THE FRUIT FLY.

Dominion, Volume 1, Issue 78, 26 December 1907, Page 8

THE FRUIT FLY.

WHAT IT IS LIKE, AND HOW IT ' ■ ■ , WORKS. , SOME NEW ZEALAND SPECIMENS. At the time of the outbreak of Mediterranean fruit fly at Hastings and Blenheim last year, tho sketches here reproduced were obtained (by permission of tho Government Biologist's staff) from flies "hatched" from maggots in' fruit gathered in the Hastings gardens. They are specially interesting-, as a record of New Zealand's memorable narrow escape from the grip of this little scourge. The illustrations show tho fly in its natural size and in the characteristic attitudes it adopts while at work depositing eggs into tho fruit. Fig. 7 shows tho fly in tho act of digging her sting (or oripositor) into an apple. By means of the sting an egg will bo injected beneath tho skin to hatch into a. maggot." .

The' Mediterranean fruit fly is a very small creature indeed for the extent of tho evil 'it can work, and it has a,large namo. It is a dipterous (two-winged) • insect, belonging to tho muscidae, and is called halterophora

capitata (or ceratitis capitata). It is only about a quarter of an inch in width , and length, and very sluggish in its movements. Its demeanour is ono of deep pensiveness and utter fatigue. It is round shouldered. Tho. drooping carriage of the abdomen suggests that it is too lazy to lift up its tail from dragging on tho ground; and the outspread wings, like those of a wounded bird, indicate a panting for breath. Tho transparency of tho wings is conspicuous, and imparts a singular air of lightness, which, with tho distinct dark blots visible on each wing, renders it almost impossible to forget tho insect's appearance after it has been once properly, examined;- Liko its relative tho house My, the Mediterranean fruit fly feeds on sweet things, principally the saccharine matter exuded through the skins of fruits.' It licks up this nutriment by means of a tongue or proboscis resembling that with which a house tiy engages a particle of sugar. It is this habit that has suggested the use of poisoned sweet fluids on leaves as a means of destroying the fruit fly. But it is not in taking its meals that the fruit fly does its damage. Its characteristic injury is 'done in the process of reproduction. i Probably while it is idly rubbing its nose on a fruiterer's window-pane, or.gyrating on the skin of an apple, he—or, rather, she, for we are coming to eggs now—is chuckling over the pear yonder, which she has filled with maggot eggs. Mr. French, the Victorian Government entomologist, is jreported to have estimated tho egg crop at only ten, but this appears to be a strangely low estimate. The ordinary house fly lays about 150 eggs, and the dacus' oleao (tho fruit fly of the olive), lays between 3000 and 4000. However,! if the "Mediterranean fruit fly lays only ten eggs, wo must fall back upon the-assump-tion that many of tho ' eggs are' double yolked. For who has yet discovered an infected fruitwhich contained only one solitary maggot? Usually there are a dozen; all in a heap. ' , . ■ The fly deposits the eggs just beneath the skin of tho apple, pear, tomato, orange, peach, or whatovor. fruit she happens to bo attacking; and her mothod of inserting an egg is interesting, though by no means uncommon among insects. It is inserted by means of an ovipositor (or egg-placer), which we usually call a sting. XJnliko tho sting of a mosquito—which is attached to tho mosquito's head, and is used for lancing to facilitate- blood-sucking—tho sting of tho : fruit fly is attached to the insect's posterior, and is used for depositing eggs. In contrast with 'the long, glistening ovipositor of tho ichneumon fly, which goes nbout tho world spearing codlin moth grubs, tho ovipositor of the fruit fly is so short as to bo scarcely visible. But it is none tho loss essentially sharp and effective. It is, of courso, nn adornment peculiar to females. Alighting, with egg-lnying intentions, on a selected fruit,.tho fruit fly settles herself firmly down, presses her tail on tho spot whore sho decides to place tho egg, pricks tho fruit, injects an egg to tho tip of tho inserted ovipositor, and then, with a joyous spasm of tho wings, withdraws tho organ, and no doubt mentally declarea the egg to be well and truly laid.

Apparently her body is then filled with an emotion of satisfaction—a senso of something accomplished—for she immediately cleans her face, with tho aid of her forelegs, in a leisurely, contented way. A fewhours after an egg has been laid, a maggot emerges from it and begins to burrow into tho fruit. The fact that the maggot hatches, not on the surface of the fruit, but beneath tho skin, is a circumstance that has confounded the efforts of fruit-growers 'for many years past, for it renders the pest almost invulnerable. It places the egg beyond the reach _ of poisonous sprays, such as are used against eodlin moth and other insect trifles. ; A codlin moth's egg, being laid on tho out* side of a fruit, is accessible to bird attacks, scorching sun, ■ torre'nts of rain, insect enqjnies, and poisonous sprays. And the maturo larva, on leaving the fruit, accommodatingly pupates in .bandages fastened round tho tree trunks as traps where tho orchardist can kill it; or in niches in the bark, where birds and spiders and beetles can drag it forth or the ichneumon fly spear it. But the fruit fly, in happy contrast, leads a charmed life. Snugly hatched beneath the fruit skin, the maggot studiously avoids tho open air, and does not leave the interior of the fruit til], on the threshold of tho pupal stage, it seeks a hiding-place in the soil. No bandage can lure it to destruction, no bird neck and mutilate it in the rough-barked branches, and no ruthless ichneumon fly easily convert its soft, warm body into an incubator. Nevertheless, it is not entirely immuno from danger. Tho newly-fallen maggots and the pupae (or chrysalides) may be turned up to poultry by frequent harrowings of the soil near the trees, and no doubt various insects on tho ground occasionally attack them as they tumble- down. The spraying of kerosene on 'the soil beneath tho trees is also destructive to the pupae, and—if overdone—to the trees also. The latest and strangest discovery to bo reported is that ,the fruit flies aro attracted in large numhers into vessels of kerosene suspended in the orchard, and ars thus destroyed. Tho discovery is alleged to have been well proved, and it is now being officially recommended by the Australian Government experts. If effective, this euro presents enormous potentialities. _ In tho matter of insect enemies, some hopes are centrod in Mr. George', tho \\ est Australian entomologist, . who has boe.n visiting northern countries, particularly India, to collect specimens of a natural pnemy of tho fruit fly, which he is convinced abounds there. Tho specimens will bo sent to Western Australia for multiplication: for California, whero the codlin moth parasite was propagated,' has no fruit flies to feed thorn on. Mr. Compere last year sent a consignment of the natural enemy homo to Westorn Australia, but thej; arrived and hatched at a time when fruit fly maggots

were unobtainable, aud they • were consequently liberated to . tako their chanceprobably to die "childless." Mr.. Compere, it. is said, was let! to the discovery of'the natural ouomy by tho fact of the striking scarcity of tho fruit fly in, India.., If tho fruit fly exists there, but only in very small numbers, it is, of course, fair to concludo that an onomy is'keeping it in check. The usual practice of an entomologist seeking to identify a parasite is to collect' samples of its victim in the stages in which thoy are parasitised—that is, filled witti the eggs of the euomy —and develop them. From the imparasitised specimens fruit flies, of course, would result; but" from those that are parasitised tho progeny of the enemy might bo expected to hatch. Thoro may be several specimens of "strangers" thus obtained —sometimes a score of different species emorge from one chrysalis, in which a score of foreign eggs had been deposited— and it is_ then that the troubles of the entomologist really begin. For lie has to determine which of these "strangers". is the one for which lie is searching. Wo shall all wish Mr. Compere success. ' • ■■

As to. the number of broods of the' fruit fly developed per season, there are certainly several. The maggots, which hatch from the eggs in a fow hours, becomo pupae, two weeks lator, and .are sometimes maturo egglaying flies within a month from egg to egg. If « female lays only ten eggs,, and half 'of the progeny are fomales,' then tho possible increaso during four months is as follows:—First generation, 5 females; second, 25; third, 125; fourth, (325. Tho males need not be counted, because they do not oviposit, and therefore do no damage.