THE CASE OF THE CRIMSON KINNIKINNICK

 Kinnikinnick leafgall aphid
Photo by: R.S. Byther (link)

Digger Spade, MG Plant Detective

In July, a client brought in several terminal branches of kinnikinnick (Arctostaphylos spp.) that looked quite healthy apart form the fact that the terminal leaves looked like bright scarlet gouty toes. Her questions: What is this? And what can be done to get rid of it?

The brilliant terminals looked like the plant’s response to an insect infestation and we quite quickly found the answer: an aphid that feeds on kinnikinnick and triggers this response by the plant.  The aphids are members of the Tamalia genus.  Some members of the group induce gall formation, and others are freeloaders attracted to an easy source of nutrients inside the developed gall.  Infestations seem to wax and wane, so the client may not see the same level of damage next year.  But she was anxious to know what could be done now. How about an insecticide?

Well, our Hortsense source doesn’t list one, and the Picol database indicates none are available to homeowners that list the specific aphid. One option is to remove all of the galls and dispose of them before a new cycle of infestation gets under way.  That reduces the likelihood of another invasion next year.  However, the flying phase of the life cycle may bring in another wave of invaders.
"But," the client said, "there are systemic insecticides on the market. Why can’t I use one of those?"

The short answer is that wouldn’t get rid of the galls. Using that approach would have been most effective as soon as there was any sign of the problem. There are several insecticides that list aphids (not specifying species) and indicate “plants such as” ground covers. Which, it seems, she would be able to use. If she does that it is important to note that she is entering into a contract to use it only as described on the label and only on listed plants and insects. In addition, it may also kill unlisted insects such as the Syrphid fly larvae that feed on the aphids in the gall, and some will still be in the plant next year in small amounts for nectar feeding insects such as bees to consume. Non-systemic  insecticides don’t get at the aphids in a gall but can get at any insects on the outside, even the good guys such as ladybugs and their larvae. Getting non-systemic insecticide on the aphids requires getting at the underside of the leaves and the gall, a tricky endeavor with kinnikinnick. Insecticidal soap would be a good choice if she wanted to do that.

However, since killing the aphids will not get rid of the galls and the galls will turn from red to brown, the aesthetic result of insecticide use would not be very satisfactory. That brought us back to the beginning.  Cut them off and dispose of them or leave them alone and relish the red phase.  That may result in somewhat slower spread of the plant, but it will not die.

Not all investigations lead to a solution, at least not to one that the client thinks is ideal.

Resources
Pacific Northwest Insect Management Handbook: Kinnikinnick (Arctostaphylos)-Aphid