Willow trees are among the trees most frequently plagued by wood-boring insects, according to the Texas Agricultural Extension Service. Although many species of wood borers attack willow trees, the University of Minnesota Extension highlights two in particular that pose a threat to willows in Minnesota: the flat-headed apple tree borer and the poplar and willow borer.
Despite the name, the flat-headed apple tree borer also afflicts willow trees in Minnesota. The insect is greenish-bronze in color with two wavy lines on the wings. As the name suggests, the shape of the insect's head area appears noticeably broad and flat. The poplar and willow borer is a weevil, primarily black except for a grayish patch near the back of the wings. Both species burrow into the trunk and stems of willow trees, where they interrupt the flow of water and nutrients through the trees.
When insects burrow into the trunk or branch of the tree, they disrupt a layer of tissue just below the bark called the cambium. The cambium contains vessels that transport water and nutrients throughout the tree, and when the insect penetrates this, the tree can no longer use the vessel for transport. Badly infested trees show dieback and, eventually, die of starvation or dehydration.
Identification of Flat-Headed Apple Tree Borer
Although the insects bore into the tree, you will be able to observe external signs of damage on your willow tree. The University of Minnesota recommends looking out for the metallic-colored adults on sunny patches of bark during the summer. If you see sap oozing from the bark of your tree, peel it back to detect larvae beneath. You may also find exit holes left when mature adults exit the tree.
Identification of Poplar and Willow Borer
In the spring, check your willow trees for evidence of adult weevils feeding on new shoots. If your tree shows dead, dry bark, you may have an infestation of this weevil. Look for holes in the stems, focusing on the lower parts of branches, and look out for the sawdust-like material produced by the weevil as it burrows into the stem.
Both types of borers can kill trees, especially young trees or those susceptible due to stress or poor planting. Keeping trees healthy will help to prevent many pest problems. If your trees do end up with an infestation of borer insects, remove infested branches to control the poplar and willow borer. Insecticides can control both species. The flat-headed apple tree borer is susceptible to 12 species of parasitic wasps. You can also control both species with the chemical insecticides imidacloprid and permethrin, according to the University of Minnesota Extension. Spray for weevils in the spring and midsummer, when you find adults on the plant. Spray for beetles in mid-May, early June and again in early July.