Part 18 (2/2)

Several pests vex the gardener in growing grapes indoors. Of these, mealy-bug, red-spider, thrips and mildew are most troublesome. In a well-conducted grapery, there is never an intermission in the warfare against these pests.

Mealy-bug is usually a sign of sloth on the part of the gardener. In a grapery devoted exclusively to grape-growing, it should never be seen, but, since gardeners must often grow other plants in the grapery, mealy-bug sooner or later appears and is often hard to dislodge. It is best repelled by removing the loose bark on the trunks which harbor the pest and then was.h.i.+ng with kerosene emulsion. When this becomes necessary, not only the vines but the rafters and all parts of the house should be sprayed with the emulsion.

Red-spider is another pest usually found in the grapery, but it thrives only in a dry atmosphere and is easily gotten rid of by syringing. As soon as red-spider appears in a house its appearance is usually known by the reddish tinge on the foliage; syringing should be kept up until the pest is disposed of, keeping the house damp in all except dull weather. Syringing is done only when plenty of air can be given and when it can be followed by sunlight so that the water remains on the vines as short a time as possible.

Thrips, another small insect, is sometimes troublesome but not often and is now easily controlled by applications of nicotine. Much care must be taken in the application of nicotine late in the season, otherwise the fruit will be injured.

The only fungous disease of the grape troublesome in the greenhouse is mildew. Mildew is usually brought on by a sudden change of temperature or by draughts in the grapery. Gardeners are of the opinion that east winds, in particular, give unfavorable conditions for mildew and prefer to open the ventilators to the west. If taken in time, mildew is easily kept in check by preventing the conditions which favor it, and by dusting the vines in dry suns.h.i.+ne with sulfur.

[Ill.u.s.tration: PLATE XVI.--Elvira (2/3).]

CHAPTER XII

GRAPE PESTS AND THEIR CONTROL

In common with other cultivated fruits, grapes are at the mercy of numerous insect and fungous pests unless man intervenes with remedial or preventive treatment. Happily for viticulture, knowledge of the pests of the vine has made such advancement in recent years that practically all are now controlled by remedial or preventive measures.

Possibly no field of agriculture has had greater need, or received greater aid from science in the study and control of insects and diseases than grape-growing. A separate treatise would be required to treat the pathological troubles of the grape fully; only such details of the life histories of the several pests to be discussed as are essential to a proper understanding of the control of the parasites can be given here.

INSECT PESTS

Insects troubling the grapes are numerous, at least 200 having been described in America, most of which have their habitat on the wild prototypes of the cultivated vines of this continent. For this reason, with a few exceptions, the insect pests of the grape in America are widely distributed, abundant, and, therefore, often very destructive to vineyards unless vigorously combated. The many pestiferous species vary greatly in importance, depending on locality, weather and the variety. Phylloxera, however, the country over, is most common and deserves first attention.

_Phylloxera._

This minute sucking insect (_Phylloxera vastatrix_), injures the grape by feeding on its roots. Decay usually follows its work on the roots and is often more injurious than the harm done directly by the parasite. This decay is always much more serious on European vines than on those of our native species. The phylloxera is a native of the United States east of the Rocky Mountains, from whence it was introduced into France and from France into California, where it causes much greater damage than elsewhere in the United States.

Wherever the pest is found, it is more injurious in heavy than in sandy soils. In fact, in very sandy soils the vines are often sufficiently resistant to be practically immune.

[Ill.u.s.tration: FIG. 36. Leaf-galls of the phylloxera.]

The life history of the phylloxera is very complex where the different forms of the insect appear and need not be entered into in detail here. East of the Rockies, the most evident indication of the presence of the pest is great numbers of leaf-galls on the under side of the leaves of the grape as shown in Fig. 36. These galls, however, are seldom to be seen in California and are not present on Concords and some other varieties in the East. The winter egg may be taken as the beginning of the life cycle of the phylloxera. From a single winter egg a colony may arise, the first insect after hatching making its way to the leaves where it becomes a gall-maker and gives rise to a new generation of egg-laying root-feeders. On varieties and in regions where the gall form is not found, the insect probably goes directly from the winter egg to the roots. Once the pest is established on the roots, generation follows generation throughout the growing period of the vines, as many as seven or eight occurring in one season.

From midsummer until the close of the growing season, some of the eggs deposited by the root-feeders develop into nymphs which acquire wings and emerge from the soil to form new colonies from eggs deposited on the under side of the leaf. An individual insect deposits from three to six eggs of two sizes, from the larger of which come the females and these, after fertilization, move to the rough bark of the vine and deposit the winter egg for the renewal of the cycle.

Several methods of control have been employed in Europe and California, as treatment by carbon bisulfide injected in the soil; flooding in vineyards that can be irrigated; confining the vines to sandy soils; and, most important, planting vines grafted on resistant stocks, there being great variation in immunity of species of American grapes to phylloxera. The subject of stocks resistant to this pest has been discussed in Chapter IV and need not be taken up again. East of the Rockies, treatment is not necessary with American grapes.

_The grape root-worm._

The grape root-worm is the most harmful of the insect pests of grapes in the grape-belt along the sh.o.r.es of Lake Erie in Ohio, Pennsylvania and New York. This root-worm (Fig. 37) is the larva of a grayish-brown beetle (_Fidia viticida_), shown in Fig. 38. The worms feed at first on the rootlets and later on the bark of the larger roots of the vines so that the injured plants show roots devoid of rootlets and bark channeled by the pest. So plain is the work of the root-worm that the grower never need be at a loss as to the cause of vines injured by this pest. The worms feed during the latter part of the growing season, reaching full growth at this time. The next June they transform into pupae and in late June or early July emerge as adult beetles.

[Ill.u.s.tration: FIG. 37. The grape root-worm.]

[Ill.u.s.tration: FIG. 38. Root-worm beetle.]

The presence of the adult beetles is more easily detected on the foliage than is that of the larvae on the roots, for the feeding beetles ravenously devour the upper sides of the leaves, leaving chain-like markings, shown in Fig. 39, their destructiveness decreasing somewhat after a few days from their first appearance. A fortnight after the beetles begin their attack on the foliage the female begins laying her eggs, to the number of 200, placing them under the rough bark of trunk and cane. These hatch in late July or August and the young grubs at once seek the roots.

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