Bronzing along the veins of this oak leaf is due to feeding mites.

In August and September, I observed bronzing due to mites feeding on some young swamp white oaks. The tops of the leaves were very bronzed along the main veins, while the undersides of the leaves remained unaffected. When looking at the leaves with my hand lens and under the microscope, I saw a very heavy infestation of mites. Mites suck plant juices from the cells of the leaf.

Continue reading “Spider mites cause bronzing on oak leaves” →

The round hole at the edge of the fallen acorn’s cap was created when an acorn weevil larva chewed its way out to find a place on the ground to overwinter.

In some areas of Oconto County, a large percentage of the acorns on northern red oaks dropped early. Most of those acorns were infested with acorn weevil, although at the time they dropped they looked whole. After sitting for a few days, acorn weevil larvae started to emerge in hopes of burrowing into the soil to find a place to spend the winter.

Adult acorn weevils have very long narrow snouts and feed on acorns that are still attached to the tree. They chew a small hole in the acorn, then lay an egg (or several eggs) in the hole. They then plug the hole with fecal pellets, camouflaging their work. The eggs hatch and larvae begin feeding within the acorns. In the fall, the acorn drops to the ground and the larvae chew their way out, after which they bore into the soil to spend the winter underground. There is a different species of acorn weevil that feeds on acorns that have already fallen.

Even though the acorns looked perfect when they dropped, the insects had already been at work on them. Floating acorns in water will allow you to sort out the ones that are damaged, even though you may not see damage on the outside.

Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.

Fall webworm started showing up around the middle of July.

Fall webworm web

This is a native insect that feeds on deciduous trees and shrubs, and it is makes an appearance every year in yards and forests. Fall webworm forms loose webbing over branch tips and if it is a small tree, the entire tree can be completely webbed.

Inside the webbing you’ll find caterpillars (alive and dead), partially eaten leaves and frass (caterpillar poop).

Fall webworm caterpillars

Fall webworm is more of a cosmetic problem than a tree health problem, but if you want to control them, the easiest way to do that is to open up the webbing. You can take a rake, fishing pole, long stick, or whatever and open up the webbing. This will allow predators to get at the caterpillars inside the webbing.  Or you can use the rake, fishing pole, etc. and roll the webbing up. Then peel the rolled webbing off and place the entire web in a container of soapy water for a couple of days. If you want to use an insecticide, you need to make sure the insecticide is labelled for caterpillars/fall webworm and the spray needs to penetrate inside the webbing. With all pesticides, the user needs to read and follow label directions. There is no need to prune off the branch. If the tree is healthy, the defoliation should not harm the tree.  You can find more information on fall webworm here. 

Written by: Todd Lanigan, forest health specialist, Eau Claire (Todd.Lanigan@wisconsin.gov)
715-839-1632

Our native pine sawyer has a “spot” between the elytra (yellow arrow) that Asian longhorned beetle does not have. They also will appear dusty or pitted.

White spotted sawyer, sometimes called Pine Sawyer, is a native longhorn beetle. It is often mistaken for Asian Longhorned Beetle (ALB). How can you tell the difference between our native beetle and ALB?  First of all, size: ALB is a big burly beetle, while our native sawyer beetle looks slim in comparison. Second, ALB has a very smooth shiny appearance with distinct white spots on black wing covers, whereas our native sawyer beetle will appear pitted or dusty, and the white spots may be less distinct or absent. Finally, our native beetle will have a nice white dot “between its shoulders” where the wing covers meet, and ALB does not have this. 

Adult pine sawyer beetles feed on the bark of twigs which can cause branch tip mortality.

Pine sawyer larvae develop in weakened, recently dead, or recently harvested conifers. Larvae first feed in the phloem layer then progress to inner wood. They will pupate within the tree and adults will chew their way out leaving large round exit holes. Adults feed on needles and the bark of twigs. Areas this year which will attract white spotted sawyers include areas of storm damaged pine, and areas of conifer decline due to high water levels, as well as any other areas where conifers are stressed. 

If you find a beetle and are unsure whether it’s ALB or our native sawyer, please take some photos to send for identification. 

Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.

Poplar vagabond aphid feeding on the buds of aspen or cottonwood cause the tree to form a fleshy, hollow gall around the insects.

Galls caused by poplar vagabond aphid form at the ends of aspen and cottonwood branches. The galls are caused by aphids feeding at the tips of twigs. This feeding causes the tree to grow an elaborate structure that the aphids can live inside. One of the galls in the photo is broken open so you can see the aphids inside. This damage does not usually kill the tree, but reduces branch growth because the formation kills the terminal buds. The aphids feed during the spring and early summer within the gall, and then leave to feed on an unknown second host plant. When the aphids leave the gall it will turn brown and woody, and will remain on the tree for several years before weathering off. Adult aphids return later in the fall and lay eggs on the woody gall or in crevices in the bark. Eggs will hatch the following spring and repeat the process. For control, prune the galls prior to egg hatch early in the spring. Because the aphids return to the same trees with the original galls it is common to see a single tree heavily infested while a nearby tree will have no galls at all. I have always seen this problem in trees that are open grown, either along the edge of a stand, along a roadway or fence row, or in a yard. I’m not sure how much of a problem it is in the interior of a stand.

Poplar vagabond aphids, which are covered in a white waxy material, are protected inside the large hollow galls formed by the tree. They suck the trees’ sap.

Written by: Linda Williams, forest health specialist, Woodruff, (Linda.Williams@wisconsin.gov), 715-356-5211 x232.