THE “TURNIP FEY” AND ITS ASSOCIATES.

Otago Witness, Issue 3662, 20 May 1924, Page 13

THE “TURNIP FEY” AND ITS ASSOCIATES.

“FLY" PROVED TO BE THE GRASSGRUB BEETLE. Of the crops most intimately associated with animal husbandry in New Zealand, turnips and rape are perhaps the most outstanding and universally cultivated, but they are more or less severely assailed by disease and insect depredations. Among the insect pests one of the great drawbacks to successful turnip-growing is the socalled “turnip-fly,” considerable areas of turnips in most, parts of the Dominion heing annually laid waste during (he seedleaf stage, necessitating one or more resowings of the crons. Prior to the investigation of this problem last spring, nothing definite and has been established as to the identity and nature of the insect or insects responsible, and the term “turnipflv” has been universally used by farmers with reference to the trouble. —The Fly and its Habits.— The reports of damage from various tur-nip-growing districts of the Dominion, showed that most of the loss occurs during November and early December, when considerable areas of turnips are in the first leaf. This pointed to the probability of the grass-grub .beetle (Odontria zeatandica White), notorious for its depredations upon the foliage of various plants such as

orchard-trees, willows, &<., being the culprit, since at that period of the year large swarms of the beetle are on the wing. Subsequent observations proved this to be the case, although other factors were found responsible for part of the damage. The attack of the "fly”—the grass-grub beetle—is confined to the above-ground portion of the plants affected, and practically any crop in the seed-leaf stage during tlie flight of the beetle is liable to destruction. 1 11 many districts the damage to rape is just as serious as to turnips, but the former grows more rapidly and has better chances of recovery. At times ttie seedling crop is damaged only within half a chain or so of headlands, particularly in the vicinity of hedgerows and tall grass, hu! though the beetle is responsible for this to a certain extent, it is mostly the work of the cutworm, which ivill be dealt with later. The characteristic feature of the beetle infestation is that the whole crop is completely destroyed or large patches are eaten out over the whole area. The injured individual plant show 3 (hat tlie* seed-leaves arid stalk may be completely destroyed, nothing being left but the underground portion, or one or both of the seed-leaves may be partly or wholly removed from tlie stalk. The e'ffect of the beetle on crops in full leaf is not so noticeable, as there is more leaf-surface to come and go on, but the attacks are nevertheless apparent, and may even result in more or less complete defoliation. The beetles eat the leaves along the edges, so that the latter, if not destroyed, are noticeable on account of their serrated appearance. The beetle, as mentioned, is on the wing during November and the first three weeks of December each year, though it may be active in smaller numbers during the latter part of October and into January. It is nocturnal, flying usually close to the ground at dusk, when it creates the droning sound characteristic of its activity. During large flights the beetles may be seen skimming, over pasture. Later in the evening they settle upon turnip and rape crops anil other vegetation, and commence to feed, and it is thus during the night that young crops are destroyed. On the approach of daylight the beetles cease feeding and seek shelter among the denser foliage of plants, or burrow into the ground whether covered or not by pasture. Numbers of dead beetles are often turned up when cultivated land is ploughed after large flights. In"the case of roughly cultivated or stony ground the clods or other objects strewing the surface act as shelter during the day. The insect is not so active on bright moonlight nights or during windy weather, but is most lively during dull, mild, and calm evenings. The female beetle-s entering the ground do so not only to shelter but also to lay the eggs. These are spherical, glisteningwhite, and about the size of a pin's head. Though usually laid in ground well stocked with roots, such as pasture, the eggs are not infrequently placed in the comparatively bare ground carrying a turnip or rape crop, with the result that the grassgrubs on hatching concentrate upon and devour the tender roots, a considerable check to the developing crop taking plica. In the case of turnips particularly, the bulb itself is frequently attacked, and although many of the attacked plants are noticeably unhealthy the greater proportion, a.s a rule, are able to recover by throwing out new roots. This type of damage is prevalent in young crops from December to March. A characteristic feature of grass-grub infestation is the pulverization of the surface soil, which becomes quite spongy in extreme cases. On the approach of winter the grass-grubs cease feeding, and burrow downwards, each constructing an earthen cocoon in which to hvbernale. Activity recommences at the surface during August, when the grub becomes fully grown. During September a great number, and in October, practically all, have again descended and constructed cocoons for the second time. In these cocoons pupation takes place, and the van of the main November and December flights of beetles is on the wing by the end of October. —Control. — The key to control should be one of avoidance. As it is not practicable to stop the flight of beetles, since they are widely distributed throughout the country, and their grubs breed in ground stocked with the roots of grass —both native and sown -and in many other natural and cultivated plants, the process of control lies in regulation of the sowings of turnips and rape so that as few crops as practicable will be in the seed-leaf stage during the beetle flights. That is, in order to avoid damage, the crops should not be sown till after this period, or should be sown sufficiently early before the flights to be .so far developed in the second leaf as to bo able to withstand attack. Since the beetle’s most destructive period is during November and the first three weeks of December, crops sown in October are sufficiently advanced to be free from serious injury later. Sowings made during November and the first half of December are likely lo bo most seriously damaged and often destroyed. From the last week in December onwards, crops can be started with safety, though minor attacks occur by late-flying beetles. Owing to the absence of crops on bare ground, the beetles will not bo attracted thereto, except to a limited extent for sheltering purposes. In consequence. such ground being free, or practically free, of eggs, the chances of rootinjury by grubs to subsequent crops will be proportionately lessened. When early crops havp been sown in October, prior to (ho beetle-flight, there will, of course, he a considerable amount of egg-laying in such occupied land, but the crop will be at that period both far enough advanced in leaf and root development to withstand leaf attack by the beetle, and to a great extent root-attack by the grubs shortly after. Though the November-December period, during which crops are likely to be seriously damaged, holds good throughout New Zealand, there are slight variations. For example, in certain parts of Canterbury, towards the foothills, fly activity does not generally become seriously apparent until the last week in November, and continue* to the third week of December; while along the coastal districts from North Canterbury to Otago extensive damage is wrought from the beginning of November till th* first fortnight of December. In Southland, again, owing to the comparatively lati beetle emergence, the crops, as a whole, are well advanced in the second leaf before the attack commences. Tll ihe North Island, the November and r.arlv December period holds Ihrnr'dio't with but little j v "•'■■■■ I 'on : in Tarniinki. f.»r exanple, the 1 1923 flight occurred throughout November,

and was practically over by 12th December. Naturally one of the greatest aids to successful crop-development, in the. presence of insect-depredations, is the establishment of vigorous crops, and one of the serious and too-coinmon hindrances to such is poor cultivation. Roughly cultivated land not only affords attractive anil essv sb<-Ber for the beetles, but also retards the growth of crops grown therein. Under sue conditions crops sown even early enough before beetle-flight may not develop sufficiently in leaf to withstand assault, and certainly are very likely to suffer from attack by the grubs, owing to tardy development of the root-system. Late winter and early spring cultivation of land has a beneficial and direct effect to a certain extent on the control of the grubs, since the nearly mature grubs are exposed on the surface and devoured by birds, which almost invariably follow the plough. NTr J. G. McKay, of the Ashburton Experimental Farm, stated that when ploughing to a depth of sin during August, enormous numbers of the beetle grubs were turned up and devoured by swarms of sparrows. In this respect starlings are also helpful, and will dig up the grubs from infested ground when the insects are active near the surface. However, though great numbers of grubs must thus he destroyed, and as many adult beetles prevented from developing, the general effect on the fly situation is but limited, since beetles sufficient to carry on destruction develop in the extreme areas of neighbouring unciSlt i valed land. A common practice In Canterbury is to light fires at night Jn order to attract and destroy the beetles,

but such a method gives but an unsatisfactory control at best. The grass-grub is apparently comparatively free from insect enemies, though predaceous subterranean larvae of various insects no doubt prey upon it. One type of such was found to do so, this being the larvae of two species «f native robber‘hes. These larvae are more or less maggotlike m colour and shape, but much longer, and have delicate white hairs projecting from the rather large terminal segment When fully grown they transform in the ground to pupae, and the adult flies finally emerge therefrom and leave the ground. The flies are two-winged insects, and are known as “robber-flies” from their habit of capturing other insects and sucking them dry. They are long and narrow bodied ; the larger (Itamus varius) measures about 4-5 m long and is black; the smaller (fearapogon antipodus) is about 3-sin long and is reddish with black and yellow markings. The larger captures such insects as the house-fly, stable-fly, and even the honevbee. J —The Cutworm.— Reference has already been made to seedling turnips being injured in the characteristic “fly” manner along the headlands of the fields. Unlike the beetleattack, such injury is not restricted to the November-December period, but appeal's among the earliest-sown crops. It is the work of the “cutworm,” the blackish caterpillar of a. moth. A feature of this phase is that such damage is most prevalent in the vicinity cf hedgerows and coarse grass. This is to be expected, since it is in such places that the moths shelter and lay their eggs, and the caterpillars feeding upon the grasses migrate on to a crop as soon as it appears above ground. These caterpillars are nocturnal, and during the day gyc usually found just beneath the surface of the ground, curled up at the roots of the plants attacked the night before; as a rule there is a single caterpillar to one plant. Vi hen in the seedling stage, turnips, and practically any other crop just above ground, are completely devoured, the stems being nipped off close to the ground. The injury, though most apparent in early growth, is not confined to it, but may continue throughout the life of the plant. At later stages of growth a leaf-petiole is eaten through, so that the leaf falls to the ground and is there devoured. The cutworm is a ground feeder, and very often drags a . leaf, or a portion of it, underground into its resting-place. Damage to turnips when well formed is not apparent on the foliage, but is very marked on the root itself where it stands above ground, in many cases the cutworm having “ringbarked” the root just above the soilsurface. It is the spring brood of cutworms that damage the foliage of young turnips, but the “ring-barking” is the work of a second or third generation. The caterpillars transform to pupa 3 in the

ground, and from these the moths untimately emerge. '1 he depreciations of cutworms are difficult to control owing to the prevalence of hedges with their associated growth of coarse grass. If it were practicable to keep the dense growth of grass in such places within bounds, there would be a marked improvement, as the cutworms would have to find quarters elsewhere. But autumn cultivation should help to reduce the numbers of moths, since the subterranean pupae would be destroyed in considerable numbers. 'these cutworms are parasitized by the grub of a tachina-fly (Plicrocera margmata). This is a two-winged insect and resembles a bluebottle in general appearance and size, but the body is much more bristly. The egg of this .fly is apparently laid on the caterpillar, and the grub on hatching bores into the body of its victim. ’When the parasite is full-fed it kills the cutworm, cuts it way out of the latter, and transforms to a blackish barrel-shaped pupa in the ground. Hie adult fly emerges by pushing olf the end of the pupa-case. Starlings are great hunters of cutworms, and it is no uncommon sight to see these birds at work fossicking under the soil about the roots of damaged plants, in practically every case a caterpillar being found and destroyed. A notable feature is that the birds almost invariably confine their attention to plants showing signs of cutworm injury. —Other Causes of Turnip-injury.—-In some cases rabbits will attack young turnip-crops, but their depredations, though certainly sensibly extensive at times in certain localities, are, as a whole, of minor importance, compared with those of tile fly. The diamond-black moth (Plutella cruciferarum) and the cabbage-aphis (Rrevicoryne brassicae) must rank as two of the most serious pests of turnips, rape, and other cruciferous crops. A considerable amount, of investigation has been carried out with regard to these insects, and they will form the subject of future work. It may be mentioned here that the urgent need of investigating the problem of the “turnip-fly” u'as brought up by farmers at the Ashburton Farm School last year—David Miller, in Journal of Agriculture.