Trees and Insects

Trees and Insects: Bagworms

When I saw my first bagworm casings, I thought they were some diseased seed cone on our junipers.  Bagworms, a native moth and not a worm, feed on many species of trees beginning in late May. When they hatch, the larvae cover their bodies with a silken bag camouflaged with pieces of the plant they are eating.

At first this is tiny and may go unnoticed but eventually the bag is up to 2” long.  By late August they are fully grown and pupate inside their bags. Females remain in their bags for their entire life and don’t look like a moth, having no wings, eyes or antennae.

The males have short ½” long clear wings and hairy black bodies.  The females emit strong sex pheromones so the males fly to them and mate.  The female then lays 500+ eggs in her case.  She drops to the ground, dies and leaves the case to the eggs which overwinter in it.  In spring they hatch as 1/25” caterpillars.  They spin a silk thread and disperse to a nearby tree.  The cycle starts anew.  They can be numerous enough to defoliate a tree (deadly to evergreens) so pick them off when you see them. ―Nancy Weiss

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Trees and Insects: Galls

The first gall I noticed grew on a witch hazel leaf. It was shaped like a witch’s hat. I thought that was how the plant got its name. Totally wrong, but that’s another story. This gall is caused by an aphid, Hormaphis hamamelidis. It always forms a cone on the upper side of the leaf.

A gall is a plant deformity caused by another organism. It may be an insect, mite, fungus, bacterium, virus or nematode. There are over 1000 different creatures that cause plants to form galls. Most do so in such a specific way that the organism can be identified even if it is too small to see.

Insects and mites cause galls by entering young plant tissue. They stimulate the tissue to modify its growth and to produce more nutrients. As a result, galls serve as shelter and food for the developing larvae. The damage is contained to a small space, and although unsightly to us, rarely causes significant damage.

Galls present in many shapes and colors and occur on every part of the plant. Although the ones we notice most often occur on leaves, they also can form on the bud, fruit, stem or root. Even when they are on the leaf, you should notice if it is on the petiole or blade, in the middle or the end. If on the blade, is it the upper side, underside or edge? And of course, which tree is it on? Galls definitely can make you look carefully.  ―Nancy Weiss

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Trees and Insects: Friends and Foes ― Leaf Miners

The first summer I moved to Virginia, I thought all the black locust trees (Robinia pseudoacacia) were dying. But they didn’t. Yet, each year about this time, I would see so many of these leaves turn brown that as I drove along, I could pick out the locusts just by the pattern of their brown leaves. Finally I decided to find out why.

It was leaf miners! Leaf mining is caused by the immature form of many different species of insects from many families. The larva feed within the center of the leaf which has the least cellulose. You can often tell which insect is causing the damage by studying the pattern of the leaf damage. The black locust leaf miner culprit is the larva of Odontota dorsalis, a beetle.

The adult beetle hibernates, usually in the leaf litter of its host tree. In spring, it emerges and begins feeding on the black locust’s developing foliage. After a short time, it dies but not before depositing flat oval eggs on the undersides of the leaves. These eggs hatch and the larvae then eat into the inner layer of leaf tissue, forming the mine. The edge of the leaf is the preferred feeding site. As the larva grows so grows the mine. The larva then pupates.

Upon emerging from its pupal case, the beetle skeletonizes the undersurface of the leaf. (See the accompanying photo which shows an adult beetle next to the brown mine of the larva and the skeletonized leaf where the beetle is eating). The tree then grows a second set of leaves.If these get eaten too, the tree can be so weakened it will soon die. In early summer these brown mines can so consume the leaves that it looks as though a fire has swept through. Fortunately, there are many enemies of Odontota to keep them in check. These include several wasp parasites but the most common here is the wheel bug, Arilus cristatus. It feeds on leaf miners while they are still inside the leaf tissue. Between the wheel bugs and the various parasitic wasps, our black locusts live on and their blooms will continue to infuse the spring air with their wonderful fragrance. ―Nancy Weiss