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ISSUE 6 June 5, 2003

YELLOW HEADED SPRUCE SAWFLY

If you have spruce showing partial, ragged-edged needle loss at the ends of branches, and/or if you are seeing straw-colored, ragged needles on last year’s growth on spruce, one possible cause may be Yellow Headed Spruce Sawfly, Pikonema alaskensis. This pest may also produce symptoms of top dieback on spruce, where the branches at the top of the tree are brown or bare. Some other possible causes of top dieback include drought or main stem cankers, and all three should be considered when examining a tree with top dieback symptoms.

Yellow-headed spruce sawfly was identified in some areas of ND in June of 2000 (www.ag.ndsu.nodak.edu/aginfo/trees/problems/pr061500.htm). All native and introduced spruce species are hosts to this pest. Larvae of the yellow-headed spruce sawfly first feed on new needles in the spring, leaving jagged-edged stubs that eventually turn brown. Older needles will be chewed only after the new growth is devoured. By July, trees may appear ragged and yellow-ish brown, especially near the tops.

The yellow-headed spruce sawfly emerge in the spring as adults just as the buds are beginning to swell. Females deposit eggs into the base of new needles. Within 2 weeks, usually the first or second week of June, larvae emerge and begin feeding on new needles. These larvae will be fully grown (about 20 mm) by mid-July, when they fall to the ground to overwinter in cocoons in the soil. There is only one generation each year. Young larvae are quite small with a yellow-ish body and a reddish-brown head. When disturbed, they arch both their head and rear in an s-shape. The mature larvae are dark green with lighter stripes down the sides, and still have a reddish-brown head.

Trees may withstand light defoliation associated with larval feeding but extensive defoliation or consecutive years of defoliation will kill trees. On isolated trees, it is possible to simply knock the larvae to the ground or blast them out with a stream of water, where they will be attacked by natural predators. Chemical insecticides are quite effective if the yellow-headed spruce sawfly is determined to be the cause of the symptoms. Acephate (Isotox and Orthene), carbaryl (Sevin), and cyfluthrin (Tempo) are all labeled for this pest. Yellow-headed spruce sawfly feeds in groups so treatment need only be done on clumps of feeding larvae. Since this pest is 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 swell in the spring.

Chemical treatment is not warranted unless the larvae of the yellow-headed spruce sawfly are identified on the tree. To try to confirm this insect pest, look for chewed and jagged needles still on the tree. It is important to note that the symptoms will also appear in July. If the tree in questions looked good all summer and only now show the browning symptoms, the cause is likely not the sawfly pest. Bare branches where the needles of the tree are completely gone, or brown needles that are intact and clinging to the branches are also not indications of yellow-headed spruce sawfly. This is more typical of drought or a canker, possibly Cytospora canker. We are trying to define areas of yellow-headed spruce sawfly infestation so please call or email the lab if you believe you that this is the case in your spruce trees.

Cheryl Biller
NDSU Plant Diagnostician
diaglab@ndsuext.nodak.edu


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