Some rose pests can be effectively controlled by other insects. The beneficial insects, like ladybugs, lacewings and hoverflies, arrive to feed once a pest population has built up to the point necessary to feed the beneficials. If you can wait out the early season assault of aphids and rose slugs, you will probably see insects arrive to defend your roses and feast on the pests. Make these helpful insect welcome in your garden by limiting your use of chemicals. Attracting birds to your yard increases the team working with you for balance and beauty in the garden.
Japanese beetles (Popillia japonica) are a very destructive landscape pest that has been present in the US since their accidental introduction in the early 20th century. While they have mostly been a problem for gardeners in the eastern United States, the populations are now spread more widely across the country. The beetle adult attacks the flowers and foliage of hundreds of plant species while the larvae eat away at grass roots. It is a heartbreak to see them feeding in large numbers on your roses.
The best long-term control of future populations is to target the grubs, which overwinter and spend most of the year about three inches deep in the soil, before emerging as adults in spring. Milky spore (a bacterium), applied to the lawn, causes a lethal disease specific to the Japanese beetle grubs. It does take a few years to become effective however.
The adult beetles produce a pheromone trail to your roses in their droppings so remove them as quickly as possible. It is not unreasonable to prune and dispose of heavily infested plant parts to discourage newcomers from finding their way to your plants. Hand pick the beetles and discard them in a pail of soapy water. They release easily when startled and may even fall off the plant into a bucket of water in an attempt to escape you.
Keep in mind that many birds such as cardinals, catbirds and grackles are the natural enemies of Japanese beetles and favor the grubs or beetles, or both, so support birds in your garden. Avoid the use of pheromone-emitting Japanese beetle traps. As satisfying as it is to remove a trap full of beetles, the attractant in the trap may be luring more beetles to your garden.
While they may not be kind to your roses, we hesitate to call leafcutter bees a pest. The damage that these bees cause is very identifiable, smooth-edged and circular holes in rose leaves. But the bees are not feeding on the leaves, they are using the cuttings to line their nests, usually in rotting wood, and their activity is brief.
These bees are important native pollinators and many rose gardeners choose to leave them alone. That is what we recommend. Any product that you attempt to apply to them will equally affect beneficial insects.
These yellow-green, olive green or black, slug-like pests are the larvae of several varieties of sawflies, non-stinging wasp family insects. Their appearance varies by type and they are frequently mistaken for caterpillars. These damaging larvae are active mostly at night and leave behind windows of papery, translucent damage on your rose leaves that later turns brown. Their feeding damage is rarely a threat to the health of the rose though the results are not pretty. Occasionally, a heavy infestation can defoliate a plant to the point of serious decline.
Begin looking for these pests on the underside of leaves in early spring. Larvae actively feed on plants for a period of about a month before dropping to the ground. The larvae cannot return to a rose leaf once they have fallen to the ground so spraying with a hose or brushing them off the plant is very effective. Some types of rose slugs may have multiple generations in a year so keep an eye on your roses for a reappearance later in the season.
You have many natural allies who will happily consume these insects. Birds, beneficial insects and even small mammals will help to control rose slugs if you make them welcome in your garden and limit the use of chemicals that can cause them harm. More gentle chemical approaches, like neem oil, horticultural oil and horticultural soap will all be effective in controlling rose slugs if applied to the insects while they are active.
If you have watched an entire season of rose blooms deform, blacken and fall off the plant then you are familiar with the damage that can be caused by rose midges. It is the larvae left by these tiny flies inside of rose buds that feed on the plant and cause withering and destruction of buds and stem tips.
By the time you notice the damage to the plant or see the white grubs at work, the injury to your roses may be nearly accomplished. The worm-like larvae move quickly and then fall from the plant to pupate on the ground. Once a rose cane is damaged, it will stop growing and must be pruned back to undamaged tissue to resume development in the following growing season.
If you have had rose midges damage your plants in the past, it is likely that they will over-winter in the rose bed and you should take early precautions to control them. In summer, once you identify the problem, place horticultural fabric under the rose plant to catch falling larvae for disposal so that they don't reach the protection of your garden soil. Practice careful garden hygiene in the fall, removing damaged plant materials, replacing mulch and disposing of all fallen rose debris (away from your compost area). Sadly, there are no known biological controls effective against rose midges. Contact your county's cooperative extension office for information on the chemical preparations that are licensed and effective as preventatives in your area. Chemical applications must be timed before the damage begins in the late winter or early spring and repeated every week to ten days to be effective. Both the soil and the plant should be treated.