Monitor grape buds for climbing cutworm and flea beetles

The cool spring may cause buds to swell for an extended period placing buds at increased feeding risk from cutworms.

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. The cool weather this spring may result in buds swelling over an extended period, and this can place buds at risk of feeding by climbing cutworm for a longer period than normal. Cool nights are ideal for activity of cutworms, with warm sunny days being more suited for activity of flea beetles. The coming weeks will be good times to scout vineyards for these two pests, to determine whether management is needed.

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 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 2 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 (see below).

Climbing cutworm damage.
Photo 1. Climbing cutworm damage. Photo credit: Tom Zabadal.

Flea beetles (Steely beetles)

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.

Flea beetle adult
(left) and larva (right).
Photos 2 and 3. Flea beetle adult (left) and larva (right). View more images of flea beetles.

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 above cutworm description).

Wherever possible, cleaning up overwintering sites (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 wine grapes 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.

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 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.

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 cutworm at 1 quart per acre, in at least 50 gallons of water 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 Statevineyards has shown excellent protection against cutworms using only trunksprays 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. Thisapproach significantly reduces the cost of application, but it is important torealize that this will not protect the upper canopy from flea beetle feeding.

For more information on grape production, visit grapes.msu.edu

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