Insect Management

Cutworm, Variagated

CUTWORMS ON SWEET CORN SEEDLINGS

    Some growers are reporting serious cutworm damage in sweet corn that is at the seedling stage now. Cutworms can also damage in lettuce, brassicas. peppers, tomatoes, eggplants and other seedlings. On top of crops lost to flooding and soggy soils, this loss is  especially discouraging.  Corn is especially susceptible at the one to two leaf stage; by four to five leaves, cutworm feeding is not a problem.

    Damage and identification. Feeding takes place at night.  There may be some leaf feeding, but most often the stems are clipped. There are many species of cutworms that attack vegetables including black, variegated, dingy, spotted, dark-sided, or glassy cutworm.  Spotted and variegated cutworms climb and feed above-ground, and black cutworms climb when they are young (less than half an inch long).  Cutworms can be difficult to identify.  Variegated cutworm has some pale yellow markings on its back, while black cutworm is nearly uniform gray to black with a greasy, rough appearance. (See Pest ID Supplement for photos). The cutworms we took from an infested sweet corn field this week were black cutworms, one of the most common in this area. 

    Adults of all cutworms are moths with dark gray forewings, often with various lighter or darker markings, and lighter hindwings.  They are in the same group of moths (noctuids) as the corn earworm, fall armyworm, and cabbage looper, but with very different life cycles and habits. Usually we don’t see them since they fly at night.

    Moths are capable of migrating long distances. Black cutworm moths are reported to overwinter no farther north than Tennessee but migrate north very early in the season (March, April) and lay eggs.  Thus, we see damage from their offspring in June.

    Some cutworms spend the winter in the larval stage as deep as 5 inches (12 cm) down in the soil and they may be  present in the soil at planting time, ready to feed on early spring transplants and seedlings.  Other cutworms winter in the pupal stage.  Adults from these emerge in May or June. Moths emerge from pupae of spring-feeding larvae later in the season.  There may be one to two generations per year.

     Where are cutworms a problem? Cutworms occur where moths chose to lay eggs and where conditions are good for survival. Females lay eggs on grass leaves, weeds, or the crop residude.  Attractive habitats include weedy or grassy areas, and alfalfa. Corn and soybeans are among the least attractive egg laying sites.  Black cutworm moths are reported to select low spots in the field that has been waterlogged or flooded. We have had plenty of those type of areas this year!  Eggs are also concentrated on low-lying weeds such as chickweed, curly dock, shepherd’s purse, peppergrass, mustards such as yellow rocket, or plant residue from the previous year’s crop.  Note that the above list includes lots of winter annual weeds that grow in the fall – weeds that we often overlook. Larvae feed on weeds – and destruction of weeds just before planting can make the crop more vulnerable, since all other food has been just taken away from the existing population of larvae.

    Monitoring.  Pheromone traps can be used to monitor the adult flight. To sample cutworms, scout fields when seedlings are young to look for damage. Sample 50-100 plants in groups of 10 or 20. Larvae burrow in the soil and are difficult to find, though searching in the soil near a clipped stem usually turns up a cutworm.  Estimate % stand loss to determine need for a spray.  Damage may be more concentrated around field edges or in low areas.

    Cultural Management.  Weedy land harbors the most cutworms, as the adult moths seem to prefer dense plant cover for egg laying.  Crop residues may also attract higher populations.  Therefore, crops that follow weedy crops, alfalfa, or no-till crops are more likely to be damaged by cutworms.  Plant early transplants into fields that had low weed pressure the previous year, especially in the fall, or where crop residue was tilled under in the fall.  There seems to be little information about specific relationships between what cover crops are planted in the fall, and cutworm infestations in the following year.

    Plow fields in spring and keep weed free for at least two weeks before planting to starve young larvae and reduce egg-laying.  Avoid planting susceptible crops close to sod, alfalfa or fallow areas.  Summer plowing disturbs eggs and larvae and raises them to the soil surface where they are more vulnerable to predation and desiccation.  Fall plowing will do the same.  Plan rotations to avoid planting vulnerable crops after a grassy sod, and plow sod fields in later summer or early fall.  Cultivate frequently to injure and expose hiding cutworms to predators. 

    Chemical Management.  Post planting rescue treatments for corn include several pyrethroids, carbamates, and one organosphosphate (Lorsban).  Direct sprays over the row and toward the base of the plant.  For best results apply in the evening just prior to active feeding.  Consult the 2006-2007 New England Vegetable Management Guide for more details (www.nevegetable.org or in hard copy; call 413-545-2717 to order) and for products labeled for each crop group.

    Organic options.  Insecticide baits made of a concentrated solution of an allowed insecticide, mixed with bran and a bit of molasses have been reported to work. The bait can be sprinkled on the ground near the crops or made into patties that are placed along the rows.  Bt sprays (ie, Bt aizawi or kurstaki directed at plant stems and foliage) work sometimes, but not consistently, possibly because large cutworms do not ingest a big enough dose of BT to stop feeding and die before they have caused damage. Collars around each seedling work on a garden scale but are impractical for field scale crop systems. Daily search and destroy missions -- by hand -- are a time-honored “biological” management for small plantings!

    Biological management.  Probably the most promising biocontrol organisms are beneficial nematodaes, which are available commercially and can attack soil insects such as cutworms.  It is important to use the correct species.  Two species of  nematodes, Steinernema carpocapsae (Sc), or Heterorhabditis bacteriophora (Hb) are often used in combination because they attack insects in different levels sof the soil. This works well for cutworms that move up and down. The nematodes are shipped to you on a sponge. You mix them with water and apply to the soil.  Follow the instructions that come with them. They can reduce cutworms and last for anywhere from 8 days to several weeks, according to various research trials.  The nematodes reproduce in the soil and, if the conditions are good for them, large populations of the nematodes will build up and these will hold the population of cutworms down.  Good soil moisture favors survival.  Repeat applications may be needed.

--Ruth Hazzard (with thanks to the following sources: Capinera, Handbook of Vegetable Pests; Vern Grubinger (UVM), Brian Caldwell (NYS-NOFA),Eric Sideman(MOFGA), Rex Dufour (ATTRA)) Updated June 2006)

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