Meet the Locust Bore
In August (2014) we spotted a few beautiful black and yellow beetles mating on a tree in one of our landscapes – nothing quite like straightening up from weeding the garden to find yourself nose to nose with a Megacyllene robiniae… or six!
You’ve probably seen them yourself. Megacyllene robiniae, more commonly known as Locust Bores, enjoy a widespread distribution that extends over most of the continental US and Eastern Canada. These long horned beetles (referring to their long antennae) are a flashy member of the Cerambycidae family. Locust Bores are wood boring beetles that specifically target Black Locust trees (and their cultivars) to complete their life cycle. In our region, the average lifespan of a Black Locust tree is approximately 12 years – in part due to M. robiniae.
Life Cycle of the Locust Bore
In the fall (August-September) adult locust bore beetles emerge and mate. Females then lay their eggs in the cracks, crevices, and pruning wounds in the bark of living Black locust trees. About a week later, larvae emerge from the eggs and burrow into the sapwood where they will hibernate in galleries (short tunnels) throughout the winter. The following spring, any larvae that survives the initial vigorous sap flows begin to burrow in earnest through both sapwood and heartwood, slowly cutting off water and nutrient flow to the tree canopy. The larvae remain active in the tree until it’s time to pupate (morph into adult beetles), typically in mid to late summer. By August, last year’s eggs are emerging as sexually mature locust bores, ready to mate!
- Early Spring – oozing sap and wet spots on the bark while leaf buds swell
- Late Summer – Sawdust, as larvae clear their galleries
- Knotty swellings around tunnels – the tree attempts to protect itself
It’s possible for trees to survive a Locust bore infestation (large trees more so than small), but particularly severe or repeated infestations can weaken the structural integrity of branches and trunk alike, leaving trees more prone to windbreak, invasion by secondary pathogens, and loss of canopy/general die back due to restriction of nutrient flow.
Ultimately, the best defense against Locust Bores is a healthy tree. So if you’re thinking of installing black locusts on your property, take the time to learn about the tree’s ideal growing conditions and ask yourself: “will this tree be happy here?” If the answer is anything other than “yes!” then there’s likely a different tree that would prove to be a better investment of your time and money. Plants and trees that have been stressed from drought, damage, or other conditions are most susceptible.
Natural predators of the Locust Bore are Woodpeckers and Wheel bugs. These species serve as biological controls, but also indicators of tree health – if your tree is covered with either predator, the good news is they’re working to control pests for you! The bad news is, you had pests in the first place and the damage may be fatal – you won’t know until you take a closer look. If you’re concerned that a tree located close to your home has been infected, we recommend having an experienced arborist come out to assess the tree. Many companies offer a free consultation, so be sure to shop around!
Insecticides can be an effective form of control for this bore, when applied correctly at the right time of year. See the resource links below to learn more!