We live in the Toronto Beach area, and at the back end of our garden, under our neighbour’s very large weeping willow tree, there are literally hundreds, and hundreds, of bugs crawling around the rims of our potted plants, and along the fencing ! They are dark purple in colour, oval in shape, and leave a purple dye when squished. Also, there are ants following them around, and even wasps are landing where these bugs have been. They move around very quickly, but just go in circles, and, thankfully, don’t seem to be attacking any of our garden plants.
Should we be trying to eradicate these bugs? Please let us know what they are, and are any of our garden plants, such as azaleas, rhododendrons and Japanese maples at risk? Thank you !
Thank you for writing, and, of course, finding such hoards invading your garden must be a shocker! But in fact you’re not alone, as many equally puzzled gardeners have written queries about similar discoveries. Anecdotally, one such gardener south-east of Toronto, wrote: ” We live in Philadelphia, and over the past few months part of our back yard fence has been colonized by these small (3mm) insects. There are hundreds of them. The fence runs underneath a weeping willow tree. The bugs move fairly quickly. I’ve tried sweeping/blowing them away, and they return hours later. When you squish them it leaves a purple residue. ”
Sound familiar? The common denominator is the presence of weeping willow trees. And there is one insect that favours the weeping willow, and that is the Black Willow Aphid. Aphids are one of the most common bugs found on trees and shrubs. Aphids can range in color including green, black, red, yellow, brown, or gray. They have soft, pear-shaped bodies, and fully mature aphids can have wings, or be wingless (apterous).
The Toronto Entomologist Association posts that: “aphids are true bugs, of the order Hemiptera, suborder Homoptera: their lower lip is modified into a sucking tube that is inserted into plant or animal tissues on order to feed. “
Entomologist Jeffrey Hahn, of the University of Minnesota, describes Black Willow Aphids as “…having orangish or brownish legs, and cornicles, that is, the tail pipes of an aphid. They are large —for an aphid — reaching up to 3/16th inch in length. They can be quite abundant in August and September. These aphids are common on willows, and may also be found occasionally on poplars, and silver maples.”
So what about the sheer numbers of these “true bugs” ? Well, the Black Willow Aphids drop to the ground, either with the falling leaves, rain, or perhaps knocked off by feeding birds, and they attempt to get back on their tree, by climbing fences, or traveling around the rims of your pots. Aphids secrete a sticky, sugary honeydew, which coats the area under the infestation. This, in turn, attracts ants, and wasps, as you reported.
The good news is this: aphids on most landscape plants are “host specific”, meaning they will feed only on a particular tree or shrub, and are not interested in others types of plants. So for example, aphids infesting a quaking aspen would not be expected to colonize an oak tree. So, as you have found, these Black Willow Aphids are not tucking in voraciously on your azaleas. And healthy, vigorously-growing trees, like your neighbour’s weeping willow, tend to suffer very little damage. The even better news is that in Ontario, populations build quickly during hot, dry weather, but are often kept below worrisome levels, where predators such as lady beetles, lacewings, minute pirate bugs, etc. are doing their work.
So my best advice would be to keep your whisk handy, if they land on your garden chairs, squish as little as possible, as the purple dye is apparently permanent, and wait for the fall weather to bring their adult life cycle to a close. No action required !