June Beetle Swarm

2022 July 17

On July 15, 2022, Sam and I went out to the front yard just at sunset. As we walked past the pear tree in the yard, we noticed that it was buzzing. Looking a bit closer, we saw that the tree was surrounded by dozens of insects about the size and color of a honey bee, hovering around the leaves without landing. At first we thought that maybe they were honey bees, and maybe one of our beehives was getting ready to swarm and was scouting out the tree first. But there were two problems with this: (1) honeybees don’t normally fly at dusk, and (2) while it was a bit dark and hard to see them clearly, they buzzed just a bit too loudly to be honey bees, and didn’t look quite right. So, I grabbed an insect net to snag one out of the air, and this was what we caught:

This is clearly a small scarab beetle, most likely one of the June Beetles in the subfamily Melolonthinae. Although these are on the smaller size for this subfamily, only about have the size of the big June Beetles that one normally sees lying around under porch lights in the morning.

It has the particular type of spiny legs and claws that are characteristic of scarab beetles, and it has the “fur vest” that is commonly seen on June beetles.

The antennae tuck under the chin when not in use. We can see the antennae exposed while it was on its back, the bulb at the end of the antennae would separate into “fingers” to provide more surface area to sniff out mating pheromones.

After seeing these beetles around one of our trees, we checked out the other trees too. And they were swarming around the other pear tree, a plum tree, the stand of maples, our row of small Balsam Fir, the two big spruce trees near the barn . . . basically, every isolated tree that could work as a landmark was surrounded by clouds of these beetles. There must have been thousands of them in our yard alone.

We don’t think they were coming to feed on the trees, as the trees were completely undamaged the next morning. And, in any case, I am not even sure that these beetles even have functioning mouthparts.

This was clearly a mating aggregation. It looks like they emerged from the ground as adults, and then instead of flying around at random looking for a mate, they flew directly to the nearest prominent object where they could have a better chance of meeting other beetles who had also flown to that same object. They came out just late enough in the evening that the birds had mostly called it a night, while it was still a bit early for bats to come out. In that narrow window of opportunity, they had a chance to get together and mate largely undisturbed.

The whole event only lasted about an hour, by the time the sun had gone down completely they had mostly gone somewhere to hide out for the rest of the night.

We did see them again the next night at about the same time, but in much smaller numbers. I expect that these are one of the insect species that come out to mate over a period of just a few days, and this just happened to be the first time we had ever caught them doing it.

One might wonder, just how do they schedule things accurately enough for all of them to emerge in the same period of just a few days? I expect there are several things they use. Probably a combination of elapsed time, day length, and temperature to get into the right week, and then they probably use pheromones, with the first ones emerging basically emitting scent to call out the rest of them.

Anyway, it was pretty impressive, and I need to mark my calendar to see if they come out on July 15 every year.

2 Responses
  1. August 17, 2022

    I love it. You’re running a singles bar for beetles. Only you, Tim. Only you.

    As for the scheduling, if I was programming it, I’d set a few simple triggers that would all have to be met. Light, temperature and humidity would probably do the trick. If they all come out nearly simultaneously every year, then they would mature at nearly the same time every year. Give their maturity a normal distribution and you’ve got the big swarm at the mean and smaller swarms on adjacent days.

    Or something like that.

  2. August 17, 2022

    Well, I have an entry on my calendar to check for them every summer starting on July 11, so we’ll see if they come back on schedule or not.

    There was quite a plague of them all around town, I found their bodies lying about for some time afterwards. A number of our friends have noted that something has been killing their lawns all spring, which is what the larvae of beetles like this do. I am suspecting that their primary predator is normally bats, and that with the crash in the local bat populations due to White-Nose Syndrome[1], they have gotten out of control. Although, there is a patch of our yard that got torn to ribbons by the rapidly-expanding wild turkey flock scratching up the ground looking for something. Probably the grubs of these beetles. So maybe the turkeys are going to be their primary predators until the bat population comes back

    [1] I have started seeing bats roosting around the cave-like areas on some of the campus buildings, after not seeing any for the last few years. Hopefully it means that there were enough bats resistant to white-nose syndrome that they will now be able to build their population back up.

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