There's a sticky residue covering driveways, garbage cans and, perhaps most annoyingly, cars in Austin neighborhoods these days. But the stuff falling from the canopies isn’t tree sap.
The substance isn’t a sign of a sickly tree or “sap season,” as some have guessed. It’s actually a sugary liquid known as “honeydew,” and it’s secreted by aphids, small sap-sucking insects living in our trees.
“Euphemistically, we like to call it honeydew because people don’t like to think about what it really is,” City of Austin Forester Keith Babberney said. “It’s basically bug poop.”
It may feel as if this stuff is everywhere, and that’s because it is. The aphid population boomed this year, with thousands living in trees at a time. The bug has fewer predators because of Austin’s historic drought.
“A lot of the insects that would’ve been the predators eating aphids are not having a very good year," Babberney said.
The “honeydew” may be familiar to folks who lived in Austin back in 2011 — the driest year on record in Texas. The insect’s population exploded then, leaving sticky surfaces all over town.
Babberney said aphids and their predators come and go in cycles.
“So this coming spring, predator reproduction may be bigger and, probably, the aphids will not be as severe,” he said.
To reduce the number of aphids hanging out over your car, Babberney recommends spraying down your trees with a water hose. Otherwise, you'll just have to wait for rain.
“The more rain we get, the more of them are gonna wash out,” he said. “We’re basically looking toward the end of summer and milder weather, when some of those predator insects will probably boom back.”
Babberney said treating trees with chemicals should be a last resort, as that could damage the entire ecosystem and throw it further out of balance. He said any chemicals that hurt aphids will likely kill other insects, too.
“We might be forced into a cycle of coming back and treating with chemicals again and again," he said, "because we also killed the ladybugs, wasps and whatever else may have been eating those aphids.”
Aphid excrement is nontoxic and doesn’t damage decks or cars. Babberney said the sap washes off easily with a quick rinse.
”If you went out just about any summer and stood under a canopy of trees, you’d probably notice little droplets and think, 'It’s misting a little bit today,'” he said. “This is just nature doing its thing… and we need to let nature run its course.”
