- Scientific Name: Nephelodes minians Guenee [Phylum Arthropoda: Class Insecta: Order Lepidoptera: Family Noctuidae]
- Common Names: bronzed cutworm
- Climatic Zone: temperate
- Geographic Distribution: northern North America
- Damaging Stage: larva
- Hosts: Virtually all grasses, including those used for pastures.
- Damage Symptoms: This cutworm occasionally builds up large populations that may feed under the cover of snow on the leaf blades of grasses. When the snow melt, all that remains is the thatch surface with a few green stems. Lesser populations cause general thinning of the turf in the spring until mid-June.
- Description of Stages: This pest has stages typical of noctuids (cutworms and armyworms) with thick-bodied caterpillars.
Eggs: The eggs are light green, round, about 3/32-inch (0.5mm) in diameter, flat on the lower surface, bluntly pointed at the top and with fine sculpturing ridges on the surface.
Larvae: Larvae have five pale stripes down the bodies with three on the pronotal shield (just behind the head). The first three instars are green, but the later instars take on a deep chocolate-brown color, often with a bronze sheen. Mature larvae are often 1 3/8 to 1 3/4 inch (35 to 45mm) long and 3/8 inch (4.5mm) wide.
Pupae: Pupae are typically dark brown and elongate oval with a pointed abdominal tip. They are about 3/4 inch (20 mm) long.
Adults: The adult moths range from a light bronze-brown to a dark chocolate-brown. The wings are mottled with few distinctive features. Wing span ranges from 1 1/8 to 1 1/2 inch (30 to 38mm).
- Life Cycle and Habits: Adult moths emerge from pupae that were formed in the soil below the turf from late August through September. The moths fly during warm nights and are commonly attracted to lights. Females attach small clusters of eggs to grass blades. About half of these eggs hatch in about two weeks but other eggs remain dormant until spring conditions. Larvae that hatch in the fall will feed on grass blades until the soil freezes. Under the cover of snow, these larvae are often able to continue feeding. The first instar larvae are green and first feed within the fold of a leaf blade, but soon they move into the thatch. The first three instars remain green and feed primarily on grass blades. The larger larvae hide during the day in the thatch but feed on grass leaf blades and stems at night. Overwintered larvae tend to finish their development and pupate within the soil under the turf by early June. Larvae that hatch in April may not finish their development until mid to late June. The pupae remain dormant for two to three months below the thatch.
- Control Approaches:
Bronze cutworms are usually controlled by applying one of the contact or stomach insecticides. Since this species is a "cool-season" caterpillar, few biological controls have been discovered or developed. Turf managers often mistake early spring bronze cutworm damage with winter kill or snow mold. To ensure that you are dealing with a bronze cutworm infestation, use a soap disclosing solution to determine actual populations.
Cultural Control - Use Resistant Turfgrasses - Perennial ryegrasses and fescues that contain endophytes are known to slow development or are toxic to most turf-infesting caterpillars except for black cutworm. There are no data evaluating endophytic turf against bronze cutworms.
Biological Control - Use BT - Products containing the toxin derived from the bacterium, Bacillus thruingiensis (BT), are commercially available and some of the newer strains should affect bronze cutworms if enough material is ingested by young larvae. Unfortunately, most bronze cutworm damage is not discovered until after the larvae have nearly matured.
Chemical Control - Insecticides - Since bronze cutworms tend to feed at or near ground level, insecticides should probably be irrigated lightly after application to move the residue into the stem and thatch zone.
Curative Treatments. Numerous contact and stomach insecticides in various chemical categories effectively control cutworms in turf after the caterpillars have been detected and are still feeding.
Preventive Treatments. Where bronze cutworm is a regular problem, applications of insecticides in late October into early November may be effective in eliminating the new generation of small larvae that will feed under the cover of snow during the winter months.
Regular monitoring of turf areas for evidence of cutworm infestation and applying treatment only when larvae are present and/or damage seems eminent, is part of the control process. Monitoring includes looking for larvae, damage and/or evidence of birds (starlings) probing the turf for larvae. To determine if larvae are actually present, an effective monitoring method is to use a solution of liquid soap and water (two tablespoons of liquid Joy® dishwashing detergent in two gallons of water) spread over a one square yard area to flush larvae to the surface.
Determination of whether treatment is needed depends upon the type of turf area concerned and the standard of quality desired or required. For golf courses, damage in roughs is more tolerable than damage in fairways, and damage on tees and greens may not be tolerated at all. Many books and extension publications on turf insect management often mention “population thresholds.” Thresholds are the numbers of pest insects present in a given area (usually per square foot, yard, or meter) that warrant control. The concept behind using thresholds is to reinforce the IPM principle that application of a pesticide is not always warranted simply if pests are present. Pests must be present at populations high enough to eventually cause damage. In high cut turf, 5 to 10 cutworms or armyworms per yd2 may just begin to show damage.