Friday, July 22, 2016

Fall Webworms of Summer

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."  Wikipedia
OK, 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
H. cunea overwinter as a pupa (that is in cocoons), and the moth emerges to lay eggs the next year.  In Missouri that has been in late summer but with Global Warming (yes, I actually said it), they likely are emerging earlier leading to the two generation latitude moving north from 40° to our 37° N.  Contrary to Wikipedia, in southern Florida they may have 4 generations a year!

Moth - Wikipedia
The moth is small but kind of cute with a bushy head of fuzz.  The wings are white in the northern range with black speckled fore wings in the south.  It will be interesting to see if there is a change in wing coloration with time if multiple generations occur in Missouri.
Cocoon - Andrei Sourakov

Pupae - Andrei Sourakov

H. cunea is a "reverse invasive species," introduced into Yugoslavia in the 1940s it is now over all of continental Europe as well as in China and Japan.  A cynic might say this is a fair trade for Japanese Honeysuckle and Garlic Mustard but as usual we brought it on ourselves.

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.