The Locust Borer is a fascinating type of long-horned beetle that has piqued the interest of entomologists and nature lovers alike. Known by its scientific name, Megacyllene robiniae, this beetle’s appearance mimics a wasp or hornet, with its black body adorned by bright yellow markings and a distinctive “W” shape across the wing covers (). At about an inch in length, these beetles also have reddish legs, adding to their intimidating and striking presence.
Significant damage can be caused by the Locust Borer to black locust trees (Robinia pseudoacacia) as their larvae burrow into the trunks (). Although these infestations often weaken or kill damaged trees, the adult beetles contribute to pollination, feeding on the pollen of goldenrod and other flowers (). This beetle’s deceptive appearance may serve as a form of protection, as it closely resembles stinging insects, thereby deterring potential predators.
Identification and Description
Adult Locust Borer
The Adult Locust Borer (Megacyllene robiniae) is a type of longhorn beetle. They are approximately 3/4 inch in length with striking black and yellow markings on their thorax and wing covers, known as elytra 1.
- Color: black with brilliant yellow markings
- Distinctive “W” mark on elytra
- Reddish legs
- Moderately long and black antennae
These beetles are part of the longhorned beetle family and can often be seen feeding on the pollen of goldenrod plants^[2^], providing them with natural camouflage.
During the larval stage, these insects can reach up to 1 inch in length and are white, legless grubs2.
- Color: white
- Legless grubs
- Up to 1 inch long
The larvae feed on the xylem of host trees, such as black locust and its cultivars (e.g. purple robe locust)^[3^]. Extensive tunneling in the wood can result in structural weakness of branches or trunks.
Comparison between adult and larval stages:
|black with yellow markings
|Reddish legs, long antennae, “W” mark on elytra
|Up to 1 inch
Life Cycle and Behavior
- Locust borer eggs are laid by adult beetles on the bark of black locust trees (Robinia pseudoacacia) in late summer, usually around August1.
- The female beetle deposits eggs in crevices, knots, or bark injuries to protect them from predators and harsh environmental conditions3.
Larval Feeding and Tunneling
- Once the eggs hatch, white, legless larvae begin feeding on the tree’s xylem and creating tunnel-like galleries within the tree2.
- Larvae primarily target stressed or drought-affected black locust trees, limiting their growth and overall health4.
- As larvae grow larger, they move deeper into the tree’s heartwood, causing significant damage to the tree5.
- Larval feeding creates visible holes and frass (a combination of sawdust and droppings) on the tree bark6.
Comparison Table: Locust Borer vs. Hickory Borer
|Black with bright yellow markings and reddish legs7
|Similar coloration but active on hickory trees8
|Black locust tree (Robinia pseudoacacia)9
|Late summer, around August11
|Xylem and heartwood of black locust trees13
|Xylem and heartwood of hickory trees14
Pupation and Adult Emergence
- In the late winter or early spring, locust borer larvae create pupal chambers within the tree and undergo pupation15.
- Adult beetles emerge from the pupal chambers in the fall, usually with one generation per year16.
- They resemble wasps or hornets due to their similar coloration and markings17.
Pros and cons of locust borer attack on black locust trees
- None (locust borer is a destructive pest for black locust trees)
- Reduces tree growth and overall health18
- Causes structural damage to infested trees19
- Facilitates the entry of pathogens and other pests20
Damage and Impact
Effects on Black Locust Trees
The Locust Borer (Megacyllene robiniae) is a type of long-horned beetle that mainly targets Black Locust trees (Robinia pseudoacacia). Adult beetles lay eggs in the fall, and the larvae burrow into the tree’s bark, creating tunnels. These tunnels weaken the tree’s trunk and branches, making them more susceptible to breakage.
Black Locust trees that are already stressed or damaged (for example, from drought or cracks in the bark) are more likely to be attacked by Locust Borers. The damage can be severe, and in some cases, lead to the death of the tree.
Economic and Ecological Impact
Locust Borers can have both economic and ecological consequences due to their impact on Black Locust trees. Black Locust trees are valuable for their timber, as they are resistant to rot and are used for posts, fences, and other outdoor structures. The damage caused by Locust Borers can significantly reduce the quality and yield of this timber.
Ecologically, Black Locust trees provide habitat and food for various species of animals. As the trees are weakened or killed by Locust Borer infestations, their ability to provide these benefits is compromised. In addition, fallen or weakened branches can pose a hazard to both humans and animals.
Affected trees may attract both Locust Borers and Painted Hickory Borers, but Painted Hickory Borers are active in the spring, while Locust Borers are active from late summer to early fall, usually around September.
Comparison Table: Locust Borer vs. Painted Hickory Borer
|Painted Hickory Borer
|Black Locust trees
|Late summer to early fall
|Black with yellow markings, reddish legs
|Similar to Locust Borer, but with different timings
Management methods for Locust Borer infestations include improving tree health, pruning infested branches, and applying insecticide to the bark in late July or early August before eggs are laid. These measures can help mitigate the damage and impact of Locust Borers on Black Locust trees and the surrounding ecosystems.
Relationship with Goldenrod and Pollination
The Locust Borer (Megacyllene robiniae) is a longhorned beetle known for its relationship with goldenrod and pollination. This beetle belongs to the order Coleoptera. They are distinctly black and yellow, often mimicking the color pattern of stinging insects like bees1.
Adult locust borers are typically observed in late summer, feeding on the pollen of goldenrod1 and other flowers. Goldenrods are part of the Solidago genus and thrive in full to partial sun with dry to average soil moisture4.
Key features of locust borers and goldenrods:
- Black and yellow in color
- Feed on pollen of goldenrod and other flowers
- Help in pollination of flowers
- Mimic bees
- Grow in full to partial sun
- Dry to average soil moisture requirements
- Attract pollinators like locust borers and bees
Beetles like locust borers play an essential role in pollination, ensuring the survival of plant species such as goldenrods3. However, their larvae are known to tunnel into the trunks of black locust trees and may cause damage or death1.
Prevention and Management Strategies
Cultural Control Methods
To minimize locust borer damage to trees, promoting overall tree health is essential. Locust borers are more likely to attack weakened or stressed black locust trees. It is advisable to:
- Remove and destroy infested branches promptly
- Keep trees well-watered and properly fertilized
- Monitor the habitat and location of black locust trees
- Avoid planting new black locust trees in high-risk areas
Chemical Control Options
In cases where locust borers are causing significant damage, chemical control methods can be used. Examples include:
- Carbamates: A group of insecticides proven effective against locust borers
- Pyrethroids: Another class of insecticides known for their efficacy against locust borer infestations
Pros and Cons:
|Effective for locust borer control; moderately toxic to mammals
|Can harm beneficial insects and may lead to resistance
|Highly effective; less toxic to mammals
|Negative impact on aquatic organisms and non-target organisms
When using insecticides, it’s crucial to follow label instructions and apply them during late July or early August to target the adult locust borers before they lay eggs.
In conclusion, locust borer management involves a combination of cultural and chemical control methods. By maintaining tree health, monitoring infestations, and applying insecticides when needed, you can protect your black locust trees from these destructive pests.
Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.
Letter 1 – Locust Borer, but in Oregon?
Black and Yellow Beetle
Just found your site. Hope you can identify this beetle for me. We have a nursery and don’t like to kill "good bugs" so we kind of like to know which category they fall into. I couldn’t find this one listed anywhere and thought you might be able to help.
This looks to be a Locust Borer, Megacyllene robiniae. This is a common beetle in the East and South, but we are totally unaware of it being in Oregon. The larvae bore into the wood of black locust trees and adults feed on pollen, especially goldenrod.
Well that explains it. The Maupin City Park replaced about 30 Black Locust trees this year that had died. I don’t have Black Locust on this property but do have lots of Honey Locust. I’m about three miles from the city park. So I guess we know they’re in Oregon now. Thanks for identifying it.
(10/04/2005) This just in from Eric Eaton: “Apparently the locust borer is now found throughout most of the U.S., as its host is used as an ornamental far outside its native range. “
Letter 2 – Locust Borer on Goldenrod
Subject: Locust Borer
Geographic location of the bug: Benton Harbor, MI
Time: 05:25 PM EDT
Your letter to the bugman: I know you already have a lot of locust borer pictures, but thought, since your description in one of the pictures starts with “Adult Locust Borers are often found on goldenrod in the autumn”, you would like to see this shot of one sitting in the goldenrod we spotted while we were out walking the dogs….
How you want your letter signed: pat
We love your image of a Locust Borer on goldenrod and we are thrilled to post it, especially as we just posted another image of a Locust Borer from Washington state, not part of its native range, an expansion made possible because of the cultivation of black locust trees for landscaping and other reasons.
Letter 3 – Probably Hickory Borer (possibly emerged from firewood)
Looks like yellowjacket or paper wasp but without wings…
Fri, Feb 6, 2009 at 10:39 AM
Hi. My mom mailed me these pictures of this insect. I am actually an entomologist, but I work with mosquitoes, bed bugs and other biting and stinging public health pests. So beetles are a bit beyond me. I can’t get a good look at it, because she can’t mail me the specimen. (she lives in Indianapolis, and I live in Honolulu) . All I have is a handful of out of focus pictures. I am 99% sure it is a beetle from cerambycidae, and leaning toward something similar to Megacyllene robiniae.
Since it is such a colorful fellow (and looks like a wasp to most people) I thought I’d send it to you.
Indianpolis, Indiana. Winter (with 12+ inches of snow on the ground!
We believe this is either Megacyllene robiniae, the Locust Borer as you have surmised, or a closely related species, the Hickory Borer, Megacyllene caryae. We are curious about the sighting with 12 inches of snow on the ground, and are guessing that the insect in question may have emerged from some firewood that was stored indoors.
Update: From Eric Eaton
Monday, February 6, 2009
The “locust borer” from Indiana is almost certainly a hickory borer. It is not at all atypical to get them indoors at this time of year. The locust borer emerges in the fall. Period. There seems to be virtually no overlap in emergence times between the two species.
Letter 4 – Locust Borer or Hickory Borer
What’s this bug?
My husband and I keep finding this little critter inside our home. We live in southern Missouri, close to Springfield, in a very wooded area. I am normally real careful about bringing in firewood in the winter so I don’t end up with a house full of unwanted visitors so I’m not sure how they all got in here. They started showing up around the first of March. Sometimes they just fly up to our windows, I’m not sure if it is for extra warmth or light. When we pick them up they make a little squeaking noise, of course, I would be squeaking big time if someone were throwing me out into the cold too!
This is one of two possible beetles in the genus Megacyllene. It is either the Locust Borer, Megacyllene robiniae, or the Hickory Borer, Megacyllene caryae. We have a very difficult time telling them apart. Locust Borers usually appear in the fall and feed on goldenrod pollen. Hickory Borers are found in the sring. Both have larvae that bore into wood, and they most probably came in on firewood. The warmth indoors sped up the metamorphosis. Since their typical life cycle was altered, we cannot even guess which of the two species this is.
Letter 5 – Locust Borer??? or Hickory Borer???: Emerging from stored firewood
mystery bug in New York
I found this feller lying dead near my oven this evening. It was about 1.25″ inches long, maybe. I am not sure what it is. It looks like a cockroach in “layout”, yet the markings seem very unusual for a roach. Do you know what it is? If so, should I call an exterminator, or is it probably just a bug that came in and didn’t bring his whole family? Thanks very much!
Bradley in NYC
This is one of two species in the genus Megacyllene. It is either the Locust Borer, Megacyllene robiniae, or the Hickory Borer, Megacyllene caryae. The Locust Borer is generally found in the fall when the goldenrod blooms, while the Hickory Borer is most common in the spring. It is possible that this specimen has been dead in your house for some time, or it is possible that it was hibernating in the pupal form inside some cut firewood, and emerged in the warm house.
Thanks so much for your response. I wondered how such a bug could have gotten in here, but we do have a lot of cut firewood in our basement, which is right below my kitchen. Now it makes sense. My only other question is: can these things infest my place, or should I not really worry? There might be a few more, but there won’t be thousands, right? Thanks again for your kind help!
Hi again Bradley,
Individuals may continue to emerge from the firewood, but they will not infest your home.
Letter 6 – One of the Locust Borers, probably
Some kind of borer?
I found this attractive beetle sitting on my car a few mornings ago. I believe this is a member of the family Carambycidae (longhorn beetles) because its antennae appear to be growing out of its eyes. After I took the photos, it flew away straight up, disappearing quickly. I haven’t been able to find a picture like it, and I’ve looked at every western North America beetle website I could find. The distinctive cross on its back should make it easy to identify for someone who has one in their collection. Perhaps it is an exotic intruder? This is the best insect site on the web for casual naturalists like me. Thank you for the great service
you are providing.
We were delayed because we didn’t recognize your borer either. We checked with Eric Eaton, and it seems he doesn’t exactly recognize it either. Here is his reply: “I don’t know, but I’d put money on it being in the genus Megacyllene, the Locust Borer genus. Pretty sure it is not simply covered in pollen, but is marked with that much yellow. Neat! ” Perhaps it is an exotic import. Why don’t you check with the Department of Agriculture.
Letter 7 – Mesquite Borer with Phoretic Mites
Location: Central Texas
August 12, 2010 11:58 am
Could you please identify this bug. I live in central Texas. 100 degree weather right now. Was on our wood pile
We wish your photos were not so blurry and that they had more detail. This looks like a Locust Borer, Megacyllene robiniae, and it appears to be covered with Mites. We suspect they are Phoretic Mites and not Parasitic Mites. Phoretic Mites are opportunistic, and they use other insects for transportation purposes. We have never seen such a large quantity of Phoretic Mites on any insect other than a Carrion Beetle. Was the wood in your wood pile black locust? Knowing that would add evidence to our assumption that the beetle in your photo is a Locust Borer.
Update: Nina provided us with a comment that the wood pile is 90% mesquite and 10% oak, and that made a huge difference. Her borer is a Mesquite Borer, Placosternus difficilis, which looks very similar to the Locust Borer. BugGuide even provides a visual comparison.