Aircraft and Airport Insect Control

Duguet (1949) outlines the results of an inquiry by the Interim Commission of the World Health Organization into the methods and spirit of aircraft insect control as enforced under the terms of the International Sanitary Convention for Aerial Navigation of 1933–44 by various States and organizations having international aviation commitments. It is only too apparent from the results of this inquiry that many States do not prescribe satisfactory control measures at all, while others enforce a diversity of measures of varying degrees of efficiency. Furthermore, very few States require aircraft leaving their territories to be sprayed out with insecticides, emphasis usually being placed on the treatment of arriving aircraft. The danger of aerial exportation as well as importation of noxious insects has already been indicated in the preceding sections of this account.

Insects in aircraft may be controlled by means of fumigation, aerosol mists, and residual insecticides. Duguet (1949) discusses these methods, pointing out that fumigation, although highly effective insecticidally, is too time-consuming for consideration to be given it as a routine means of treatment. A combination of pre-flight aerosol spraying and monthly treatment with residual DDT ensures a very high level of protection against insect introductions, but until these measures can be universally enforced, and not merely recommended, by some international body, individual States desiring to guard against such introductions must rely chiefly on their own quarantine measures for their protection.

The insecticidal efficiency of chemical sprays is intimately bound up with droplet size and diffusibility, and the relatively large liquid droplets dispersed in the form of a limited cloud by outmoded pump-type spray guns render the latter of very little value for use in aircraft. The only sprays which can be considered of sufficiently high insecticidal value for use in aircraft are those dispersed in aerosol form—David and Tew (1949) defining an aerosol as "a dispersion in air of insecticide solution as particles, the diameter of which falls within the range of 5–25 microns." Aerosol mists penetrate into all parts of enclosed spaces within which they are used in approved dosages, and because of their small particle size page 25 they remain in suspension for considerable periods. An ideal aerosol for use in aircraft should combine maximum toxicity to insects and maximum diffusibility with a freedom from danger and minimum of inconvenience to humans on board; it should not be capable of contaminating foodstuffs intended for human consumption or of damaging fabrics, papers, etc., and it should be completely free from inflammability hazards; finally, it should not contain any substances liable to cause the "crazing" or cracking of stressed perspex windows in pressurized aircraft. Whittingham and Galley (1949) state that fitting stresses alone may sometimes be responsible for crazing, but that this process is accelerated in varying degrees by atomized solvents—methylene chloride, for example.

Aerosol dispensers may be portable and manually operated or built in and either manually or automatically controlled. While built-in spraying systems (see Duguet, 1949) would offer an ideal means of pre-flight spraying if used on a universal basis, manually operated portable dispensers must continue to be the chief means of aircraft insect control employed in countries relying on their own efforts for protection against insect introductions. Various types of disposable container Freon-propelled aerosol "bombs" of American manufacture were investigated by David and Tew (1949) and earlier workers. It was found that these "bombs" are subject to considerable inconstancy in performance. In consequence of temperature and other variables, the rate of insecticide dispersal varies not only for different brands of "bombs" but also among dispensers of the same brand, even when valve capillary tubes are of standard diameter. The performance of even an individual "bomb" may also be subject to considerable fluctuations. These factors are, of course, of great importance in that it is essential that, for maximum insecticidal efficiency, an accurately measurable dose of insecticide must be delivered. Although the danger of inadequate insecticidal efficiency consequent on adherence to an arbitrarily prescribed dosage may be overcome by dispensing a gross overdose, this practice, besides being uneconomical, may lead to undue inconvenience to human beings on board the aircraft concerned. David and Tew (1949), in an effort to solve the problem, studied the "Porton-sparklets" CO₂-propelled refillable aerosol dispenser. While it was found that this apparatus is of very uniform performance and permits the projection of an accurately measured dose, it proved that the insecticide involved had the grave disadvantage of containing an important crazing agent, methylene chloride.

Tew et al. (1951) described tests with a British-made low-pressure "bomb," which proved to have higher insecticidal efficiency than old-type American high-pressure "bombs." This "bomb" (manufactured by Cooper, McDougall, & Robertson, Ltd.) has minimal tainting, crazing, and inflammability risks. Being considered to be the most satisfactory of the aerosol "bombs" currently available, it has been adopted as the standard for aircraft spraying in the RNZAF until such time as page 26 efficient insecticides free from crazing hazards have been developed for use in the accurately controllable "Porton-sparklets" type of refillable dispenser.

All aircraft arriving at Whenuapai Airport from overseas are boarded by an RNZAF orderly, who proceeds to spray them throughout by means of an aerosol "bomb." Following the completion of spraying, the doors and windows of the aircraft must remain closed for a period of five minutes, to ensure that the aerosol mist is not dispersed prematurely, before disembarkation and unloading take place. Before boarding the aircraft, the orderly directs spray into the wheel cavities, and following disembarkation he also sprays any external baggage compartments and makes a visual inspection of the exterior of the fuselage with the object of locating any insect nests or egg masses which may be present. In addition, he thoroughly searches the aircraft for insect remains, which are forwarded to Air Department for entomological examination (see page 3).

The above measures afford a high degree of protection against accidental introductions of insects being made by means of aircraft. As long as spraying on arrival continues to be our chief means of protection against airborne introductions of insects, however, the chance remains that insects carried on the outside surfaces and in wheel cavities of aircraft might escape during, or immediately following, landing. Furthermore, there is a danger that winged insects travelling within the aircraft might escape through the door as it is opened to admit the spraying orderly. That these risks are of significance is shown by the fact that flight experiments carried out with Aëdes notoscriptus proved that this mosquito is active in the period during and immediately following landing in response to the stimuli of a sudden increase in air temperature and air pressure followed by a cessation of vibration as the motors are stopped (Laird, 1948).

In order to provide a further measure of protection by ensuring that any mosquitoes which might thus escape from arriving aircraft are denied nearby larval habitats, all actual and potential mosquito-breeding places within the bounds of RNZAF Station, Whenuapai, are regularly treated with DDT-base larvicides. A monthly mosquito-control report is forwarded to Air Department, together with specimens of any batches of mosquito larvae located. To date, the only larvae resulting from these collections have been those of the common local mosquitoes, Aëdes notoscriptus, Culex fatigans. and C. pervigilans Berg. An additional measure which might lead to the discovery of an insect introduction at an early enough stage for effective remedial action to be taken is that RNZAF personnel at this airport are encouraged to send forward specimens of any unusual insects which they see at Whenuapai so that these can be identified. Once again, all such insects so far identified have proved to be local species.

page 27

The control of insects at international airports is usually confined to mosquitoes, and quite frequently to one particular species of mosquito known to be dangerous locally. Thus, at Nandi, Fiji's international airport, particular attention is paid to the control of Aëdes aegypti, a species which the author failed to discover there during a visit in June, 1949. However, larvae of both Culex annulirostris and C. fatigans were found to be very abundant in ponded drainage ditches near the airstrips at Nandi; and during the evenings adults of both these species, also of Mansonia crassipes which breed in the extensive swamps on the Nandi Plains, were collected in the airport sleeping quarters. C. annulirostris, C. fatigans, and A. aegypti were all found to be breeding at Nausori Airfield, which is used as a landing field for RNZAF aircraft. The collection of examples of all four of the species named above from aircraft which reached Whenuapai from Fiji in 1950 serves to indicate how closely the danger of insect introductions being made into New Zealand is linked with the state of insect control prevailing at overseas airports used by aircraft during flights to this country.