How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Cutworms are inconspicously marked, dull-colored caterpillars ranging from 0.6 to 2.0 inch (1.5–5 cm) in length. Positive identification is important as behavioral differences affect control actions.

Mature variegated cutworm larvae are 1.5 to 2.0 inch (3.8–5 cm) long with smooth skin. Body color varies from pale gray to dark mottled brown intermixed with red and yellow dots along the dorsum.

Mature spotted cutworms are about 1.3 inch (3.5 cm) long and are a dull gray brown. A row of dark or black triangular markings are found on each side of the dorsal body surface.

Mature brassy cutworms are 1.0 to 1.2 inch (2.5–3 cm) long and are reddish or brassy in appearance. Of the cutworm species that attack grapes, brassy cutworm is the only one with hairs protruding from the compound eye area. A hand lens is needed to detect these hairs.

Variegated cutworm is the predominant species in the San Joaquin Valley and North Coast, while spotted cutworm is predominant in the Central Coast counties. In the North Coast, the variegated cutworm normally returns to the ground during the day but may also remain under the bark of the vine. In the San Joaquin Valley variegated cutworm larvae do not return to the soil but rather move under the bark. Spotted cutworms routinely remain under grapevine bark in all production areas.

DAMAGE

Feeding on grapevines occurs from bud swell to when shoots are several inches long. Injured buds may fail to develop. Grapevines can compensate for early season damage to buds or shoots to some extent by the growth of secondary buds. The fruitfulness of secondary buds, however, varies according to variety, and some varieties such as Thompson Seedless and Chardonnay have unfruitful or significantly less fruitful secondary buds respectively. In these varieties, destruction of primary buds can be expected to reduce the number of clusters in proportion to the number of buds destroyed.

MANAGEMENT

Historical records of cutworm infestations or damage are useful in developing monitoring strategies for individual vineyards because cutworm problems are normally spotty or localized. Many varieties of grapes can tolerate a significant amount of damage without any economic loss. No chemicals are highly effective in controlling cutworms, so frequently treatments may not be economically justified.

Biological Control
Natural enemies of cutworms include predaceous or parasitic insects, mammals, parasitic nematodes, pathogens, birds, and reptiles. The hymenopteran (wasp) parasites, including ichneumonids, chalcids, braconids, and sphecids, are the most important group of cutworm natural enemies. Predaceous beetles (often found under bark) and tachinid flies are also factors in biological control.

Cultural Control
Cultural practices have not been demonstrated to successfully control cutworms; however, some practices do affect their population abundance. Weed removal in late summer or fall may be beneficial in disrupting cutworm life cycles. Plowing or discing of weeds is not recommended before or soon after bud swell in spring where cutworms are a problem because it can cause movement of cutworms to the grapevines. Furrow and flood irrigation can be manipulated to bring cutworm larvae to the soil surface, exposing them to adverse weather and predators.

Organically Acceptable Methods
Biological and cultural controls and the Entrust formulation of spinosad are organically acceptable methods.

Monitoring and Treatment Decisions
Begin to monitor bud feeding by cutworms during bud swell in early spring. Cutworms can be monitored along with other pests following the procedures discussed in DELAYED-DORMANT AND BUDBREAK MONITORING (wine/raisin grapes or table grapes). Because cutworm infestations are clumped, many vines must be examined to detect an infestation. In spring cutworms leave the soil and climb up the vines. During the day they hid under loose bark towards the base of the vine and come out to feed at night. Randomly select five locations in the vineyard to observe, concentrating on areas known to be chronically infested. Check 4 vines within each location for damaged buds (total 20 vines). On each vine examine 5 buds for damage (total 25 buds per location).

In cool growing regions with a long period between bud swell and shoot growth, monitoring may be needed over a 2- to 3-week period. Record results on a monitoring form (example form—81 KB, PDF).

The number of damaged buds that can be tolerated depends on variety. If secondary buds are highly fruitful, little yield loss will result even when a large proportion of buds are damaged. If less than 4% of the buds are damaged, treatment may be unnecessary. Treating an entire vineyard is seldom necessary because infestations are usually localized; consider spot treatments. Cutworm feeding after shoots are about 6 inches long does not result in significant injury.

To make sure cutworms are causing the damage, return to damaged vines at night to look for cutworm larvae. Other species of insects (grape bud beetle, click beetles, branch and twig borers, orange tortix larvae) also cause similar injury.

IMPORTANT LINKS

• Delayed/dormand budbreak monitoring form (81 KB, PDF)

PUBLICATION

UC IPM Pest Management Guidelines: Grape
UC ANR Publication 3448
Insects and Mites
W. J. Bentley, UC IPM Program, Kearney Agricultural Research Center, Parlier
L. G. Varela, UC IPM Program, Sonoma County
F. G. Zalom, Entomology, UC Davis
R. J. Smith, UC Cooperative Extension, Sonoma County
A. H. Purcell, Environmental Science, Policy and Management, UC Berkeley
P. A. Phillips, UC IPM Program, Ventura County
D. R. Haviland, UC IPM Program, Kern County
K. M. Daane, Kearney Agricultural Research Center, Parlier
M. C. Battany, UC Cooperative Extension, San Luis Obispo County
Acknowledgment for contributions to Insects and Mites:
J. Granett, Entomology, UC Davis

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