Rose aphids are one of the most common pests, and one of the first, we see on our roses in the spring. Although I, like most rosarians, can speak about aphids from first-hand knowledge, some of the following information was gathered from extension service reports and other sources.
Aphids are tiny (1/8″) green or pink insects with plump, pear-shaped bodies and two tubes, or cornicles, which project like exhaust pipes from their abdomens.
Aphids have three stages of life: egg, nymph and adult. They overwinter as eggs and in the spring, the newly hatched insects are all female. They give birth continuously to live nymphs, as many as 10 per day. These nymphs can begin to produce in 7 to 10 days and populations can reach high numbers very quickly. In the fall when temperatures go down, some males are produced and they mate with females who lay eggs on plants for overwintering, and the cycle continues. Applying dormant oil in the late fall will help kill the overwintering eggs.
Aphids cluster in dense colonies on new succulent growth and buds and can also be found on the undersides of young leaves. Damage occurs when the aphids suck the juices from the rose stems and buds. Unable to fully digest all the sugar in this plant sap, the aphid excretes the excess in a fluid which is called “honeydew,” and this honeydew drops onto the leaves below. A sooty mold fungus may develop on the honeydew, which causes the rose plants to appear black and dirty. The sooty molds in themselves are not harmful, but they prevent light getting to the leaves, which causes premature leaf fall and spoils the appearance of the plant. Ants feed on this sticky substance and often are present where there is an aphid infestation. Although healthy plants can usually tolerate fairly high populations without much effect, aphids are extremely prolific and populations can quickly build up to damaging numbers during the growing season. Then flower quality and quantity are reduced and the buds are usually deformed and fail to open properly.
Early detection is key to reducing aphid infestations. Weekly examination of plants will help to determine the need for control. Examine the bud area and undersides of new leaves for clusters of small aphids. If aphids are found, this is an indication that they are established on the plant and their numbers will increase rapidly. If there are just a few of these small colonies, they can be crushed by hand or removed by pruning.
Natural enemies of the aphid, such as the lacewings, lady beetles, adult wasps, insect larvae, earwigs, birds, etc., help reduce heavy populations. They may eat large numbers of aphids, but usually they do not appear in significant numbers until the aphids are already numerous.
Therefore, these natural predators may not be enough to keep the aphids in control.
Before using insecticide sprays, try knocking the aphids off with a strong spray of water to get them in control (and this will also wash off the honeydew, if present). Most of the aphids will not be able to return to the plant. It is best to do this early in the day to allow plants to dry more rapidly and avoid conditions for fungal diseases.
As a last resort and as control measures are warranted, use insecticidal sprays — Malathion, Orthene, Merit, etc. Reapplication is often needed. Always follow product label directions for safe use.