Monday, June 30, 2014

Fall Webworms

Prairie Dock doesn’t typically suffer much insect predation, so a clump of brown leaves in the Prairie Dock patch easily caught my attention.  I arrived at the affected plants to find a hoard of Fall Webworms consuming the thick, leathery leaves.

I’m used to seeing Fall Webworms at Blue Jay Barrens, but they have always been on trees or shrubs.  There don’t seem to be many tree species that they won’t eat, so I guess it’s not unlikely that there are many plants they would find palatable.  If that’s where the female laid her eggs, the larvae really don’t have much choice other than eat or die.

Fall Webworms are communal feeders that build a network of webbing that covers their feeding area.  New silk strands have been stretched over the midrib of the dock leaf to offer protection to those caterpillars heading out to begin feeding on a new section of leaf.

I think this is the webworm equivalent of painting yourself into a corner.  It won’t take long to eat this small edge piece.  Then it will be a long trek to the other side of the leaf to find some more green.

As the name suggests, Fall Webworms are normally encountered towards the end of summer.  In Ohio, this species can produce two generations in a summer, especially if warm temperatures arrive early in the year and hasten the spring emergence of the adult moths.  This year we seemed to shift from winter directly into summer, with little transition.  Snow fell on April 15 and that was the last time we had any freeze or frost.  By April’s end the temperatures were in the seventies.  Any species whose development was triggered by temperature was quick to emerge.

Several of the webworms didn’t remain within the community.  Bunching together is supposed to afford some measure of protection from predators.  Maybe predators are more attracted to the mass of potential prey items and fail to recognize these lone individuals.

When I checked back a couple of days later, the webworms were gone.  I looked around, but couldn’t find them anywhere nearby.

All they left behind were shed skins and frass caught in the old webbing.  Fall Webworms typically leave the webs and go to the ground to pupate, but I wouldn’t think they would do that immediately after casting off their old skins. 

The following day I found several individual webworms.  These were all the next size up, a little larger and hairier than what I had seen on the Prairie Dock.  Some were on the ground and others were munching on various plant leaves.  Perhaps there is some type of dispersal that takes place prior to pupation, so the pupae aren’t all confined to the same small area.  I’ll have to pay attention when more of these guys show up later in the summer.  Maybe I’ll learn whether or not this is typical behavior for the species.


  1. This is interesting to read about. I have always seen fall webworms on the apple trees in my yard and they are always in large groups. This year I have seen four final instar individuals feeding on bindweed (Polygonum sp.), milkweed, raspberry, and chicory. The one on the milkweed stayed there for two days feeding on leaves.

    I wonder if fall webworms need to eat other plant foods for some reason. There certainly is no lack of deciduous trees in the area so it wasn't a case of starvation for them.

    1. Hi, Gary. That's a good question. Right now, I'm seeing both large groups still in their webs and individuals on their own. The individuals are feeding on a wide variety of trees, shrubs and forbs, including milkweed.