Several species of thrips can be found feeding, at least occasionally, on grapes. The three most common are the western flower thrips (WFT) (Frankliniella occidentalis), the grape thrips (GT) (Drepanothrips reuteri), and the flower thrips (FT) (F. tritici).
Thrips are generally minor pests of wine grapes, but are important pests of table grapes due to scarring of fruit. Their populations often increase following applications of broad-spectrum insecticides that reduce numbers of beneficial insects.
All species feed in a similar manner, but differences in biology and timing of infestations results in differential damage to grapes.
All species have long, slender bodies with dark protruding eyes and relatively short beaded antennae. Adults have two pairs of wings that are heavily fringed along the edges.
WFT are dark yellow to brown and 1-1.5 mm long. GT and FT are both around 1 mm long. The former is amber yellow with a yellowy-orange thorax, and the latter is uniformly pale yellow to nearly white.
Nymphs of all species are similar in appearance to the adults except that they are smaller, slightly paler in colour and lack wings.
Thrips tend to be very active. Adults not only fly but also run and hop rapidly.
Plant-feeding thrips should not be confused with predaceous thrips. Adults of beneficial thrips may be dark black or purplish black with clear wings or wings that appear light grey with white bands, or they are pale with dark bands on the wings. They are slightly larger to nearly twice as large as the pest species.
Nymphs of predaceous thrips have bodies that are banded yellow and orange, are reddish purple in colour or are pale with the joints of the appendages dark.
Compared with phytophagous species, predaceous thrips have front legs that are enlarged and slightly curved that they use for grasping prey. These features and the slightly larger mouthparts are best observed under a dissecting microscope.
Adult female thrips overwinter under leaves and plant debris on the ground. In spring, they deposit their eggs into developing leaf, stem and flower tissues. The time from egg to adult is about three weeks during warm weather, and several generations are produced yearly.
Most thrips feed on a wide range of host plants, and large numbers will often move to a succession of new succulent hosts throughout the year. Both adults and nymphs feed by rasping the leaf surface and sucking up the escaping fluids.
WFT adults enter vineyards in early spring, often in large numbers, when saskatoon bushes have finished flowering and other native vegetation is beginning to dry down. In addition to ground cover plants, WFT feed on flower and fruit tissues of grapes from bloom to just after fruit set. Their feeding under caps and persistent flower parts causes berry scarring and russeting.
White and green grape varieties are also damaged when females insert their eggs into fruit, causing dark spots or halo spots. Feeding on developing shoots is occasionally so heavy that shoots are stunted and leaves deformed, but damage of this sort often only occurs around the perimeter of vineyards. WFT usually decline greatly in numbers following fruit set.
FT have a similar biology and contribute to the damage caused to fruit in spring, but they are not as numerous as WFT.
GT are generally only found in significant numbers on grape during the summer months, and their feeding is largely confined to leaves and shoots. Numbers can build up rapidly, however, particularly following insecticide treatments for other grape pests.
Leaf veins can be scarred and fail to expand properly, leaf edges can become necrotic and shoot growth inhibited, resulting in shortened internodes.
WFT often invade vineyards in large numbers for only brief periods in spring, and they and FT are not normally an economic concern for wine grapes.
Scarring of fruit has not been shown to affect wine quality, and damage to leaves and shoots is usually minor. Damage usually occurs over a short period of time, which makes early detection and control difficult.
Inspect table grapes for the presence of WFT and FT in spring from around 75% bloom to the end of fruit set and apply insecticides as required. Verify the presence of thrips by sharply striking fruit clusters or leaves against a dull white or green coloured surface. The decision to spray should be based partly on the table grape variety, records of past damage, and the intended market for the fruit. There are no firm thresholds for thrips infesting grapes in B.C.
Watch for GT damage to young leaves and shoots during summer and early fall. Use an eyepiece or magnifying glass to verify that thrips are present. As for fruit clusters, leaves and shoots can be held over a piece of white cardboard and sharply hit to dislodge the thrips.
Control on mature vines is usually only warranted when growth is stunted sufficiently early in the summer to delay ripening and affect growth the following season.
For more information about chemical control, see the Pesticides page.
Predatory bugs such as the minute pirate bug (Orius tristicolor), anthocorids (Anthocorus melanocerus) and deraeocoris (Deraeocoris brevis) are all known to feed on thrips.
Studies in the Okanagan and Similkameen valleys have indicated that predatory thrips are important regulators of pest thrips populations. In the laboratory, predatory thrips at a density of one every 10-15 leaves is sufficient to control WFT.
Avoid mowing cover crops during bloom and fruit set so that thrips are not forced to feed on table grapes when they are most susceptible to fruit damage.
