Two major Cutworm species, the western and the army cutworm, are serious but sporadic pests for Montana producers. Outbreaks can occur when cutworm populations are high and weather conditions are favorable for survival. Outbreaks do not necessarily occur in successive years.
Larvae of either species generally are not active above ground on bright, sunny days. They can be found just below ground by scraping the soil surface lightly. Small larvae are well-camouflaged and can be difficult to see in the soil. Larval densities can be assessed by digging and screening the soil from one foot of row or a square foot at different sites in the field. Soil samples should be dug to a depth of at least 3 inches. Soil can either be sieved through a fine mesh screen or placed on a piece of white paper; the white background can aid in detecting cutworm larvae. However, soil sieving is both labor and time intensive and has limited practicality as a routine sampling method. Examining plants for signs of cutworm larval activity and for cumulative larval damage is a more practical approach to assessing larval populations. Damage by cutworms can be distinguished from winter kill by looking carefully for evidence of feeding damage. Canola and pulses are more susceptible to cutworm damage than small grains because cut plants do not grow back. If a foliar insecticide treatment is warranted, an evening application is best since cutworms actively feed at night.
Small Grains thresholds for Montana: If plants are healthy and vigorous, insecticide treatment is justified when there are four to five army cutworm larvae per square foot. Treatment is justified for pale western cutworms when two to three larvae per foot of row are present. Prior to making a decision to treat a field it is important to determine the size of the larvae. Large larvae indicate
completion of the feeding or damaging stage of cutworms.
Canola thresholds for Montana: Treatment can be justified at 0.3 to 0.5 cutworm per square foot.
Peas / Lentils – 2 to 3 cutworms per square meter (North Dakota State University)
Information for this article came from Montguide MT200005AG.
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