The Introduction Of Mosquitoes From Overseas.
Two mosquitoes, Culex fatigans and Aedes notoscriptus have been found in New Zealand only in vicinity of the oversea ports of Auckland, Whangarei and Nelson, from which it may be fairly inferred that they have been introduced since European occupation.
In the case of A. notoscriptus there is evidence that the conditions exist for its repeated introduction into Auckland from Sydney, and that it is still being introduced. As it is already well established here, the introduction of a few more from time to time may not be of great importance. The danger lies, of course, in the possible introduction of a disease-bearing species from overseas, a possibility rendered more likely in view of the recently-established malaria-carrying propensity of a Sydney species.
The duration of the voyage from Sydney to Auckland is entirely in favour of imported mosquitoes breeding here, for the usual, though not invariable, procedure of a female after emergence is for it to be fertilized, then to seek blood for the development of the eggs, and within one or two days to settle down and remain quiescent for three or four days before seeking water in which to lay the now mature eggs.
The four or five days' trans-Tasman journey is just sufficient for this normal procedure, and female mosquitoes on arrival in Auckland would be ready to fly ashore seeking water in which to breed.
Between May and November, 1929, an examination was made of a number of vessels arriving at Auckland from New York, Cuba, Singapore, Sydney, Suva, Niue, and Rarotonga. Officers and seamen on several of these vessels stated that they had been pestered by mosquitoes for several days after leaving tropical ports. Officers of the s.s. Lambeth, from Cuba, stated that whenever they have called there during the wet season, when mosquitoes are particularly numerous, they have been troubled with them for at least ten days after sailing. Firemen on the s.s. Waipahi, from Rarotonga, said they were pestered during the whole ten days' voyage to Auckland. Numbers of the crews of other ships made similar statements, some adding that after a few days out the mosquitoes did not bite, but remained settled on the walls, ceilings or curtains.
On the 24th May, 1929, the s.s. Maunganui was inspected immediately on its arrival at Auckland from Sydney and three live Aedes notoscriptus were secured, one in the vegetable house on the upper deck, and two between decks where horses had been accommodated on the voyage.
On the 10th June, 1929, on the s.s. Ulimaroa, from Sydney, two live female A. notoscriptus were caught in No. 2 hold. On the same day on the s.s. Tofua, from Fiji, a half-barrel with over an inch of water at the bottom was found in between decks of the No. 2 hold, in which there were several thousand larvae and pupae of Culex annulirostris, and from which adult mosquitoes were continually emerging.
C. annulirostris is a common Fijian and New Hebridean species which, fortunately, is not a disease carrier. It is, however, a persistent biter that breeds in a variety of situations, including water of considerable salinity. Two months later a barrel containing water in which a considerable number of larvae of C. annulirostris were actively swimming was discovered on the waterfront, about 300 yards west of the berthing place of the s.s. Tofua. (Pl. 20, fig. 3.) The larvae were all secured and preserved, and a thorough though fruitless search was made in other receptacles along the waterfront in which they might be breeding.
This case illustrates the danger of water-containers on the water-front as giving opportunity for live mosquitoes brought by ships to commence breeding here, and also shows that even our two coldest months, July and August, are not too severe for the larvae and adults of tropical species to withstand.
The importance of this will be realised in connection with the following two cases:—
On May 27, 1929, a live female Anopheles maculipennis, a transmitter of malaria, was secured in between decks on the s.s. Sussex, from Singapore. It was in a quiescent state and there was no blood in its stomach. A fire had occurred on the voyage and no doubt smoke had penetrated the holds, so possibly other individuals of A. maculipennis had either been driven away or killed.
On September 4 of the same year, a live female A. maculipennis was caught on the s.s. Narbada, from Samarang. Another was seen but not secured. These were found in the insulated holds, which, when loaded with rice and sago at the eastern ports, are closed down for the voyage. The holds are not refrigerated and maintain temperatures between 71° and 81° F., thus providing ideal conditions for the survival of tropical mosquitoes during the long voyage.
We do not know if these disease-bearing mosquitoes would be ripe for breeding after a long voyage, but if both sexes should arrive together the conditions for their establishment certainly exist, at present, on the Auckland waterfront.
In view of these ascertained facts—that mosquitoes certainly do arrive at Auckland from overseas, that introduced mosquitoes have been breeding on the waterfront, and that conditions in North Auckland are suitable for their establishment—it is urged that active measures be taken by the authorities to prevent their introduction and to ensure that the waterfront be kept clear of all receptacles which could contain water in which they could breed.