Roses are widely planted in gardens and landscapes for their beautiful floral display and scent. However, they are susceptible to a variety of insect and mite pests, one reason roses are considered a high-maintenance plant. This article discusses some of the major rose pests, including aphid, Japanese beetle, two-spotted spider mite, rose midge, and rose sawfly.
Aphid. The predominant aphid species that attacks roses is the rose aphid, Macrosiphum rosae. However, many others also attack roses. Aphids are soft-bodied, pear-shaped insects, generally less than 1/4 inch long; they vary in color (green, pink, and red). Two tubes, cornicles, stick out from the end of the abdomen.
Aphids don’t have to mate to reproduce, and they can give birth to live offspring. Aphids are generally found feeding on roses early in the season. They re-move plant fluids with their piercing–sucking mouthparts. Aphids tend to congregate in large numbers, feeding on terminal growth (leaf and flower buds) and on leaf undersides. Their feeding causes leaves to curl upward and deforms flower buds. In fact, flower buds may abort (fall off) before opening. In addition, aphids produce honeydew, which attracts ants and serves as a growing medium for black sooty mold fungi. Aphids don’t normally cause direct plant harm to roses unless they are present in high numbers.
Aphids are susceptible to a variety of natural ene-mies (predators and parasitoids), which in sufficient numbers can provide some control. A hard stream of water removes aphids from plants without harming their natural enemies. This technique is effective as long as it doesn’t promote diseases such as black spot. If needed, pest-control materials for aphids include acephate (Orthene), diazinon, imidacloprid (Merit), and insecticidal soap. When using systemic insecticides such as imidacloprid, apply them early enough so that the active ingredient is present in new growth just as aphids start feeding.
Japanese beetle, adults of Popilla japonica, which was discussed in issue no. 10, is a major pest of roses. Japanese beetle adults are 3/8-inch-long, metallic green beetles with coppery brown wing covers. They have tufts of white hair at the end of the abdomen.
Adults are typically present from late June to early July. Japanese beetle adults congregate in large num-bers on rose flowers. They mainly feed on flowers but also on leaves. They chew holes in flowers, which causes them to not open or to drop petals prematurely. They also feed on the petals and pollen of fully opened flowers. Adult feeding on leaves gives a skeletonized appearance.
Japanese beetle adults can be managed in small rose plantings by handpicking or placing a fine net-ting over the plants. In larger plantings, pest-control materials—including carbaryl (Sevin) and cyfluthrin (Tempo)—are generally effective in killing Japanese beetle adults. Repeat applications are necessary. Sevin should not be used when bees are active, as it is extremely toxic to bees. Apply Sevin in early morning or late evening. Avoid using Japanese beetle traps, which tend to lure more beetles into an area than would normally be present.
Two-spotted spider mite, Tetranychus urticae, is orange, green, or yellow, with two dark spots on the sides of the body. Spider mites tend to feed on the underside of rose leaves, especially older leaves. They remove chlorophyll (green pigment) from rose leaves with their styletlike mouthparts. Their feeding causes leaves to appear bronze-colored and stippled. From a distance, leaves look yellow and dusty. Also, silken webbing may be present on leaf undersides. Heavily infested rose leaves turn brown, curl, and fall off.
Infestations of two-spotted spider mite normally occur under dry, hot conditions (above 70°F). During summer, spider mite populations can build up to tremendous numbers. Roses experiencing drought stress are more susceptible to spider mites. Two-spotted spider mites overwinter in protected areas such as on weeds, ground litter, or fallen leaves. They do not overwinter on roses, which means dormant oils are not effective. Spider mites can be detected on roses by holding a sheet of white paper underneath leaves and tapping the leaves sharply. Minute green, red, or yellow specks about the size of pepper grains drop on the paper and crawl around.
A hard spray of water removes spider mites from roses and preserves populations of natural enemies. Remove any fallen leaves or branches, as well as weeds, which may serve as a host for spider mites. Keep plants well watered and apply mulch to rose plants during drought conditions to minimize stress. Pest-control materials include abamectin (Avid), bifenthrin (Talstar), dicofol (Kelthane), hexythiazox (Hexygon), insecticidal soap, and summer oil.
Rose midge, Disineura rhodophaga, can be a destructive rose pest. Adult midges are small (1/20-inch long), red to yellow–brown in color. Females lay tiny yellow eggs on new growth, beneath the sepals of flower buds, in leaf buds, or in shoots. Eggs hatch into small (1/12-inch long) white larvae (maggots) that eventually turn bright orange. The larvae possess sickle-shaped mouthparts, which they use to create deep gouges in plant tissue; and then they feed on the exuding sap. They generally feed at the base of flower buds or on the upper leaf surface and leaf petioles. A large number of larvae (10 to 25) may be present in a single bud. Their feeding causes leaf axils and buds to become twisted, deformed, and blackened; this even-tually kills new shoots. The larvae fall to the ground to pupate in small, white cocoons. Rose midge over-winters as a pupa. Adults emerge in about 5 to 7 days. The adults are rarely seen and do not feed, living for only 1 to 2 days. The life cycle from egg to adult, depending on temperature, takes 16 to 22 days.
Managing rose midge initially involves removing and destroying infected plant parts. If necessary, pest-control materials including acephate (Orthene), diazinon, and malathion can be used.
Rose sawfly. Three species of sawflies attack roses: the rose slug, Endelomyia aethiops; the bristly rose slug, Cladius difformis; and the curled rose sawfly, Allantus cinctus.
The rose slug females create slits or pockets along the edges of rose leaves with the sawlike ovipositor and insert individual eggs. The eggs hatch into larvae resembling slugs. The larvae are 1/2-inch long when fully grown and yellow–green, with an orange head. They feed on the upper leaf surface, giving the leaf a skeletonized appearance. Eventually, the larvae drop to the ground to pupate. Rose slug overwinters as a pupa. There is one generation per year.
Bristly rose slug larvae are pale green and 5/8-inch long. The body is covered with many bristlelike hairs. The larvae feed on the underside of leaves, similar to rose slug, giving the leaf a skeletonized appearance. There may be several generations per year.
The curled rose sawfly larvae are green, with white markings on the thorax and abdomen. The head is yellow, with black eye spots. This sawfly differs from the other two in that the larvae may consume an entire leaf and then bore into pruned twigs. The larvae initi-ally skeletonize leaves, but they eventually eat the entire leaf except for the main vein. The larvae then tunnel into twigs to pupate. This leads to death of stems and creates wounds, which serve as entry sites for fungal pathogens. Two generations per year may occur in Illinois.
To manage rose sawflies, clean up all debris and remove weeds. A hard spray of water knocks sawfly larvae from plants; and they are unable to crawl back onto roses. The three species of rose sawflies are susceptible to natural enemies, including predators and parasitoids; but they may not occur in numbers sufficient to prevent damage. If necessary, pest-control materials including carbaryl (Sevin) and insecticidal soap may be used.