Brown Leaves and Bags in Trees

Posted: August 21, 2012

Locust leafminer and fall webworm are active in Pennsylvania's forests. What do these insects' activities mean to the trees they impact?

This has been an interesting summer. Across Pennsylvania, temperatures have set new records, while, depending on where you live, rainfall has either exceeded or not reached expectations. In some places, trees and grass show signs of recovery, especially during the past couple of weeks as temperatures and rainfall moderated. This certainly is not the case across the larger regional landscapes, but that is another issue.

Recent travels into the central parts of the Ridge and Valley Province raised questions. Many of the hillsides displayed splotches and patches of subtle brown foliage. It was not that heavy brown of a dead oak or maple. It was more of a lacy brown, you could almost see through it. Along roadsides and field edges it was easy to see that locust leafminer had been busy hollowing out the inner workings of individual black locust leaves. Surprising was the distribution of locust across the forested landscape – they often extended from the easy to see places up the hills to show where, maybe, there were once fields.

Black locust is an interesting tree. It grows in diverse places and often seems to tolerate relatively hostile conditions – strip mines to dry stony old fields. Often, it seems to produce its leaves late in the spring and lose them early, even before fall begins. Locust leafminer often terminates its growing season. That is to say, the browning of black locust leaves is common in Pennsylvania. It seems to happen, to some extent, every year. If you time it right, you can actually watch the larvae of the leafminer as they feed between the outer leaf layers. What seemed unusual this year was the timing and extent of activity. It seemed to reach a peak several weeks earlier than usual and it was, at least in some places, very heavy. Maybe you noticed the work of the leafminer where you live – watch for it every year.
The fall webworm is currently busy decorating trees across the state. This is another late summer insect. Normally, they begin to show up about the middle of August, and they may have been evident a week or two earlier this year. You will see their webs on the tips of many branches, especially on black walnut, apples, and birches. Their brown webs or bags seemingly weigh down branches. The webs, while unsightly, are not a big deal for the trees they affect. By this time of the year, trees are really beginning to slow down in preparation for casting off their leaves. That is to say, their most productive growing days are behind them. If there were heavy webworm feeding every year, it might challenge individual trees, but that seldom happens.

Many people, especially with yard trees, want to remove the webs, which is relatively easy to do when they first begin to form. Simply snip off the twig and place it in the trash. This effectively removes the feeding larvae as they only feed inside the web, expanding the web over time to reach new leaves. Before you take them all away, spend a few minutes watching them feed in their protective homes. It is not likely you will find them so intriguing that you keep them around, but you might learn to appreciate their house building skills.

Many entomologists and ecologists pay close attention to when insects begin to feed. The timing of events like black locust browning and fall webworms provide clues to how conditions change over time and how we might need to respond to adapt or mitigate their impacts on our forests. Watch for changes in plants and insects where you live and recreate. Note, even mentally, the changes you see and try to follow them from year to year. It is fun to look forward to even brown leaves and bags in trees.

Written by: Jim Finley
Phone: 814-863-0401