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Volume 9, Issue 4
April 2011

Many wildflowers can serve as insectary plants
Toward a more sustainable Cedar Mill
Identifying and Controlling Garden Pests: Insects
by Donna Prock

Garden insects are divided into two categories: Beneficials and Pests. Beneficial insects prey on many common garden insect pests and offer the gardener a safer, natural alternative to pesticides. Some examples include Orchard Mason Bees, bumblebees, ladybugs, Predacious Ground Beetles, spiders, Green and Brown Lacewings, and parasitic wasps. OSU Extension's handy "Pocket Guide to Common Natural Enemies of Crop and Garden Pests in the Pacific Northwest" can be found here:

You can attract beneficial insects to your garden by adding "insectary plants"—flowering plants that attract and host beneficial insects. These plants provide habitat, shelter, and food (nectar and pollen) for predatory insects. Their flowers offer a flat landing place for insects and sometimes they exude scent as an attractant. Adding this diversity to your garden is a form of "companion planting," where the plants can work together for the purposes of deterring pests, acquiring nutrients or attracting natural predators.

This useful website contains lists of insectary plants and the beneficial predators they attract as well as their prey.

Some of the most common garden pests that we see in the NW are:

aphidsAphids: Aphids actually do very little damage to plants, although they do suck the juices out and can damage the appearance, and need to be removed after harvesting.

In my experience the two best methods of control are spraying with water and trap crops. Spraying the infested plants with a forceful stream of water can dislodge, injure and drown these soft-bodied pests. Trap crops are plants that lure the aphids away from other plants you're trying to grow. Two of the plants that work best for this are nasturtiums and marigolds: they are lovely flowers and aphids go crazy for them. Mix these blossoms in with your vegetables or plant them very nearby, and then when the aphids infest the flowers, pull them up and dispose of them, aphids and all. By disposal, I mean you bag up the infested plants and throw them in the garbage. If you try and compost these bugs, they'll just leave the pile and go back to your garden.

Make successive plantings of the flowers so that you have grown ones to immediately replace the infested ones that get taken out of the garden.

looperCabbage Looper (Trichoplusia ni): This insect produces several generations each year. Winter is spent as a pupa attached to a leaf. The adult emerges from the pupa in spring as a brown moth. In northern regions adults may head south for the winter.

This 1 ½"-long caterpillar is green with pale stripes down the back and is identified by the characteristic way he humps along like a measuring worm. The adult is a brownish moth with a silvery spot in the middle of each forewing and flies at night with a wingspan of 1 ½". The isolated round eggs are a greenish-white and are laid on the upper surface of the host plant leaves.

This pest will attack bean, broccoli, cabbage, cauliflower, celery, kale, lettuce, parsley, pea, potato, radish, spinach, and tomato plants, by chewing the leaves. This weakens the plant and may impair edibility as the plant responds with bitter chemicals.

The Trichogramma Wasp is a natural predator that can help control this caterpillar. For serious infestations, use Bacillus thuringiensis.

leafhoppperBeet Leafhopper (Circulifer tenellus): This insect produces many generations each year and hibernates as an adult among weeds. The leafhopper lays its eggs in March and the adults swarm from early May to June. This 1/8" to 1/5" insect is pale green or yellow, wedge-shaped, and becomes darker toward the winter. The nymph is also pale green. The eggs are yellow and are laid in the stems of plants.

This pest will attack bean, beet, potato, and tomato plants and carries "curly top" and "tomato yellows" that causes the plants to be stunted and deformed. The symptoms are warty, pronounced leaf veins, kinked petioles, rolled leaves that look cupped or ball-like, with brittle masses of hair-like growths on the taproots.

Control can be obtained by using resistant varieties of plants. Lacewing larvae will attack and eat the leafhopper nymphs. Pull affected plants as soon as the symptoms are noticed and clear the area of weeds. Replace the weeds with perennial native bunch grasses (more than one species is better), since this insect tends to avoid grasses. If problem persists, dust your plants lightly with Diatomaceous earth.


A spinach leaf damaged by a leafminer. Note the active larva visible inside the tunnel.

Leafminers (Liriomyza sp.): This insect produces several generations each year and hibernates in cocoons within the soil. This 1/10" black fly has yellow stripes and lays its eggs on the surface of leaves. The larva is yellowish and is very stout and worm-like.

This pest attacks azalea, bean, birch, blackberry, blueberry, boxwood, cabbage, chard, chrysanthemum, columbine, holly, lettuce, lilac, morning-glory, nasturtium, oak, parsnip, peppers, potato, spinach, and turnip plants. The maggots mine under the surface of leaves, resulting in the formation of white tunnels. In some related species of these pests, they will chew the leaves giving them a blotchy look.

When an infestation is noticed, remove any damaged leaves and destroy them. If this is not adequate control, you can dust with Bacillus thuringiensis.

Spider mite damage weakens a tomato plant and can cause it to die

Spider Mites (Arachnida): Numerous generations are produced each year, often requiring only a couple of days! The adults hibernate in garden debris and under the bark of orchard trees.

The body is reddish-brown to off-white, very spider-like with eight legs and no anntenae, thorax, or wings. They are about 1/150" to 1/50" long. A 5X lens will help you spot this pest on plants or look for bugs the size of salt grains. The eggs are laid at the base of plants and on the leaves and buds.

This pest feeds on leaves, fruits, and roots of most indoor plants, apple, peach, evergreens and strawberries, causing yellowing, first along the veins and then over the entire leaf surface. Fruit trees with severe Mite damage will have small, poorly colored fruit and premature drop. Leaves curl slightly; and a copper color develops on the bottom of the leaf, which is covered with a fine web.

These pests are not common on unsprayed fruit trees, as many naturally-occurring predators feed on all mite species. They only become a problem when insecticides are repeatedly used, because the insecticides also wipe out the beneficial insects. Ladybugs or the Spider-Mite Destroyer will eat the Spider Mite.

The Predatory Mite is the best mite predator and can be used indoor and out. Insecticidal soap sprays ( can be used until the Predatory Mite becomes established. In orchards, there have been successful scientific trials that show that intercropping with Alder (Alnus sp.) or mulching with Rye, Wheat, or Sorghum significantly reduces Mite damage.

Garter snakes eat slugs. Encourage them by providiing habitat where they can hide, such as woodpiles.

Slugs: Garter snakes, birds, and frogs are some of the common predators of slugs. Among domesticated animals, ducks and geese are effective predators.

Many objects, such as rocks, boards, and compost piles, provide shelter for slugs. Gardens next to areas of tall grasses and weeds are particularly susceptible to slug invasion. Eliminate these sites since they provide excellent protection for slugs.

Stale beer attracts slugs and many insects as well. Cans of beer sunk into the soil in and around the garden area have proved useful in slug control. The slugs crawl into the liquid and drown. Over the years, homeowners and popular garden articles have stated that cinders, wood chips, or sand used as borders along gardens provide effective barriers against slug invasion. This probably works because it eliminates favorable slug habitats. Picking, stabbing, cutting, and other laborious methods can be used to control slugs. However, this will be an endless task unless hiding places, such as tall grasses and debris, are eliminated.

Another helpful web resource is



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