Image by Holt Studios/Nigel cattlin
Seen clustering around young rose shoots and flower buds, the rose aphid (Macrosiphum rosae) congregates in large numbers. Aphids, which are green (or pink) in colour and measure about 3 mm (1/10 in.) in length, are sucking insects that secrete "honeydew." You may notice ants hanging around aphids, collecting the secretion to take away. In large numbers, aphids may cause plants to develop distorted shoots and flower buds to fail to open. The insect can also spread viruses from plant to plant. Aphids have a phenomenal reproductive capability, especially during warm weather. Several generations occur throughout the growing season, and the pests overwinter mainly by eggs. Adults may also develop wings for easier dispersal to other plants. You may also find aphids feeding on other host plants, such as weeds. To control rose aphids, try the following measures:
- Wash away insects with a mild jet stream of water at least twice a week, preferably early in the day to avoid wet leaves late into the evening. Using water tends to be kinder to beneficial insects. While squishing aphids by hand may be quicker, it could inadvertently kill beneficial insects at the same time.
- Remove all rose foliage in the fall.
- Apply dormant oil treatment as directed on the label during the dormant season (November to February).
- Consider planting flowers rich in pollen and nectar. These "insectivory" plants serve as a food source to attract beneficial insects into your garden. The "good bugs" (lady beetles, parasitic wasp, lace wing, etc.) will produce more offspring, which then seek out the bad bugs. Some examples of insectivory plants include Achillea (yarrow), Lobularia (sweet alyssum), Potentilla (cinquefoil), Echinacea (purple coneflower), Cosmos, Helianthus (sunflower), Coreopsis (tickseed) and Solidago (golden rod). Try to establish an area in your garden for these types of plants. Select an undisturbed, sheltered site that includes a water source (such as a saucer partially filled with pebbles and water) to attract beneficial insects.
- Restrict spraying to spot treatments in order to reduce the impact on the beneficial insect population. Always follow the directions on the label, and remember that any spray that kills insects will not discriminate between "good" and "bad" ones.
- Avoid excessive fertilizing, which tends to produce a lot of aphid-attracting succulent foliage. If you top-dress your roses regularly with organic matter, you can cut back on your fertilizing program.