Gardening & Living in Grand Style
Locust trees and the locust borer…
by Michael Johnson
Aug 29, 2013 | 2051 views | 0 0 comments | 13 13 recommendations | email to a friend | print
The world of gardening involves many different areas, including plants, insects, soils, fertility and more. As such, it’s wise when thinking of one specific area – say trees – to consider a variety of aspects such as the always important soil preparation and irrigation, but also potential pests.

In articles I have written this summer I have talked about trees to plant and also about insects that have caused problems in our landscapes. Now I want to talk about a couple of plants I happen to like and why just liking them might not be enough.

One small- to medium-sized tree I like is the purple robe locust (Robinia pseudoacacia). This tree can grow 30 to 40 feet high by 20 to 30 feet wide. It’s medium to fast growth rate means that with appropriate care you could have a nice size tree in five years or so. With its open leaf structure and smaller leaves, the purple robe locust fits our hot and dry climate reasonably well. Its leaf structure means the tree’s water needs aren’t over the top, and the rose pink flowers hanging in clusters can put on quite the show in the spring. However, this tree can have stiff spines or thorns on the branches, although many growing in our area don’t appear to have these. Also, its fast growth and upright branch structure can lead to branches splitting off and damaging the trunk. On the plus side, our tree specialist and I agree that with the growth rate of this tree you could consider it almost a perennial, which could mean if the tree was to become excessively damaged you cut it down, plant a new one and in fewer years than with most trees you would have nice shade again.

A similar tree that seems to be sold more often in our nurseries is the Idaho flowering locust (Robinia x ambigua) which grows 25 to 40 feet high and 15 to 30 feet wide. This tree usually has the short spines or thorns and also the rose pink flowers, but it features more of a medium growth rate and better branch structure.

If I could leave it there I would say that for most landscapes the benefits of this tree outweigh the problems. But alas, it’s not that simple. A lot of these trees have been planted but not given appropriate care, resulting in an increase in a pest called the locust borer. This is a pest of the black locust tree (Robinia pseudoacacia) and the purple robe and Idaho flowering locusts are part of that tree family.

Adult locust borers are three-quarters of an inch long and mostly black with bright yellow lines running across the body. On the wing covers these lines are V-shaped, while at the base of the wings the lines form a “W.” The long antennae, which make the locust borer part of the longhorned beetle family, as well as the legs are yellowish to red.

Adults emerge in late summer to early fall and feed on goldenrod and other flowers. The females will lay eggs in bark crevices or around damaged areas of the trees and when the young hatch they will bore into the inner bark to feed.

What is it about trees that draw insect pests? Say it with me now – stressed, damaged or otherwise unhealthy trees draw insects. Unfortunately, stressed trees are more common here because it seems that getting people to understand the whole root growth and water needs thing is much harder than I would wish. However, if you do care for the trees properly they can make for a nice tree.

Thought for the day: “Never swat a mosquito that flies up your nose.” —Greg Gazin

For more information ore about these topics call the Utah State University Extension Grand County office at 435-259-7558 or email Mike Johnson at

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