Also known as plant lice, aphids are small, soft-bodied, sucking insects that cluster on the stems or underside of leaves. Aphids are usually wingless and green, brown, or black in color. Symptoms of infested plants include distorted or curled leaves, presence of sticky sap (honeydew) on the infested surfaces, and misshapen new growth.
Aphids can be controlled with the use of insecticidal soap or a registered insecticide. A strong stream of water directed at the infected plants may knock aphids from the plant. Many beneficial insects such as ladybeetles and lacewings feed on aphids and if an infestation is not too severe, it may be appropriate to be patient and let nature take its course.
Caterpillars can quickly damage annual plants
Larvae of numerous species of moths and butterflies. These voracious creatures come in many sizes and colors. Plant symptoms include chewed or completely missing leaves. Some types of caterpillars will roll or fold the leaves and hide inside. Often, frass (droppings) are present on and around the plants.
A light infestation can be easily controlled by picking larvae from the plant and crushing them. Common registered insecticides will effectively kill caterpillars.
Grubs and Cutworms
Grubs and cutworms are the larval stage of many moths and beetles. Most live in the soil and feed on roots or emerge at night to feed on stems and foliage. Severely damaged plants may die from having the stem or roots severed. Other symptoms include chewed lower leaves and/or wilted or stunted plants that result from root feeding.
Grubs and cutworms can be controlled by handpicking, or using a soil drench of an approved insecticide.
Leafminers are small insect larvae that burrow under the leaf surface while feeding. Symptoms are easily recognized and exhibit themselves as zig-zag or wandering lines on the upper leaf surface that are lighter in color that the rest of the leaf surface. These are tunnels in the leaves caused by leafminer feeding.
A light infestation of leafminers can be controlled by removing and destroying damaged leaves. A heavy infestation will require the use of a registered systemic type insecticide.
Sometimes covering a crop with a floating row cover will exclude the insects from entering the plant.
Mealybugs are sucking insects that infest stems of many plants. Mealybugs are easily recognized by the presence of a cotton-like white substance they deposit for protection.
Control of mealybugs can be had by spraying the plants with a direct stream of water, using an insecticidal soap, or applying a registered insecticide.
Not actually insects, these miniscule pests are related to spiders. They spin protective webs on the underside of leaves and feed by sucking juice from the leaves. Symptoms include color mottling that, at a distance, may appear as a general yellowing of older leaves. Webbing will be present on the underside of infested leaves. The mites, to small to be easily visible, can be detected by shaking a leaf over piece of clean white paper.
Spider mites prefer dry, dusty environments. Sprinkler irrigation or routine washing of leaves with water usually keep them at bay. A severe infestation may require the use of a registered miticide. Most common insecticides are ineffective against spider mites.
Slugs prefer damp places in the garden
Slugs and Snails
Slugs and snails prefer damp soil and humid conditions. Slugs and snails often hide during the day and feed at night. Symptoms include chewed leafs and glistening slime trails on plant surfaces.
Control snails and slugs with baits.
Damage is caused by the larva of this small, four-winged insect. Thrips reside on the underside of leaves and use their rasping mouthparts to scrape away the surface of the leaf after which they feed on the sap. Symptoms appear as small white streaks and blotches, more prominent on the underside of the leaf.
A light infestation does little permanent damage to the plant and can be ignored. A heavy infestation will likely require the use of a registered insecticide.
In Idaho, whiteflies are more commonly a problem in greenhouses than they are outdoors. They are small insects with distinct bright white wings that reside and feed on the underside of leaves. Symptoms include the presence of honeydew on leaf surfaces, often accompanied by a black sooty mold. When disturbed, clouds of the white, rapidly flying insects will rise above the foliage, then quickly resettle.
Trap the flies with yellow sticky boards or use a registered insecticide.