Mosquitoes

Their Habits and Control

Letter to the Editor MY attention has been drawn to an article “Mosquitoes—The Pervading Pest of the Tropics” in the Febru ary issue of the “Pacific Islands Monthly” which calls for some correction.

Mosquitoes have always been present in Great Britain and are well-known in Norway, Sweden and Labrador. Malaria was, at one time, endemic in England.

During 1914-1918 there were original cases of malaria in Flanders and there still are numerous cases of endemic malaria in Holland.

Mosquitoes the world over are found breeding in stagnant or slowly moving water on which they lay their eggs.

Specialised species lay eggs in rot holes of trees, leaf axils of taro, pandanus trees, other suitable plants, coconut husks, etc. Domestic and semi-domestic species breed in water in tins carelessly dis carded by the householder.

Stegomyia aegypti, the intermediary host of dengue and yellow fevers, is purely a domestic mosquito. It breeds only in house tanks, flower vases, sagging roof guttering, discarded tins, meat safe pro tectors against ants. etc. This mosquito will breed in rot holes in trees near houses when all domestic water is denied to it. and the eggs will withstand desic cating and freezing.

All tanks should be securely protected with brass, bronze or rust proof steel gauze of 18 mesh and 33 gauge. All other water receptacles must be emptied at least once per week and all old tins com

pletely destroyed. Sawdust impregnated with coal tar placed in rot holes and de pressions in trees is an infallible pre ventive against mosquitoes breeding in such situations.

Your correspondent appears to be con fused concerning the flight of mosquitoes.

They will travel more than two miles without the aid of wind, and many miles with it. Mosquitoes are common, accord ing to my own observations, at altitudes of over 6.000 ft., and have been found at elevations of over 10,000 ft.

Trees of any description are not the natural home of mosquitoes, but only of the specialised few kinds which breed in rot holes, leaf axils and depressions in trees.

The normal feeding time of mosquitoes is any time from daylight to dark, al though there are exceptions, i.e., Culex fatigans, a domestic mosquito and inter mediary host of the periodic type of filaria, generally feeds in the dark at night, though it can be induced to feed in the light. Anopheles mosquitoes bite more commonly from about dusk until daylight.

Your correspondent is vague and con fused when he calls a mosquito the

“Culex Anopheles” because there is no such mosquito or other insect. Also, it may be stated that blackwater fever is not conveyed by any mosquito.

Anopheles mosquitoes of any kind are definitely not present in Fiji. Anopheles, for the most part, prefer to breed in sunlit situations, others are found breed ing in partial shade. Very few species in deed breed in “the thick damp jungles and forests”.

Elephantiasis is one of the after effects of parasitism by the filarial worm. Num erous young forms are discharged into the blood stream and elsewhere from the adult female worms. These young forms are the infective form for the mosquito.

Many of the dramatic symptoms of filariasis, including elephantiasis, are due to mechanical effects such as obstruction of blood vessels caused by these young forms and their subsequent degeneration.

The breeding of mosquitoes in swamps may be controlled, and sometimes com pletely stopped, by the use of the Top Minnow (.Gambusia Affinis). This fish should never, on any account, be put into creeks or running water. Where it is not possible to employ this fish, effective use may be made of sawdust, impregnated with crude oil, tied in a bag and the bag securely fastened to a stake, driven into the ground at the head or intake, so that the bag lies wholly in the water.

The use of petroleum or other light oils is both expensive and more or less futile on account of rapid evaporation in hot climates, tropical and temperate.

Personally I think too much weight is placed upon the value of the utility of the “Giant Mosquito” (a species of Megarhinus) since this mosquito is not a ground water breeding species. It breeds in rot holes and depressions in trees and occasionally in discarded tins and bottles.

In my experience I have never seen it even in coconut husks, much less ground water.

I am etc.,

Frank H. Taylor.

Entomologist, School of Public Health and Tropical Medicine, Sydney. 31/7/1939.