Wednesday, September 9, 2015

Fall Webworm outbreak

   The white webs are everywhere. Street trees, yard trees, and trees along the highway all seem to be covered by numerous colonies of the fall webworm. Last month, I photo graphed some fall webworm egg masses and predicted that we would be seeing an severe outbreak of this pest this fall. And sure enough, if you didn't spray your trees back in August, your trees are probably covered with webs.
    The photo at right shows a young tree in covered with webworm colonies. When I look at this picture, two questions pop into my mind : 1) Will webworms cause long term damage to this tree? and, 2) Why do the webs look so much bigger than usual?
   Fall webworm larvae can totally defoliate a young tree by mid-September. However, branches and dormant buds are untouched by the caterpillars. This means that limbs, defoliated in early fall, will typically remain in a dormant condition until next spring. The true long-term impact of webworm defoliation is a little harder to see.
    Losing leaves 6-8 weeks early means less time to build carbohydrates via photosynthesis and ultimately less stored energy for budbreak the following spring. The result is a weaker spring flush of growth and a slow down of tree growth rate. Fortunately, trees can overcome this lag in growth rate by supplying trees with ample soil nitrogen in early spring. 

    This fall's webworm colonies appear larger because the colonies actually have more caterpillars within each web. Over the years, I've looked at a lot of fall webworm egg masses and they always seem to be roughly the same size. This would mean the size of a colony's  web should also be about the same size each year. However, this year, there were so many egg masses laid that sometimes two egg masses appeared on a single leaflet (photo at left).  When two egg masses hatch in close proximity, caterpillars eventually coalesce and become like one big colony.  This year, I've seen young trees totally covered by webs in what looks like one huge webworm colony. However, what I'm actually seeing is the result of multiple webworm colonies competing to consume every last bit of green leaf tissue from the tree.