Monitor grape buds for climbing cutworm and flea beetle damage
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Two important insect pests of grapevines become active around the time of bud swell, and both have the potential to cause damage to early growth if populations are high. Warm weather has brought bud swell to south Michigan vineyards in the past week, and Mark Longstroth is reporting cutworm damage in this region. The weather predicted over the next week will bring some vineyards through the risk period during bud swell, but cool nights are ideal for activity of cutworms, and warm sunny days can bring activity of flea beetles. This week and next will be a good time to scout vineyards for these two pests to determine whether management is needed.
The term cutworm covers many species in the moth family Noctuidae, and as their name suggests, these insects are nocturnal. Vineyards on light-textured soils are often the most heavily infested. Both the adults and the larvae are only active at night, and the larvae can climb up onto vines during very cool night-time conditions. During the day, cutworms hide in the soil or leaf litter, and can be found in the top layer of soil. Many of these insects feed on weeds, but some climb the stems of plants to feed on buds and other young foliage. These climbing cutworms are the ones causing damage to grapevines. Direct observation of feeding by the larvae requires a late-night trip to the vineyard, but their damage is quite easy to see.
In Michigan vineyards, the spotted cutworm, Amathes c-nigrum, is our main pest species, and the larvae feed on buds and may also feed on leaves until the shoots are 10 to 15 cm long. However, it is the feeding on small buds that has the greatest potential for economic damage.
Cutworm feeding on a bud can reduce the crop by one to two clusters so the high potential for rapid damage by cutworms requires that growers make good management decisions. Even two percent bud injury is an economic threshold for an insecticide treatment to prevent further damage, so vineyards should be scouted during the period of bud swell to identify regions with cutworm pressure.
Flea beetle (Steely beetle)
This insect attacks buds of both wild and cultivated grape, and is another early season grape pest. The adult insects move to the vines at bud swell, and usually are localized within the vineyard. Sites near overwintering habitats such as woods or abandoned vineyards are especially at risk. Beetles are most easily seen during warm sunny weather when they tend to be on the top of vines, usually mating or feeding on canes and buds.
Adults are shiny blue, about 4-5 mm long, and have strong hind legs that enable them to jump if disturbed (hence the name). The overwintering adults cause the greatest damage by boring into the developing bud and hollowing out the inside, while the larvae and summer adults feed on leaf tissues. Bud feeding is similar to that caused by cutworms, with similar effects to the vine (see cutworm description).
Wherever possible, cleaning up overwintering sites (wasteland and woodland) near to vineyards can help combat grape flea beetle. Early sprays for grape berry moth will also help to reduce the later larval populations of this insect.
Scouting for bud damage
Growers should watch for damage by cutworms and flea beetle, especially if the vines remain in the susceptible bud swell stage for a while with cooler weather. Cutworms tend to be more of a problem in sandy sites, so these should be prioritized for scouting. Both cutworms and steely beetle can cause damage quickly if the temperatures warm up, and since they are difficult to catch “in the act,” regular scouting for the first signs of damage is essential to prevent significant bud loss.
An action threshold of 2 percent damaged buds is recommended in juice grapes, and this can be determined by sampling 10 buds on each of 10 vines spread through the vineyard. Thresholds in winegrapes may be lower due to the higher value of the crop, but there has been little formal research on this topic. Still, it is clear that the potential damage justifies scouting and management if cutworm damage is detected.
Once the shoots get past bud burst and into the 1-3 inch range the danger from flea beetles and cutworms is diminished significantly.
Vineyards that are weedy tend to have more cutworm problems, presumably because the larvae have more places to hide and conditions are better for them. Weedy vineyards also provide more places for the cutworms to hide from sprays applied for their control, so improving weed control is one component of an IPM program to reduce cutworm damage.
Although it may be too late for this year if you have finished pruning, leaving some extra buds is a potential strategy for hedging your risk against cutworm (and frost) injury. Scouting is still required though, to make sure the damage doesn’t exceed the number of extra buds left behind.
An appropriate insecticide application should be considered if scouting shows significant damage is occurring, and assessments of damage should include wooded borders where flea beetle pressure may be higher, and areas where cutworms have been a recurring problem.
The Special Local Needs label for Lorsban 4E for cutworm control in Michigan expired in 2009, but Lorsban Advanced is labeled for cutworm at 1 quart per acre. There are also a number of pyrethroid insecticides registered for use against cutworms including Mustang Max (2-4oz/acre), Danitol (10.6oz/acre), and Brigade (3.2-6.4oz/acre) that provide excellent control of cutworms and flea beetle. Sevin is also registered for use against flea beetles and has performed very well in observations of treated vineyards at 2 qts/acre.
Recent research in Washington State vineyards has shown excellent protection against cutworms using only trunk sprays of a pyrethroid. This approach targets the spray on the surface that larvae have to climb up to reach the buds and it also reduces the cost of application. However, it is important to realize that this approach will not protect the upper canopy from flea beetle feeding.
For photos of grape flea beetle and cutworm damage to grapes, see the pages at the grapes.msu.edu website: