and C. p. fatigans the least; the latter was particularly insusceptible to DDT.
Heavier doses of dieldrin might be effective against the less susceptible species, and a dose of 100 mg. per sq. ft. has been found to remain effective for six months against Mansonia.
Except for the trial with light doses, dieldrin gave the best results against all species. When fresh it gave kills as high as or higher than those of BHC, and it remained effective longer than DDT or BHC.
Dieldrin and BHC when fresh gave complete or nearly complete kills of all species, but the rate of decline in the kills with time varied widely, and was quickest with the least susceptible species. With DDT, on the other hand, the rate of decline in the kill seemed to be roughly the same with all species, and it was the initial kill which varied. For the least susceptible group of species this was well under 50 per cent, in the first month, and was only 5 per cent, with one strain of C. p. fatigans; with the most susceptible it was between 80 and j 90 per cent.; it never reached 100 per cent.
Few mosquitos were found dead in treated huts, the great majority escaped into the window traps, especially with the slow-acting dieldrin. With A. maculatus in the first four months after treatment with DDT, only 25 per cent, of those dying in 24 hours were found dead in the hut. With BHC and dieldrin the corresponding figures were 14 and 2 per cent. The immediate kill (total found dead in the hut and window trap in the morning) was commonly less than half of the 24-hour kill. These results clearly show the importance of using window traps and holding mosquitos, if possible for 24 hours, if a reliable estimate of the effect of the insecticides is required.
Light doses (100 mg. DDT, 10 mg. γ BHC or 10 mg. dieldrin per sq. ft.) were tried against A. maculatus; DDT and BHC remained effective for three months, but dieldrin only for one month.
The performance of the insecticides at the higher doses can be characterised by saying that DDT is irritant and persistent, but not toxic enough except to the most susceptible of the species tested. BHC is irritant when fresh, though it kills at the same time; it is very toxic to all species, but does not remain effective long enough except against the most susceptible ones. Dieldrin is slower acting than DDT and BHC, but is non-irritant, very toxic, and remains effective longer; against less susceptible species higher doses than 40 mg. per sq. ft. will be needed. In countries where vector species rest indoors by day, dieldrin may prove particularly lasting because mosquitos will rest on treated surfaces for long periods.
The probable relation is discussed between the kills recorded in our windowtrap huts, and the degree of malaria control that may be expected when houses are sprayed. The kill necessary to control malaria depends to a large extent on how much contact the vector has with the insecticide. If its habits bring it into frequent contact with treated surfaces, a comparatively low kill on each occasion of contact may greatly reduce the population of the vector and suffice to control malaria. But if contact is infrequent, as may be expected with species of Anopheles such as those studied, which rest outdoors and feed only to a limited extent on man, then a high kill on each occasion of entry into treated houses is needed to control malaria, and there may be very little effect on the population of the vector. In these circumstances a 24-hour kill of 50 per cent., which is here considered the lower limit of effectiveness, seems a reasonable figure to adopt.
The mosquito factors which affect the degree of control achieved with residual insecticides can be thought of under three headings: (a) susceptibility to the insecticide, which largely determines the kill on each occasion of contact, and which we have shown may vary widely with different species, and is often quite low; (b) frequency of contact, which depends on habits, as these determine frequency of entry into treated shelters; (c) duration of contact on each occasion of entry, which is important when the insecticide deposit is no longer fresh, and depends mainly on whether the mosquito rests indoors by day and whether it is irritated by the insecticide.