By analogy with these cases, and considering that Anopheles thrives to a latitude of 40° S. and beyond, there is every reason to suppose that an introduction of this mosquito into New Zealand could have disastrous results. It has now been shown that mosquitoes are unaffected in any significant degree by exposure to the conditions of vibration, reduced pressure and temperature of the aircraft environment, even when these conditions are far more extreme than are those normally encountered in air travel. The results of the field work undertaken with Aëdes notoscriptus and Culex pipiens pallens and of the laboratory experiments on the former, point to the conclusion that adequate measures to guard against the airborne introduction of insects into this country are necessary now and will continue to be necessary in the future.

In the last analysis, New Zealand would be largely safeguarded from introductions of noxious insects by air if all overseas airfields used by planes coming to this Dominion were “sanitary airfields” in the terms of the International Sanitary Convention for Aerial Navigation (Anon., 1933). During my field investigation certain airfields on the Japan-New Zealand route were found to be in a poor state of mosquito control, and both adult and larval anophelines were captured within the bounds of air stations in Australia, the Netherlands East Indies, and the Philippines.

Even if all overseas airfields used by aircraft on their way to New Zealand were under adequate mosquito control, there would still remain the possibility that noxious insects might fly in from outside the controlled area or be brought in among passengers' effects and cargo. Thus an efficient scheme for insecticidal spraying of aircraft is essential. Such a scheme should involve carrying out insecticidal spraying on each stage of a journey so as to safeguard intermediate landing fields in other countries as well as the New Zealand terminal. There has been some controversy with regard to the best time for carrying out this spraying. Spraying after the loading of passengers and cargo but before take-off is recognised procedure in some countries. Others prefer that spraying should be carried out in flight, and still others that it should be performed by a ground-crew orderly after landing.

There are three main disadvantages attached to the last-mentioned system. The first is that there is always a possibility that any insects on board an aircraft might fly out as the spraying orderly enters. This possibility is enhanced in the light of observations that mosquitos travelling in an aircraft become active at landing, and that treatment with aerosols has the secondary effect of stimulating insects to hyper-activity (U.S.A.A.F., 1946). The second is the fact that the aircraft must remain sealed for at least five minutes after the completion of spraying, thus causing delay in disembarkation and unloading of cargo. Finally, a control system depending on ground spraying on arrival alone offers no safeguard against the risk of accidental introductions following forced landings at emergency airfields where insecticidal facilities are not available.

An important point with regard to spraying in flight arises from the present study. Using Aëdes aegypti as a test insect David

and Bracey (1944) report that this mosquito is much more susceptible to an insecticidal mist when actually in flight then when at rest. Mosquitoes in flight contact a much larger number of spray droplets than resting insects, and it appears that the rate of movement of the insect in relation to the droplet and the relative momenta are also important. David and Bracey exposed batches of Aëdes aegypti to an insecticidal mist containing pyrethrins. Some of these insects were chloroformed and inert during exposure to the insecticide, some were wingless, some had been chloroformed but had recovered, and others were quite normal. Sixty-two per cent. of the normal insects and 58 per cent, of those which had been chloroformed and had subsequently recovered, died within twenty-four hours of the experiment. Only 12 per cent, of the chloroformed mosquitoes and 13 per cent, of the wingless ones died in the same period. David (1945) shows that Aëdes aegypti, Musca domestica and Drosophila spp., all collect a very large number of spray droplets on their wings when in flight. These droplets are later distributed to other parts of the body when the insects clean their wings with their legs. When the wings of Aëdes aegypti and Musca domestica, are removed just after exposure to an oily spray mist, the kill as recorded twenty-four hours later is reduced by about 50 per cent, as compared with a group in which the wings have not been removed.

In view of the fact that Aëdes notoscriptus at least will not fly voluntarily at temperatures below 10° C. or at altitudes above 10,000 feet, insecticidal spraying carried out during flight beyond these limits is unlikely to be effective. This objection to spraying in flight still holds good in the case of aircraft with pressurized cabins, for these have many unpressurized spaces in which insects might travel. A second objection to carrying out insecticidal treatments during flight is that air currents set up within many types of aircraft may tend to dissipate the spray mist before it has had time to take effect.

On the whole, the system with least disadvantages seems to be that of spraying before take-off. Besides guarding against the dangers of insects leaving aircraft before spraying has been carried out and of accidental introductions of insects attendant upon emergency landings at airfields where insect control facilities are not available, this system does away with unnecessary delays after landing. As already remarked, mosquitoes on board aircraft tend to become active when the motors are first fully opened out. It would appear that the most favourable time for spraying is after the motors have been tested at full revolutions during “cockpit drill” and before commencing the take-off run, as air currents set up within the fuselage when the plane is moving at speed might cause premature dispersion of the spray mist.

Insecticidal spraying in aircraft may be carried out with some form of manually operated spray-gun or by means of a fixed automatic system. The former system has been favoured to date. Pyrethrum aerosol bombs as developed a few years ago for use by the armed forces of the U.S.A. (Anon., 1942) offer a convenient and satisfactory means of spraying when in the hands of competent operators. A serious drawback attached to manually-operated

systems, however, is the factor of unreliability of the human element. Too often the men undertaking insecticidal spraying have not a proper understanding of the nature or importance of their task, and their attitude results in the treatment being carried out in an unsatisfactory manner. This applies particularly to members of aircrews detailed to carry out spraying, as these men often see the task as being a little beneath the dignity of their office.

Furthermore, certain parts of aircraft which may harbour insects are inaccessible to hand-operated spraying equipment. These parts are notably the interiors of wings and tail, and the cavities into which the undercarriage is retracted.

An automatic spraying system delivering a measured quantity of spray to all the enclosed spaces of aircraft would appear to offer the only fully satisfactory solution. Such a system, incorporating a reservoir of insecticide under pressure from which spray would be distributed through an arrangement of narrow pipes, could be controlled from the instrument panel in the cockpit. Routine insecticidal spraying could thus be incorporated into the pilot's “cockpit drill” before take-off. This system would have the double advantage of rendering all the enclosed spaces of aircraft accessible to the spray, and of greatly reducing the factor of unreliability of the human element. Its chief disadvantage would be the possibility of mechanical breakdown, but this could largely be eliminated by incorporating an examination of the spraying equipment into the routine inspections of the aircraft.

Mackay (1938) and Snow (1945) deal with practicable automatic spraying apparatus for use in aircraft.

In addition to the insecticidal techniques already discussed, treatments of all enclosed spaces of aircraft with residual D.D.T. should be given consideration. Madden (1945) considers such treatments to be of definite value, greatly reinforcing but not replacing normal insecticidal spraying. The United States Army Air Force Board (1946) concludes that “disinsection of aircraft can be accomplished by the combined used of properly applied D.D.T.-pyrethrum aerosols and D.D.T. residual treatments, but not by either of these methods alone.”

Despite every care taken to guard against the carriage of insects by aircraft, there remains a slender possibility that some insects may be transported on parts of a machine quite inaccessible to any form of spraying. Only recently, for instance, a beetle was observed to travel from Fiji to Aitutaki on the outside of the fuselage of an R.N.Z.A.F. Dakota aircraft. New Zealand airfields handling overseas traffic should be kept under strictly supervised mosquito control to guard against the risk of noxious insects being introduced in this way, and against the much more likely contingency that foreign aircraft without adequate insecticidal apparatus might introduce such insects. As a further safeguard against the latter contingency an efficient ground organisation for carrying out insecticidal spraying in incoming planes should be maintained at the airfields concerned. All aircraft arriving from overseas should be required to halt at a designated place on the taxiway and remain there until