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ISSUE 7   June 15, 2000

 

TREES - YELLOWHEADED 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.

Marcus Jackson
Extension Forester
mjackson@ndsuext.nodak.edu
http://www.ag.ndsu.nodak.edu/aginfo/trees/ndtreinf.htm


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