Horrors! It’s Borers!

There’s the tell-tale hole where the borer entered the pith. Very bad news.

Last week while admiring my tiger eyes sumac, I noticed a hole in the pith in one of the stems.  Pith, in case you’re not aware, is the spongy material inside each stem.

I then examined the pith in two other stems that had been pruned before I bought the plant six weeks ago.  All three stems had a hole.

Omigaw! I realized that my baby tree was under attack by borers—currant borers, most likely, which attack currants, gooseberries and black elders, as well as sumacs.

According to Colorado State University, currant borers lay their eggs on the plant’s bark in June and early July.  Then the caterpillar larvae bore into the plant, drilling down into the pith and wood.  The nearly adult pest spends the winter in the base of the canes before pupating and later emerging as a full-grown adult in late May or early June.

Before I pruned it, the tree had two nice, leafy branches. Notice the two upright bare branches? That’s where the borers attacked. They also attacked at the lowest crotch of the tree.

Make no mistake—these bugs can kill the plant, if left untreated.

So I whipped out my pruners and began amputating lengths of branches until I reached non-holey pith.  The worst damage occurred on an auxiliary branch, where the borer had drilled all the way down that branch and into one of the tree’s two main branches.  I could even see the borer in the hole.  This meant I had to make the unkindest cut of all to save the plant’s life—removing a main leafy branch, leaving just one leafy branch on this 30-inch-tall tree.

It wasn’t easy, but I did what had to be done, disfiguring the tree in the process.  Fortunately, sumacs produce new branches relatively quickly, so I expect the plant to recover in the next couple of years.

After pruning, the looks disfigured, but the tree wlll likely grow plenty of new branches in the next couple of years. Notice the healthy pith where the branches have been cut. I sealed the cuts with fingernail polish.

I then sealed the three pruned canes with colored nail polish to prevent further infestation.  Some gardeners think sealing is unnecessary.  Personally, I seal canes larger than 1/8 inch.

Borers attack a broad range of trees and shrubs.  The most infamous borer in Colorado right now is the emerald ash borer (EAB), which began attacking ash trees in Boulder in 2013, and has spread to Longmont and Lafayette.  It’s just a matter of time before the EAB will reach Denver and Fort Collins.

Treatments for borers vary, depending the variety and size of a plant.  For information on borers and their treatment, visit Insects that Feed on Colorado Plants and Shrubs.

Next time you’re strolling through your garden, check pruned branches on your shrubs for borers.  Once the autumn leaves have fallen, pruned canes will be easier to spot.

As for trees, learn to recognize borer entrance and exit holes in the bark, so you can keep an eye out for borers there.