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Introduction.
Observations on the value of various aquatic animals as agents in the natural control of the Culicidae, were made in the course of a mosquito survey at Jacquinot Bay R.N.Z.A.F. Station, New Britain, during 1945.
At Palmalmal, Jacquinot Bay, it was observed that water bodies which outwardly bore a close resemblance to one another often held mosquito populations of markedly different constitution. When the mosquito faunas of two outwardly similar water bodies were compared, differences between them were often apparent both in the actual species present and in the relative number of individuals representing some particular species. While some pools held many larvae and pupae of Culicidae, others contained but few of these insects. There were pools that were quite uninhabited by mosquitoes, although superficially they seemed ideal mosquito breeding places.
Water bodies studied ranged from artificial temporary pools, such as those in wheel-ruts of recent origin, to natural permanent pools. Those which by reason of their temporary nature had no macroflora or predacious macrofauna often supported heavy populations of Anopheles punctulatus Dönitz (Laird, 1946b). Many kinds of aquatic insects inhabited longer-established pools, which generally supported a diverse macroflora. The composition of the macrofauna seemed to have an important bearing on the numbers and species of Culicidae present in such pools. Extreme cases were encountered where pools with heavy populations of other aquatic animals harboured no mosquitoes at all.
Much has been written on the subject of mosquito enemies. Some writers, like Howard (1900), claim that little practical importance can be attached to such enemies. This view has to be somewhat modified in view of the subsequent realization of the value of certain surface-feeding fish, such as top-minnows of the genus Gambusia, as agents in the biological control of anopheline mosquitoes (Van Dine, 1907; Hildebrand, 1921). Hamlyn-Harris (1929) sums up the position by declaring that: “The idea of controlling mosquitoes by means of their natural enemies sounds so exceedingly attractive that many are tempted to place an unwarranted amount of confidence in what has rarely proved to be of any great value. That natural enemies, under natural conditions, are capable of taking a large toll of mosquito life cannot be denied, and for that reason alone they
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are worthy of considerable encouragement.” After commenting on the presence of many natural factors operating against the undue preponderance of most larval destructors, Hamlyn-Harris stresses the importance of our gaining a thorough knowledge of the natural enemies of mosquitoes in order to be able “to apportion them as opportunity affords, and to prevent their ruthless destruction due to ignorance.”
The results of an investigation into the significance of various arthropods and of a few animals of other groups as factors in the natural control of mosquitoes at Palmalmal, are discussed in the following pages.
Most of the aquatic animals mentioned in this paper were captured with the aid of a small dip-net. Very small creatures were caught with a long-handled 250 cc. dipper in the same manner as mosquito larvae, and were transferred to the collecting bottle by means of a pipette.
Some difficulty was at first experienced in capturing water-striders, particularly the larger Gerridae, when these insects inhabited pools too shallow to admit a dip-net. It was subsequently found that the capture of such insects was rendered easy if they were first anaesthetized by spraying a little ether on the water surface.
Field estimations of hydrogen-ion concentrations were made with nitrazine indicator paper. This paper covers a range of pH 4.5 to pH 7.5, and allows estimations to be made to within 0.5 of a pH.
A light wooden press was used for the preparation of plant specimens. The extremely high humidity experienced during the survey favoured the rapid growth of moulds, and the satisfactory preservation of such specimens was thus rendered difficult.
Whenever opportunity afforded the press was hung out in the sun to dry. The newspaper used to aborb moisture from the drying plant material was changed as soon as it showed signs of dampness. When thoroughly dry the specimens were sprinkled with thymol and stored in a metal ammunition box containing silica gel as a dessicant. These steps served to keep plant specimens in fit state for identification.
MacGregor's solution (MacGregor, 1924b) proved a most satisfactory preservative for small aquatic animals. Lightly-chitinized delicate forms remained in excellent condition in this medium. Storeboxes holding insects preserved as dry mounts were impregnated before use with naphthalene dissolved in chloroform and thymol dissolved in alcohol. This treatment kept stored specimens free from mould, and excluded such pests as ants and mites from the boxes.
Some trouble was experienced in making permanent mounts of material for microscopic examination. The heat was such as to prevent Canada balsam from setting, and the high humidity made it hard to keep alcohol and xylol free from water. The following technique was developed to overcome the first of these difficulties. Discs of light glazed card were cut to the size of the coverslips used when making permanent slides. From one to three holes were pierced in each card disc with a 9/32 in. cork borer. The prepared cards were then fastened to glass slides with a thin smear of gum. In mounting, each of the cells so formed was filled with balsam, and a
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specimen was positioned in it. The cover slip was then applied, so that its rim coincided with that of the card disc. The latter materially increased the friction between cover slip and slide, thus preventing damage to the mount by movements of the cover slip in handling. At the same time it kept the weight of the cover slip from crushing the preparation. Slides prepared in this manner were much more convenient to handle than those made in the usual way.
A collection of the aquatic predators discussed in the following pages has been deposited at the Cawthron Institute, Nelson, New Zealand.
My thanks are due to Major-General Sir Fred Bowerbank, D.G.M.S. (Army and Air), Wing Commander Howard Marsh, R.N.Z.A.F., D.M.S. (Air), and Professor L. R. Richardson, of Victoria University College, for encouragement and assistance in the preparation of this paper; to the Royal Society of New Zealand, the New Zealand Department of Health, and the Cawthron Institute, for generous library facilities; to Professor J. Balfour-Browne, of the British Museum (Department of Entomology), for identifying the aquatic Coleoptera; to Mr. H. Womersley, Entomologist at the South Australian Museum, for his identification of the water-mite Limnesia jamurensis Oudemans; to Mr. R. R. Forster, of the Dominion Museum, Wellington, for his generic identification of the Araneida; and to Messrs. V. D. Zotov and A. J. Healy, of the Botany Division (N.Z.D.S.I.R.), for their identification of botanical material collected at Palmalmal.