Insect-related soot vexes Cape homeowners

Article from the Cape Cod Times

Lawn furniture covered in dark soot caused by Lecanium scale.

SANDWICH — Earlier this summer, John Recker installed a new plastic fence on his deck. Within a couple of weeks he noticed it was black with what appeared to be soot.

“New white plastic should be clean,” he said.

His wife attacked it with cleansers, but it came back. Then, he noticed it on his cars.

“I washed them, and the next day it was just as bad,” he said. But his wasn’t the only home afflicted.

“Anywhere you go outside, I see it on similar decks, at the golf course,” Recker said. “It’s everywhere.”

No one knew where it came from, but they had their theories. William Lundholm, of Cummaquid, wondered if the black/green soot that coated his decks, lawn furniture, plants, window sills and roof came from the jet exhaust from nearby Barnstable Municipal Airport.

“They come and go right over our house, kicking the old thrusters in when they’re going up,” Lundholm said.

He resorted to bleach and a garden hose.

“We’ve been here 19 years and never seen anything like this,” Lundholm said.

Its been a common complaint this summer, from Bourne to Brewster as well as on Martha’s Vineyard.

But entomologists and plant scientists say the problem isn’t jet exhaust, but bug exhaust. They cite an outbreak of sap-sucking lecanium scale insects that poop out what is known in the plant world as honeydew, a sugary substance that is a perfect growing medium for mold.

You might not even notice lecanium scale insects unless you looked closely at the twiggy end of oak branches and saw what appeared to be tiny shells lined up like an apocalyptic traffic jam on leaves. These “shells” are actually the armor the female scale insect builds to shelter her brood, which can number anywhere from dozens to thousands.

Lecanium scale feed exclusively on woody plants and trees. They have specialized parts in the mouth that can pierce the outer layer of a leaf or twig and tap into the phloem, tiny vessels in the leaves that carry sugars and soluble organic compounds produced by the leaves during photosynthesis, which are essential for plant growth and health.

But this sap, while nutritious for the plant, is low in the nutrients required by the insects. Therefore, insects must eat a lot to get what they need.

The insects excrete most of what they take in as a sticky cloud of water, sugars, amino acids and minerals that drops to the ground coating everything in its path. It is fertile ground for sooty mold, a saprophytic fungi that feeds on dead and decaying matter.

Sooty mold can damage plants. Dense growth blocks out sunlight to the leaves and can cause them to wilt. Because it is black, it can also retain the heat of the sun, burning leaves.

Although they won’t kill healthy trees, a heavy infestation of lecanium scale can stunt their growth and cause twig death and dieback, especially when drought, or stress from other insect invaders, have already weakened it.

These insects can be hard to kill, as males and females are armored. The female undergoes a spectacular metamorphosis. As she feeds, she grows much larger in size and a large cavity develops along her underside that hardens into a hollow helmet. She dies, and her brood hatches and begins feeding on the plant, safe under her protective dome.

It’s a nearly foolproof defense against spraying, according to arborists.

“My car is white and it’s black every morning, and you can imagine what I’ve sprayed up there,” said David Chalker of Mashpee, an arborist with Bartlett Tree Experts in Orleans.

“You have to treat it during the crawler phase,” he said.

Eggs are laid in late May into June and hatch within a month. The crawlers are the first stage to emerge from under the protective helmet of the female. They feed on the underside of leaves along the main veins, according to the University of Massachusetts Center for Agriculture in Amherst. By late summer, they migrate to twigs for winter.

Insecticidal oil can be sprayed onto a tree to kill them then.

“Nothing is 100%,” Chalker said. He expected the honeydew would continue to rain down through September when photosynthesis in the leaves grinds to a halt as trees prepare for winter.

The good news is that this is not an invasive species like the gypsy moths and winter moths that have devastated large stands of oak forests in the region in recent years.

It’s not yet known if the black oak gall wasp that also decimated oak forests is an invasive or native, said Joseph Elkinton, a professor in the Department of Environmental Conservation at the University of Massachusetts Amherst. Elkinton did the genetic testing to determine that these were native oak lecanium scale insects on the Cape and Martha’s Vineyard.

Trees have their own defenses, such as emitting volatile compounds that attract the parasites and predators of the invading insects. As a native species, lecanium has a lot of natural enemies that will eventually catch up to the population boom and knock it down to lower levels, said Russell Norton, horticulture and agriculture educator for the Barnstable County Cooperative Extension.

Ironically, some research has linked spraying for the moths and wasps to possible die-off of these beneficial parasites and predators, allowing the scale populations to explode.

Chalker and Norton advised homeowners to hold off on spraying for the average oak in their yard and to direct treatments, if any, at a few valuable trees that are exhibiting signs of infestation.

This is the second year of the infestation, Norton said.

Peter Wild, owner of Cape Tree Preservation, an organic plant health care company, is hoping it will be a non-problem in five years or less, but he worries that the Cape has a high percentage of older trees that have already been hit hard in recent years by successive waves of gypsy moths, winter moths and black oak gall wasps.

“The same three insects hammered the oaks,” Elkinton said.

But the forest trees have been given a reprieve. Recent wet springs encouraged the growth of fungi that decimate gypsy moth caterpillars. Natural predators caught up to the gall wasps, and Elkinton and his team at UMass, with help from the USDA, raised and released enough of the parasitic flies that specifically target winter moths that they have been reduced to just background noise.

“Hopefully, that will maintain itself for some years into the future,” he said.

— Follow Doug Fraser on Twitter:@dougfrasercct.