Sticky shiny goop has started covering trees, dripping on anything underneath. Folks are calling wondering what this is. They are also noticing lots of ants on the plants. Are the ants causing this goo? The answer is no. Ants don’t cause the goo, but they ‘herd’ the aphids generating the honeydew. Plants most affected are ash, cherry and plum trees; roses and dogwoods.
Aphids are generally small critters with soft bodies although willow aphids can be large. With long mouthparts, aphids pierce stems, leaves and other tender plant parts to suck out plant juices. They come in a multitude of colors: not only green or black, but also brown, yellow or red. Some are woolly such as the woolly apple aphid, which looks like cotton on a tree. The main identifier for aphids is the tubes on their backside called cornicles.
Most adult aphids are wingless, but in spring, many will have wings to facilitate travel to different host species. As in the case of the giant willow aphid, aphids usually feed in large colonies. One of the challenges of aphids is that females can produce live young without mating. This allows for many generations of aphids in a year. They can go from newborn to reproducing adults in only seven days; when they can produce 80 “babies” in a matter of a week. Populations can boom!
Aphids in moderate numbers rarely cause significant damage to healthy plants. The sticky honeydew that they exude as they suck out plant juices can be an annoying problem. It can even take the finish off a car. An additional concern is that aphids carry and transmit viral diseases that affect a large number of plants adversely.
Aphids aren’t difficult to control. For most aphids, I recommended hosing them off. You have to do this weekly for a few weeks, then as the hot weather hits, the aphid populations will start to decline. Or, you can squish the aphids you can reach with your fingers. Since natural predators are important in aphid control, avoid using chemical sprays, except as a last resort. Insecticidal soaps work quite well. Horticultural oils are another alternative, but pay attention to labels to avoid damaging sensitive plants or discoloring blue evergreens. I have noticed that the goldfinches, pine siskins and other birds I attracted and fed all winter keep the aphid population low on most of my trees.