Those strange white bumps on your rose bush stems might look like a plant disease. Is it a fungus? A canker? Some type of virus? Maybe it’s some other form of damage?
What you’re likely actually seeing is rose scale – an immobile, sap-sucking insect that lives under a protective shell. Once they become adults, these insects spend their entire lives in one spot, sucking juices from plants.
When the population gets high enough, these insects can cause plant decline, decreased flowering, and stem and twig dieback. Sometimes heavy infestations can kill your rose bush.
There are thousands of species of scale insects in the world, most of them preferring certain varieties of plants. The species that prefer roses, Aulacaspis rosae, also feeds on raspberries, blackberries, hydrangeas, and pear trees.
We will be focusing on Aulacaspis rosae in this article. There are other scales, such as Chionaspis furfura (scurfy scale), that also attacks roses. Confusingly, sometimes aulacaspis rosae is also called scurfy rose scale, but these are two different scale pests. Luckily, the way you control them is similar.
How to Identify Rose Scale
You can recognize this pest by the white to light gray bumps up and down your rose stems. These ¼ inch “bumps” are the waxy shell made by this armored scale and provide the insect under it with protection from predators. It also protects it from any insecticide you may try to spray.
Under the armor are a small orange insects. They tend to live in colonies, which is why you’ll tend to see multiple scales on your rose stems. As they form a mass on the stems, it oftentimes leads gardeners to think what they see is a disease.
Life Cycle: How Do They Grow
Females lay eggs under their shells. They can lay anywhere between 50 to 150 eggs in their lifetime.
When the eggs hatch in spring, the immature scales, or nymphs, emerge. These nymphs are tiny and are difficult to see. If you suspect nymphs, you can shake a branch over a piece of white paper and look for tiny, moving orange dots.
This is the same scouting method you would use to look for spider mites, which require a different control method. A strong magnifying glass will help you determine if it’s mites or scale. Remember, mites have eight legs, insects have six.
The baby scales will travel to a suitable feeding spot. Wind can blow the nymphs onto other rose bushes, or they can also travel to other plants by hitching a ride on animals, people, and garden tools.
Within a day or two of hatching, the nymphs find a permanent spot. They settle in, pierce their mouthparts into the stem, and begin feeding. Soon after that, their waxy protective covering develops over them.
Under their armor, female rose scales are about 1/16th of an inch in diameter. Males are white and elongated and much smaller than females. The males can fly, while the females cannot.
1-2 generations of these insects can coexist at the same time in the northern United States, but more generations can coexist in warmer areas.
It’s a good idea to contact your county extension service if you have a large rose scale problem. They can help you determine exactly what to do about this pest in your specific area.
What Kind of Damage Do They Cause
Small infestations are unsightly but generally do not cause much damage. When the colonies are big, severe twig and cane dieback can occur. Shrubs can lose their vigor, flowering is not as abundant, and a very heavy infestation can kill a rose bush.
Small infestations generally do not cause significant damage to healthy roses. It is up to the individual gardener to decide if the infestation is problematic enough to warrant controls.
Always start with the method that will have the least impact on your garden’s health. Start with mechanical and cultural controls first, and only use chemical controls if all else fails.
Keep in mind that the waxy shell will persist even though the insect under it is dead. These shells will flake off easily.
Cultural and Mechanical Control Methods
Grow in your roses in full sun. Scale infestations can be more severe in shady locations and high humidity.
Keep your plants healthy with proper watering and fertilization. Healthy roses will be able to better survive an insect attack than weak or sickly plants.
Removal is the best way to deal with this pest. Simply cut out the infested stems and dispose of them. Do not attempt to compost them. Clean your clippers or loppers as you work so that you do not accidentally move the insects from one location to another or from shrub to shrub.
Small infestations can be rubbed away or picked off. Watch out for the thorns, though!
Scale insects have many natural predators, including parasitic wasps, praying mantis, and ladybugs. Encourage these “garden warriors” by limiting your use of pesticides, planting a bit wildly, and letting nature do what she does so well.
When you decide to use chemical control on any plant, please read the label carefully before you buy. Be sure that the product you want to use is appropriate for the pest you are trying to control.
Use only the recommended amount, apply it at the right time, and wear any recommended personal protective equipment (PPE). Sometimes pesticides can cause more harm than good.
Horticultural oil can be applied in late winter or early spring before the leaves start to bud out. Horticultural oil smothers the insects and the eggs under them. Coat the rose canes thoroughly and only use them when temperatures are above freezing and below 85F° (29C°).
A systemic insecticide can be used, but only after other control methods have failed. A systemic insecticide is one that is applied to the soil around the root zone of a plant. After you water it in, it is taken up by the plant’s vascular system, which makes the plant poisonous to the insect feeding on it. Do not use a systemic insecticide when the plant is in bloom, otherwise, it will be harmful to the bees and other pollinators that visit the flowers.
Do not try to control rose scale with other types of chemical controls, such as “all-purpose” garden sprays. Insecticides are only effective against rose scale when the insects are in the nypmh stage, and it’s challenging to determine when that is, especially when there can be multiple generations in a season. Because the waxy coating protects them, eggs and adults are invulnerable to chemical controls.
Additionally, insecticides kill both beneficial insects and pests. If you use insecticides indiscriminately, you can kill natural predators and pollinators. You will only be creating more headaches for yourself by killing insects that are helpful in your garden.
Homemade sprays will also be ineffective since they cannot penetrate the protective shell.
There aren’t any known measures to prevent this pest. But there are some steps you can take to reduce populations and keep the damage they cause low.
While there aren’t any rose bushes that are resistant to scale, buying strong, hardy, time-tested varieties of any plant is the first step in pest management. Pests are attracted to weak and sickly plants.
Keeping your roses healthy will help them weather a scale infestation. Planted in a good sunny spot in good soil and providing proper watering and fertilization will help your roses resist all kinds of pest and disease problems.
Going out in your garden and looking for pests before populations can get large and cause significant damage is crucial to pest management. Now that you know, keep your eyes open for those white bumps on your rose canes and prune them out right away.