Sticky shiny goop has started covering trees, dripping on anything underneath. Folks are calling to ask 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.
Most adult aphids are wingless, but in spring, many will have wings to facilitate travel to different host species. 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. At that point, they can produce 80 "babies" in a week.
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.
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.