Monday, April 5, 2021

Worms in Trees this Spring

Every so often, we see larger population outbreaks of certain insects, for seemingly no real explainable reason.  This year is one of those years with either cankerworms or oak leaf rollers.  I'm seeing both species around the San Antonio area, but there's really not need to panic!

Spring Cankerworm in dark color form
Photo Credit - Mike Merchant,
Texas A&M AgriLife Extension

Cankerworms, also called inchworms, can be bright green in color or greyish brown.  They inch up when they walk and when disturbed will drop from the trees on a silk strand.  They can be seen on the trunks of trees, dropping from limbs, on the roof, car, anywhere else the wind may have blown them.  

Oak leaf rollers also drop from silk strands and are green in color, but have a dark blackish head capsule.  

Oak leaf roller on leaf.
Photo Credit - Bart Drees,
Texas A&M AgriLife Extension

Neither caterpillars are harmful to humans and often are not harmful to the tree.  In high population outbreaks they can cause some defoliation, but often the tree will leaf back out again.

Should you feel your trees are in danger, there are some options to decrease the population.  Bt foliar sprays early in the infestation or Spinosad foliar applications are organic options.  Bt is specific for caterpillars and will not harm other animals or insects, but be careful around your milkweed and other butterfly garden plants!  Permethrin is another option that is not organic.  Ready to use formulations make application easy - just plug into the hose and spray up into the trees.  Trees that are incredible tall may need the help from a pest management professional or arborist.

Often times, the best course of action is just to wait them out!  We may find that in two weeks, the caterpillars just disappear.