Borers

Borers chew tunnels in woody plant material.

Borers kill rose bushes and other plants

​Which insects are borers?

Boring into wood provides safe habitat and food for several different beetles, moths and even some wasps! Most borers transform into pupa and then adult insects within this safe haven, only to emerge and start the cycle again. According to the Colorado State Extension (and a few other sources), the following are the most commonly found wood boring insects in the United States:

Beetles:

• Flathead borers/Metallic wood borers (Coleoptera Buprestidea)
• Roundheaded borers/Longhorned beetles

Moths:

• Clearwing borers
• Carpenter worms
• Pyralid borers

Wasps & Sawflies:

• Rose stem sawflies (Hartigia trimaculata)
• Horntails

California flathead borers

Flathead borers

I found the pictured borers in one of my rose bushes. The bush hadn’t been performing well for a while, but I had neglected taking a really close look at it until it was too late. When I dug it up, the weakened main stem broke in half and I was able to see two rather large (1” long), creepy looking larvae. A little research helped me see just how little I knew about borers, but now I have a better idea of what symptoms might indicate a borer infestation.

​Flathead borers make zig-zagging paths under the bark, interrupting the flow of critical fluids in the cambium layer. You will not see sawdust expelled by these insects. Instead, the sawdust is crammed into the paths created by the larva. In trees, the upper branches, or crown, will thin first when a flathead borer is at work. Adults emerge from their woody home through a D-shaped exit hole. Metallic wood borers are a variation of Flathead borers and they can be recognized by, you guessed it, their metallic bodies. The adult Flathead beetle is a rough-looking, blackish beetles with gray splotches - thoroughly unimpressive, but very destructive.

Rose stem girdlers

Rose stem girdlers (Agrilus cuprescens Ménétriés) are the boring insects responsible for hollow stems. The larva tend to be fatter and more segmented than the flathead borers, so that wasn't what killed my rose bush. However, rose stem girdlers can kill raspberries and blackberries, along with roses, so watch out! The adult beetles is rust colored and it is related to the bronze birch borer. The fascinating thing about these particular pests is that the larva chew through the bottom of the egg they start out in and then into the cane of bramble fruits or roses, where they chew a spiral pattern into the cambium layer, killing off the branch.

Rose stem girdler ("Indian forest insects" 1914) Public Domain

Rose stem sawfly larva (James Solomon - USDA Forest Service) CC BY 3.0 US

Rose stem sawflies

Sawflies are cousins to wasps and bees and this particular variety saws holes in the canes of rose bushes in order to lay eggs. The adult sawflies are black or brightly colored, yellow or orange, with black wings. Adults have a narrow body and each variety has a signature method of feeding destruction. The bristly rose slug sawfly (How’s that for a name?) skeletonize leaves from the underside before chewing large holes all the way through the leaf.

There are several other wood boring insects, which we will learn about later. One in particular, the Emerald Ash Borer, may result in the loss of nearly all ash trees in the U.S., if effective counter measures are not found. This invasive pest came from Eurasia and has spread across most of the U.S. on infested firewood.

How to prevent borer damage (before it's too late)

The best way to prevent borer damage is to keep plants healthy in the first place. Healthy plants are better able to defend themselves. This means proper irrigation, sunburn protection, and the removal of diseased plants. Using sticky barriers can reduce beetle migrations, but it can't do much about moths...