When insect eggs hatch, they become larva, immature insects, which is often the most plant-destructive period of an insect’s life. Many insects go through a stage between larva and adult when they form a cocoon or hard shell around themselves. They don’t eat or cause damage at this stage, but they don’t succumb easily to predators or pesticides, either. Adult insects usually have wings and are at their most mobile life stage. Some adult insects feed on plants by piercing or rasping holes and sucking or sponging up the plant juices. Others chew on plant parts.

Butterflies and moths don’t damage plants as adults and actually help pollinate plants.

The following list includes the worst offenders of the insect world. (Many more insects cause damage, of course, and you can get more information about the ones to watch out for in your area from your local cooperative extension office.)

• Aphids: These tiny (up to 1/8 inch), pear-shaped pests come in many colors, including black, green, and red. They pierce holes in plant tissue and suck the juices, leaving behind sticky sap droppings, called honeydew, which attract ants and may turn black if covered with sooty mold. Aphids can proliferate quickly on weakened plants and tend to congregate on the newest leaves and buds.
• Bagworm: Adults lay eggs in bags in the fall. After hatching in late spring, bagworm caterpillars feed on the leaves and twigs of many trees and shrubs, especially arborvitae and juniper.
• Bean leaf beetles: Adult beetles chew large holes in bean leaves and the larvae attack the roots. 
• Billbugs: The adult beetles have a long snout and eat turf grass leaves, while the grubs consume the grass roots and lower stems.
• Black Turfgrass Ataenius (Ataenius spretulus): These 14-inch-long black beetles lay eggs in turf grass in the spring. The eggs hatch into small white grubs, which feed on grass roots until midsummer.
• Borers: Some beetle and moth larvae or grubs tunnel into the wood, canes, and stems of various trees and shrubs. The tunneling weakens the plant, makes it more disease-prone, and can cut off sap circulation, causing wilting and twig or cane death.
• Cabbage loopers: The 1-inch-long gray adult moths lay eggs on cabbages and similar types of crops in late spring to early summer.
• Chinch bug: Both the immature nymphs and the black-and-white, 1/6-inch-long winged adult bugs cause significant damage to lawns and grain crops by sucking the juice from grasses.
• Colorado potato beetle: The yellow and black-striped adults emerge and lay orange eggs on the underside of potato-family leaves, such as potato, eggplant, and tomato. The reddish grubs devour the plant leaves, mature, and lay a second generation of eggs later in the summer.
• Cucumber beetles: Striped and spotted cucumber beetle species — adult and larvae — cause significant damage by chewing large holes in leaves and vegetables, and eating their roots. They can also carry viral and bacterial wilt diseases, and spread them throughout your garden.
• Cutworms and armyworms: The 1- to 2-inch-long cutworm caterpillars chew through the stems of young plants at night, kill them, and then spend the day curled in the soil nearby. Armyworms also feed at night, usually in early summer, stripping the leaves from grasses, grains, and vegetable crops.
• Flea beetles: These highly mobile, shiny blackish beetles are only 1/10-inch long, but they tend to feed in large groups, skeletonizing leaves in a few days’ time. Adults emerge in spring and do most of their damage by midsummer.  ....read more