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Carpenterworms Are Spreading in the Phoenix Metro Area

Andrew Backhaus, Ph.D.

In the October 2004 newsletter we reported on our discovery of carpenterworms in the neighborhood surrounding 34th St and Osborn Rd in Phoenix. In that article we cautioned home owners to be on the lookout for the potential spread of these insect pests into other neighborhoods. Well, our worst fears were realized when Dave Spilsbury called us about some "ash borers" he found on a giant ash tree growing near 51st St. in the Arcadia area of Phoenix in early December. While the tree (see below) did not appear terribly damaged or at risk, it had a few flagging branches (note the dried, yellowed leaves on the photo below). Closer inspection of those limbs revealed severe borer damage with a notable Swiss-cheesing of much of the wood (see below). The real destructive nature of these insects was realized when Dave inadvertently caused a number of the limbs to come crashing to the ground after barely touching them while he was up in the tree. Up to 70% of the cross-sectional area of the limbs was gone! Another danger sign was the huge amount of frass that accumulated at the base of the tree (see below). It managed to pile up to a foot around the trunk. Yet, the tree still looked attractive from a distance.

The unsettling truth was that the damage was caused by carpenterworms and NOT leopard moth, as was originally suspected. I brought a bunch of logs back to the lab and stored them inside a large metal can. After just one day I noticed several tablespoons-worth of fresh sawdust in the bottom of the can and thought I could actually hear the munching of wood within. Later, with the help of Bob Celaya of the State Dept of Land Management, I managed to extract a few of these large critters and was amazed at the speed at which they could re-enter the worm holes they created. Bob also verified that these large caterpillars were, indeed, carpenterworms, Prionoxystus robiniae.

I am now rearing the larvae in our lab to see how long it takes for them to emerge. After watching them feed I can now see how they have the reputation of being the most aggressive wood boring insects known in North America. Until now, Arizona HAD been relatively free of this pest. However, with the outbreak we uncovered we should all be on the alert and monitor their spread in our state.