Identifying Insect Damage

Phil Peters
Adam's County Master Gardener Program

Wondering why that plant looks so bad? The cause might be insects, one of the easiest to diagnose.

Controlling insect damage begins with the correct identification of the insect causing the damage. The insect’s life cycle will reveal when it is most susceptible to control, either biological or chemical. A treatment applied at the wrong time or to the wrong part of the plant or for the wrong pest is a waste of money and frustrates the gardener. And it will destroy the wrong insects.

Insects attack all parts of the plant. Each plant-loving insect variety has a favorite species of plant and particular part of the plant where it feeds or lays eggs. To find it, we have to examine the entire plant: flowers, fruit, leaves, stem and roots.

Sometimes we are lucky and the invader is obvious. We all know a Japanese beetle when we see it. The same holds true for the large nests of tent caterpillar and the fall webworm as the large webs are very noticeable. Or insects may swarm around a plant in large numbers. Capturing one will tell whether it is friend or foe.

Start with the leaves and flowers. Crumpled or distorted leaves are clear signs of sucking insects or mites (arachnids, not insects) that are weakening the plant by sucking out sap. Aphids are prime suspects. Ants guarding the aphids give away their presence. Unfurl the leaves and look underneath. You may be fortunate enough to see ladybugs or tiny wasps preying on the aphids – the cavalry to the rescue.

Mite infestations can cause evergreens to turn brown overnight. Mites are relatives of spiders, so look for minute, silken threads on the leaves. Or hold a blank piece of paper beneath a branch and smack the branch smartly. Watch the paper. If small dots begin moving, they are mites. Both aphids and mites can be controlled by a strong spray from the garden hose.

Sometimes sucking insects will leave yellow dots where they have sucked a leaf cell dry. This is called stippling. Occasionally the excretions are liquid and form sticky tar-like spots called honeydew on the leaves. These could be from aphids, plant bugs, thrips or some other kind of sucking insect. Look under the leaf and along the stem to find the culprit.

Scale insects are another common pest. They will usually be in colonies along the stem. Since pesticide control is very specific, it is often easiest to prune out the affected branch and dispose of it in the trash.

Caterpillars may devour the entire leaf or just the tasty green part between the veins. They can also drop pellets of waste material. Smaller caterpillars hide under the leaf. If you see these tell-tale signs, you have a clue to what you are looking for. Again look under the leaf and along the stem of the plant to find the insect. Pick it off, identify it, and dispose of it in a bucket of soapy water. Just remember, though, you can’t have butterflies without caterpillars! However, you must know whether your caterpillar produces a butterfly and not a moth.

Many insect larvae tunnel within the leaf where they leave tell-tale mines that radiate in various patterns. Virtually all plants are susceptible to leaf miners. The insect is invulnerable to insecticide control while it is in the leaf. If the infestation is not too severe, remove the affected branches and dispose of them in the trash. Systemic insecticides that work within the plant tissue may be effective, but they must be applied at the right time so they will be in the plant when the eggs hatch and the larvae begin to feed.

Galls are another sign of insect damage. These are strange looking growths on the surface of the leaf or in the stem that cause it to swell and distort its growth. They usually do not harm the plant. If you find them too unsightly, just prune them out and dispose of them.

Borers pose special problems. We notice them most easily in squash vines, iris, lilac, and firs and pines. We see the damage after the egg has hatched and the larva has begun to feed. Look for clearwing moths or sawflies flying around your bushes and trees. Then look for holes in the stem, especially around the base, or the needles. Control ranges from pruning out the affected branch or branch tip, in the case of sawflies, to heavy applications of an appropriate insecticide at the proper time around the base of the plant. Intimate knowledge of the borer’s lifecycle is essential for effective control.

Insects that attack the roots usually cause the entire plant to collapse, often suddenly. Probing the soil may reveal the pest.

One of the best resources for identifying the insects that attack our plants is Whitney Cranshaw’s, Garden Insects of North America (Princeton University Press, 2004). You can look up your plant in the appendix and see at a glance the insects that chew leaves, suck the sap, bore in the stem or attack any other part of the plant. An exhaustive index will direct you to the appropriate information on the specific insect and excellent photographs will help you identify the little beast. If you can catch some of the insects, you can also bring them to your local Cooperative Extension office where a master gardener can help you. Only when you know which insect is attacking your plant can you apply proper effective control that will be less costly and environmentally safe.

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