Many growers struggle with thrips management because thrips are extremely small insects (1.5 – 2mm long) that are difficult to detect. Outbreaks also tend to be sporadic and often unpredictable, complicating management even further. To improve management, it is important to understand how thrips damage table grapes, as well as what the factors are that affect their occurrence in vineyards.
How do thrips damage grapes?
Egg-laying: Various species of thrips, mainly western flower thrips (Frankliniella occidentalis), citrus thrips (Scirtothrips aurantii), onion thrips (Thrips tabaci) and blossom or “kromnek” thrips (Frankliniella schultzei) damage table grapes during the flowering period by partially inserting their eggs under the epidermis of the ovaries and developing young grape berries. This results in halo spot damage, as seen in Figure 1a (right-below) and b (right-bottom).
Figure 1. Thrips eggs in grape berry (a) and halo spot scars (b).
Feeding: All thrips have piercing-sucking mouthparts (Fig. 2) and they feed by sucking the cell contents from plant cells or pollen grains.
Figure 2. (Above) Thrips mouthparts (from “Thrips as Crop Pests”, edited by T. Lewis, CAB International,1997).
Epi = epipharyngeal sclerite, cib = cibarium, pc = precibarium, mn = mandible, ms = maxillary stylets, sal = salivarium,
hyp = hypopharyngeal sclerite, oe = oesophagus.
Scar tissue is formed where thrips larvae and adults feed on young grape berries, resulting in corky scarring (Fig. 3) as the berries expand. Thrips can also cause damage by feeding on grape berries close to harvest, usually where berries touch each other. This results in fine pin-prick scars, often in a circular pattern (Fig. 4).
Thrips feeding also causes scarring damage on grapevine shoots and leaves (Fig. 5), which can lead to inhibition of shoot growth and even shoot die-back in severe cases. This type of damage often occurs in vineyards where high numbers of citrus thrips occur later in the In the Western Cape indigenous Heliothrips sylvanus (guava thrips) sometimes causes feeding damage, which appears mainly on leaves during the latter part of the season, when its indigenous host plants become dry and unpalatable. This mostly manifests as silvering (Fig. 6). Damaged leaves are also stippled with drops of excrement which the thrips use to repel predators.season.
Figure 3. Scarring caused by thrips feeding.
Figure 4. Thrips feeding damage on grape berries close to harvest.
Figure 5. Thrips feeding damage on shoots and leaves.
Figure 6. Silvering caused by Heliothrips sylvanus and drops of brown excrement produced by thrips to repel predators.
Thrips occurrence and management in vineyards
• Citrus thrips are indigenous and occur on many host plants in and around vineyards and orchards. They prefer to feed on new growth, and where citrus and table grapes grow near each other. Citrus thrips will move back and forth between the orchards and vineyards as the growth flushes occur.
• Although they are exotic species, blossom thrips, onion thrips and especially western flower thrips have become established on a wide range of local plants and weeds. • Therefore, it’s impossible to “spray out” or eliminate thrips from a vineyard, as thrips will come in from the surroundings as soon as the effect of the insecticide wears off. In other words, finding thrips in a vineyard two to three or more weeks after spraying does not mean that the product was ineffective.
• The aim with thrips management is to protect the grape berries and new growth from thrips damage during those times in the season when they are vulnerable.
• Monitoring is critical to ensure timely sprays. Blue and/or yellow sticky traps, as well as tapping of flower clusters and growth tips over a white container (preferably with a small amount of water or surgical spirits) can be used to determine if thrips are present.
• Not all thrips cause damage and to prevent unnecessary sprays for harmless or beneficial (predatory) species, thrips caught during monitoring should be identified by an expert.
• To protect young grape berries during flowering and berry set, monitoring should commence as soon as the inflorescences appear, particularly in vine-yards with a history of thrips damage. If pest thrips are present, insecticides should be applied before the caps of the flowers start opening up. Once eggs are laid in the ovary, it is too late to prevent halo spot damage.
• If monitoring shows that pest thrips are present when growth flushes occur, e.g. development of secondary shoots just after berry set in the northern region (Mpumalanga), or the postharvest growth flush, insecticides should be applied to prevent shoot damage and die-back.
• Thrips can develop resistance to insecticides very quickly if the same product is used repeatedly during the same season. Therefore, it’s important to alternate between products with different modes of action. The IRAC group code on the product label indicates the mode of action. For resistance management, producers should alternate between products with different IRAC group codes.
• Example: According to Table 1, Delegate (IRAC 5A) can be alternated with Closer (IRAC 4C), but not with Tracer (IRAC 5A).
• Sugar is a feeding stimulant only and adding it to an insecticide could result in the thrips spending more time feeding, thus increasing exposure to the contact insecticide. The sugar does not attract thrips over a distance, therefore it won’t draw in thrips from surrounding vegetation.