What are those nests in the trees along Interstate 83?

As you've been driving around York County, especially along Interstate 83, you might have noticed thick webs in some of the trees.

Some trees are covered with the nests while others only have a few.

So what are they?

They are called fall webworms. 

Here's what you need to know:

Q. Are they native?

A. Yes, fall webworms are a common, native species of moths, said Kelsey Frey, manager of education with York County Parks. The nests of the caterpillars are pretty easy to spot in trees, such as walnut, cherry and willow, at the end of summer and beginning of fall.

Fall webworms make their nests around the entire branch, and the caterpillars eat the leaves while inside the protection of the web, she said.

That's different from the eastern tent caterpillars, which are another native species. Tent caterpillars make their nests in the spring in the crotch of a tree, which is where branches meet together or connect with the tree trunk. The caterpillars leave the nest at night to feed and stay inside the nest during the day for protection.

Visitors to York County parks often mistake the fall webworms for gypsy moths, which are an invasive species, Frey said. Gypsy moths do not make webs like that.

Q. Are they worse this year?

A. The garden hotline at the Penn State Cooperative Extension in York County has recently received several calls about the fall webworm, said Connie Schmotzer, consumer horticulture educator for the office. 

She has seen a report that they seem to be heavier this year. The population will spike some years, but then return to more level numbers.

It often goes in cycles, said Michael Skvarla, insect identification lab director at Penn State University. During an outbreak year, more predators and parasites breed, and then in a couple of years, the fall webworm is well-controlled because there is an increase in natural enemies.  

Natural enemies lag behind because they do not breed as quickly, which allows outbreaks of fall webworms occasionally, he said.

Q. Will they hurt the trees?

A. No. Fall webworms can defoliate trees, but it rarely kill them, Skvarla said.

It's almost time for the trees to lose their leaves, anyway. It's not like defoliating the tree at the beginning of the season when it has to grow a whole new set of leaves, expending a lot of energy.

The trees have gotten what they are going to get out of their leaves already.

"Usually, it's just an eyesore," Frey said.

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Q. How do I get rid of them?

A. If you want to get rid of them, all you need to do is take a stick and poke some holes in the nest. That will allow natural predators, such as birds, bees and wasps, to get into webs and eat them.

"There's no need to cut down that branch. There's no need to burn it or spray chemicals," Frey said. "All you got to do is cut open a hole in the nest, and it lets predators get into them easier."

Skvarla said he just leaves the ones in his trees alone.

"People don't like that it's unsightly, but the benefits of killing them often don't outweigh the costs," he said.

In some cases, however, homeowners may want to protect a tree that has suffered outbreaks of fall webworm several years in a row or has been weakened by another pathogen, he said. 

Homeowners can prune the branches with the webs and dispose of them.

Q. What's the life cycle of a fall webworm?

A. The eggs hatch in mid-summer, Skvarla said. Some people will notice the young larvae in July or early August. The young take about six weeks to mature, and during that time, they live in a web while feeding gregariously.

The mature caterpillars will descend from the web and spin a cocoon in leaf litter, soil or another area to pupate over winter, Skvarla said. That's when people are likely to encounter them.

The adults emerge next season, usually in mid-June, he said.

The webs will persist long after the caterpillars leave.

The weather, however, will help to remove the nests from the trees, Schmotzer said.

Q. Can I have a little fun with them?

A. Yes. These caterpillars — if they are still in the nest — have a defense mechanism where they will dance or wiggle all together to make themselves look like something big and scary to predators, Frey said.

Anyone can wave at the nest or poke it, and the caterpillars will shake all together, she said. It's happened while she has been taking a photograph of the web.

Q. How do I find out more?

A. If you are looking for more information on moths and butterflies, visit the website: www.butterfliesandmoths.org. Another resource is a book, "Tracks & Sign of Insects and Other Invertebrates: A Guide to North American Species."