Grape Phylloxera - Pest Management Program for Grape Series
Table of Contents
Grape phylloxera, Daktulosphaira vitifoliae (Fitch) is an insect pest which attacks many grape species both wild and cultivated. During feeding, phylloxera secrete a chemical which causes plant tissue to grow near the feeding site, resulting in the characteristic galls. Phylloxera is an indirect pest of grapes, damaging vines by feeding on plant sap from roots, leaves and tendrils, but not causing actual fruit injury. It is often described as an "aphid-like", sucking insect or a "vine louse".
Figure 1. Phylloxera leaf galls on De Chaunac grapes.
Above-ground feeding by phylloxera occurs mainly on the foliage (Figure 1), but it can also occur on stems and tendrils if population densities are high. French hybrids and a few North American cultivars are very susceptible to leaf gall formation. Light to moderate levels of leaf gall formation can occur on Vitis vinifera. While plant sap in the leaves provides nourishment for the insect, the affected areas of the leaf are induced to provide protection as well. Leaf galls are hollow, pea-sized swellings of plant tissue, usually red or green in color, that form on the undersurface of the leaf, a result of the growth inducing chemical secreted by the phylloxera insect. Unlike root feeding, leaf feeding occurs from inside the leaf gall. The hollow, thick-walled leaf gall is both a perfect incubation chamber for eggs and a barrier to the harsh external environment of insecticides, as well as dessication, predators, and diseases.
On cultivars susceptible to root infestation, mainly ungrafted V. vinifera, phylloxera infestations may be lethal to the vine. Infested roots swell to form root galls while the phylloxera continue to feed on the outer surface of the swollen area. Large galls on older roots are often attacked by root rot diseases, which usually results first in decline, and then death of the vine, three or four years following a phylloxera infestation. In major grape growing regions, phylloxera-tolerant American rootstocks are used to cultivate V. vinifera. In Ontario vineyards, the grape phylloxera inhabits most root systems, but has no lethal effects due to low winter temperatures which prevent excessive buildup of the phylloxera population. The root form in Ontario is currently classified as a minor pest.
Grape phylloxera adult females, both root and leaf feeding forms, are wingless and oval, 0.7 mm to 1.0 mm long and about 0.5 mm wide. On the leaves, young adults are bright yellow to orange becoming brown with age. On roots, they are pale green, light brown or orange. Newly deposited eggs are oval, bright yellow, approximately 0.4 mm long and 0.2 mm wide. Just prior to hatching. the eggs turn dark yellow with 2 visible red eye spots at one end. Emerging nymphs are similar in size to the egg. The nymphs progress through 4 developmental stages before reaching the adult stage. The winged adult female, emerging from the soil in late summer and early fall, is orange with a grey-black head and thorax with two pairs of lightly veined wings.
A. Root Cycle
On roots, phylloxera overwinter as first instar nymphs (Figure 2). In the spring, as soil temperature rises. nymphs begin feeding on root sap and mature to adults in 15 to 20 days. The spring and summer feeding adults, which are strictly females, reproduce without male fertilization. One female can produce 100 to 150 eggs over a period of approximately 45 days. New nymphs move to other root areas, begin feeding and cause gall formation. When mature, they will start producing the next generation of eggs. Five to nine overlapping generations can occur during a growing season. In September and October, newly hatched first instar nymphs begin hibernation for the winter.
Figure 2. Instar nymphs.
B. Leaf Cycle
The phylloxera leaf feeding cycle begins later in the growing season. In late July through to October, some of the nymphs on the roots develop wings and emerge from the soil when mature (Figure 2). Winged adult females lay eggs on the upper vine. The eggs hatch to give wingless male and female phylloxera. Mating occurs and one egg per mated female is deposited under the bark of the grapevine. This egg is the overwintering stage of the leaf infesting cycle of phylloxera.
The overwintering egg under the bark hatches early in the spring as grape buds break dormancy and begin to open. The wingless female nymph crawls to a new shoot up to the growing point. From the upper surface of a young, unexpanded leaf, the nymph begins feeding on plant fluids by inserting its stylet into the leaf cell tissue. As the leaf expands, the leaf gall develops around the phylloxera, forming a hollow, pea-sized growth on the undersurface of the leaf in which feeding continues. If the nymph begins feeding on a mature leaf, incomplete gall formation results. This first generation of female adults causes the first appearance of leaf galls about mid-May.
Figure 3. Phylloxera gall on lower leaf surface cut open to reveal adult female and eggs
Figure 4. Phylloxera nymphs emerging from a gall through the exit hole on the upper leaf surface
Figure 5a. Grape shoot tip attacked by phylloxera nymphs. New leaf galls are becoming visible on lower leaf surface.
Figure 5b. Phylloxera nymphs on upper leaf surface beginning to feed and form galls on a young unexpanded leaf.
When the female reaches maturity, 10 to 15 days after feeding initiation, and when gall formation is complete, eggs are deposited inside the gall. Up to 200 to 300 eggs may be deposited by a female over a period of 30 to 40 days (Figure 3). As eggs hatch, the new nymphs immediately emerge from the gall through a small exit hole on the upper leaf surface (Figure 4). The nymphs crawl from the leaf gall to the stem. Many of the nymphs move upwards along the shoot, again, to feed on unexpanded leaves at the shoot growing point and begin forming new galls of their own (Figures 5a, 5b). Other nymphs crawl or are windblown to other shoots on the vine and form galls there. This new formation of leaf galls represents injury caused by the second generation of phylloxera in which third-generation eggs are deposited. The lifecycle is repeated as the shoot grows and the season progresses. Therefore, early generations of leaf phylloxera can generally be distinguished by the location of galls along the shoot with the youngest generation being nearest to the growing point. Five to seven generations can occur over a season, which begin to overlap by the third generation, making it difficult to distinguish between generations by mid-summer. As many as 200 phylloxera galls can be found on a leaf during heavy infestations. In the fall, some hatching nymphs migrate, are windblown or fall to the ground and crawl down to the root system to hibernate through the winter.
Grape phylloxera causes indirect damage to susceptible grapevines. With foliar infestations, heavily galled leaves are malformed and rolled, causing a decrease in leaf surface area (Figure 6). Leaf gall tissue may contain up to 90% less chlorophyll than healthy leaf tissue, suggesting a reduction of photosynthesis. The loss of carbohydrates, due to severe infestations, could result in reduced fruit sugar levels at harvest. The vine may also be unable to adequately store carbohydrates as food reserves, which would otherwise promote winter hardiness and supply energy for new growth the following spring. Severe infestations of phylloxera galls on the foliage will also cause defoliation and retard shoot growth.
Figure 6. Healthy grape shot (left), compared to shoot with severely leaves (right).
*Phylloxera are able to cause variable levels of gall formation on some V. vinifera.
Monitoring and Control of Leaf Phylloxera
Monitoring the leaf feeding cycle of grape phylloxera depends on timely visual observations. Leaf galls caused by first-generation phylloxera generally appear about mid-May, usually as one or two galls on the first to third new expanding leaves. Samples of these galls should be opened and checked with a hand lens, or better, dissected under a low-power microscope to determine when second-generation eggs begin to hatch. Two to three days after egg hatch begins is an ideal time to apply chemical control, as the new nymphs emerge from galls and crawl to the young, unexpanded leaves. The crawling and early gall development phase is the stage that phylloxera are most vulnerable to an insecticide because once new gall formation is complete, chemical control becomes difficult due to the protection offered by the leaf gall structure. Systemic insecticides may control the feeding females inside the galls; however, deposited eggs are often unaffected. Similarly, leaf galls caused by second-generation phylloxera can be sampled to determine when egg hatch begins and subsequent nymph crawling for spray timing.
Early-season chemical control may be more effective than mid- to late-season control in problem vineyards. A split-age structure in the phylloxera population develops by the beginning of the third generation, making spray timing more difficult as the season progresses. Research indicates that grapevines are able to withstand light to moderate amounts of leaf gall formation without having adverse effects on fruit quality or vine health. Therefore, chemical control is only recommended in vineyard blocks that have a history of serious leaf phylloxera problems.
Common predators, including lacewing nymphs (Figure 7) and a predatory fly larvae (Figure 8) will help to reduce phylloxera populations.
Figure 7. Lacewing nymphs are common predators of grape phylloxera.
Figure 8. Predatory fly larvae feeding on phylloxera adult and eggs inside a leaf gall.
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