Recognizing and Controlling Insects Harmful to Rose Bushes
By T. Jeff Williams
As beautiful as roses may be, they also seem to attract an uncommon number and variety of pests that do them harm. There is nothing personal, it’s just what happens when certain insects meet irresistible rose bushes.
While there are many harmful insects out there—and the types and control measures are described below—there are also many beneficial insects in the rose garden. Chief among these are the many different types of bees, all of which are vital to pollinating plants around the world. While insecticides may be necessary in some cases, limiting their use will further protect beneficial insects. There are viable alternatives to insecticides, as outlined below.
Giving your roses prime growing conditions and keeping the ground under the bushes clean to prevent insect breeding and nesting grounds are essential to developing healthy roses that are resistant to many insect attacks.
Insects and Controls
Aphids: These tiny (1/8-inch) insects cluster on rose bushes and literally suck the life out of them. As aphids draw increasing amounts of fluid from a rose bush the leaves wither and turn yellow, which in turn weakens the entire plant. Small, newly transplanted rose bushes can be weakened to the extent they will die.
Aphids often appear seemingly from nowhere in late spring early summer. One moment your rose bush looks fine and the next time you look it is coated with aphids. Although commonly a gray to light green in color, they may also be yellow, black, brown or even red. As they cluster on stems and buds they excrete a sticky honeydew substance, which attracts ants. The honeydew can also promote the growth of sooty black mold, which leaves black spots on the leaves.
Control: The first line of defense is to knock aphids off the bush with a hard spray of water. This does not kill them but delays further damage. Spraying once or twice a week will keep your bushes largely free of aphids.
An organic approach is to introduce ladybugs into your garden. You can often buy ladybugs at garden centers and ladybug larvae feed voraciously on aphids. For best results, release the ladybugs in the evening when they are less inclined to fly. Another beneficial insect that will feed on aphids is the lacewing.
If the problem persists, spray with insecticidal soap.
Cane Borers: Cane borers are the larvae of saw flies and carpenter bees. They often enter where rose canes have been cut or damaged. Once the eggs hatch, the larvae begin eating their way down the center of the cane, usually killing all growth above.
A likely sign of cane borers is a small hole in the cane and wilting or yellowing leaves above that. As the larvae begin working down inside the cane, that stem will turn brown and leaves will likely fall off.
Control: Prevention is the best measure here. After pruning your plants, put a dab of waterproof glue or fingernail polish on top of each stem to seal the opening. This will sharply reduce cane borers means of getting into the cane in the first place. If you see a hole in a cane, spray in some insecticide such as Malathion. It may be too late, but you might get lucky.
Prune all canes at least six inches below any signs of damage and discard the canes.
Thrips: Generally too tiny to see, thrips are insects that can do significant damage to rose bushes. Evidence of these nearly microscopic bugs includes curling leaves and leaf margins turning white, yellow or brown. The flower buds usually become discolored and bend over as they weaken. Flowers may not open and those that do may have bumpy petals streaked with brown. White and pastel flowers seem more susceptible to attacks than other varieties.
Control: First remove all infected leaves, buds and canes. Discard them completely rather than adding to a compost pile you may have. Shake diazinon powder on the ground under the bushes where young adults mature. Spray the tops of the bushes with insecticidal soap, diazinon, Malathion or Sevin, and spray the ground as well.
Rose Scale: Rose scale is an insect attack that causes the growing tips to die back and stop further plant growth. The rose bushes do not flower and leaves turn yellow and fall to the ground. A careful inspection will show clusters of round or oval tiny insects with crusty gray or white shells bunched on the canes.
The insects often also hide under gray scales on old canes. They feed by sucking juices from the plant, weakening it. Small cluster can simply be scraped off with you fingernail.
Control: As prevention, spray the canes in early spring before they leaf out with a horticultural oil to smother the insects before they develop. Once the problem develops, spray the canes with insecticidal soap until the symptoms are gone.
Rose Slugs: The larvae of sawfly wasps, these small slugs are widespread in the U.S. Evidence of rose slugs include small, rounded holes in the leaves and then the whole leaf becoming skeletonized, with all but the veins eaten. Generally the slugs will be found on the underside of the leaves. They have a yellowish head, black eye spots, white dots down the length of the body and short legs. They resemble a caterpillar.
Control: Remove damaged leaves and spray the remaining leaves with insecticidal soap. Be sure to spray the undersides of the leaves as thoroughly as the tops. If necessary, spray with Malathion or Sevin.
Spider Mites: Technically an arachnid instead of an insect, they can still do major damage to a rose bush. Spider mites usually form large colonies on the underside of leaves. They are recognizable by their mottled, salt-and-pepper appearance. As the infestation advances, you will see spider webbing on the leaves. The problem becomes more intense in hot and dry weather.
Control: First, keep the ground under the rose bushes cleaned up because spider mites hibernate in weeds. If you see colonies, wash them away with water. You have to mist regularly under the leaves to get at them, but they will not survive when regularly dampened. You can also apply chemical miticides or Malathion.
Leaf Miner: Evidence of this insect on the leaves are irregular white blisters that contain the tiny grub. As it develops, the leaf miner will do what the name implies, eating his way through the leaf.
Control: Because the leaf miner is protected by the leaf once inside, insecticides are not effective. Simply remove and discard all infected leaves.
Caterpillars: If you see large oblong holes in leaves and holes in the bottoms of rose buds, you may have caterpillars. They are the larvae of various insects and can do considerable damage. You may also see evidence of them as leaf rollers, which use silken thread to pull the leaf around the caterpillar.
Control: Caterpillars come in various sizes, but many are large enough for you to handpick after closely inspecting your plant. You can also use Bt (Bacillus thuringiensis), which is a bacterium that will kill caterpillars but is harmless to plants and animals. Bt is often found in a variety of commercial sprays for insect control. For leaf rollers, simply pull the leaves off and discard.
Leafcutter Bees: The holes made by leafcutter bees differ from caterpillars in that they are cut into the leaf margins and are almost perfectly round. The small bees, perhaps 1/2-inch long, do not eat the leaves but use the little leaf segment to build their nests.
Control: Damage is slight and not long lasting, so no real control is necessary. Any insecticides would possibly kill honey bees as well.
Japanese Beetles: These small insects, usually about 1/2-inch long or less, are readily recognized by their hard copper or green shells. If you see round or oblong holes in the rose petals and sometimes in the leaves, suspect Japanese beetles. These insects, however, are much more of a pest in the eastern half of the U.S. than in the west.
The Japanese beetle grubs live in the ground under the rose bushes and feed on grass roots in the winter. By spring, they hatch and fly up to the roses. They prefer light colored blooms.
Control: In the early morning before the sun warms them, you can hand pick Japanese beetles off your roses. Drop them into a container of soapy water to kill them. Equally effective is to put some newspaper under the bushes and then tap the canes smartly to knock the beetles loose. When they fall onto the paper, slide them into the soapy water.
A particularly effective measure against these beetles is to apply milky spore disease powder on the ground under the rose bushes. This is a naturally occurring bacterium that is lethal to the grubs but will not harm birds, bees or other animals.
Chemical insecticides such as diazinon will also kill the grubs once they emerge on the ground.
T. Jeff Williams is veteran builder, landscaper, gardener and former rancher who brings years of practical experience and a journalist’s background to his subjects. He has authored and co-authored some 15 books on these subjects, including books for Sunset, This Old House, Better Homes and Gardens and Ortho Books.