Roses and the Insects of Summer
Master Rosarian, Orange County Rose Society
This article is a 2019 Award of Merit winner
Roses & You, July 2020
Spring provides conditions in which mildews thrive. However, when summer arrives and temperatures rise, insects become dominant in the rose garden. Heat increases insect reproduction and keeps them active, flying and traveling on summer breezes. Some of these insects are rose pests and some of them can be quite destructive. Mother Nature provides a balance with other insects that prey on the ones that harm our plants. Here is a rundown of the most common insects pests and beneficials you may see in your garden. Aphids are not included as they appear first thing in the spring.
Western flower thrips are small, slender, dark colored insects that rasp the juices from blooms and buds. Females create small slits in petals and lay their eggs within; after a few days the eggs hatch into ravenous larvae. They are difficult to control because they live deep within the buds and blooms of roses and they spend their pupae stage in the soil beneath. Damage shows as brown and silvery streaks, and red spots on the petals. Blossoms may look ragged while buds may look as they have been chewed. If the buds become deformed, they may ball or fall off the stem.
Unfortunately, sprays that eradicate this pest will also kill any beneficial insects so most home gardeners try to tolerate flower thrips. They may make your some of flowers look bad, but they won’t kill the plant. There are organic solutions such as insecticidal soap or neem oil, but they have to be applied directly to the flowers. Remove any infected buds or flowers, and if you have a bloom that is important to you, consider covering it lightly with a very fine mesh that these tiny insects cannot penetrate. The thrips rely on wind currents to transport them. Remove any weeds or overgrown grasses where these thrips often start breeding. Flooding the area under the rose may help by destroying pupaes.
Chilli thrips are a whole different challenge. These pests rasp new growth on the plants, leaves, canes, buds and blooms. They are invisible but their damage is severe. Leaves are stunted and twisted and canes are stripped. Buds are deformed. These thrips also travel on the wind and they multiply rapidly in hot weather. Because you can’t see the insects, the damage they do to your bushes may seem sudden. Spraying is required; they will not go away and will spread quickly unless you spray. According to current research, the most effective products to use contain spinosad or imidacloprid. You can find products that contain spinosad in local big box stores - look for it on the label. Stronger concentrations of spinosad can be found in products available online. Spray weekly until thrips are under control.
Because they spread easily between plants, it is essential to cut off any infected growth and dispose of it immediately. This provides a challenge because the infected growth is new and has the buds you’ve been waiting for. But it is necessary because chilli thrips are persistent and will multiply quickly.
Rose slugs are not really slugs or worms; they are the larvae of a little black fly, the sawfly. They rasp leaf surfaces, sometimes enough to make holes in the leaves. They hide beneath the leaves, but they are quite visible. Although their damage is unsightly, they will not kill a plant or stunt it like other pests such as chilli thrips and spider mites. There are several species that attack rose leaves, and one of them is active during the summer months. If their damage is intolerable, spinosad is effective in killing them. Their arrival is usually periodic, they do not live in colonies, and they will not multiply like other summer pests.
Spider mites used to be the biggest summer threat before the arrival of chilli thrips. Once again, these insects are miniscule and infection is indicated by the damage they do. Because they do not have wings, damage often appears first on the lower parts of the bushes. Rasping under the leaves causes brown spots and streaks on the surfaces and dirty looking specks of black excrement create a “salt and pepper” appearance. Webbing may occur in heavy infestations. Spider mites love hot temperatures and gravitate towards drought stressed plants. Hard water sprays under the leaves every four days for one to two weeks may control them, but if their damage is overwhelming, rosarians may resort to using chemical sprays, such as abamectin (Avid). Neem oil or insecticidal soap may be effective, but requires insect contact. Spraying them should be repeated frequently, such as twice a week, to take care of eggs and new hatchlings. Hot, dusty, dry areas are their favorite so keep your plants hydrated and clean.
Scale is a pest that also enjoys warm temperatures and we do sometimes see it in the rose garden. There are many types of scale insects. They are tiny and protect themselves with a hard or soft shell. What is actually visible are these shells, appearing as a collection of bumps on the canes. The actual insects are under the shells. As a result the easiest way to get rid of them is to cut out the part of the plant they inhabit. If this is not possible or sufficient, light horticultural oils or neem oil will smother pests, but don’t use them on hot days.
Leaf cutter bees leave very visible damage on the leaves. They cut well formed circles in the foliage because they use the material to line their nests. Often gardeners think caterpillars or beetles are eating the leaves. Leaf cutter bee damage is easy to identify because of the symmetry of the chewed holes. These bees are pollinators and are not feeding on the leaves, so most gardeners leave them alone. Whatever spray product that would eradicate them will also kill beneficial insects.
Several themes emerge here. Most insect pests thrive and multiply rapidly as temperatures rise. Some of them travel on breezes between plants. The most destructive ones are very difficult to see. Spinosad is effective in controlling many of them because it contains a bacterium that paralyzes their digestive system and kills them in one to two days. It is only effective when rasped from the leaves, but if bees get it on their bodies while its wet they may somehow ingest it. Use spinosad when bees are not foraging. Once it is dry, the bees are safe.
Beneficial insects usually arrive after the insect pests have reached a level that will feed their offspring. The problem, particularly at the beginning of the rose blooming season is to resist using pesticides because beneficial insects will not come to a garden without prey to feed on. In a spring that is heavy with aphids the gardener must have lots of patience because it seems lady beetles arrive much later, but once they do, you will find larvae all over the aphid infested bushes.
Lady beetle adults eat primarily aphids, as many as 50 to 60 a day. Their larvae also eat aphids, but they also feast on mites, scale insects, and insect eggs. The mature larvae are usually around 1/8 inch long and are black with orange markings.
Lady beetles are one of the most visible and popular predators in the garden.
Syrphid flies are also easily seen; many times people think they are bees or wasps because of their black and yellow striping. Careful observation will discover they only have one pair of wings instead of the two pairs that these other insects have. Also known as hover flies, they drift around the garden, looking for areas where they can lay their eggs near a prey source. The adults do not eat insects but their larvae do; they feed on aphids, mites, thrips, mealybugs, and insect eggs. The larvae can grow to 1/2 inch long, and are green or light brown in color. They may look similar to rose slugs, but they are lighter in color and have small spines along their bodies.
Parasitic wasps and flies tend to be small and are easy to overlook in the garden. There are many varieties, but chances are, if you see a small flies or wasplike insects on your roses’ leaves, they are beneficial and are looking for prey. These insects actually deposit their eggs inside an insect pest; the larvae hatch and then eat their host from the inside. What’s left is the dried shell of the insect with a hole in it made by the emerging predator. These beneficials will feed on aphids, scale insects and sawflies.
Lacewings are one of the most elusive and beautiful beneficials. Green or brown, their wings are spun of thin fragile fibers. Their bodies are very slight so they blend into the foliage. The adults often fly at night and eat nectar and pollen. Their larvae are voracious insect eaters; they eat aphids, thrips, mites, whiteflies, leaf hoppers, and insect eggs. They are about 3/8 inch in length and are speckled or striped brown and yellow with hairy spines. They are ferocious looking but they are the gardener’s friend.
Assassin bugs, minute pirate bugs, and damsel bugs are considered “true bugs” that have mouthparts designed to pierce and suck other insects. They are generalists; they eat a variety of insects, including aphids, thrips, mites, and even small caterpillars. The adults, as well as their larvae, are the predators in this case.
There are a number of other beneficial insects such as mites, spiders, and beetles. If you want your garden to be an environment that encourages pest predators to visit, you can do several things. Stay away from pesticides, particularly chemical ones. Many adult insects feed on pollen and nectar; companion plants with small flowers will meet their needs. Like all living things, these insects need water. Include sources such as fountains, or even a spray of water to make puddles on the ground and drops on the leaves.
Rose lovers who are garden lovers should lower their standards on what their garden should look like. An extremely tidy garden that is well manicured and devoid of insects defies the balance that nature intends. There is a natural stability in a garden where insects interact and maintain an ongoing equilibrium. Avoid pesticides if you want to enjoy a garden full of life. i