Q What are cutworms?
A Soil-dwelling nocturnal caterpillars of several moths, which eat plant roots in summer. The turnip moth (Agrotis segetum) is the most common.
Many other species such as the garden dart moth (Euxoa nigricans), white line moth (Euxoa tritici) and large yellow underwing moth (Noctua pronuba), are also involved.
Caption: Cutworms damage plants by eating roots in summer
Q How do I recognise cutworm damage?
A The worst damage is done to young plants with tap roots. Lettuce suffers especially badly. The cutworms eat the stem at or just below the soil surface. The plant eventually collapses, but by then the cutworm is attacking another plant.
This can happen suddenly, but early warning signs are wilting and stunted growth. In fact they often work along rows, killing one plant after another.
Q What do the cutworms look like?
A The adults are small brown moths with brown front wings and white to yellow hind wings. The caterpillars are dull brown or green with darker lines and dots on their backs.
They are hard to spot in soil. They are 35-50mm long and have three pairs of true legs at the front followed by five pairs of abdominal legs. Other soil pests like compost-fly larvae do not possess these.
Q When should I expect trouble from cutworms?
A The moths emerge from late spring to August. They lay their eggs on stems and leaves of weeds and cultivated plants.
The dart moth's eggs don't hatch until the following May or June, but the eggs of the other moths hatch within a few weeks, depending on the weather. The resulting caterpillars immediately start feeding on roots and stems, but are not ready to pupate until the autumn.
The damage occurs when they get larger, in the summer, and not in the spring when they are too small to do serious harm.
Rain and irrigation harm the turnip-moth caterpillars, but the other kinds are less affected. Wet weather in June and July leads to the death of many caterpillars. Irrigation at these times will have a similar effect. They spend the winter as chrysalids in the soil.
Q What other pests could I confuse cutworms with?
A Chafer-beetle grubs will damage the stem and roots in a similar way – you will find this swollen comma-shaped white grub with a dark head and legs in the soil.
Wireworms can cause plant collapse: look for thin, tube-like golden brown grubs up to 5cm long in the soil beneath the plant.
Slugs will do similar damage to potato roots and seedlings, but they leave a tell-tale slime trail and are more prevalent in wet weather, while cutworms are more common in warm, dry weather. Slugs are worst in moist, heavy soils, and cutworms thrive in lighter, dry soils.
Q Are there varieties that are resistant to cutworms?
A We don't know of any resistant varieties.
Q Can cutworm damage be avoided?
A Weedy soil supports more cutworms than cultivated soil. So if you let weeds grow on ground before clearing and planting it, your new plants are more likely to be attacked by cutworms. If your garden plants regularly suffer from cutworm damage, it’s worth making a special effort to keep weeds down with regular hoeing or applying a weed-suppressing mulch. On large areas, black polythene or pieces of carpet will save you money. Plants grown under insect-proof mesh to exclude other commoner pests (eg cabbage-root fly or lettuce-root aphid), will be protected from cutworms too. Costly mesh is seldom worthwhile for cutworm protection alone.
Q Can chemicals be used to control cutworms?
A A thorough application of a contact insecticide containing pyrethrin every 14 days in June and July may control young caterpillars. However, as the cutworms get older and live deeper in the soil, insecticides become less effective. Also, it is hard to know when to spray, so you could miss the vulnerable stage. Unless your site seems prone to regular attacks, it is probably best to trust to frequent watering to keep the caterpillars under control. If attacks develop, search out and destroy the caterpillars by hand
in the soil around damaged plants.
Q What organic controls for cutworms are there?
A Frequent watering of beetroot, lettuce and other plants will make attacks much less likely. Some biological controls recommended for other soil pests are said to control cutworms, but we have yet to assess their effectiveness.