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~ Control of the Locust Borer, Megacyllene robiniae ~

The Locust Borer is a native North American pest that is extremely destructive of the locust tree, Robinia pseudoacacia, and its cultivars. If you own a black locust, you must be on the lookout for this insect whose larva tunnels into the bark and heartwood, sometimes in such great numbers as to reduce the tree to a sprouting stump. Unfortunately, all advice you will find calls for the use of insecticides now prohibited for domestic use or by many municipal bylaws. Here I propose a method that can be of use to small scale control my homeowners.

Learn to detect the presence of the insect. The most easily recognized sign of the borer is fine "saw dust" on the bark, branch crotches, or, in heavily infested trees, on the ground around the trunk.

Find the insects. Carefully examine the bark for the presence of damage, in particular, particular, small holes that are often plugged with "sawdust." With the help of a bobbypin, small key, or other improvised "stick," enlarge the hole a bit and clean up the "sawdust" from the entrance.

You will need a can of foam wasp and hornet killer insecticide, the type that comes with a small straw to better fill cavities with the expanding foam. Allethrin is an acceptable insecticide for this purpose. Use the straw to inject the insecticide into the borer's cavities. The borers die soon after contact, and might occasionally be observed attempting to crawl out of the hole, or fall in a cradle at the bottom of the hole, near the entrance. The foam will soon disappear.

You will need a some spray fungicide, because the injured wood of the black locust is very susceptible to rot. After the insecticidal foam is no longer visible, spray the fungicide inside each cavity

Lastly, cover all cavities with a generous layer of tree wound dressing. While tree wound dressing is not generally recommended, the damaged bark will attract adult borers to lay their eggs in late summer. The wound dressing will, hopefully, discourage some insects to exploit the entry points left by borers from the previous generation. The dressing will not prevent rot, or promote healing.

Life Cycle

Eggs are laid in bark crevices in late summer, around the time of golderod flowering (the adults feed on golderod flowers). The white, smooth eggs (about 28 mm long) are oviposited either singly in V-shaped crevices in the bark, or in loose groupings of several eggs in an old wound. The base of the trees should be kept free of debris as it is a preferred spot for egg laying. About a week later, the larvae hatch, crawl into the tree's innver bark where they will spend the winter. In early spring they begin to feed and grow, burrowing into the sap and heart woods. Entrances to the larval tunnels are marked by sap oozing from the tree and sawdust-like granular exudate (frass) pushed out by the larvae. Larval mines can literally honeycomb both sapwood and heartwood, and can make the tree susceptible to deadly heart rot fungus. The general course of their L-shaped tunnels is initially in an upward and inward direction from the point of entrance, then angles sharply and goes directly down the trunk. Feeding stops for pupation in mid-July. The adults emerge about the last week of August and first week of September. They can be found feeding on goldenrod.

Special note
Young trees and branches with diameters greater than 2.5 cm are preferred by the pest, which rarely attacks hosts with trunks greater than 15 cm in diameter, unless they are previously damaged "brood trees."

Weakened wood will make trees more susceptible to wind damage/breakage, branch death, or even trunk girdling.

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