Any guesses?

At the end of May 2014 and the beginning of June 2014, I observed a swarm of Harpalus katiae in Victoria city, TX. The phenomenon happened after a very heavy rain storm. I assume that the rain caused the beetles to come to the surface. But it was probably another factor, too. Maybe the rain affected the food resources of the beetles or something related to pheromones and the time of the year. There were heavy storms with a lot of rain, but only then I did notice this phenomenon
Beetles were everywhere, around the buildings, on roads and I even noticed them in some shops. In a corner store, a guy tries to sweep them outdoor with a broom.
Alongside the roads, there were thousands of beetles.
Associate Harpalus katiae, there were also specimens of Stenomorphus californicus rufipes, and one or two other species. But, by far the majority were Harpalus katiae. Without H.katiae it could not have been called a swarm.
After a while, they became rare to see, and I have never witnessed such a phenomenon on such a large scale. I was in Victoria, for about 3 years. In fact, they seem to be pretty rare to be seen.

… Iustin Cret, 9 July, 2022 - 4:07pm

Swarms of beetle marauders are falling from the sky in Arizona.

I wish to help in the species identification, but can do so only with fresh specimens mailed to me. Here is what I communicated today to my colleagues interested in Carabidae:

In the past couple of days some entomologists in the Arizona area sent me field photos. They appear like Discoderus amazingly enough. One of the best photos is attached [not yet here]. I asked for immediate shipping of fresh specimens to me. I haven't heard confirmation from anyone about this request. One of the entomologists is thinking about writing a scientific note about this unusual swarming event. There was concern expressed for a beetle not native to Arizona that is exploding in population. Absolutely no diet information is mentioned for any of the Discoderus species in "A Natural History of the Ground Beetles of America north of Mexico" by Larochelle & Lariviere (2003).

What are the odds that our present revisionary work on Discoderus might become helpful for accurate species identification during such a major swarming event? I welcome your opinions.

… Peter W. Messer, 2 July, 2022 - 10:54am

"Discoderus obsidianus"

appears to be the culprit thanks to an image posted by Andrew Meeds here: https://www.inaturalist.org/observations/125829685.

… Peter W. Messer, 12 July, 2022 - 9:25am

MYSTERY SOLVED!!

Thanks to Andrew's efforts and your attentive help/expertise, we now know that D. obsidianus has the potential to build large populations in this part of the country.

… Salvador Vitanza, 12 July, 2022 - 9:51am

What are reasons driving one beetle species but not others

to suddenly swarm like this? Sudden heavy precipitation after a relative drought period appears to be a necessary element. I would like to know what the literature says about the swarming of beetles more than for other insect groups.

… Peter W. Messer, 12 July, 2022 - 10:10am

A couple of other local notes

- The only other species that I noticed with any sort of numbers during the same swarming event were what I think were hybosorids. I took one photo at the time but also have a handful of specimens: https://www.inaturalist.org/observations/124822698

- We did have a massive emergence of Hyles lineata caterpillars last year following some heavy rains (literal millions came into the neighborhood from the neighboring preserve). I'm not sure how widespread that event was and it seems to be something that happens with some regularity in desert environments, so probably not relevant. But wanted to mention since that was the last population explosion that I remember.

… Andrew Meeds, 12 July, 2022 - 10:24am

Specimen condition

How fresh do you need? Do they need to be in good enough shape for molecular work? I pinned several but I could see if more show up. I'm down in SE AZ right now but I'll be back home tomorrow. The images of Discoderus do look really similar to what I collected.

… Andrew Meeds, 3 July, 2022 - 10:08am

This situation is not happening in

Cochise or Santa Cruz counties as far as I can tell. I live in Nogales, and yesterday I went bug-hunting (using sweep net in the day and lights at night) in Patagonia, Willcox Playa, Whitewater Draw Wildlife Area, and the San Pedro River. There was no over-abundance of ground beetles. Although the monsoon rains have barely started and some areas look extremely dry, insects are very active and this might be a good season for arthropods.

… Salvador Vitanza, 3 July, 2022 - 10:38am

Fresh = collected during current swarm & in good condition.

I won't be doing molecular studies, so no special precautions. Flexible specimens are best mailed wet (unpinned), grouped according to collection data (penciled paper tag) and wrapped together in the same alcohol-dampened paper towel strip. Additional such wrappings occur for other data. All wrappings can then be placed inside a snack or sandwich plastic ziploc bag which is secured inside a small sturdy cardboard box for mailing. I'm not sure if smallest flat-rate USPS container is the least expensive route. One may also send specimens in alcohol vials with penciled tags, but the wrapping approach weighs less and it may be easier for most. Thank you for donating specimens for species identification during this unusual event. Specimens of Discoderus will also contribute to our revision of the genus which is still in manuscript form.

… Peter W. Messer, 3 July, 2022 - 10:30am

Small carabids

I live in north Phoenix (right next to the North Mountain Preserve). I had just flown back into town, so I'm not positive, but I think there had been some light rain earlier in the day or the day prior. I actually set up my blacklight that night (for a short time- it just turned into a carabid party) and collected quite a series. I can probably image a few next week. Have plenty of specimens if anyone wants any as well.

I know nothing at all about carabid ID, but they did have quite a rounded, distinct pronotum.

Had a couple of palo verde borers show up to the campus lights the same couple of days, so I do think there must have been some recent moisture.

… Andrew Meeds, 2 July, 2022 - 10:05am

That reminds me of similar

Ground beetle outbreaks in El Paso, TX

… Salvador Vitanza, 2 July, 2022 - 10:51am

NM too

I UV light every night here in Albuquerque and Discoderus are showing up in large numbers the last several days. I may be getting about 20 each night with two low wattage bulbs over water trays. Other years there have been swarms of Selenophorus as well.

… Rick Buss, 2 July, 2022 - 7:56pm