May 19, 2005 at 8:59 a.m.
The spruce sawfly can be found from Maine to Alaska in northern United States and throughout southern Canada. It feeds on many species of spruce and is usually more of a problem on open-grown trees, such as those grown as ornamental or in a shelterbelt.
Young trees suffer more severe attacks than those that are more than 15 feet tall, and sawflies rarely attack natural forest stands. In Minnesota, the flies feed on white, black, Colorado, and Norway spruce. Of these, white spruce trees usually experience the heaviest needle-loss.
The yellow-headed spruce sawfly overwinters as a full-grown larva in a tough, papery dark brown cocoon. The larvae pupate in the spring and wasp-like adults emerge in May, about the time the lilacs are in full bloom.
After mating, the adult female uses its sharp-toothed ovipositor which is on the end of the abdomen. They cut a slit in plant tissue where they deposit their eggs. This saw-like appendage gives the insect groups its name. Within a week or two, the larvae hatch. They are olive-green with gray-green longitudinal stripes along their back and sides. When young, their heads are yellow, but become reddish-brown when mature and are about 18 mm long.
The larvae feed for about a month and are full-grown by July. Then they drop to the ground to spin a cocoon and overwinter in the needle duff or top layer of soil. There is one generation per year of this pest in Minnesota.
Damage ranges from slight to complete defoliation. Young larvae feed in large groups on the edges of new spruce needles. Older larval stages devour both new and old needles, often leaving only a short brown stub. Damage is usually first observed in the topmost growth of the tree.
During and after feeding, the ground under trees is littered with partly digested needles and insect droppings. Once defoliated, spruce trees will not recover.
If caught before damage is extensive, usually in early to mid-June, spruce sawflies can be controlled with Insecticidal soap, permethrin, imidocloprid, or carbaryl. In most cases spraying should start when larvae begin to feed about 10 days after bud caps fall off. As with any pesticide, be sure to follow label instructions. Do not treat if the larvae are nearly full grown. You may also control smaller populations by hand picking.
At the beginning of June, start inspecting trees for telltale feeding injury and small larvae. Trees suffering from drought stress or recent transplanting may be more vulnerable to sawfly damage. Much of this article is taken from research developed by Katharine Wildin who is a plant pathologist. You can view a picture of the larvae at: http://www.agr.gc.ca/ pfra/shelterbelt/pest/pest17.htm.
Now is the time to:
•Cool season vegetables can be planted now––peas, carrots, beets, other root crops, salad greens such as lettuce and spinach and cole crops, including Chinese cabbage and Chinese celery. Hold off on peppers, tomatoes, eggplant, corn, beans and vining crops until the weather is consistently warmer. Cool weather will actually set back greenhouse transplants of these heat-loving plants. They don't do well when their roots are stuck in cool soil. If you want to speed things up, put down some black plastic to warm the soil for a few days before planting.
•Check for asparagus beetles as asparagus spears start to emerge.
•When forsythia flowers begin to drop and lilac buds show color is the time to put down pre-emergence for crabgrass.
•You can get all of your lawn care questions answered at a free class with Jerry Spetzman from the Minnesota Department of Agriculture. On May 24, from 6:30-7:30 p.m., Spetzman will be speaking on the new lawn fertilizer laws, weed control, seasonal lawn care and lawn renovation at the North Branch Senior Center.
This is the first of a series of free classes the Master Gardeners will be sponsoring this summer. They will be held the fourth Tuesday of each month, from 6:30-7:30 p.m., at the Senior Center, at 6th and Maple in North Branch.
Ways to access information
Master Gardeners will be offering free plant clinics every Monday night, from 4:30-8 p.m., at the Extension office in North Branch.
Stop by 38780 Eighth Ave. (the Anderson Chiropractic Building) or call 651-674-4417 to speak with a Master Gardener.
•Online: www.extension.umn.edu/ county/chisago
•Check out the 'Hot Topics' box in the middle of the page for current Chisago County Master Gardener news and events.
•You can also click on 'Ask a Master Gardener' next to the cute little flower on the right hand side of the page. Here you can search 1000's of answers from Master Gardeners around the state. If you don't find your answer you can submit a question online or search for University publications.
•Bell Museum of Natural History
•For information about snakes, skunks, raccoons or other wildlife around your yard, call the wildlife information line at (612) 624-1374 or www.bellmuseum.org.
•To see the latest Yard and Garden newsletter, go to: http://www.extension.umn.edu/yardandgarden/YGLNews/YGLNews.html
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