THE TURNIP FLY.

Wairarapa Age, 8 May 1924, Page 7

THE TURNIP FLY.

AND ITS ASSOCIATES.

BY DAVID MILLER, ENTOMOLOGIST. DEPARTMENT OF AGRICULTURE, (in the “Journal of Agriculture."/ (Continued.) In the Nbrth Island the November December period, during wmeh tne crops are likely to be seriously damaged, hold good with but Tittle variation in Taranaki, for example, the 1923 flight occurred throughout November, and was practically over by December 12. Naturally one of the greatest aids to successful crop-development in the presence of insect-depredations is the establishment of vigorous crops. One of the serious and too-common hindrances is poor cultivation. Roughly cultivated land not only affords attractive and easy shelter for the beetles, but also retards the growth of crops grown therein. Under such conditions crops sown even early enough before beetle-flight may not develop su&ciently in leaf to withstand assault, and certainly are very likely to suffer from attack by the grubs owing to tardy development of the root-sys-tem. 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 tfie nearly mature grubs are exposed on the surface and devoured by birds, which almost invariably follow the plough. Mr J. G. McKay, of the Ashburton Experimental Farm, stated tUat when ploughing to a depth of sin. during August enormous numbers of the beetle grubs were turned up and devoured by swarms of sparrows. 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 be destroyed, and as many adult beetles prevented from developing, the general effect of tho fly situation it but limited, since beetles sufficient to carry on destruction develop in the extreme areas of neighbouring uncultivated land. A common practice in Canterbury is to light fires at night in 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 which was found to do so is the larvae of two species of native robber-flies. These larvae are more or less maggot-like in colour and shape, but much longer. When fully grown they transform in the ground to pupae, and the adult fiies finally emerge and leave the ground. They are known as “robber-flies” from their habit of capturing other insects and sucking them dry. Reference has already been made to seedling turnips being injured in the characteristic “fly” manner along the headlands of fields. Unlike the beetleattack, such injury is not restricted to the November-December period, but appears among the earliest-sown crops. It is the work of the “cutworm,” the blackish caterpillar of a moth. Such damage is most prevalent in the vicnity of hedgerows and coarse grass, where moths shelter and lay their eggs. 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 are usually found just beneath the surface of the ground curled up at the roots of the plants attacked the night before. When in the seedling stage turnips, and any other crop just above ground, are completely devoured, the stems being nipped off close to the ground. The injury may continue throughout the lire of the plant. At later stages of growth a leaf-petiole is eaten through, and falls to the ground. The etttworm is a ground feeder, and very often drags a leaf or a portion of it underground. Damage to turnips when well formed is not apparent on the foliage, but rs very marked on the root itself, where it stands above ground, in many cases tho cutworm having “ring-barked” the root just above the soil. The depredations of cutworms are difficult to control owing to the prevalence of hedges and coarse gras£ If it were practicable to keep the dense grass in such places within bounds there would be a marked improvement, as the cutworms would have to find quarters elsewhere. Autumn cultivation should help to reduce the numbers of moths, since the subterranean pupae would be destroyed in considerable numbers.

Starlings arc ’great hunters of cutworms, and it is no uncommon sight to see them fossicking under the soil about the roots of damaged plants, in practically every case a caterpillar being found and destroyed. The birds almost invariably confine their attention to plants showing signs of cutworm injury .

It may be mentioned her e that the urgent need of investigating the problem of the “turnip-fly” was brought up by farmers at the Ashburton Farm School last year.