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Common insects and associated pests: attacking bedding plants and perennials

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By Master Gardener, Brunswick County Extension

Charlie Spencer 

Master Gardener

Sowbugs and Pillbugs (Isopods)

Isopods (commonly called pillbugs) are not insects but relatives of the crab and shrimp. They have a head with obvious antennae and a trunk region with 11 pairs of legs. They tend to hide during the day and emerge at night to eat irregular holes in leaves of young plants. These pests are easily detected at night with a flashlight or by pulling back mulch around the plants. 

Under normal conditions, these general feeders rarely cause much damage to living plants since they prefer to feed on decaying organic matter. During rainy weather or where gardens are mulched too heavily and watered constantly, isopods can build up large populations and cause visible damage. 

The best control for isopods is to remove excess mulch, use irrigation sparingly, and remove any old leaves or dead plants immediately. This will reduce habitat and food necessary for large populations. Some of the general insecticides also have sowbugs or pillbugs on the labels and these can be used during wet seasons when cultural controls are not effective. 

Millipedes 

Millipedes are often confused with their fast running, predatory cousins: the centipedes. Millipedes have heads with antennae and elongate trunks with 20 or more segments and two pairs of legs per visible segment. These slow moving animals are usually scavengers but occasionally feed on living plants, causing damage similar to isopods. Centipedes have only one pair of legs per body segment, run rapidly and are beneficial predators. Control millipedes with the same techniques used against isopods. 

Spider Mites 

The most common spider mite attacking bedding plants and perennials is the two-spotted spider mite. This is a common pest in greenhouses and is often transplanted into the garden with bedding plants. This mite, however, can overwinter as an adult female hiding in protected areas. 

The tiny mites are about 1/50-inch long and usually feed on the undersides of leaves. You will need a 10x hand lens to see these pests. They make tiny cuts into plant cells and suck out the contents. This results in tiny yellow or white speckles on the upper leaf surface. Spider mites also produce fine webbing that may coat the plant when populations are extremely high. This is often easy to observe in the morning dew. 

The two-spotted spider mite is a sun and heat-loving pest that can complete a cycle from egg to adult in less than two weeks. Therefore, populations tend to be a real problem in the heat of summer. 

Since spider mites are not insects, most insecticides are not effective for control. Beware of standard insecticides that claim “mite suppression” on the label. Look for true miticides or pesticides that claim “mite control.”

Spider mites are best managed by selecting bedding plants or new perennials that are not infested. Since the mites are very tiny and prefer dry, sunny weather, attempt to grow mite susceptible plants in the shade. A regular washing with a firm jet of water (syringing) can help keep populations down. 

Several of the insecticidal soaps are also registered for mite control and can be effective if thoroughly applied to the upper and lower leaf surfaces. Most soap and miticide applications will have to be repeated two to three times in order to kill resistant eggs and resting stages of this pest. 

Plant Bugs 

The tarnished plant bug and four-lined plant bug are common sucking pests that attack a variety of bedding and perennial plants. The daisy and mint families are especially susceptible to attack. Both bugs are quick to fly and the nymphs quickly run to the under surface of leaves when approached. They damage plants by causing small (1/16-inch), round, sunken spots on the leaves. These spots occur when the leaf bugs kill the leaf tissues during feeding. When these spots are numerous, the entire leaf may curl and wither. 

The tarnished plant bug has a light-green nymph and the adult has mottled brown colors. The four-lined plant bug has a bright red-orange nymph and the adults are lime green with four black stripes. 

Damage to plants usually occurs in the late spring and early summer when the nymphs are active. If this activity is several weeks before flower bud initiation, no damage will be evident at the time of flowering; however, early flowering plants can be severely damaged. These are the plants that need protection. 

Since the plant bug nymphs cause most of the damage, control of this stage is suggested. Inspect plants early and try to detect the first signs of the sucking damage. Small numbers of nymphs can be dislodged from the plants into a container of soapy water. Higher populations are best controlled with a registered pesticide or insecticidal soap. Check the plants again in two weeks to catch any late emerging nymphs. 

Aphids 

Every plant seems to have some type of aphid that may feed on it. These pear-shaped, soft-bodied insects with long legs and antennae may cause young, soft leaves and stems to twist and curl. Aphids also produce considerable honeydew, a sugary excrement, which is attractive to ants and may allow for the growth of black sooty mold. Aphids may be any color with winged and non-winged forms present in a colony. Most of the reproduction is asexual, with females giving live birth to the nymphs. Populations can therefore explode in a short period of time. 

Fortunately, there are numerous predators and parasites that attack aphids. Lady beetles, lacewings and aphid wasps are good examples. Learn how to identify these beneficials and do not apply pesticides when they are active. Often, the beneficials arrive after an aphid population has built up to alarming numbers. In order to reduce these populations, the aphids can be washed off the plants with a strong jet of water (syringing) or insecticidal soaps can be used. 

Whiteflies 

Whiteflies are not true flies but relatives of aphids and scales. These 1/16-inch long pests are usually first detected when the plant is touched and the small white insects take flight. They damage plants by discoloring the leaves and depositing honeydew. 

Most of its life cycle is spent as a sessile nymph and pupa attached to a leaf. Female whiteflies attach eggs on leaf surfaces and the nymphs (called crawlers) move to find a suitable spot to insert their sucking mouthparts and begin feeding as a sessile nymph. The nymphs feed for about two weeks and excrete honeydew. They then form a pupal stage from which the adult emerges. 

Whiteflies are very difficult to control because only the adults and crawlers are susceptible to normal pesticides; however, insecticidal soaps seem to control some of the recently settled nymphs. It is best to avoid whiteflies. The greenhouse whitefly normally gets started in the garden after being brought in on bedding plants. Check new plants carefully for signs of whiteflies. 

Japanese Beetle 

Japanese beetles are bright, metallic-green and eat the flowers or skeletonize the foliage of ornamental plants. These pests begin to emerge in late-June and continue their activity until early-fall. It is best to use plants that are not attractive to adults. Control is a difficult process and several applications of an appropriate pesticide are usually needed to protect susceptible plants. Refer to: HYG-Fact Sheet 2002, Control of Japanese Beetle Adults and Grubs in Home Lawns. 

Cutworms and Caterpillars

There are numerous caterpillars that may be found attacking bedding plants and perennials; however, the most damaging group is the cutworms that may kill newly transplanted seedlings. These thick-bodied caterpillars live in the soil during the day and emerge at night to find seedling plants. When a seedling is found, the cutworm chews into the plant’s stem at ground level and fells it like a miniature lumberjack. The caterpillar may drag the plant back to its burrow or may crawl under the dead plant to feed. When seedling stems get too hard to cut off, the cutworms may crawl up to the leaves and dine until daybreak. They then return to the soil to hide. Examining the soil near recently felled plants often exposes the culprit and allows one to eliminate it by crushing or drowning in soapy water. 

Cutworms can be controlled by placing a plastic or cardboard collar around newly transplanted seedlings. This ring should extend one to two inches into the soil and be three to four inches tall. Several pesticides are registered for control of these pests and the dusts applied to the soil seem to be most effective. 

Thanks to David J. Shetlar for this information.

This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, North Carolina State University and North Carolina State University Extension assume no liability resulting from the use of these recommendations. 

Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.