Fighting Common Rose Pests

Roses are always a popular topic for Master Gardeners’ free gardening helpline, operated by the University of California Cooperative Extension, as well as various workshops and educational programs. 

In this second column of a multi-part Q&A series, we discuss how to tackle some of the most common pests that attack roses, and their natural predators.

Question 

Help!  My roses are being eaten by insects.  How can I protect my roses without frequent applications of pesticides?

Answer  

Insects and mite pests can cause havoc in a rose garden.  They can blacken leaves, chew blossoms and leaves, distort blossoms and cause leaves to stipple or yellow.  Pests can even make rose canes die back.  That’s the bad news. 

The good news is many rose owners maintain strong, healthy plants bursting with blossoms, with little to no insecticides.  This is especially true of Ventura County’s dry interior valleys.  The secret lies in selecting the right varieties for your area, tolerating minor insect damage and adopting good gardening practices. 

Number-One Nuisance:  Among roses, aphids are the most common pest.  These hungry insects feast on rapidly growing plant parts such as buds and shoots, especially in spring and early summer.   In moderate to high levels, aphids secrete large amounts of honeydew, which can cause a sooty mold that blackens leaves.  In very high levels, aphids can kill rose buds or reduce flower size.

Tackle aphids before they become a serious problem.  First, wash aphids off plant with a strong stream of water early in the day, so the foliage dries before evening.  If that doesn’t work, try insecticidal soaps or neem oils, which only moderately affect the aphid’s natural enemies.

Fortunately, aphids have many natural enemies, including lady beetles, green lacewigs and parasitic wasps.   Control ants around your roses with sticky barriers on canes to encourage more lady beetles.   If you purchase lady beetles at the nursery, mist beetles and plants with water before releasing the insects.  For best results, release beetles at dusk and place them on canes at the base of plants.  Don’t be surprised when most fly away, especially after the aphid population is reduced.

Stippled or Yellow Leaves:   Are your leaves bleached or stippled, with webbing?  You may have spider mites.  Often when the spider mites’ natural enemies have been killed by broad-spectrum pesticides, these tiny mites will increase in number.  To reduce this pest, wash leaves periodically.  If stronger controls are needed, try insecticidal soap, horticultural oil or neem oil. 

In some areas, rose leafhoppers can be a problem.  The stippling is larger in this case, and there is no webbing on the leaves’ undersides.  Insecticidal soap will help.

Distorted Blossoms:  If blossoms become distorted or streaked with brown, the culprit may be thrips.  These tiny yellow or black insects prefer fragrant, light-colored or white roses.  They also like numerous rose bushes planted together, because this provides a continuous-blooming habitat for their dining pleasure.   To reduce these pests, clip and remove spent blossoms regularly.

Chewed Blossoms and Leaves:   Several pests chew flowers and leaves – from Fuller rose beetles that leave ragged edges on leaves to leafcutter bees that cut semicircular holes.  Rose slugs look like tiny caterpillars, but are the sluglike larva of a sawfly.  Young rose slugs can skeletonize lower leaves, while larger ones can chew large holes.  A strong stream of water or insecticidal soap will help reduce these pests.

Dead Rose Canes:   Flatheaded borers lay eggs on diseased and stressed rose canes.  The white larvae can kill canes or an entire plant.  Another pest is the raspberry horntail, which also lays eggs on rose plants.  The larvae can cause canes to wilt and die in spring, reducing the second bloom cycle.  Scale insects cause small, grayish, round to oval encrustations on rose canes.  For all these pests, remove and destroy any infested plant material.  Insecticidal soap on remaining canes helps protect plant from further damage.

Gardener’s Friends:  Minute pirate bugs, lacewigs and lady beetles are some of the natural predators of rose pests.  Keeping a healthy garden, encouraging plant diversity among your roses and tolerating minor insect damage can go a long way towards attracting more of these natural enemies.

More Help:  Want more rose gardening tips?  The University of California Division of Agriculture and Natural Resources has information on beneficial insects as well as common rose pests and mites (Pest Notes, Publication 7466).  Visit www.ipm.ucdavis.edu for a copy.

Free gardening advice is available by calling the Master Gardener Helpline at 805/645-1455.  The helpline is staffed Tuesdays and Thursdays, 1-4 p.m.

Author:  Teresa O’Connor is a Master Gardener with the Ventura County Cooperative Extension.  For more information on the Master Gardener program, contact 805/645-1455.