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Feeding injury to a spruce shelterbelt (375KB)

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Feeding injury to a spruce branch (280KB)

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Spruce sawfly larvae feeding on spruce (1,240KB)


Spruce Sawflies

There have been several reports of injury from yellowheaded spruce sawflies in north-central North Dakota this week. All native and introduced species of spruce grown in North Dakota are potential hosts to yellowheaded spruce sawflies. The sawflies are more common in northern North Dakota than in southern areas of the state. These insects are often discovered as late-instar larvae feeding on older needles, at which time much of the damage has been done and chemical control becomes difficult. Treatments are most effective if they are applied when the larvae are small.

Larvae have yellowish- to reddish-brown heads and olive-green bodies with six gray-green stripes running the length of the body. They will rear up in a characteristic "s" when disturbed. Larvae will reach a length of about 3/4 inch before they drop to the ground in July and spin a cocoon where they will overwinter. In early spring, tan to straw-yellow adults emerge, mate, and lay eggs in current year’s needles at approximately the same time as the young shoots lose their bud caps. The eggs hatch 4 to 12 days after they are deposited in the needles; therefore, we should be monitoring now for the insect in areas where it has been a problem in the past. Young larvae will begin feeding on new needles and will move to older needles as they mature. Open grown trees that are 5 to 9 years old (3 to 18 feet in height) are more vulnerable to yellowheaded spruce sawfly damage than are older trees or trees in dense stands.

Healthy trees will often survive minor needle loss from yellowheaded spruce sawflies, but complete defoliation can kill trees. The first year of defoliation should be taken as a forewarning to monitor and treat, when necessary, for the insect in subsequent years.

Although rodents will feed on the cocoons 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. Acephate (Orthene and Isotox), carbaryl (Sevin), chlorpyrifos (Dursban), and cyfluthrin (Tempo) are labeled for use against sawfly larvae. Read and follow pesticide labels. 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. This should be done after the larvae have finished feeding, but before the spruce buds begin to swell in the spring.

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