Rose Pest Prevention

Have your rose leaves become ragged, stippled or covered with orange pustules? It may be a fungal disease called mildew.

Check plants for slugs, which leave translucent windows in the leaf between veins. Regular inspections are essential, as feeding can quickly progress to extensive leaf skeletonization. Use traps or insecticidal soap sprays with a high toxicity.

Aphids

Aphids feed by sucking the sap of rose plants and other tender garden plants. This results in stunted growth, twisted and curled leaves, and distorted or yellowing stems. Infected plants produce a sugary liquid called honeydew that attracts beetles and other insects, which then spread the aphids to other plants.

Inspect plants regularly for aphids, and be quick to catch an infestation. Spraying the aphids with water interrupts feeding and causes many of their mouth-pieces to break off, so they starve. It also disorients them so they are easier to remove by hand or with the garden hose.

Chemical controls include insecticidal soaps and horticultural oils, which kill the pests on contact. There are also products that contain parasitic fungi that infect and kill aphids when they come into direct contact with them (be sure to read the label carefully). Some of these fungi are now being grown as a biological control, which means they don’t harm bees or other beneficial insects.

Leafhoppers

Leafhoppers attack many types of plants, including grapes, ornamentals, vegetable garden crops, and roses. The insect’s nymphs and adults pierce leaves and suck juice, leaving behind white spots on the leaves. In some areas, several generations may develop per year. The leafhoppers overwinter as eggs that hatch in late winter or spring. Treatment is not necessary unless 20 or more leafhopper nymphs are found per leaf. Use of neem or horticultural oils can prevent leafhopper damage. If spraying is required, look for products with Piperonyl butoxide (PBO), an additive to pyrethrum that increases its efficacy and reduces the number of sprays needed per season.

Monitor populations using yellow sticky traps and sweep nets starting in mid-April, depending on location and economic thresholds. Attract chickadees, purple finches, sparrows, titmice, and swallows to the vineyard; these birds eat leafhoppers.

Rose Curculio

SCOUTING: When flowers have ragged edges and the stems are bending over, this is a sign of rose curculio. Adults are red or black-bodied and about a quarter inch long. They have black heads and a long, narrow snout.

These weevils drill holes into rose buds to feed and lay eggs. Once the larvae hatch they eat the reproductive parts of the buds and skeletonize the petals. They drop to the ground in late summer where they hibernate over winter.

Preventing rose curculio starts in the spring with diligent disbudding and removing spent rose blossoms to deny these beetles a host. If a serious outbreak occurs, spray the roses with insecticides that contain carbaryl or spinosad (Sevin). These will kill the adult beetles and protect the buds from damage.

Hoplia Beetle

Hoplia beetle (Hoplia callipyge, family Scarabaeidae) is a spring pest that feeds on light colored flowers including roses, calla lilies and hydrangeas. They burrow into flower petals, leaving chewing damage. They do not feed on foliage or roots and their life cycle takes a full year.

These beetles can be confused with Japanese beetles or rose chafer beetle, but their feeding damage is distinctive. If you have these beetles on your lilies or roses, you can simply pick them off by hand and drop them in a bucket of soapy water. This is a good strategy because spraying with broad spectrum insecticides near bloom runs the risk of harming pollinators and bees.

To prevent hoplia beetle, you can also use nematodes in October to kill the beetles in their grub stage before they can lay eggs. You can find these nematodes at garden centers and online.

Rose Midges

Rose midges (Dasineura rhodophaga) are dark mosquito-like flies that damage flower buds and leaf shoot tips. They hatch from eggs laid inside the sepals of new flower buds and feed in these areas, causing them to blacken or abort. Heavy infestations kill or skeletonize leaves and stems.

Look for notched or ragged edges on flowers and foliage and small larvae chewing on the undersides of leaves. In spring, midge feeding causes the tips of rose canes to wilt and die and reduces second-cycle blooms.

To prevent midge problems, do not fertilize or mulch around rose plants in early spring when the adults are emerging from overwintering pupal cocoons. Foliar sprays of the systemic insecticide acetamiprid (Ortho Bug B Gon Systemic Insect Killer concentrate or Ortho Flower, Fruit and Vegetable Insect Killer Ready-to-Use) can be effective when applied in late March to early April before buds develop.

Two-Spotted Spider Mites

Unlike other mites, which feed by sucking the fluid out of cells, two-spotted spider mites chew. Heavy infestations leave leaves ragged and notched. They can also skeletonize stems and buds, stunt plant growth and cause overall defoliation.

The mite carries a virus that causes rosette disease, which smothers plants and reduces bloom production. Monitor susceptible plants and destroy rosette-forming canes as soon as they appear. Soil drench treatments using pyrethroids (cyfluthrin, cyhalothrin and bifenthrin) can help control the eriophyid mite.

Look for signs of infestation on the undersides of leaves, including webbing, eggs and shed skins. Begin monitoring in late May as the weather warms, and check plants weekly. For best results, use a hose-end sprayer to apply sprays. Follow treatment intervals recommended by the label and avoid applying a spray when the plants are stressed.

Southern Red Mites

For the most part, disease prevention in roses relies on careful plant selection (varieties differ significantly in susceptibility), attentive cultural practices and, when necessary, thorough spray coverage. Avoid the use of fungicides that also kill beneficial insects and mites.

Rose midges, Aphidoletes aphidimyza and Dasineura rosae, feed within stem or bud surfaces and can leave blossoms ragged or blackened and deformed, with heavy infestations reducing second-cycle bloom production. Foliar sprays containing spinosad and soil drenches with imidacloprid provide effective control.

Western flower thrips, Frankliniella occidentalis and madrone thrips, can cause severe injury to flowers with distorted petals or streaking with brown color, especially during peak bloom periods. Scouting for thrips requires close examination, sometimes with a hand lens. Regular scouting also helps with mite monitoring. Look for bronzing and stippling of leaves and shaking foliage over a white piece of paper makes it easier to see mites, their eggs or cast skins.