When is a locust borer not a locust borer?

…when it is a hickory borer!

Hickory borer (Megacyllene caryae) mating pair on trunk of fallen mockernut hickory (Carya alba).

The hickory borer, Megacyllene caryae, is perhaps the most frequently misidentified beetle in eastern North America due to its almost perfect resemblance to the closely related locust borer, M. robiniae.  Unlike the latter species, however, which is encountered abundantly during the fall on flowers of goldenrod (Solidago spp.) and attacks living black locust (Robinia pseudoacacia), the hickory borer is active only during the spring and breeds in the dead wood of hickories (Carya spp.).  Adults emerge from the wood as soon as temperatures begin to warm in early spring, a fact which causes it to be most frequently encountered during winter when it emerges indoors from firewood brought in from outdoors.  Many times this causes the alarmed homeowner to post a photo of the insect on BugGuide and ask if it will cause damage to their home.  So close is its resemblance to the locust borer that novice insect enthusiasts often identify it as such based on comparison to photos and refuse to believe it is not that species, even when told otherwise.

Of course, there are distinguishing characters that, with a little practice, become quite obvious – the legs of the hickory borer are often distinctly reddish (as seen in the above photo), and the bands of the elytra will many times show an alternating pattern of yellow and white (not quite so apparent in the above photo).  The elytral bands are also slightly narrower and often broken and incomplete in this species, while in the locust borer they are wider and nearly always extend completely across the elytra.  Lastly, the pronotum of the locust borer is narrowly margined with yellow on the anterior edge, while in the hickory borer the anterior margin is black.  That’s a tough character to see without magnification, and all of these characters really are only necessary when examining specimens in a collection (and even then only if there is no date on the collection label).  Season is the easiest distinguishing character – if it occurs during spring it is the hickory borer, and if it occurs during fall it is the locust borer.  There are several other species in the genus that can be confused with these two, but they do not occur in eastern parts of North America.

This mating pair was encountered on the trunk of a recently wind-thrown mockernut hickory (Carya alba) during our early April hike of the lower Wappapello Section of the Ozark Trail.

Photo Details: Canon 50D (ISO 100, 1/250 sec, f/14), Canon 100mm macro lens, Canon MT-24EX flash (1/4 ratio) w/ Sto-Fen diffusers. Typical post-processing (levels, unsharp mask).

Copyright © Ted C. MacRae 2010

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About Ted C. MacRae

Ted C. MacRae is a research entomologist by vocation and beetle taxonomist by avocation. Areas of expertise in the latter include worldwide jewel beetles (Buprestidae) and North American longhorned beetles (Cerambycidae). More recent work has focused on North American tiger beetles (Cicindelidae) and their distribution, ecology, and conservation.
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19 Responses to When is a locust borer not a locust borer?

  1. That is a stunning longhorn. With that aposematic coloring, is it known to be distasteful or poisonous?

    “Please don’t move this image again.” …she moved it!, even after hearing about the seasonal difference…

    • I don’t think it’s aposematic, but simple mimicry. As abundant as the locust borers are on goldenrod flowers, they are much less common than the wasps that could be their models. It might also have a crypsis function (despite the apparent conspicuousness of the beetles in the photo) – the alternating bands actually break up the outline of the beetles and make them hard to see on the wood or flowers.

  2. peteryeeles says:

    Whatever it is… that’s an attractive beetle!

  3. Kurt says:

    Great shot. Haven’t seen anything like these yet where I am.

  4. jason says:

    They’re quite attractive. It’s easy to see how a novice might run across the wrong species first and come up with a bad ID (especially if their photo didn’t capture all the right details).

    And I laughed about the BugGuide image being moved back and forth! When asking an expert–or more than one expert, it’s usually wise to accept their help without challenging them on it.

  5. DougT says:

    Nice photo. I laughed when I read the part about the firewood. I have but one specimen in my collection. It emerged from firewood that was stored in a friend’s garage.

  6. Modoc Charlie says:

    Nice photo of a beautiful species. I don’t know how far west either of the ones you mention occur.
    We do occasionally get Megacyllene antennata in Southeast Arizona in mesquite areas. Although quite interesting, it is not as colorful. There are records of Megacyllene snowi on the east side of the Chiracahua Mtns. on goldenrod and Mexican locust. We have looked, but have not been fortunate as yet to find any.

    • Thanks, Charlie. Both species are distributed throughout much of eastern North America – caryae is limited by the range of its host to the eastern edge of the Great Plains, while robiniae gets a little further west (as far as Idaho and Colorado). We’ve also got another gorgeous species – decora, which is larger and more boldly marked than the other two species.

      There are several Great Plains-only species – angulifera, comanchei, and powersi.

      I’ve found antennata in Arizona and Texas, but I sure would love to run into snowi or robusta in Arizona someday – I don’t have either one of those.

  7. ourowntime says:

    Thanks for this great explanation. I was just asked about a lovely beetle found on my workplace grounds – and your explanation helped me to feel relatively confident of my ID. It is April, so I’m relatively certain it is Hickory borer.

  8. john rausch says:

    I recently encountered an infestation of the Hickory borer in a load of freshly cut Shagbark Hickory for firewood. They can consume a lot of wood in a short period of time. I was hoping that they would have been finished after one season, but after splitting some remaining logs in March 2013, I found them flourishing still. I can only hope they stay in the Hickory!

  9. Andrew says:

    Are the locust borers able to be in adult stage in the spring? I encountered a large swarm of these yesterday (late April) but it was in the broken trunk of a locust tree that I cut down just the day prior. I think locust borer because of the tree and the fact that they appear to be coming out of it. On the other hand, I think hickory borer because of the time of the year. Can anyone add some incite?

    • By “locust” do you mean black locust (Robinia pseudoacacia) or honey locust (Gleditsia triacanthos)? Locust borer breeds only in the former, while the hickory borer is known to breed in the latter as well as some other genera besides hickory.

      I am not aware of any instance where locust borers occur naturally during the spring.

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