Gay Wilhelm, Master Gardeners of Placer County
Q: What can I do to keep the roses insect free?
A: Start by selection of rose varieties that are less susceptible to insect and disease problems. Healthy and vigorous plants can produce beautiful blooms with little or no use of insecticides. Appropriate cultural practices,
like watering and fertilizing, are your best defense against insects and mites. Occasional handpicking or forceful sprays of water can also decrease populations. Learn to identify natural predators than can also assist your fight against pests.
Aphids are the most common insect pests on roses. With new spring growth, they could be the first pests you will see. Low to moderate populations do little damage. High populations can secrete honeydew that could lead to sooty mold, which can blacken leaves. Bud formation may also be reduced. Lady beetles, soldier beetles, and syrphid flies are natural enemies that can significantly reduce their numbers. Look for packages of lady beetles at nurseries this spring. The good news is that aphids are usually around only 4-6 weeks in spring and early summer. Hot weather naturally sends them packing. Strong sprays of water will decrease the populations as well as insecticidal soap. These methods have only moderate impact on natural enemies. Soil-applied systemic insecticides as well as other insecticidal sprays may be effective but seldom necessary.
Spider mites are associated with dry and dusty conditions. Leaves may be stippled or bleached and dry up and fall off. Mites are tiny, less than the size of a pinhead. Populations usually increase after broad-spectrum insecticides are used to control other pests. Once again sprays of water or insecticidal soaps are usually all that is needed.
Thrips are usually found within the flower petals. They can cause petals to streak with brown or become distorted. Fragrant and light colored flowers are most susceptible. Thrips can usually be tolerated. Removal of spent blooms can decrease populations. Thrips are also beneficial as a predator to spider mites.
Leafcutter bees, leaving tell tale semicircular holes in the margins of leaves, are important pollinators. Learn to tolerate this minor annoyance. Fuller rose beetles chew flowers and foliage. Rose curculio is a red to black snout weevil that prefers white to yellow roses. Handpicking is the most effective method for both these pests.
Insect damage to roses can be controlled with observation, cultural and something as simple as a spray of water.