Friday, August 8, 2014

Fall Webworm in Kitsap County

Fall Webworm "tent."
Clemson University - USDA Cooperative Extension
Slide Series,
This spring the resurgence of Western Tent Caterpillar (Malacosoma californicum) became more noticeable here in Kitsap.  I wrote about it here.  If you're worried about your trees because of it, seeing caterpillar tents in your trees now in summer might give you the jitters.  Fortunately, there is good news:  new "tents" in your trees at this time of the year are nothing to worry about.  They are caused by a far-less troublesome moth species called Fall Webworm (Hyphantria cunea).  Right now, the rascally Western Tent Caterpillar exists only as sleeping egg masses on the twigs of your prized fruit tree.  Next April they'll awaken and wreak havoc.

It is possibly a bit early to see Fall Webworm tents.  If they've even hatched out yet, their tents should be pretty small.  So far I've not seen any, and some years I don't see any at all.  But that is what makes Fall Webworm much less troublesome than Western Tent Caterpillar:  they don't normally build
up to destructive population levels.  In any given year they produce comparatively fewer nests.  Their numbers are kept low by more than 50 species of parasites and 36 species of predators known in their native North America (1).
Fall Webworm caterpillars.
Steven Katovich, USDA Forest Service,

Depending on where you are in North America, the Fall Webworm may have one to four generations per year.  In warmer climates you can have up to four.  Here in Kitsap county, one generation per year is typical and the larvae appear in late summer to early fall.  After hatching out the larva begin feeding and building their protective "tents."  Over 600 hosts species have been recorded for this species.  It feeds on deciduous broad leaf trees like alder, willow, cottonwood, and various fruit trees to name a few.  Unlike the troublesome Western Tent Caterpillar, Fall Webworm larvae feed within their tent.

The caterpillars are quite color variable.  Two genetic races can be found:  Blackheads and redheads.  Both differ from each other not just in head color but also body color variations as well.  You can find both types in Kitsap county.

After feeding for a few weeks, the caterpillars pupate and go dormant to survive the winter.  While in the pupal stage, the larva develops into an adult white moth.  The moths emerge from their cocoons in late spring to early summer in our area and eventually they mate and lay eggs with the eggs hatching within one to two weeks.  The new caterpillars then begin to feed and build tents, starting the cycle all over again.

Adult Fall Webworm moth.
Jerry A. Payne, USDA Agricultural Research Service,
So what should you do if you find Fall Webworm tents in your trees?  As I see it you've got two reasonable options:

  1. Do nothing.  They are not going to be very damaging and it's unlikely that you've got very many nests in your trees.  It will be only a  minor, temporary inconvenience.  If you think you've got a large number of tents, something else may be going on (send me pics).
  2. Prune out the tents.  If your trees are small enough, pruning them out should be relatively easy.  If a small ladder is not adequate, see option #1 above.  Or if you absolutely must get the tents out, hire a professional ISA certified arborist to help out.

If you notice "tents" in your trees, don't panic.  Just think "Cool!  They're not Western Tent Caterpillars!"


  1. Johnson, Warren T., Howard H. Lyon.  1991.  Insects That Feed On Trees and Shrubs. Cornell University Press, Ithaca and London.  p 166.

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