Monitor vineyards for cutworms and flea beetles during bud swell growth stage

Time spent monitoring vineyards for cutworms and flea beetles can avoid costly pest damage.

Bud swell of grapes is being reported in Southwest Michigan, and I have seen wild grape buds starting to reveal some green tissue this week (as of April 29, 2014). This means we are approaching the time when growers should monitor for cutworms and flea beetles, two early season pests that can feed on buds and limit crop yield. There are good rules of thumb for deciding whether damage from these insects warrants control, and information on scouting and other management components from Michigan State University Extension is provided below.

Cutworms

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 typically most at risk. Both the adults and the larvae are only active at night, and the larvae can climb up onto vines during very cool, nighttime 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 10-15 centimeters 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 2 percent bud injury is an action 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 (see below).

Flea beetles or steely beetles

This insect attacks buds of both wild and cultivated grapes 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 buds. This insect can arrive in large numbers when conditions are right for their activity, so be sure to check vines on good weather days during bud swell.

Adults are shiny blue, about 4-5 millimeters 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 above cutworm description).

Wherever possible, cleaning up overwintering sites, both wasteland and woodland, near to vineyards can help combat grape flea beetle.

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 beetles 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.

As mentioned above, once the shoots get past bud burst and into the 1- to 3-inch range, the danger from flea beetles and cutworms is diminished significantly.

Cultural control

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 integrated pest management program to reduce cutworm damage.

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.

Chemical control

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.

Lorsban Advanced is labeled for cutworms at 1 quart per acre in at least 50 gallons of water per acre. Delegate is a reduced risk option registered for cutworm control at 3-5 ounces per acre. There are also a number of pyrethroid insecticides registered for use against cutworms that provide excellent control of cutworms and flea beetle, including Mustang Max at 2-4 ounces per acre, Danitol at 10.6 ounces per acre, and Brigade at 3.2-6.4 ounces per acre.

Research in Washington State vineyards has shown excellent protection against cutworms using only trunk sprays of a pyrethroid. This approach targets the spray to the trunk surface and larvae have to climb up through the residue to reach the buds. This significantly reduces the cost of application, but it is important to realize that this will not protect the upper canopy from flea beetle feeding.

For photos of grape flea beetle and cutworm damage to grapes, see the Identifying Insects pages at the recently updated MSU Viticulture and Enology website at www.grapes.msu.edu.

Dr. Isaacs’ work is funded in part by MSU’s AgBioResearch.

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