Agrotis ipsilon (soybean)
Authors: Buyung Hadi, Jeffrey Bradshaw, Jan Knodel, Ken Ostlie
The larvae of black cutworm (Agrotis ipsilon) range in size between 0.2-1.5 in. (6-38 mm) long. The color varies from greasy gray to black above the spiracles (small circular openings on the caterpillar's sides) with lighter shades on the underside. A broad light gray band runs down the middle of the back. Convex and shiny skin granules of various sizes appear conspicuously on the upper side of the body.
Life Cycle and Seasonal History
Adult black cutworms migrate to states such as Kansas, Iowa and other states further north from southern latitudes in early spring. These migrants deposit eggs singly or in clusters on grasses, broadleaf weeds and crop residues before soybean are planted. Female migrants seem to prefer dense low-lying weeds such as curly dock and yellow rocket to lay their eggs. Thus, agronomic practices and field characteristics that allow for crop residues or weed growth on a particular field in early spring may increase the risk of black cutworm infestation in the subsequent crop.
Black cutworm larva develop through six to nine instars depending on temperature and diet. The larvae rest underground during the day and feed on plant parts at night. Last instar larvae dig deep underground and pupate in earthen cells. Adult moths crawl up to the surface through the tunnel made by the last instar larvae.
Pupal developmental temperature is thought to trigger long range migration behavior of the resulting adult moths. It is hypothesized that most black cutworm pupae have a range of temperatures (around 32 to 96.8°F or 0 to 36°C) within which the resulting adults will stay within a region. However, if the pupae are exposed to higher temperatures, for an extended period of time, (as in early spring in the southern latitudes) the adults will enter nocturnal low-level jet winds that blow south to north; thereby migrating to the northern states. On the other hand, if the pupae are exposed to temperatures lower than this range for a period of time, as expected during fall in the northern latitudes, adults actively enter the airflow related to cold fronts that move southward; thereby initiating a north to south migration.
Plant Injury and Damage
On soybean the black cutworm is considered an unusual pest with the damage more pronounced in years with heavy cutworm infestation on corn.
The larva feeds on plant parts immediately above ground surface, moving from weeds to soybean when the weed hosts are no longer available. Most of the plant is not consumed, yet enough stem tissue may be eaten to cause the whole plant to fall or wilt. The last instar larva was reported to produce significantly higher damage on corn than any other instar.
In situations where the soil is dry and crusted, black cutworm larvae of advanced instar are more likely to feed on the stalk below ground, causing plant wilting. Wilted plants due to this injury may remain standing yet easily pulled. When pulled, this plants showed cutting on the stem just below the soil surface.
As soybean matures, the damage caused by black cutworm larval feeding is subsequently reduced due to the increase in plant robustness. Thus although there are more than one generation per year, only the first generation cause significant damage on soybean since the emergence of the second larvae usually happen when soybean plants are already in advance developmental stages.
Scouting and Threshold
Black cutworm larvae should be scouted once or twice a week beginning with plant emergence. Observe plants at several locations in each field for evidence of leaf feeding or stem cutting. Pay special attention to poorly drained areas, areas where weeds were present before planting and areas with soybean stubble. If cut plants were found, examine the soil around the plants for cutworms. Cutworms maybe found on the ground, below the surface or under debris during the day. Take note of the size and species of cutworms found. If it is determined that a given field has black cutworm problem, it may help to monitor the same set of plant during the scouting period. To do this, mark off 100 plants in a row with stakes or flags. Do this at several locations across the field, and continue to monitor these marked plants for injuries. Monitoring the same set of plants a more reliable picture of black cutworm activity can be developed.
Adult pheromone traps can be used to estimate the timing critical scouting phase of larvae later in the season. Commercial pheromone traps are available in the market. The timing to setup pheromone trap varies with regions, but generally it falls between March and April. The number of nightly capture of adult moth of black cutworm can be recorded and the time when significant capture occur can be pinpointed. A significant capture is defined as eight or more moths caught per trap over two nights. The timing of significant capture of black cutworm adult moths is used as the starting point to count degree days to estimate the time of critical larval scouting. When 300 degree days (using the base temperature 50°F or 10°C) have accumulated after this date, fourth instar larvae are expected to be present. This is the first stage that is capable of cutting corn stems. At this time more intensive sampling for black cutworms larvae is warranted. This method cannot predict the severity of cutworm related damage. This must be determined by field scouting. The absence of significant black cutworm activity does not mean that field scouting is not necessary since there are other cutworm species that can cause damage.
Soybean can tolerate substantial stand reduction without yield loss, thus management action is only required in very large cutworms populations.
Weedy or residue covered fields may attract oviposition of mated female adult moths in early spring. Consequently, later in the season high number of black cutworms may emerge. Planting in fields with this characteristics may expose the crops to high risk of cutworm infestation and, when possible, should be avoided. Tillage and herbicide application just before planting do not always help since the eggs would have already been laid. A delay of seven days or more in planting after seedbed preparation has been recommended in states with routine cutworm problems. The delay period is intended to give time for the cutworm larvae to emerge, starve and die due to the absence of food source.
Opportunistic natural enemies help reduce black cutworm population in the field. These enemies include birds, ground beetles, small insect-feeding mamals, caterpillar-feeding insects and ground-dwelling spiders. Endemic soil-predator complex has been shown to be factor that regulates black cutworm population in the field.
Neonicotinoid-based insecticides applied as seed treatment (e.g. clothianidin and thiamethoxam) were reported to reduce feeding damage caused by black cutworm, albeit erratically. Insecticide application as seed treatment requires growers to make decisions before confirming the actual black cutworm infestation in the field. In areas where black cutworm problems are sporadic, confirmation of cutworm infestation in economically damaging level is recommended. Such confirmation can only be made through field scouting. In situations where economic threshold are attained, post-emergence rescue treatment can be carried out.
Applying treatment in the evening increases the chance of exposure since the black cutworm larvae are active at nights. Under dry soil conditions, a light surface tillage such as rotary hoeing may help move some insecticide into the soil zone. Pyrethroid insecticides should not be incorporated into the soil.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com