More than 25 species of cutworm have been identified feeding on the buds of grapevines in the southern interior of B.C. Some of the more common include Barnes’ (well-marked) climbing cutworm (Abagrotis orbis) Reed’s dart (A. reedi) and Abagrotis nefascia.
The W-marked cutworm (Spaelotis clandestina), old man dart (Abagrotis vetusta), dark-sided cutworm (Euxoa messoria), and others, can also cause considerable damage in certain locations in some years. The invasive lesser yellow underwing (Noctua comes) has increased in numbers and distribution since it was first found in the Okanagan in 2001.
Cutworm feeding damage to the buds of grapes 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.
Cutworm larvae are smooth, soft, thick-bodied caterpillars measuring about 4-5 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 grey or brown and marked with spots and stripes.
Adults are mostly drab moths with bodies around 2-2.5 cm long. The hind wings, which are sometimes distinctly marked, are usually lighter in colour than the forewings.
The lesser yellow underwing moth is most noticeable by the bright orange hind wings that are edged in black.
Cutworm moths are active at night and rest during the day in sheltered locations.
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 developing buds and new shoots.
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 sizeable, sometimes ragged, holes and remove a significant 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 find refuge under loose bark of older vines.
Moths emerge in June and remain in a quiescent state throughout the summer. In fall, they become sexually mature and begin laying eggs on the surface of the soil that will hatch into the overwintering larvae. There is a single generation each year.
Unlike the Abagrotis species, the underwing moth lays eggs in clusters on the upper surfaces of leaves.
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. Damage is often higher in areas with dry gravelly or sandy soils that do not support good stands of broadleaf weeds, or when weeds are controlled too early in the spring before shoots have elongated.
The new invasive yellow underwing moth is an exception in that it prefers heavier soils. Maintaining a record of previous damage can help suggest which areas need to be monitored more closely and where damage is likely to occur first. However, bud damage needs to be assessed throughout the vineyard.
New vines planted in untreated land previously in pasture are prone to damage. Cutworm 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 limit 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. It is especially important to frequently monitor for damaged buds 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, method of pruning, presence of predators and parasitoids and tolerance level of the individual vineyard manager.
Suggested economic thresholds provided by 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.
For more information about chemical control, see the Pesticides page.
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 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 hundreds of 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.
Damage is generally much lower in vineyards with mixed ground cover vegetation and where broadleaf weeds — particularly winter annual mustards such as shepherd’s purse (Capsella bursa-pastoris) — are present in the vine rows and drive rows in spring. Where possible, broadleaf weeds growing in the vine rows should not be controlled in spring until shoots have elongated and the first leaves have expanded.
With the exception of lesser yellow underwing that prefer heavier soils, cutworm damage is 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, cultivation or establishment of a suitable cover crop the year prior to planting will help reduce damage to newly planted vines by cutworm, flea beetles, root weevils and June beetles.
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. However, there will be added costs related to removal of extra shoots. Most grape varieties produce fruit on secondary shoots, but yields will still be reduced and ripening can be delayed or uneven.
