ISSUE 5   June 7, 2007



This is the Prairie/Western tent caterpillar. It feeds on many hardwood trees - chokecherry, apple. There is one generation annually and it overwinters as fully developed embryos in eggs. Larvae hatch in early spring, usually coinciding with flushing of their host's foliage. The fully mature larvae (2 inches long) have black backs with a white mid-dorsal line and light blue sides. Larvae live in colonies in large silk tents in forks of trees. The protective webs are conspicuous and unsightly. After feeding, larvae spin silken cocoons in folded leaves, bark or litter, and adult moths emerge in mid-summer. Larvae feed outside of the web nest, which makes control with insecticides easy. Bt works well to control young larvae, while pyrethrins or synthetic insecticides are needed for older caterpillars.



On June 5th, the first signs of defoliation from the Yellowheaded spruce sawfly were observed on the currents year’s shoots of spruce in the southern half of the state. This is a damaging defoliating insect of spruce plantings in North Dakota. The insect is most damaging in the northern tier counties and counties of the north central region.

Description/Biology: Adult sawflies are not flies but stingless wasps. The insect is called a sawfly because the female's ovipositor has serrated teeth resembling a wood saw. Adults are reddish-brown in color and 1/3 inch long. There is only one generation per year. Sawflies overwinter as cocoons. In late May to mid June, adults emerge, mate and females begin to lay eggs. A single egg is deposited at the base of a needle. Eggs hatch in five to10 days. Larvae are 1/8 inch when they first hatch. Mature larvae are 3/4 inch long, and dark glossy green with a light lateral stripe and reddish brown head (see photo). Larvae feed for 30 to 40 days consuming the new foliage first and then the older needles. When mature, larvae drop to the ground and spin cocoons for overwintering.

Damage/Symptoms (see photo): Defoliation is caused by larval feeding. Heavily infested trees appear ragged, especially near the top, and can be completely stripped of foliage. Severe infestations over one to several years can kill trees directly or make trees susceptible to attack by other insects or adverse weather conditions.

Pest Management: Open grown trees that are five to nine years old (3 to 18 feet tall) are more vulnerable to yellowheaded spruce sawfly damage than are older trees or trees in dense stands. Although rodents will feed on the prepupae and birds on sawfly larvae and adults, these predators, in addition to various parasites, are not always effective in keeping yellowheaded spruce sawfly populations at acceptable levels. If infestations are light, adequate control may be achieved by simply removing young larvae by hand. When an isolated ornamental tree is infested, spraying young larvae off of the tree with a strong jet of water will often be effective in reducing insect numbers. Yellowheaded spruce sawflies tend to attack the same trees repeatedly; therefore, chemical control often becomes necessary as sawfly populations increase. Some of the insecticides labeled for sawfly control include: Acephate, cyfluthrin (Bayer Advanced Garden PowerForce), esfenvalerate (Bug-B-Gon), imidaclporid (Bayer Advanced Tree and Shrub) carbaryl (Sevin), and permethrin (Astro). Always read and follow label instructions! Since most yellowheaded spruce sawflies are believed to overwinter very near the soil surface, removing the duff beneath infested trees may reduce the impact of this insect.

Janet Knodel
Extension Entomologist

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