When I saw the first webs the first webworms in little clusters in June I didn't think it could be "Fall Webworms", Hyphantria cunea. I was used to the large webs covering a whole branch and these were just on a couple of leaves. I was wrong, they were just warming up.
The web was full of caterpillars and their frass which is exactly what you think it is. The cats were moving about very slowly even though the brisk breeze was whipping the branch around. I grabbed the branch and they immediately sensed the difference and began to crawl around frantically (for a caterpillar), showing evasive behaviors as seen in this real time video.
"One generation per year emerges in the northern part of North America, with larvae appearing in late summer through early fall. South of an approximate latitude of 40°N there are two or more generations annually, with webs appearing progressively earlier further south." WikipediaOK, so what are they doing here so early? I suspect there are two factors. We had a record population last fall, prompting lots of newspaper and TV reports. This means there is a larger graduating class of adults in the spring.
As temperatures rise some moths may emerge earlier and lay their eggs ahead of schedule. Successful reproduction requires that a food source be available when caterpillars emerge from the eggs. Studies show that the first leaf out time is occurring earlier in recent years, so there is now food for the young caterpillars available earlier.
|Second instar - REK|
|Moth - Wikipedia|
|Cocoon - Andrei Sourakov|
|Pupae - Andrei Sourakov|
|Fourth instar - Shelly Cox|
One element of their success is their wide range of food plants. They have been reported to eat at least 636 species of leaves. Fortunately they haven't developed a taste for animals or we might all be in trouble.