Cutworm

Pests of Field Crops in Southern Africa

CUTWORMS

(Lepidoptera: Noctuidae)

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Text Box: Cutworms are notorious world-wide for their habit of severing, or partly severing, growing plants at soil level. The most common species in Southern Africa is Agrotis segetum, but A. ipsilon sometimes causes severe damage, and A. longidentifiera and A. spinifera have also been recorded. The adults are inconspicuous moths with mottled greyish-brown forewings (span of about 45 mm) and pale creamy hindwings. They are nocturnal in habit and are often attracted to lights. The larvae, or cutworms themselves, are greasy-looking and greyish in colour, and grow to about 40 mm long. They readily curl up into a ring-shape when disturbed (see photos). The older larvae rest in the soil during the day, and only emerge at night to feed. Pupae are plump and reddish-brown at first, but darken before moth emergence. Cutworms must not be confused with whitegrubs, which are the fat, white, C-shaped scarabaeid beetle larvae often found in soils, and which never emerge above soil level to feed. TOP

Text Box: Cutworms are more or less indiscriminate feeders, causing damage to a variety of crops, mainly when these are young. Among the crops that commonly suffer are maize, tobacco, potatoes, young coffee, cotton plants and vines, and a number of vegetables such as onions, cabbages, tomatoes, beans, etc. Garden annuals may also be heavily attacked. TOP

Text Box: After hatching, and until they are in the third instar, cutworms feed above the soil surface. They consume tender foliage, particularly that which is close to or on the soil surface, and this damage is not usually very noticeable. Older larvae are able to attack woodier material and usually feed on young stems just above soil level. This habit of cutting of plants is the better known feeding habit, and is very wasteful as far more plant material is destroyed than the cutworms actually consume. Serious damage to fairly tall maize plants has been seen by the author. In this case, sturdy plants that were knee-high had had their bases bored into by the cutworms, many of which were found in situ, with the result that the plants exhibited the "dead heart" symptom. In potatoes and root vegetables, damage can take the form of unsightly holes in the subterranean tubers, which may allow the penetration of secondary fungi. TOP

Text Box: Cutworm moths fly at dusk, and eggs are laid in random clusters or singly on low-lying vegetation or clumps of soil. In general a female lays between 600 and 800 eggs which are whitish and about 0.5 mm in diameter. They hatch in a few days and the young cutworms feed on vegetation near the ground. After 10 to 14 days they avoid daylight and become nocturnal feeders, remaining in the soil during the daylight hours. The larval stage lasts four to six weeks and when fully fed, the cutworms rest in the soil for about a week before pupating about 40 mm below soil level. This stage lasts two to three weeks in the summer, before the moths emerge. It may take up to two months during the winter. Irrigated winter crops are particularly attractive to egg-laying moths as other green plant material is in short supply at this time, but in fact, any growing plants in a land will encourage egg-laying. Thus, the presence of green weed material prior to planting will play an important part in determining the size of the problem. TOP

Text Box: While cutworms have a subterranean habit, they are not nearly as difficult to control as other pests of the soil, because they spend a significant part of their time roaming exposed on the soil surface. Scouting for damaged plants early in the season will enable timely and effective control measures to be taken, and these usually take the form either of baits or of synthetic pyrethroids applied as a band over the rows or an overall spray. Included here are deltamethrin, cypermethrin, fluvalinate, flucythrin, lambda-cyhalothrin, and probably many others. They have a rapid knock-down, and work much more effectively if the soil is damp. The bait formulations also work well, but may be cumbersome to apply in a field situation. A commonly used home-made one is with maize-meal and sugar, to which either trichlorfon or endosulfan may be added. The bait must be moistened with water to a crumbly texture and applied to the field as late in the day as possible to avoid drying out. This bait also has the advantage of controlling crickets and surface beetles, but a disadvantage is that it may be attractive to small fowl and animals, and for this reason, less toxic chemicals are preferred. Ready-made formulations of quinalfos and sodium fluosilicate baits are also available in South Africa. For tobacco, in addition to some of the pyrethroids, applications of chlorpyrifos, acephate and some other organophosphates are also recommended. They are applied either as sprays over the ridges or as drenches cupped onto the plant bases. For vegetables, overall sprays of profenofos may be applied soon after seedling emergence. If an infested land can be left fallow and strictly weed-free for six to seven weeks prior to planting, cutworms would have little chance of surviving. This would ensure that the mature and most harmful cutworms are not present when the crop is in the susceptible germination stage. TOP

Cutworms attacking maize

Tomato seedling ‘cut’ by cutworm which is usually found in soil nearby.