Agronomists in Montana have warned farmers to be on the lookout for cutworms this spring, and Jim Broatch, pest management specialist with Alberta Agriculture, says that should serve as a warning for Alberta.
Montana State University extension entomologist Kevin Wanner has already received reports of cutworm damage in winter wheat fields. During fall 2008, MSU researchers, using pheromone traps to monitor the pest, found larger numbers of adult army and pale western cutworms than usual. Wanner says that in some cases populations were as much as double or even quadruple the numbers found in fall 2007. Such high populations plus the right kind of weather conditions can trigger an outbreak.
MSU researchers think the long dry fall in Montana has allowed the army cutworm larvae, which hatch in the fall, to enter spring more developed than usual. (Pale western cutworms, on the other hand, lay their eggs in the fall and emerge in the spring).
A wet spring will knock back both populations. According to data compiled by researchers with Alberta Agriculture and Rural Development, who have been monitoring cutworm populations in southern Alberta since 1978, an outbreak year usually comes after a year with an unusually dry July and a wet autumn.
SCOUT YOUR FIELDS
There are several kinds of cutworms that attack a wide variety of crops including cereals, oilseeds, and forage crops. To avoid cutworm damage, Broatch encourages growers in southern Alberta to be aware of the heightened concern and to scout their fields for symptoms and damage early on in the season. While army cutworms tend to feed on the leaves, pale western cutworms feed on the crown and stem. The first signs of damage that growers will notice are holes and semi-circular notches around the edges of the leaves. Producers may also find seedlings, and eventually larger plants, that have been severed just at or below the level of the soil. When conditions are dry, growers may mistakenly attribute the wilted or dead plants to the lack of moisture.
To scout for cutworms, growers can look in the soil on the edges of the damaged areas. Cutworms are nocturnal so during the day they are found 5-7 cm under the soil and you must lightly scrape the soil surface in order to find them. Then either count the number of larvae found per 30 cm of row to identify the threshold or count the number of larvae found in a 50 cm by 50 cm area and multiply that number by four in order to identify the concentration per square metre. To get a picture of an entire field, count the larvae in several different spots.
WHEN TO SPRAY
Broatch says there are different economic thresholds for spraying, depending on the crop and its condition. Weedy fields tend to suffer less damage than clean fields. Crops with sufficient moisture usually have a higher threshold. Older crops usually suffer less damage than younger crops. In fact, crops that come up after the cutworm has moved on to the pupa stage usually escape any damage at all. This happens if the cutworms are well advanced by the time the crops are put in the ground, for instance, if the crops are seeded later in the spring due to wet weather.
In cereals or corn, AARD recommends that army cutworm be treated if there are one to two larvae per 30-cm row when the plants are less than 10 cm tall. If the plants are 12-15 cm tall and have adequate moisture, the threshold is four larvae per 30-cm row.
For pale western or redbacked cutworm in cereals or corn, AARD identifies the economic thresholds for treatment at three to four larvae/m2. However, “Wellestablished fall-seeded crops or spring-seeded crops with good moisture conditions can tolerate higher numbers.” Canola tends to be more susceptible than cereals or alfalfa. In oilseeds, whose seedlings have a much harder time bouncing back if damaged, crops should be treated if there are less than 5/m2.
There are several registered pesticides to choose from to deal with cutworm damage including Matador, Decis, Pounce, and Lorsban.. AARD’s online insecticide selector is also useful in decision-making. It can be found at http://info.agric.gov.ab.ca/app23/pesticides/insecticides/getcalcproduct.jsp
Because cutworms emerge at night, sprays must be applied late afternoon or early evening. During the day, cutworms are under the soil so chemical controls will not be effective. Growers may find it more cost-effective to restrict spraying to the most heavily damaged patches rather than the whole field. If army cutworms are marching, AARD describes another control method, which involves ploughing a steep trench in their path and lining it with a plastic sheet. Cutworms fall into the trench, are unable to emerge, and may be easily and economically sprayed.