Bud swell is the time for monitoring vineyards for cutworms and flea beetles

Time spent monitoring grape vineyards now can avoid costly pest damage.

Comparison of the bud damage caused by grape cutworm (left) and flea beetle (right)
Comparison of the bud damage caused by grape cutworm (left) and flea beetle (right). Photos by Rufus Isaacs, MSU Entomology.

Bud swell of grapes is underway in southwest Michigan and in our Marquette vineyard on campus in East Lansing, Michigan, that we have been pruning this week. This growth stage is 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 is provided below.

Cutworms

The term cutworm covers many species in the moth family Noctuidae, and as their name suggests, these insects are nocturnal so they are easily missed. Vineyards on light-textured soils are typically most at risk from cutworms. Larvae overwinter in the soil and weeds under the vines, and the climbing cutworms walk up the trunks during cool night-time conditions to munch on buds. 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 centimeters long.

Cutworm feeding on a bud can reduce the crop by one to three clusters, so the high potential for rapid damage by cutworms requires growers make good management decisions. Even 2% 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 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 and/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 damage is more of a hole drilled into the bud compared to cutworm’s ragged munching damage, but with similar effects to the vine.

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

Scouting for bud damage

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. Both pests 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.

The action threshold of 2% damaged buds can be assessed by sampling 10 buds on each of 10 vines spread through the vineyard. Or, you might want to do one sample at the edge and the other inside the vineyard to determine if a border treatment is sufficient. 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 the insects or their damage are detected.

Once 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 (IPM) 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 thoug 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. Lorsban Advanced is labeled for cutworm at 1 quart per acre in at least 50 gallons of water per acre. Note that Corteva is no longer manufacturing new product but material with the label is still registered and there are generic sources of this insecticide. Delegate is a reduced risk option registered for cutworm control (3-5 ounces per acre). There are also a number of pyrethroid insecticides registered for use against cutworms including Mustang Max (2-4 ounces per acre), Danitol (10.6 ounces per acre) and Brigade (3.2-6.4 ounces per acre) that provide excellent control of cutworms and flea beetle.

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, as well as insect identification pages, see the Grapes Insect Pest Management page from Michigan State University Extension.

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