Archived Story

Insects cause damage to all manner of plant life [UPDATED]

Published 4:02am Monday, May 6, 2013 Updated 6:06am Monday, May 6, 2013

Everyone knows about the elm borer and emerald ash borer, but you probably don’t know about some other bothersome insects. For instance, the honeylocust plant bug.

This bug overwinters as an egg under the bark or two and three year old twigs. They hatch soon after bud break and begin to feed on the unfolding leaves.

This is a tiny green insect so small that to find it you will have to shake a branch over a piece of paper. The nymph appears first as a pale green oval crawler. As it ages it will develop yellow spots on its back. They will become adults in about a month. They and the nymphs feed until mid-summer. They cause severe leaf distortion, dwarfed leaves, chlorosis, and yellow, brown leaf spots. A heavy infestation can cause a failure to leaf out, or leaves to fall off.

To get rid of the critters, spray the tree with a strong spray of water while they are small. You may need insecticidal soaps or an insecticide if you don’t get them early and they get bigger and tougher.

Even our tough basswood trees have a worm that eats their leaves. Called a linden looper; this guy will also attack maples, oaks, apples, birches and hawthorn trees.

It is a worm with a rusty brown head, yellow body and wavy black lines on the body. The young ones eat holes in the leaves. As they get older, they eat all but the mid-ribs of the leaves. They can defoliate a tree. Spraying with B.T. (basillius thuringiensisi var. kurstaki) is effective on the young worms, but infestations are usually small enough that that is not needed.

Elms have to put up with damage from inchworms that eat their leaves, the elm beetle, and a flea weevil. The weevil eats holes in the leaves. Heavy feeding can cause the leaves to wither and turn brown. The adult lays an egg in a cavity she makes in the leaf mid vein. This shows up as a blotch at the tip of the leaf. This can be controlled by an insecticidal spray. If the tree is 40 feet high, however, you may have to rethink that.

Every year, someone brings in an oak leaf to the Extension Office with big bumps on it. These are galls. They are caused when an insect lays an egg in a developing leaf. This causes the plant cells around the damage to grow and engulf the egg. A good deal for the egg as now it is not only sheltered, it feeds on the inside of the gall. One gall is caused by a cynipid wasp, but there are other culprits, too.

Although these galls look terrible, they don’t damage a tree unless they totally infect every leaf. This will slow down photosynthesis but won’t kill the tree.

One good disease is the rust that shows up on buckthorn. Unfortunately, this doesn’t hurt the tree much. The best control is to prune the tree to soil level then paint with full strength brush killer. It is an invasive weed. The fruit isn’t even good bird feed, and it is a laxative for the poor things.

Do keep a sharp eye on your trees. A mature tree adds value to your home, cools it in summer and reduces the wind in the winter saving on both heating and cooling dollars.


Bev Johnson is a master gardener in Otter Tail County.


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