Cutworms: prevention & control

Cutworms can play a valuable part in your garden’s ecosystem but can equally be a pest. Learn all about cutworms and how to prevent them.

Cutworms can damage your precious plants’ foliage, roots and even your vegetable crops. Read on to find out how to identify cutworms and how to prevent cutworms from harming your plants.

Cutworms: what are they?

If you are wondering what cutworms are, they are the caterpillars of several different moth species in the Noctuidae family. Here in the United Kingdom, you are most likely to find the destructive cutworms of the turnip moth (Agrotis segetum). However, the cutworms of the garden dart moth (Euxoa nigricans) and large yellow underwing moth (Noctua pronuba) are also prevalent here.

Female moths lay large numbers of cutworm eggs on plants or the soil in early summer, which hatch either within a few weeks or the following year. Feeding on the foliage of nearby plants, these young caterpillars then mature into their third instar as cutworms, which then eat the stems or root systems underground. From October onwards, they then overwinter deep under the soil as larvae, before pupating the following year in early summer.

Although moths are important for pollination and as a source of food for other garden insects and animals, cutworms can cause immense damage to your garden and vegetable patch. Feeding on lower leaves and soil-level roots, cutworms can damage your vegetable plants, such as lettuce (Lactuca sativa), potatoes (Solanum tuberosum), spinach (Spinacia oleracea) and many more.

Identifying cutworms

Moths can be hard to identify as they can vary greatly in appearance, even within their own species. For example, the turnip moth’s forewings can differ from light grey to dark brown, and have a wingspan of around 30 to 40mm. However, you can identify them by their white hindwings and lightly fringed forewings. With two broods produced each year, you can spot the turnip moth in both early and late summer. Whereas the larger yellow underwing moth has a large wingspan of 40 to 50mm and is named after its black-bordered yellow hindwings. Often seen mid-flight from July until September, the large yellow underwing is widespread, especially in the south of the UK.

You can spot clusters of cutworm moth eggs during the summer months. Often laid in large numbers on dense plant material or the soil, the tiny round eggs start off white before turning brown. However, the cutworm eggs do not stick around long, as they tend to hatch within 1 to 3 weeks.

Like the adult moths, the cutworms also vary in appearance. Usually growing no more than 4cm long, they can be brown, grey or even green with dark dots on their back and have three prominent pairs of legs at the front. However, when startled they tend to curl up in defence. The caterpillars in their early stages feed on foliage, leaving behind holes in the leaves or around the margins. Having matured, the cutworms then feed under the soil, causing damage to root systems and crops.

After 2 to 3 months or the following spring, the caterpillars pupate. The cutworm pupae are red-brown, have a segmented and pointed tail end and can be found on top of or just below the soil.

Cutworm control

You can find cutworms in any garden. However, they tend to only cause significant plant or crop damage in the summer when they are old enough to go under the soil and feed on roots. At the beginning of summer, it is advisable to check if any plants, especially lettuces, have had their stems or roots eaten signifying that they are present. Being beneficial for biodiversity some damage can be tolerated, but if more than a few plants or crops are affected, you may want to use some of the following control measures:

• Hoe or dig over the soil to disturb the larvae and either remove by hand or leave for wildlife to find.
• Water young plants regularly from spring onwards as the young larvae are susceptible to heavy rain and watering.
• Keep growing areas well weeded and free of fallen plant debris to remove dense vegetation for eggs to be laid.
• Place homemade collars around plant stems to create a barrier against cutworms. A plastic pot with the bottom cut off can prove surprisingly effective.
• Grow plants under fleece or fine mesh to provide protection against cutworms.

There are no insecticides for cutworms available for the home gardener. However, you can also control cutworms in the garden by applying nematodes (Steinernema species) to the soil to kill the caterpillars. The nematodes work by entering the larvae and producing bacteria, which then kill the cutworms. These nematodes are also effective for controlling cabbage root fly (Delia radicum), onion fly (Delia antiqua), and gooseberry sawfly (Euura ribesii). What is more, they are completely safe for garden use and are target-specific and residue-less.

Cutworm nematode application steps:

• On purchase keep refrigerated as instructed
• Apply between June and September, or after planting but only when the soil temperature is 12 °C or above
• Mix as per instructions and use a watering can to apply the nematode solution as a root and soil drench
• Repeat applications every 2 weeks to ensure current and future infestations are managed

Tip: in Europe, the microorganism Bacillus thuringiensis is available as a biological insecticide against cutworms and box tree caterpillars (Cydalima perspectalis), which are now widespread in southern England. However, it is not currently available or approved for home use in the UK.

Cutworm prevention

There is no surefire way of preventing cutworms in your garden. However, creating a wildlife-friendly garden and practising companion planting and crop rotation can go a long way in helping prevent cutworms. Creating a natural habitat in your garden will help encourage cutworm predators, which include birds and hedgehogs (Erinaceus europaeus). Cutworms can also be put off by a damp soil, which you can promote by mulching annually to prevent moisture loss from the soil. Keeping an eye out for any caterpillar foliage damage in spring is also advisable; if spotted, implement the above control measures and apply cutworm nematodes.

Encouraging natural predators in your garden can help prevent all manner of pests. Learn more about beneficial creatures for your garden in our separate article.