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Fall webworms

Webbed branches formed by the fall webworm, Hyphantria cunea, were found on species of cherry and serviceberry. In Region I, there were fewer reports of this pest this year and it was not as commonly seen along roadsides compared to the 1996 and 1997 general surveys.

Fall webworms

Webworms can cause some branch deformity but are mainly an aesthetic concern in ornamental plantings in yards or parks. Webworms over winter in the pupal stage and adults emerge and lay eggs from late June to mid-July. By August, caterpillars feed in colonies and spin webs around the branches while feeding on the leaves. The web expands as the webworms devour leaves already engulfed in the webbing. The leaves are skeletonized by the caterpillars and then they shrivel up and turn brown. Braconid and Ichneumonid wasps usually keep webworm populations under control.

Branch flagging and discoloration of aspen

Branch flagging and discoloration of aspen are common this year in northeastern Minnesota. A variety of causes are likely. Much of the flagging and discoloration appears to be a result of branch death caused by Hypoxylon canker. Hypoxylon is always common but the added stress of drought is probably making more branches and tree tops flag out this summer than usual. The tan or brown leaves on these branches and trees tend to stick out against the background of surrounding green aspen. Other damaging agents may also be involved, such as, other canker diseases and feeding or egg-laying wounds of insects.

In late July, many aspen trees in northeastern and central Minnesota appeared to be losing their green coloration by turning yellow to white to brown in blotchy patterns. The cause of this discoloration is the aspen blotch leaf miner, Phyllonorycter salicifoliella, which has been noticeable for the last four years. The blotch miner is a small caterpillar that feeds inside the leaf between the upper and lower surfaces. As its name implies, its pattern of feeding results in circular area or blotch being mined out. This area turns color first yellow then tan or brown. Heavy feeding can give entire trees an off color appearance. This damage is often heaviest on the smaller trees along roadsides but can affect entire large trees, as has occurred this year in some stands near Duluth. The caterpillars change to a pupae in the mine and tiny moths emerge in August. The tiny moths are able to survive the winter by hibernating under the bark of pine and spruce trees and then lays eggs on aspen leaves the next spring.

Still another possible cause of discoloration is leaf bronzing of aspen. This is a condition that has been reported in the Lake States since about 1973. In late July and early August, leaves on branches in the lower crown turn yellow and then bronze and finally a dark brown color. The petioles of the discolored leaves stay lime green or yellow. Additional branches may be involved the following years. Branch death and eventually tree death may occur. The cause of this condition has yet not been determined. It is not very common and is not considered a serious threat to the aspen resource but is occasionally observed.