LEAFLTTS FOR GARDENERS AND FRUITGROWERS, NO. 2.

New Zealand Mail, Issue 1205, 5 April 1895, Page 7

LEAFLTTS FOR GARDENERS AND FRUITGROWERS, NO. 2.

New Zealand Department of Agriculture. John D. Ritchie, Secretary. CINERARIA FLY, OR MARGUERITE FLY. (Phytomyza nigricornis.) By T. W. Kirk, F.L.S., &c., London. Last season it was noticed that many of the marguerite and cineraria plants in the gardens round Wellington were marked, as though tiny snails had crawled* over them, leaving white tracks behind. A further examination, however, showed that the damage was not oh the surface of the 1 leaves, but had been caused by a small grub, or, rather, numerous small grubs, tunnelling between the two surfaces, and eating out the flesh in all directions. Later on these grubs turned to chrysalids, arid from them emerged tiny flies, which to the unaided eye look black, but which the microscope shows are somewhat highly coloured. This season the insect has greatly extended its field of operations, until there is scarcely a garden, or indeed a roadside, whei'e it may not be seen. Cinerariagrowers suffer most from its depredation^. In Europe the fly also attacks turnips and peas, but so far does not appear to have done much damage to those crops in New Zealand. > The eggs are usually laid on the underside of the leaf, but at times the females seem careless, and place them on either upper or lower surface, and sometimes on the stem. 1 The period elapsing before the eggs hatch varies with the position of the plant, and apparently is longer or shorter according to the amount of light and warmth received. Some, in one experimental shade, hatched five days after being laid, and others were double that time. Shortly after the young grubs appear, the leaves, if carefully examined, will be found to have small dots where the . grubs have eaten through the epidermis, and from these dots the mines will extend in all directions, crossing and reerossing till in many places nothing but the cuticle remains. The leaves, of course, soon wither and die. Before this occurs the grubs have reached maturity, eaten close to the surface, and entered the chrysalis state, The mines are of course

in places partly filled with the excreta of the grubs, but at various points will be seen larger objects* some greenish, others brown, and others black. These are the chrysalids* and in favourable weather the flies will emerge in about a week after the chrysalids have assumed the last-men-tioned colour. The above cuts will give a good idea of the form of both grub and fiy; The latter will well repay the trouble of microscopic examination. The coloUr of the body varies from rich-black to slate colour ; the head is ochreous, With a greenish-yellow spbt oii the drowns The poisers arid thighs are yellowish, and the wonderfully iridescent wiilgs make this insect a most beautiful object Under a moderate power. It is a species of FhytomyZa (evidently closely allied to P. iiigrieornis) , a genus the members of which are notorious for disfiguring garden-plants. [Since the above Was written, I have received from Mr C. French, Government Entomologist, Victoria, a note ill Which he says that specimens sent by me have been submitted to Mr SkUse, Who

pronounces them undoubtedly P. nigriGornis .] The difficulties encountered in fighting this pest would not be so great were its attacks confined to garden-plants—namely, cinerarias, peas, chrysanthemums, etc.- — but unfortunately it is found on a number of wild plants also. I have reared specimens from groundsel, meliolotus, and the recently-introduced Noogoora burr, while the common sow-thistle or rauriki is everywhere attacked, and seems to be the favourite food-plant.

I have taken flies reared on cineraria and placed some in a breeding-cage with rauriki, and others in a separate cage with cineraria. The progeny of the former throve much better than the others. The insects being miners, it -is very difficult to get at them when once the plant i 3 attacked; defensive rather than offensive measures must therefore be employed. In greenhouses the plants should be carefully watched at the commencement of an attack, and all affected leaves picked off and burned. If a plant is set in a tub of water, so that the leaves are immersed,

and left there for some hours, it will be found that many of the grubs have come to the entrance of the mines and been drowned. This, of course, will not kill the chrysalidsi The above plan cannot be adopted with outdoor plants, but I have proved experimentally that the. female will not lay her eggs oh a plant which has been carefully sprayed with arsenical spray or With tar Water (41b coal-tar, boiled in 1 gallon of water, and, when boiled for some time, diluted with 50 gallons fresh water/ This must be stirred till well mixed; it is then ready for Use. Tar is obnoxious to all insects, and they will get away from it it possible), preferring to deposit them on the glass sides of the breeding-cage. This, then, seems to point to an easilyapplied preventive. The proposed remedy, however, needs more extended trial, and this is being Undertaken; The spraying will certainly need repeating from time to time, as the insect is a very free breeder. Indeed, I have some specimens of the third generation now in the breeding-cages.

In Europe there is a small ichneumon fly which helps to keep P\ nigricornis in check. This has now reached the Colony, but is not a sufficiently effective check to be relied on solely.