Ministry of Agriculture
Grape Insect and Mite Pests
Climbing Cutworms
(Lepidoptera: Noctuidae)
- Identification
- Life Cycle and Damage
- Monitoring and Spray Thresholds
- Biological Control
- Cultural Control
- Chemical Control
Prepared by D. Thomas Lowery, Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, Summerland.
To date, 17 species of cutworm have been identified feeding on the buds of grapevines in the southern interior of British Columbia; the three most common species being Abagrotis orbis (Grote), A. reedi Buckett,and A. nefascia (Smith). Damage can vary greatly between years and locations and it is not uncommon for only parts of vineyards to be seriously damaged. Because populations fluctuate greatly and monitoring of cutworm larvae is difficult, damage can be considerable.
Identification
Cutworm larvae are smooth, soft, thick-bodied caterpillars measuring about 4 to 5 cm when fully grown. Larvae typically live in the soil or under debris and will curl up when disturbed or handled. Depending on the species, larvae are most often light to dark brown, and are usually marked with spots or stripes. Adults are drab moths with bodies around 2 to 2.5 cm long. The hind wings, which are sometimes distinctly marked, are usually lighter in colour than the forewings. Adults are active at night and rest during the day in sheltered locations under bark, debris, or leaves.
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Cutworm larva Photo courtesy of M. Weis, Agriculture and Agri-Food Canada, Summerland. |
Life Cycle and Damage
Cutworms that feed on grapes spend the winter as small larvae in the soil or under fallen leaves or debris. When the weather warms in spring they resume feeding and complete their development. Damage to grapes occurs during this time when the rapidly growing larvae feed on unopened buds and new shoots. Moths emerge in June and remain in a quiescent state throughout the summer until fall when they become sexually mature and begin laying eggs that hatch into the overwintering larvae. There is a single generation each year.
Damage to grape buds varies with the size of the cutworm larvae. Small larvae will often burrow into a bud, creating nearly circular holes that were previously attributed to feeding by adult click beetles. Larger cutworm larvae will chew large, somewhat ragged holes and remove a large portion or all of the bud. Feeding on new, tender shoots can cause the shoots to break or wilt. Larvae climb the vine trunks or support posts each night and return to the soil during the day, or they will find refuge under loose bark of older vines.
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| Cutworm damage to grape bud Photo courtesy of H. Buchler. |
Cutworm damage to young shoot. Photo courtesy of M. Weis, Agriculture and Agri-Food Canada, Summerland. |
Monitoring and Spray Thresholds
Cutworm numbers vary over time and between and often within vineyards. Some vineyards rarely experience significant levels of cutworm feeding damage while others are damaged yearly. Mild, dry conditions during the winter increase survival rates, while outbreaks of viral and other diseases that affect cutworms can reduce numbers to low levels. Cutworm damage is often higher in areas with dry gravelly or sandy soils that do not support good stands of broadleaf weeds, with drip irrigation systems, or as a result of weeds being controlled too early in the spring before shoots have elongated. Maintaining a record of previous damage can help suggest which areas need to be monitored most closely and where damage is likely to occur first, but bud damage needs to be assessed throughout the vineyard.
New vines planted in untreated land previously in pasture are prone to damage, and larvae are often sheltered and somewhat confined within the protective milk cartons or sleeves placed around new vines. Loss of buds on new vines can retard growth and delay fruiting and they should be closely inspected in spring for signs of damage and treated when necessary.
Significant amounts of damage can occur over a period of two or three days when the nights remain warm, and it is especially important to monitor for damaged buds frequently during this time. At other times it is usually sufficient to monitor vines weekly or twice weekly until shoots have elongated. An accurate threshold based on the degree of bud damage is difficult to determine, as it is influenced by the variety, training system and method of pruning, presence of predators and parasites, and tolerance level of the individual vineyard manager.
Suggested economic thresholds from growers range from virtually zero to over 15%; a value around 3% bud loss is likely for most vinifera varieties. Establishment of spray thresholds should also consider the potential for secondary pest outbreaks that will require additional insecticide sprays later in the season.
Biological Control
Cutworm eggs are consumed by various predators and are parasitized by small wasps. Larvae are eaten by ground beetles, hunting spiders and birds, and are parasitized by wasps and flies. Fungal and bacterial diseases that attack cutworm larvae are very common, particularly when conditions are cool and wet. The combined effect of these natural controls is to eliminate most cutworm eggs and larvae. Of the 300 or more eggs laid by a female moth, only a few survive to become reproductive adults. Even so, damage resulting from the few remaining larvae can be considerable.
Cultural Control
Broadleaf weeds should not be controlled in spring until shoots have elongated and the first leaves have expanded. Damage is generally much lower in vineyards with mixed ground cover vegetation and where broadleaf weeds, particularly winter annual mustards such as Draba verna and Shepherd’s purse, Capsella bursa-pastoris, are present in the vine rows in spring. Larvae are usually more common in areas with well-drained sandy or sandy-loam soils that support fewer broadleaf weeds. Diseases that infect larvae are also likely to be more prevalent in soils that retain moisture. Control of vegetation with herbicides or repeated discing during the fall prior to the establishment of a new vineyard will help reduce damage to newly-planted vines.
Cutworm damage is partially related to pruning practices and grape varieties. Vines with a greater number of retained buds in spring will suffer marginally less damage, but there will be added costs related to removal of extra shoots. Some grape varieties will produce fruit on secondary shoots, but yields will still be reduced and ripening can be delayed or uneven.
Chemical Control
Sprays of the pyrethroid insecticide Pounce (permethrin) should be directed to the soil at the base of vines, to vine trunks, and to support posts so that larvae contact the insecticide while moving from the soil to the canopy. Do not cultivate or disturb the soil for several days after treatment. For young vines, sprays should be directed into the milk cartons or other protective structures applied around newly planted vines. Avoid spraying insecticides unnecessarily, as these are very damaging to beneficial insects and can result in outbreaks of secondary pests later in the season.
Altacor™ (rynaxypyr) was recently registered for cutworms on grapes. Rapid cessation of feeding and toxicity result largely through ingestion and sprays should be directed to the developing buds. Altacor is considered a reduced–risk material due to its greater selectivity and reduced toxicity to non-target organisms. Follow the label instructions and wear appropriate protective clothing as for all pesticides. Refer to the most recent edition of Pest Control Products Recommended for Use on Grapes in British Columbia for appropriate and registered materials.
September 2010
