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aphis reduces his yields and also to ascertain if the protection obtained pays for the additional expense of spraying materials to combat the insect. The results from such a test ought to demonstrate to the grower the feasibility of undertaking larger operations as a regular procedure in the upkeep of his apple plantings.
THE RADISH MAGGOT:*
P. J. PARROTT AND H. GLASGOW.
The radish or cabbage maggot is a common pest of radishes. The insect varies in importance from year to year. During some seasons it is very destructive, reducing the yield and quality of marketable roots.
Injuries to radishes occur during the latter part of May and early June. As grown in most home gardens the time of maturing of roots coincides with the period of egg deposition and development of larvæ of first brood. Plantings which attain marketable size as first-brood larvæ are maturing usually sustain the most serious damage.
Experiments during the past four years to determine the influence of time of planting upon the numbers of insects and extent of injury showed, in 1914, severe infestation in plantings of radishes maturing about June 9; for 1915 and 1916, considerable infestation in plants harvested about June 1 and June 15 respectively; and, for 1917, slight injuries only during June. In general, radishes pulled before the latter part of May have been free from important injuries.
Screening of radish beds is an efficient method of affording protection from maggot injury. Under normal seasonal conditions plants raised in screened frames grow more rapidly and have generally been superior in size, succulence and tenderness to those produced in open beds. In 1917 when temperature and precipitation during growing months were abnormal only slight differences existed in the conditions of plants produced by the two methods.
Practical measures for production of radishes that are largely exempt from insect attack are early sowing and growing of plants in frames screened with cheesecloth.
* Reprint of Bulletin No. 442, November, 1917.
The radish or cabbage maggot, Phorbia brassica Bouché, is of three-fold importance: (1) The insect dwarfs and destroys early cabbage; (2) .it attacks seedlings of late cabbage, resulting during some seasons in a serious shrinkage in the output of seed-beds; and (3) it causes maggoty radishes and, in addition to reducing the quality of the roots, may either stunt or destroy the plants. The destructive activities of the pest with reference to the culture of early and late cabbage are discussed in Bulletin No. 419 of this Station. In the study here presented attention is directed to the injurious work of the insect on radishes and to experiments with screening as a means of protecting radish beds.
THE INSECT AND ITS RELATIONS TO RADISH CULTURE.
The adult (Plate XXXI, fig. 4) of the radish maggot is a fly, resembling the common house fly, but smaller, measuring about onefourth of an inch in length and with a narrower body and proportionately larger wings. The thorax is ash gray in color with three distinct longitudinal lines on dorsum. The body is covered with numerous hairs or bristles. The legs are black and bristly, with a tuft of short bristles at the base of the posterior femur, which serves to distinguish the males of this species from males of closely allied forms. The eggs (Plate XXXI, fig. 1) are white, glistening and marked with irregular longitudinal furrows. They measure about one twenty-fifth of an inch in length. The larva or maggot (Plate XXXI, fig. 2) is a white, fleshy creature, at first cylindrical and tapering anteriorly, but later, toward the end of the feeding period, with middle segments much enlarged. The third stage of the insect from the egg is a puparium (Plate XXXI, fig. 3). This is a brownish body, elongate ovate in form and bluntly rounded at the ends. Large individuals measure about one-fourth of an inch in length. The outer protective covering is formed by the hardening and contraction of the integument of the third larval instar, and within this the pupa is formed. It is the inactive stage of the species and comparable to the chrysalis of moths and butterflies.
Studies on the life history of the insect at this Station have shown that the first of the adults or flies to emerge in the spring make their appearance during the first two weeks of May, the actual time of their emergence depending somewhat on the weather. The flies apparently do not appear simultaneously, as in one observation they emerged for a considerable period, from May 9 to June 14 or about thirty-six days. Within a few days after their escape from the ground they commence to deposit eggs. In this operation the female generally alights on the outer margin of a leaf. This she examines, running quickly from one area of inspection to another. She then crawls down the stalk of the plant to the ground, and upon finding a crevice or chink she lays her eggs. Oviposition is usually very active during the latter part of May and during early June. The eggs hatch within three to five days. The larvæ or maggots attack the roots, and it is this stage which is responsible for injuries to radishes, early cabbage and seedlings of late-maturing varieties of cabbage. The creature may be found in hidden tunnels, penetrating to varying depths, or in more or less open grooves along the surface of the roots. The laceration of the tissues is often accompanied by decay of the injured areas. The destructive work of the maggot is generally most evident in June, although the time of appearance of adults and the period of egg-laying and damage by the insect are hastened by warm weather during April and May. Upon maturity the larva generally abandons its host and tunnels the ground in the immediate vicinity of the plants, when it transforms to the pupal stage. A large percentage of the puparia are within three inches of the injured plant and in the first three inches of soil. Occasionally puparia may be found in the plants themselves when these have a tuber-like root, as radishes or turnips. The pupal stage lasts from twelve to eighteen days, although it may be prolonged for an indefinite period of several months. When favorable conditions exist there are at least three broods and perhaps a partial fourth brood.
Mention has just been made of the number of broods. It should be stated in this connection that, while with a proper selection of varieties radishes may be grown in this latitude over an extended season, yet in actual practice the culture of the radish in open beds out of doors, in the region of Geneva, is limited largely to a short period during the months of April, May and June. As a rule, comparatively few radishes are grown much later than this period as the plants suffer from drought and heat incident to the summer,
CHART 1.- RELATION OF TIME FOR GROWING RADISHES TO DATES OF APPEARANCE
OF THE MAGGOT IN ITS VARIOUS STAGES.
Time of planting and growth
period of radishes.
Time of appearance of maggot.
Time of planting seed.
Growth period of radishes.....
Eggs of flies in greatest numbers and first appearance of maggots of first brood.
Maggots full grown and period of
greatest injury to radishes.