Bedding plants and perennials provide beauty and tranquility to homeowners and landscapers. They also encourage biodiversity by attracting butterflies and other beneficial pollinators. Unfortunately, many other insects, mites and other invertebrates (ie., slugs, sowbugs and millipedes) are also attracted to these plants as a food source. It is important to differentiate between the actual insect pest causing damage to your plants, and the pollinators, predators and parasites that are helping maintain a healthy, balanced landscape.
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| Beneficial honeybee visiting aster. |
In order to manage the activity and damage caused by these pests, you must understand "pest management" and learn how to properly identify each insect. Each plant has a number of key pests that may attack the flowers, foliage, stems or roots. Some of these insects will only attack a certain kind of plant, while others are considered generalists and will feed on a variety of plants. This fact sheet will attempt to help you identify the common "generalists" and will suggest methods of keeping them under control.
Integrated Pest Management
We now use the term "integrated pest management" because we know from experience that there is no magic bullet to rid the garden of unwanted pests. We can merely attempt to keep pests at tolerable levels and minimize their damage—this is management. When using integrated pest management, remember these four important principles:
- The mere presence of a pest does not warrant control; determine a threshold for damage.
- Monitoring is essential, as is proper identification of the insect.
- Reliance on a single control technique will eventually fail. Use cultural, mechanical and biological control methods first; if those prove ineffective, choose the least toxic chemical control available.
- Go back to see if your methods were successful; if not, make appropriate adjustments.
Insects will always be present in a landscape; the key is to understand which insects are a threat, and which are merely pollinating or preying on other insects.
Complete reliance on pesticides, whether organic or not, can be harmful to beneficial insects and can actually make a problem worse by killing off the natural enemies. In order to manage pests and their damage, integrate your approach and use cultural (and mechanical) control (i.e., resistant plants, traps, crushing and sanitation) and biological control (i.e., predators, parasites and diseases) in conjunction with chemical control.
Diagnosis of a Problem
It is imperative to properly identify the pest(s) in question before attempting control. Various insects feed on different parts of the plant; where and how they feed will affect which control measures you use. Generally, pests have either chewing or sucking mouthparts.
Pests with chewing mouthparts eat portions of the plant. Some insects may defoliate the plant by eating all the leaves, while others only eat portions of leaves, resulting in skeletonized foliage (the leaf tissues between the veins are eaten), notched foliage (only the edge of the leaf is eaten), shot-holed foliage (tiny holes are nibbled into the leaves) or shredded foliage (most of the leaf is eaten except for the major veins). Other chewing pests feed inside leaves (leaf miners) or bore into stems and roots (borers).
Pests with sucking mouthparts feed by sucking nutrients from the plant. This type of feeding usually causes the plant to discolor or twist and curl. The plant may discolor from tiny yellow speckles (spider mites), larger darkened spots (plant bugs) or coatings of black sooty mold growing on honeydew deposits (from aphids and whiteflies). Curling of foliage or twisting of young stems is how many plants react to insect saliva introduced while feeding as well as to direct injury.
Locating the actual insect pest makes diagnosis easier. Many pests remain on the plant at all times, and a close inspection is all that is necessary. Others run or fly when disturbed, so cautious, slow inspection may be necessary. Approach the plant low to the ground and try to observe the plant's upper and lower leaf surfaces without casting a shadow. Many pests come out at night, so you will have to look for them with a flashlight.
If you are unable to properly identify the insect, try taking a photo, capturing a specimen or collecting examples of the damage. Then, take the evidence to your county's Ohio State University Extension office. Attempt to keep the insect intact by placing it in a small jar or zip-lock bag. A specimen may be kept in the freezer until you have it identified.
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| Skeletonized leaf from Japanese beetle feeding. Notice, leaf veins are intact. | Plant bug damage. |
Snails and Slugs
Snails and their "shell-less" cousins, the slugs, are common residents in the garden. Most of these feed on decaying plant matter or fungi, but many can chew the foliage of living plants, leaving ragged holes in leaves. They may also feed on ripening fruit or succulent plant material close to the ground. Snails and slugs are usually active at night or on foggy or cloudy days. Use a flashlight at night to detect these pests, or look for the slime trails on damaged plant foliage in the early morning.
These pests require high humidity or moisture and usually reach pest status during wet years or during the rainy periods of spring and fall. Control is best achieved by making the garden less suitable for snails and slugs. Eliminate debris, weeds, large stones or other places they can hide, and keep mulch levels below 3 inches in depth.
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| Garden slug. |
If vigilant, hand removal can be effective. Copper barriers are also effective when buried around ornamental beds. Be sure you are starting with a slug-free flower bed before placing the barrier. The larvae of fireflies, ground beetles and parasitic flies also feed on snails and slugs but are rarely in high enough numbers to be effective. Because slugs and snails are attracted to yeasty odors, beer traps can be placed at ground level, forcing the pests to fall in and drown. These traps need to be replaced on a regular basis.
Several baits are available, which can be quite effective when used in conjunction with other control methods. These baits often contain pesticides and can be toxic to dogs and cats, while others are toxic to earthworms. Always read and follow the pesticide label.
Sowbugs and Pillbugs (Isopods)
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| Isopod. |
Isopods (commonly called pillbugs) are not insects but are 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 plant 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 eliminate any stones, grass clippings, leaf piles or other hiding places. This will reduce the habitat and the food necessary for large populations. Some of the general insecticides also have sowbugs or pillbugs on the labels; these can be used during wet seasons when cultural controls are not effective.
Millipedes
Millipedes are often confused with their fast-running predatory cousins, centipedes. Millipedes have heads with antennae and elongate trunks with 20 or more segments, each having 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. These creatures are usually abundant in compost piles and heavily mulched ornamental plantings. Centipedes have only one pair of legs per body segment, run rapidly and are beneficial predators. Control of millipedes can be accomplished using the same techniques discussed under 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 they 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, which 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 the summer.
Because 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 and horticultural oils are also registered for mite control and can be effective if thoroughly applied to the upper and lower leaf surfaces. Use caution with soaps and oils on flowers, as the soft flower tissues can be damaged. 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. Two-spotted spider mites are notorious for developing resistance to miticides. Switch chemicals if the miticide doesn't work.
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| Millipedes in gardens can be flattened or round, but all creep rather slowly. |
Two-spotted spider mites. |
Earwigs
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| European earwigs are the most common species in Ohio gardens. |
The European earwig is a common inhabitant of home gardens. The adults and nymphs have elongate bodies ending in a pair of forceps like pinchers. Though these insects look dangerous, they are harmless and can be considered beneficial when they prey on other insects. Some of the larger males may be able to pinch soft skin when picked up, but most protect themselves by releasing a foul odor.
Unfortunately, European earwigs are omnivores that eat both plant and animal matter. When populations are low, most would never notice the occasional notch or hole in plant foliage and flowers. When mild winters are combined with above-normal rainfall, earwig populations explode and the adults and nymphs can be packed by the hundreds in flowers and in every other crack and crevice of the garden. In most years, earwigs should be ignored or appreciated for their predatory behavior.
When earwigs reach intolerable populations, they can be fairly easily controlled with standard insecticides. Nonchemical control can also include the construction of trap boards. These are two flat boards with ¼-inch grooves tied together so that the spaces are enclosed. Earwigs like to hide in these spaces, and the boards can be collected regularly and dunked in soapy water to kill the pests. Other cultural controls use the same types of habitat modifications (i.e., reducing mulch and sanitation) listed under isopods.
Plant Bugs
The tarnished plant bug and the 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 plant 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 adult is 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 may need protection. Since 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. Since four-lined plant bugs insert their eggs into old plant stems to overwinter, cut the stems of perennials to the ground and dispose of them away from the garden. 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 with insecticidal soap. Check the plants again in two weeks to catch any late-emerging nymphs.
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| Four-lined plant bug. | Tarnished plant bug. |
Aphids
Every plant seems to have some type of aphid that can 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 nonwinged forms present in a colony. In spring and summer gardens, 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 or oils can be used.
Whiteflies
Whiteflies are not true flies but are 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 garden white flies are tropical insects and are introduced each year.
Most of the whitefly's 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 sessile nymphs. 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. Inspect new plants carefully for signs of whiteflies before purchasing.
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| Winged and wingless aphid. | Whitefly adults and nymphs. |
Japanese Beetle
Japanese beetles are one of the most obvious ornamental pests in landscapes. Adult beetles are bright metallic-green, and they 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, as more adults can fly in from other locations and begin feeding. Hand removal can be effective in cases of low populations. Japanese beetle pheromone traps (e.g., Bag-A-Bug) are not recommended, as they can attract more beetles than they actually catch. Applications of an appropriate contact pesticide are used to protect susceptible plants, though systemic insecticides may be used to protect nonblooming plants (to conserve pollinators and butterflies).
Cutworms and Caterpillars
Numerous caterpillars may be found attacking bedding plants and perennials. However, the most damaging group are the cutworms, which 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 and cuts in just above the soil surface. 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 it in detergent water. Cutworms can be controlled by placing a plastic, aluminum 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 granules applied to the soil seem to be most effective.
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| Japanese beetle adults. | Back cutworm larvae. |
This fact sheet is a revision of HYG-2151.