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Desert Gardener: Roses require intensive management

January 12, 2002|By Eric Natwick, University of California-Imperial County Cooperative Extension adviser

Roses are among the most common plants in home landscapes.

Rose culture requires intensive management, including insect pest control. While insects and mites may attack roses from time to time, intensive use of insecticides is not always necessary to maintain vigorous plants that produce high quality blooms.

Rose varieties vary significantly in susceptibility to insect and disease problems and vary in tolerance to hot dry weather, so select varieties carefully.

Follow appropriate cultural practices for pruning, fertilizer and watering. Hand-picking of insect pests from rose bushes can be a painful experience, but using a high-pressure nozzle on a garden hose to spray pests such as aphids and mites away from plants can be an effective pest control practice.

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Rising natural enemy populations can rapidly help control aphids and mites, so learn to recognize beneficial insects as well as pest insects.

The most common insect pests on roses are aphids. Some aphid species observed on roses in the low desert are the rose aphid, the potato aphid and the cotton aphid.

Aphids are most commonly found on new growth such as buds and shoots but also may be found on mature leaves and green stems.

Although many gardeners are concerned with the very presence of aphids, most roses can tolerate low to moderate levels of these pests with little damage to plants. Aesthetic value and plant health decline rapidly with the development of moderate to high aphid populations. High aphid populations remove large quantities of plant sap and secrete copious amounts of honeydew, resulting in the growth of sooty mold, which blackens leaves. When aphid populations increase to high numbers, flower size is reduced and new buds may be killed.

Natural enemies of aphids can rapidly reduce increasing populations. Some of the aphid's natural enemies are lady beetles, syrphid flies, green lacewing and tiny stingless parasitic wasps.

Ants are attracted to aphid honeydew and protect aphids from natural enemies. Sticky barriers or traps can be used to discourage ant activity and to improve biological control.

Aphid populations are most common in the low desert during the spring. Lady beetle, lacewing, syrphid fly and aphid parasite populations increase when aphid populations are high. The convergent lady beetle is sold at nurseries for release against aphids and may reduce numbers when properly released. However, seven-spotted lady beetle and green lacewing migrate into towns when their populations build on alfalfa and wheat during the spring.

Green lacewings are common natural enemies of aphids and other soft-bodied insects. Green lacewing adults feed on honeydew, but the gray-green to brown alligator-shaped larvae are the predatory stage of theses insects. A single lacewing larva can devour many aphids per day.

Adult aphid parasites are tiny wasps that lay eggs singly within aphids. The parasitic wasp larva kills the aphid, which turns into a bronze crusty, bloated mummy. The wasp larva pupates within the mummy and then cuts a neat round hole and emerges as a full-grown wasp, capable of parasitizing many more aphids.

Syrphid fly larvae are important predators of aphids and very common on roses. Adult syrphid flies also are called flower flies or hover flies. Syrphid fly adults resemble small bees or wasps, feed on nectar and pollen and often are seen hovering above flowers. Syrphid fly larvae are legless and pale green or tan maggots, often found within aphid colonies, and have a voracious appetite.

The second most common arthropod pests of roses in the desert are two-spotted spider mites. Spider mites cause leaves to be stippled or bleached, often with webbing, or they may cause leaves to dry up and fall.

These nearly microscopic pests, (no larger than a period at the end of this sentence) are best seen with the use of a hand lens.

Spider mites flourish under dry, dusty conditions and increase greatly if their many natural enemies are killed by broad-spectrum insecticides applied for aphids or other pests.

An insecticide commonly used on roses, carbaryl (Sevin) frequently is followed by an increase in mite populations. To manage spider mite populations, conserve natural enemies, provide sufficient irrigation and reduce dust. Spider mites do not like wet conditions, so overhead irrigation or periodic washing of leaves with water can be effective in reducing mite numbers.

When treatment of spider mites is necessary, insecticidal soap or NEEM oil can provide control.

Tiny true bugs, called minute pirate bugs, are often among the first predators to appear in spring. Minute pirate bugs feed on mites, insect and mite eggs.

Predacious mites also are important predators of spider mites. Two-spotted spider mites have two spots on the dorsal abdomen, which are lacking in predacious mites, and predacious mites move rapidly, a characteristic spider mites lack. Spider mites and mite eggs also are preyed upon by western flower thrips.

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