My Arctostaphylos uva-ursi has suffered from galls caused by aphids. What approach would be best to combat the aphids and when is the best time in their life cycle to attack?
Kinnikinnick or Arctostaphylos uva-ursi sometimes suffers from galls caused by aphids, and is also susceptible to fungal diseases. If your plant has galls, you would see distorted, thickened, and often reddish leaves which almost don’t seem leaf-like. The aphids may also secrete honeydew which can then turn blackish with mold.
Douglas Justice, University of British Columbia Botanical Garden Associate Director offers these comments on Arctostaphylos uva-ursi:
The Arctostaphylos uva-ursi cultivar ‘Vancouver Jade’ — a UBC introduction and one of the most widely grown cultivars in temperate climates — is adapted to wetter conditions than many other cultivars, as it was selected from the Pacific Northwest. Nevertheless, like all kinnikinnicks, it is not a plant for poorly drained, shaded or high traffic areas. And unfortunately, it appears to be rather more susceptible to manzanita pod gall aphid than other cultivars. Populations of that insect pest can build up during “warm winter” periods (such as we’ve been experiencing in Vancouver over the past several years) and disfigure plants significantly.
Source: UBC Botanical Garden Forums
Oregon State University has information about leaf gall on Arctostaphylos uva-ursi in their Plant Disease management handbook online.
The following, from Pacific Northwest Insect Management Handbook (WSU, OSU and U. of Idaho, 2005) provides more information about the aphids.
Kinnikinnick Arctostaphylos – Aphids
Manzanita leafgall aphid, Tamalia coweni:
Pest description and crop damage – Manzanita leafgall aphids are grayish or greenish in color and prefer new growth. They feed on the leaves of kinnikinnick and other manzanita species (Arctostaphylos spp.). Aphid feeding causes the leaves to thicken and form bright red galls. Older galls turn brown. Severe infestations may slow the growth of the plant.
Nongall-forming aphids also may be seen occasionally on kinnikinnick. They are greenish, soft-bodied insects that may feed on leaves or stems. Honeydew, a sweet, sticky material, may be associated with aphid feeding. It may attract ants or become covered with a growth of dark, sooty mold. Severe infestations may result in leaf and twig dieback.
Management-biological control:
Syrphid fly larvae are important predators of leafgall aphids, and will feed on them inside the galls. Avoid use of broad-spectrum insecticides which kill these and other beneficial insects such as ladybird beetles and parasitic wasps.
Management-cultural control:
Prune off and destroy galls where seen. Avoid frequent shearing and overfertilization, which encourages succulent new growth favored by aphids. Wash other aphid pests from plants with a strong stream of water or by hand-wiping. Avoid excessive watering, and use slow-release or organic sources of nitrogen. Control ants, which “farm” aphids and protect them from predators in order to harvest their honeydew.