ISSUE 12 July 19, 2001
WHEAT MIDGE ACTIVITY CONTINUES . . . BUT SLOWING
Wheat midge is still active in the north and northwest areas of the state. See the report from Around the State from Jan Knodel for more detail.
CATERPILLARS CONTINUE IN THE SPOTLIGHT
Numerous caterpillars continue to be found in the region’s crops, particularly soybean, sunflower, canola, and alfalfa. Thistle caterpillar are beginning their next generation. In addition, the alfalfa caterpillar, alfalfa webworm, velvetbean caterpillar, and imported cabbageworm are being found in various crops in the region. Descriptions, crops where present, and management information follow.
For aid in identification, several good circulars are available through the internet with full color photos. They can be found under alfalfa and soybean information at:
Thistle caterpillar - new larvae, ¼ to ½ inches long, are being found in soybean and sunflower fields, indicating the start of the next generation. The painted lady butterflies are very abundant in areas where larvae were found earlier. The butterflies can be seen feeding on thistle, milkweed and other flowers during the day.
Alfalfa caterpillar - Descriptions of larvae found in southern ND counties indicate that some of these larvae are being found in soybean fields, as well as in alfalfa, where we are more likely to find them in most seasons. This larva is a grass-green color with a fine white stripe on each side of the body. The white stripe has a very fine red line running through it. When full grown, the larvae is 1½ inches long. Larvae feed for 2+ weeks. The adults of the alfalfa caterpillar are the familiar, yellow butterflies that can be seen flying over alfalfa fields, particularly in August. In alfalfa, the caterpillars are often controlled adequately through timely cutting, naturally occurring diseases, and several parasitic wasps; thresholds for treatment are relatively high at 10 larvae per sweep. In soybeans, this caterpillar should be considered along with others when assessing whether treatment thresholds of 20% defoliation will be reached. This defoliation level could occur when 4 to 8 larvae per row foot is found.
Alfalfa webworm - There have been populations of webworm found in western and south central ND. They have been reported in alfalfa, clover, soybean and other legume fields. They can be found feeding on a wide variety of plants, but are not reported to feed on small grains or grasses. Infestations are characterized by light webbing over the leaves. Beneath the web is where the larvae feed, consuming the leaves. These larvae are 1inch when full grown. They are greenish to nearly black with a light stripe that runs down the middle of the back. There are three dark spots, each with hairs, on the side of each segment. These larvae feed for about 3+ weeks.
Large numbers of the webworm moths were observed in western ND. The webworm moth is 1 to 1¼ inches in length, buff-colored with irregular markings of light grey to dark gray. When walking through infested fields, the moths are disturbed, causing them to fly up and move short distances, landing again nearby.
In alfalfa, cutting disrupts the population. Larvae are exposed to bird predators as they move from drying plants. Small plants, especially seedlings are most vulnerable. Some reports from the west indicate the biggest problems were in clover or other newly seeded legumes. Alfalfa has a number of insecticide options such as carbaryl, Baythroid, methoxychlor, methyl parathion, permethrin (Ambush or Pounce), and Warrior. Insecticide options in clover and other legumes are limited to carbaryl (Sevin and other formulations), methoxychlor, and methyl parathion.
Velvetbean caterpillar - These larvae have been found in soybeans in southern counties. They will also occur in dry edible beans. Normally, numbers are low, but with the unusual year favoring caterpillars, they are being found with greater frequency. The larvae are 1 3/4 inches when full grown. They feed for 3+ weeks before reaching this length. They are yellow-green to brown with alternate dark and yellow-white stripes running the length of the body. When observed during the day, they are seen lying flat against the leaf or stem where they are resting. When disturbed they will wriggle around to escape.
These larvae should be considered along with others found feeding in soybeans when making defoliation and caterpillars per row foot assessments.
Imported cabbageworm - This caterpillar is feeding in canola and other mustard plants, including our gardens with cabbage. The caterpillar, 1¼ inch when full grown, is green with a velvety appearance. There is a light yellow stripe down their back. The adult butterfly is similar to the alfalfa caterpillar, only they are white in color.
The caterpillars chew irregular shaped holes in leaves. They are found on the leaves during the daytime. Due to their color, they are well camouflaged. They have only affected foliage, not flowers or pods, and are not considered a pest of canola.
CANOLA CATERPILLAR PESTS TO WATCH IN NORTH CENTRAL ND
Bertha armyworm and diamondback moth will become the focus of canola scouting in the north central counties. Trap captures have indicated that some areas in western Bottineau County have reached cumulative trap captures for Bertha armyworm to place fields in the uncertain to moderate risk category. Refer to Crop and Pest Report, Issue 8, June 21, for a summary on Bertha armyworm management decisions.
Traps in Mountrail County have had high enough captures of diamondback moth to place canola fields at risk to that caterpillar. With recent showers, diamondback larval risk may decline. Larvae are susceptible to many diseases under wet or more humid conditions.
CORN BORER LARVAE BEING FOUND
It is time to intensify scouting corn for making corn borer decisions. Reports continue to come in of the earliest hatching larvae now feeding and causing shot holing in leaves. Moth emergence has peaked throughout the major corn producing areas of ND. Scouting during the next week should provide information on the need to treat or not.