Pests build up most quickly when just one or two kinds of plants grow, so a bed of mixed perennials is less likely to be decimated than a bed of roses or petunias. That’s because the pests that prefer specific plants will have a harder time finding them when they’re “hidden” among all those other perennials. That doesn’t mean perennials are pest-free, though, as gardeners who’ve encountered borers in the bearded irises, Japanese beetles on their hollyhocks, or slugs on their hostas can attest.
One cause of pests on perennial beds is the nature of perennials themselves. The plants are in place a long time, giving pest populations a chance to establish themselves over many growing seasons. Fortunately, there are five ways to keep pests at an acceptable level in your garden.
- Grow resistant species and cultivars when available.
- Use good cultural practices.
- Apply biological controls, which are alive and often self-perpetuating.
- Use manual controls, including barriers and traps.
- Use insecticidal sprays when required.
The best and easiest way to keep pests under control is to find them when they’ve just arrived, and there aren’t many of them. If you can start control early, your perennials will suffer minimal damage, and you can usually use a simple control. Handpicking a few Japanese beetles is far better than coping with bug sprays and stripped plants.
Become a garden detective. When you’re weeding, watering, or just out strolling in your garden, check your plants for pests and signs of feeding injury. Make sure you’re in the garden every day, pest populations can build up fast. Try to familiarize yourself with the major perennial pests, so you’ll know what you’re looking at, then apply appropriate controls.
1/12 – 1/5 inch long; green, reddish, or blue-black; pear-shaped; with 2 tubes projecting back from abdomen; some are winged, some wingless.
Leaves, stems, and buds distorted, sticky; look for clusters of small insects. Wash pests from plants with a strong spray of water; use insecticidal soap sprays for serious infestations.
Asiatic Garden Beetle
1/3 inch long, brown, velvety beetles.
Leaves with irregular holes in edges. Handpick adults at night into a can of soapy water; spray heavily infested plantings with pyrethrins or rotenone.
Fuller Rose Beetle
1/3 inch long, gray, long-nosed weevils.
“Ticket-punch” holes around leaf margins. Attacks Chrysanthemum, Hibiscus, Penstemon, Plumbago, Primula, Rudbeckia, Scabiosa. Handpick adults at night into a can of soapy water; shake plants over drop-cloth or sheet in the early morning and collect weevils.
1/2 inch long, metallic-blue or green beetles with coppery wing covers.
Stems exude sawdust-like material and break; leaves wilt; iris borers cause irregular tunnels in leaves, damaged or rotted rhizomes. Apply Bacillus thuringiensis (BT) at the first sign of borers. Destroy weeds where borers winter. Crush borers in iris leaves; dust the base of iris plants with pyrethrin in spring.
1/16 – 1/2 inch long, usually shield-shaped; may be brown, black, green or brilliantly colored and patterned.
Buds and leaves deformed or dwarfed. Handpick into a can of soapy water. Spray with insecticidal soap; treat severe infestation with rotenone. Destroy weeds where bugs overwinter.
Gray or brownish moths.
Seedlings your young plants cut off at soil level. Place plant collars in the soil around seedlings or transplants.
1/10 inch long wasplike, with yellow-striped black bodies and clear wings.
Leaves with tan or brown blotches or serpentine tunnels.
1/12 – 1/5 inch long, with grayish, brownish orange, reddish-brown, or cottony white shells; males are winged, females wingless.
Leaves turn yellow, drop; plants may die. Prune off infested stems and leaves. Remove scales with a cotton swab dipped in rubbing alcohol; spray severe infestations with pyrethrins or rotenone.
Slugs and Snails
1/8 – 8 inches long; gray, tan, green, black, yellow, or spotted, with eyes at the tips of small tentacles; snails have a single spiral shell; slugs are shell-less.
Leaves with large, ragged holes. Place copper strips around beds; sprinkle sawdust, ashes, or diatomaceous earth around plants. Set beer traps in the garden. Handpick at night into a can of soapy water.
1/50 inch long, reddish-brown or pale spider-like mites with 8 legs; wingless.
Leaves stippled, reddish to yellow, with fine webbing. Spray plants daily with a strong stream of water. Keep soil moist. Chemical methods of control for severe infestations include the use of a miticide such as Avid.
1/50 – 1/25 inch long, with yellow, brown, or blackish bodies and two pairs of fringed wings.
Flowers buds die; petals distorted; growth stunted. Destroy weeds where thrips overwinter. Use insecticidal soap for serious infestations.
1/12 inch long, white, moth-like insects.
Leaves yellow; plant weakened. Spray leaves with insecticidal soap. Destroy weeds to reduce whitefly populations.
Although they have wings, these insects rarely fly, preferring to run for cover when they are disturbed.
Young earwigs can be destructive in springtime, but in summer and fall they feed on decaying plant remains thus transforming garden debris into useful humus. Large infestations may be a nuisance, and the insects can inflict a painful pinch. Keep the garden clean of debris and remove earwig hiding places. Make earwig traps by placing damp, loosely rolled-up newspapers on the ground near plants. The insects will crawl into the rolls, which you can then discard. For severe infestations, spray soil around plants with diazinon in the late afternoon or evening, according to label directions.
These slim cream-colored grubs eventually develop into click beetles and are especially common in new flower beds made in grass and lawns.
The wireworms feed on fleshy roots, bulbs, and tubers, usually weakening the plant rather than killing it. Attack flowers. Cultivate a number of times before planting to give predators an opportunity to eat them. Regular hoeing of established borders will help to reduce wireworm numbers. Control weeds, which may nourish them. Protect individual plants by forking in a soil insecticide when planting or by forking it in lightly around established plants.
These are becoming increasingly troublesome pests because they have developed resistance to many of the chemicals, which were previously used to control them. They are most commonly found in warm climates.
Adult weevils eat foliage, characteristically by taking semicircular notches out of the leaf edges. Orange-headed white grubs feed on roots and tubers. They attack most types of herbaceous plants are potential hosts; especially damaging for primulas, Michaelmas, daisies, phlox, peony, cyclamens, heucheras, and tiarellas. Wash the soil off susceptible plants and repot in new potting soil. Some insecticides will control adults. Use biological control: a parasite eelworm seeks out and penetrates the vine weevil larvae; it carries a bacterium that infects and kills the larvae.