Cutworm

Key Points

Many high-value crops are particularly susceptible to economic damage from Cutworms
Potato crops grown for seed should be monitored closely
Control is only possible at first and second instar stages
Irrigation can be used to reduce the risk
Dursban WG is approved for control of Cutworms in seed potatoes
Always apply in high water volumes

Introduction

Cutworm is the name given to caterpillars of certain Noctuid moths, in particular those of the turnip moth Agrotis segetum. The older caterpillars feed underground, damaging plant roots and stems (including the storage organs that we use for food), sometimes so badly that the plant topples — hence the name.

The risk of economic damage from Cutworm to different crops varies. Leeks, salad onions, red beet and leafy salads are highly susceptible, root vegetables such as carrots, parsnips and sugar beet moderately susceptible and crops such as swede and turnip only slightly susceptible. Potatoes grown for high quality markets, such as bakers, are classified as moderately susceptible.

Biology

The adult moths fly from late May to early July, laying eggs on plants or on pieces of litter and debris in the soil. Adult moths have a wingspan of around 40mm, with the forewings being whitish-brown and carrying brownish-black or black-edged markings. The hindwings are normally pearly-white. Eggs are very small, 0.5mm across, pale grey or cream in colour and distinctly ribbed. These hatch in around 8 – 24 days, depending on temperature, and then the young caterpillars seek out and feed on the aerial parts of plants.

Caterpillars have a plump greyish-brown body, with a light yellow or pink tinge and indistinct dark lines along the back. The head is normally yellowish-brown. After a further 10 – 20 days, again depending on temperature, the caterpillars go through their second moult, becoming third instar caterpillars. At this growth stage they can reach 35mm in length. It is at this point that they adopt the Cutworm habit, becoming subterranean and feeding on roots etc.

The risk of economic damage from third instar Cutworms to different crops varies considerably. Crops with differing degrees of susceptibility to economic damage are categorised as highly susceptible, moderately susceptible or slightly susceptible.

Highly Susceptible

Forestry nursery stock
Red beet
Leeks
Salad onions
Lettuce
Leafy salads
Young transplants

Moderately Susceptible

High quality potatoes
Carrots
Parsnips
Sugar beet
Celery
Market brassicae

Slightly Susceptible

Other potatoes
Swedes
Turnips
Bulb onions

The most susceptible crops will need treatment several days before moderately susceptible crops, which in turn will need treatment several days before the least susceptible crops.

Unhatched Cutworm eggs and the older, subterranean caterpillars are largely invulnerable to the effects of the weather and insecticides. The two early caterpillar instars differ, however. If there is substantial rainfall (defined as 10mm or more of rain falling in showers of moderate intensity) whilst these caterpillars are feeding above ground then this causes a high mortality. They are also vulnerable to insecticides whilst feeding on the foliage.

Cutworms are most common on weed-infested land, as adults are attracted to the cover and caterpillars have a ready source of food. Crops immediately following heavy weed cover are much more likely to be seriously damaged than those grown on land that is subject to good weed management. Because first and second instar Cutworms can not survive in wet conditions, the frequent irrigation of susceptible crops can reduce, or even prevent, the development of damaging infestations.

Product Choice

Treatment with an insecticide is not normally necessary unless damage is seen in susceptible crops, or a Cutworm Pestwatch bulletin has been issued recommending application of Dursban* WG. Timing of treatments is very important as control measures are much more effective when young Cutworms are feeding on the foliage or at the soil surface. Control with insecticides is not possible once third instar larvae are feeding below ground.

Adult moths are trapped in late May and early June and temperature and rainfall data may be used to forecast egg development, Cutworm survival and predict the need for treatment. Where necessary, Dursban WG should be applied at the rate of 1.2kg/ha in 600 to 1000 litres of water per hectare. Water volumes at the higher end of the range are preferable. Dursban WG can be used in seed potatoes, leaf brassicae and onions. However, do not spray seed potato crops under severe drought stress as damage may occur. The variety Desiree is particularly susceptible.