FO-06594     1997

To Order

Roses for the North

Insects

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Roses are hosts to many insect pests. Aphids, mites, rose stem borers, and gall wasps were the four most common insect pests observed between 1989 and 1992 at the Minnesota Landscape Arboretum. Leaf miners, leafhoppers, thrips, and other chewing and skeletonizing insects were seen occasionally.

Aphids

These small insects are one of the more common pests of roses. They are soft-bodied, usually lime green, and are found on cane tips, flower buds, and on the bottom of new leaves, where they puncture the plant to suck juices. As they feed, they excrete a sticky and glossy residue called “honeydew.” Severe infestations can lead to drying and curling of new leaves. Pesticides and natural predators, such as ladybugs, provide control for aphid infestations.

Figure 10. Aphids feeding on Rosa ‘Robusta’. The larval stage of ladybugs, called “lions,” are feeding on the aphids.

Mites

Mites are also common on roses, but are difficult to see because of their very small size. They pierce foliage and suck plant sap from leaves. The yellowing and mottling of leaves that result are often the first symptoms observed. Mites also form fine webs on the undersides of leaves where they feed. Severe infestation causes defoliation. Mites are most often seen under dry conditions.

Cultural practices that result in vigorously growing roses are a gardener’s best defense against mites. Pesticides are commonly used to eliminate severe infestations. Dormant oil, which smothers mite eggs, is often applied to dormant bushes. Natural predators of mites can also be purchased for biological control. Repeated water sprays of infested plants, especially at the undersides of leaves, will help control but not eliminate mites.

Stippling caused by mites.

Rose Stem Borer

The rose stem borer, Agrilus aurichalceus, damages rose canes when larvae tunnel in a spiral fashion beneath the bark, girdling and killing the canes (figures below). Their presence is indicated when a cane dies above the point of borer tunneling. Leaves on the infected cane turn brown as they die, creating a “flag” among healthy, green-leaved canes. On close observation, a swelling or gall on the infected cane can be seen below the dead tissue, indicating where the borer’s tunneling occurred. The gall formation weakens canes and it is common to see infected canes broken off by wind. The quickest and most effective control is removal and disposal of infected canes in fall.

From top to bottom: rose stem borer flag, rose stem borer gall, and rose stem borer tunneling.

Mossy Rose Gall

Mossy rose galls are caused by Diplolepis spinosa, a cynipid gall wasp. These galls are common on wild roses of North America, from Ontario to Alberta in Canada and throughout most of the northern United States. They are becoming common on Rugosa cultivars.¹⁷ The presence of these insects is indicated by the formation of spherical, golf ball-size, spiny galls on the canes of host plants.

The development of these galls is stimulated in the spring by newly hatched larvae. The galls encase the larvae until adult wasps emerge the following spring. The galls are unsightly and alter the plant’s shape. They also stress the host plant, behaving like nutrient sinks, drawing nutrients away from the rest of the plant.^{17, 18} Large numbers of galls on a plant can kill the plant.

Insecticides have no effect on the wasp that causes mossy rose gall. The most effective control is physical removal and disposal of galls in autumn after leaves have dropped and galls are visible. It is important to dispose of all galls since even a single missed gall can produce and reintroduce 30 to 40 mature wasps to the garden the following spring.

Mossy rose galls caused by Diplolepis spinosa on rose canes. Hybrid Rugosas are particularly susceptible to this disfiguring gall.

Insect Observations

Observations of aphid, mite, mossy rose gall, and rose stem borer occurrence in the Arboretum’s Shrub Rose Garden from 1989 through 1992 are provided in Table 9. Insect pest occurrence was highest among Hybrid Rugosa cultivars, with cultivars among other classifications infected less often. Sixty-eight percent of the insect pest observations occurred on Hybrid Rugosa cultivars or on R. rugosa species.

Although aphids and mites often favor particular cultivars when feeding on roses, they commonly feed across all classes of roses. In contrast, mossy rose galls caused by Diplolepis spinosa were seen only on Hybrid Rugosa cultivars and Species Roses.

Rose stem borer damage occurred primarily on Hybrid Rugosa cultivars and R. rugosa species. The occurrence of the rose stem borer was, however, more widespread than this evaluation shows. Because the borer was not diagnosed until late in the study, early observations of borer symptoms were pooled with other unexplained symptoms.

Although the occurrence of the rose stem borer was highest among Hybrid Rugosa cultivars, they were also found on cultivars in other rose classes. They have been observed on ‘Lillian Gibson’, ‘Prairie Dawn’, and ‘Alba semi-plena’ since the end of the evaluation.

The Hybrid Rugosa ‘Frau Dagmar Hartopp’ has large, single, light pink, fragrant blossoms. Flowers and large red hips appear together in late summer and fall.

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