Know your crop pests: Cutworm

As moths, both sexes of the cutworm have pale brown to greyish forewings with black lines and irregular markings.

Know your crop pests: Cutworm
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Scientific name: Agrotis spp, Spodoptera littoralis.
Family: Noctuidae

Distribution: Widely distributed over the African continent.

As moths, both sexes of the cutworm have pale brown to greyish forewings with black lines and irregular markings. The hind wings are creamy- to snow-white. Body length is approximately 20mm, with a wingspan of 45mm. The cutworm itself is the caterpillar stage of the insect. Reaching a length of 40mm to 50mm, it varies in colour according to age. Young caterpillars are green, brown, grey, or yellow with longitudinal stripes; older caterpillars are much darker. They are nocturnal.

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Cutworms are considered the second-worst pest of maize. They get their name from their habit of chewing (cutting) away at the base of seedling stems, which eventually fall over. They generally attack plants during the early growing phase. While young caterpillars feed on the leaves and stems, mature caterpillars are the most destructive: they drag the felled plant back to their burrows to consume during the day.

In addition to eating the stem, buds, flowers and pods, cutworms may attack the seeds, ears and leaves.

Other susceptible crops include beans, cabbage, carrots, cauliflower, lettuce, peas, potatoes and tomatoes. Many of the caterpillars also attack roots and tubers by day. An infestation can wreak havoc within a short time.

Depending on the species, females lay their eggs singularly or in small clusters on the stems or leaves of the host plant. Initially creamy-white in colour, the eggs darken to almost black as they near hatching. A single female can lay more than 2 000 eggs during her life. Pupation occurs in the soil or in silken cocoons. The pupae are brownish-red. Just before the adults emerge, they turn black. In South Africa, the life cycle is completed in six to eight weeks.

Control measures
‘Felled’ seedlings are a sure sign that cutworms are present. A walk through the crop during the evening with a torch will confirm this.

Chemical: Sprinkling cutworm bait over the soil during the evening will reduce their numbers, as will spraying plants with a chlorpyrifos insecticide such as Lorsban.

Natural control: A number of natural predators prey on cutworms. Most are either parasitic wasps or flies – such as the Braconid and Ichneumon wasp (Enicaspilus spp) and Tachinid flies (Tachinidae), especially the cutworm fly Gonia bimaculata. Natural predators can have a success rate as high as 90%.

Other: Many adult caterpillars overwinter in the soil and pupate the following summer. Ploughing during winter exposes the caterpillars or pupae to predators such as birds, lizards, frogs and scorpions. Starvation can also be used. Seven to 10 days prior to planting, the soil should be ploughed and weed infestation reduced to a minimum. As many cutworms use weed growth to ‘migrate’ between cultivated and wild plants, the female moth will have nothing to lay its eggs on.

(Please note: this series is primarily a guide to identifying pests. The control methods discussed are merely suggestions. For help with area- or crop-specific measures, please consult your agricultural extension officer.)

Paul Donovan is a biologist based in Botswana, where he promotes the protection and use of biological control agents.