Art. XIX.—The Wattle-blight (Icerya purchasi) in Tasmania, and its Natural Enemies.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 22, 1889, Page 176

Art. XIX.—The Wattle-blight (Icerya purchasi) in Tasmania, and its Natural Enemies. By Jas. Hudson, M.B. [Read before the Nelson Philosophical Society, 4th April, 1889.] The ravages of this blight on our acacias and wattle-trees are well known. It seems to have a special liking for plants of the natural order Leguminosœ (gorse and broom, &c.), but it also attacks, and that most disastrously, orange- and lemon- trees. I know many instances of lemon-trees in Auckland and Nelson which formerly bore splendidly, but which now, owing to this blight, give their owners scarcely any fruit. You may frequently find it also in rose-bushes, and it is said to affect pines, cypresses, and grass. The insect, like most of the Coccididœ, is at first active; soon, however, it bores through the bark of some twig, which it selects, with its proboscis, and

W.M.M. delt 2d nat. COCCIDIDæ C.H.P. lith.

commences to suck up the juices of the plant. It grows rapidly, and soon begins to secrete its characteristic cottony exudation: this is in reality the ovisac, and in a fully-developed female is found full of little red eggs to the number of two or three hundred. The male is a winged insect, about ⅛in, long: it may be frequently seen in wattles infested with the Icerya. There is one peculiarity which this creature shares with many of the Hemiptera—viz., the power of secreting honeydew. You may frequently observe a drop of this exuding from its posterior extremity. Every one must have noticed the blackened stems of trees affected with this blight: this is indirectly caused by this honey-dew—in this way: the insect scatters the honey-dew over the stems of the plant, and in this honey-dew various species of black fungi grow, which give to the plant-stems the black, sooty appearance. While at Launceston, in Tasmania, in the early part of January of the present year, I took a walk along the banks of the South Esk. I could not help observing the numerous wattles, and how healthy and luxuriant they appeared; and I at first concluded that that district must be free from the Icerya. However, I soon observed one or two of the trunks having black patches on them, and, knowing this to be indicative of blight, I was induced to examine them more carefully. I soon found a group of perfect specimens, stuck on by the proboscis in the usual way; and on tearing open the posterior cottony exudation I found it full of the little red eggs of the insect, so the identity was perfect. We have long known that ladybirds are great blighteaters, and when I saw that the Iceryœ were there, and yet that they did not do much damage, I at once looked for ladybirds, and I soon found some, one of which I secured, and I now show it to the society. You will observe that it is larger than the ordinary New Zealand species; further, it is covered with rather large black spots symmetrically arranged on a reddish-orange ground. I did not see the ladybirds actually eating the Iceryœ, though I make no doubt they do so, but I came across some larvæ which were feeding on them most voraciously. I saw one with his head buried in the cottony exudation, no doubt feeding on the eggs inside. These larvæ were short, hairy, nearly black in colour, with six legs, no prolegs, and a divided spike-like projection posteriorly. I also found a small group of about twenty or thirty eggs stuck on to the bark of one of the wattles. These eggs were of peculiar shape, elongated, with their long axes vertical to the tree. I cut out the piece of bark with them on, intending to bring them to Nelson; but, alas! it was in the full heat of an Australian summer, and when I looked at them a few days later I found they had all hatched; but from what I could see of the little

black creatures that had come out they assimilated in appearance to the larvæ I had seen devouring the Iceryœ. In looking for the ladybirds I could not help observing that the wattle-trees were completely overrun with ants—small black ants, similar in appearance to what are abundant in England. I had long known how fond ants are of honey dew, and I surmised at once that they were after the honey-dew of the Iceryœ. I watched them smoothing the Iceryœ with their antennæ: this may be a way of stimulating the flow of the honey-dew. I further observed, to my astonishment, that the ants had built numerous nests, which I shall hereafter call stables, over portions of the trunks of the wattles: these were composed of the semi-decayed fine needle-like leaves of the wattle, and rubbish off the ground, loosely stuck together with the exuded gum from the wattle itself. The least crack in the bark of a wattle, such as might be caused by its natural expansion, will be followed by a gum-like exudation, and this is made use of by the ants to cement their stables. It is quite possible that the ants may purposely wound the wattle. A great many of these stables were empty (i.e., there were no living creatures inside), but lots of them were teeming with ants, and also contained a variable number—from five to thirty or more—of Iceryœ, adherent by their probosces to the bark in the usual way, but with very little cottony exudation, and never any eggs (at least, on all that I examined). My deductions from these observations are— 1. Ants are very fond of the honey-dew of the Icerya purchasi. 2. They very probably obtain it from them by a voluntary effort on the part of the ant. 3. By penning them up in these stables that I have described, they prevent the access of the winged male, so that the whole of the food of the Icerya, goes to the production of honey-dew, instead of the formation of eggs, so that the multiplication of the creature is greatly lessened, while its individual life may not improbably be prolonged. 4. The deserted stables I found were those in which all the Iceryœ had expired. 5. Probably the ants had their real geal germinating nests in the ground, only using these stables for food-supply. I have lately ascertained that the ladybird is a species of Coccinella.