This article will address only a few of the most common insect pests found across the country. If you have damage that doesn't fit these profiles, contact your local extension agent and provide him or her with a sample of the injured plant. The plant pictured at right was actually damaged by a hailstorm, but could easily be diagnosed as the result of chewing insects.
Aphids: those tiny green or black sticky blips on the stems of our favorite annuals, perennials, and woody ornamentals. Sometimes called "plant lice," these minute insects in large numbers have the capability to kill any plant by sucking out all the plant sap, thereby reducing the plant's ability to utilize food. Aphids secret a honey-like substance, and ants on your plants is a good indication of aphid infestation, as seen in photo 1 (except in the instance of peonies where ants are merely attracted to the nectar in the opening buds).
Four-lined Plant Bug: a group of these little critters (photo 2) can destroy an ornamental almost overnight. The plant is fine one day, the next morning it is peppered with tiny, transparent circles. These circles decay and the plant leaf becomes debilitated. If enough leaves are damaged, the plant has a hard time bouncing back. Photo 3 shows extensive damage to bleeding heart leaves.
Caterpillar: regardless of how beautiful the resulting butterfly or moth may be, if you have caterpillars in your garden, you'll have chewed leaves! I never was able to identify this furry one at left, but he half-destroyed a small sunflower plant. When you notice this type of chew-damage, inspect the leaves just beneath the affected leaves. If you see little black dots of excrement (known as "frass"), you have caterpillar damage. Early in the morning while the air is cool, go out and hand-pick the caterpillars off. If you wait until it warms up, they've retreated to the cool soil at the base of the plant.
Earwigs also cause similar damage by chewing the leaves and blossoms of low-growing flowers and vegetables.
The brightly colored Milkweed Tussock Moth caterpillar at right is munching on a vining milkweed plant, which is fine with me since the stuff spreads like crazy.
Cutworm: Same type of damage, only these worms are smooth and green, making them difficult to see. This cutworm made short work of a newly emerging columbine plant (photo 6). Note the black specks on the dandelion leaf underneath the worm.
Grasshoppers: these voracious eaters can consume their own weight in food in 16 hours. Leaf damage is quite distinctive, as they only eat the flesh between the veins and stems, leaving a skeleton (photo 7).
Japanese Beetles: swarms of these nasty pests will defoliate a tree or shrub in hours. The damage is easy to identify because the beetles remain on the host until nothing is left.
Japanese Beetle damage
Leafminers: distinctive tunneling between the upper and lower surfaces of the leaf shows up as light-colored trails. Unless damage is extensive, plants are not usually compromised enough to die. This columbine leaf (photo 8) is under attack by both Four-lined Plant Bugs and Leafminers. Lilac Leaf Miners mature and roll the leaves as shown in photo 9. The leaves eventually turn brown and shrivel.
Lilac Borers: also known as Ash Borers, these pests tunnel into the trunk of the shrub and over-winter in the heartwood. Signs of this pest are small round holes in the bark (photo 10).
Webworms: these horrible things are like something from a sci-fi movie! (photo 11) As soon as you see the beginnings of webbing in a tree or shrub, cut it out and burn it.
Other common garden pests that cause damage similar to that listed above include slugs, stink bugs, and squash bugs, but the key is knowing what signs to look for. Then, take action.
This lady is not a pest, but I didn't appreciate her plans to catch my hummingbirds!