Wasp Wednesday: Another Puzzler

A good many people mistake me for an expert who knows everything about every insect known to man, but I am not. Thankfully, there are entomologists like Doug Yanega who do seem to know everything, but he would be the first to deny that claim as well. Still, I have learned more from him than I can possibly recount. Case in point a wasp that had me scratching my head over the weekend.

This all started out innocently enough. I was down at Chico Basin Ranch, a sprawling 88,000 acre cattle ranch that straddles the El Paso and Pueblo County line on the high plains, looking for grasshoppers with several other members of the Mile High Bug Club and the general public. At our first stop out on the heavily-grazed shortgrass prairie, one of the first insects that got my attention was a small, maybe 20 millimeter-long, slender black-and-red wasp that was running erratically between patches of grass.

I had it pegged as either a thread-waisted wasp in the family Sphecidae, or a spider wasp in the family Pompilidae, both of which behave in this manner as they search for potential prey. I did not have the camera set for speeding Hymenoptera, so the images are a bit blurry. Now I am wishing I had spent more time with this creature, but hindsight is twenty-twenty. I do not recall the wasp ever flying, but I figured a close approach might send it fleeing permanently.

Back at home, reviewing the magnified images on my computer, I quickly decided I had no clue at all as to what I was looking at. It seemed to be a fusion of the two potential suspects I had surmised initially. So, off to the "Hymenopterists Forum" on Facebook I went. Posting the images there got me several "likes," but no one ventured an identification. Enter Doug Yanega, a good friend and colleague from the University of California at Riverside. He is always willing to help others online and in person, and the university's collection is so well organized that he can use it as a reference for cases like mine.

"I just recalled the other name I was thinking of: Pterombrus rufiventris (now in Thynnidae). I can't find any photos of it online, but I caught one in Kansas once and it had me stumped for a while," wrote Doug. Well, nice to know I'm not the only one who has been baffled by this species. Doug went on to add "Just checked our collection, this is Pterombrus rufiventris, and it attacks cicindeline larvae. Very rare but widely distributed. No photos in BugGuide."

It is no wonder that I did not recognize it, because the most common thynnid wasps are in the genus Myzinum, and they look nothing like this. See my blog post on those here.

The Large Grassland Tiger Beetle, Cicindela obsoleta is one known host for this wasp.

Cicindelines are known commonly as tiger beetles, colorful and active predators in their own right. The larval stage typically lives in a vertical burrow with a diameter just barely large enough to accommodate the grub. The larva has a flattened head that is held flush with the top of the burrow, the better to see, lunge after, and seize any unsuspecting insect that happens by. The victim is then dragged into the burrow to be consumed. A wasp has to have a lot of bravery to take on one of these voracious beasts, armed as they are with large and powerful jaws. True, the wasp does have her stinger, and I can only imagine how deftly she must wield it to be successful.

According to field observations by others, the wasp crawls down the burrow of the tiger beetle larva and stings it repeatedly under the head or thorax, before depositing an egg on the grub's abdomen. The mother wasp then plugs the burrow with a solid, compacted layer of soil, and then fills in the remainder of the burrow above the plug with loose soil particles. Her egg hatches in about 3 days, and the wasp grub that emerges grows by feeding on the tiger beetle larva for almost 9 days. The mature wasp larva then detaches from the now-deceased host and spins a cocoon in the burrow of the host. There it overwinters, emerging the following summer with the monsoon rains of July.

Indeed, this year Colorado has seen extraordinarily heavy precipitation from severe storms. Timing is everything, and I consider myself lucky to have crossed paths with this rare insect. These are perhaps the first images online of the species. Keep your eyes out for this, the western subspecies P. rufiventris hyalinatus, and the eastern subspecies P. r. rufiventris, from southern California to Texas, and on to Georgia and Virginia.

Sources: Knisley, C. Barry, Darren L. Reeves, and Gregory T. Stephens. 1989. "Behavior and Development of the Wasp Pterombrus rufiventris hyalinatus Krombein (Hymenoptera: Tiphiidae), a Parasite of Larval Tiger Beetles (Coleoptera: Cicindelidae)," Proc. Entomol. Soc. Wash. 91(2): pp. 179-184.
Krombein, Karl V., et al. 1979. Catalog of Hymenoptera in America North of Mexico Volume 2. Washington, DC: Smithsonian Institution Press. 1199-2209.

Breaking: Mexican Silverspot Butterfly Sighted in Colorado

Right place, right time, and with a little help from your friends. That is how discoveries are made. All of those factors came together to verify the spotting of a Mexican Silverspot butterfly, Dione moneta on the eastern edge of Colorado Springs, Colorado on April 18, 2017.

Sharon Milito is a geology lecturer at University of Colorado, Colorado Springs, and a retired Colorado Springs School district eleven teacher. She has also been a volunteer with the city for ten years, doing paleontology surveys and acting as a science education consultant. She has, in her volunteer capacity, access to places not open to the public, including Jimmy Camp Creek Park on the very eastern edge of the city.

Sharon and I went scouting on Tuesday, April 18, and she took me to specific locations that seem to have a slightly greater biological diversity in the sprawling former ranch. It is a unique composite of many different habitats including mostly shortgrass prairie punctuated with sandstone bluffs. Some of the bluffs are covered in Ponderosa Pine forest. There is also Jimmy Camp Creek itself, a drainage fed by numerous springs within the property. There is almost always water in the creek, and some surprisingly deep pools in the sandstone bed. Otherwise, lots of mud trampled by cattle that still graze the park.

One of the areas we went to boasted a grove of wild plum, Prunus americana, in full bloom. It was like walking into a fairy tale. Hundreds of butterflies were sipping nectar from the blossoms. Most were Painted Lady, Vanessa cardui, but there were also Red Admiral (Vanessa atalanta), Milbert's Tortoiseshell (Nymphalis milbertis), Gray Hairstreak (Strymon melinus), Marine Blue (Leptotes marina), and several Monarchs (Danaus plexippus). Most of the Monarchs seem to have had a rough migration up from the south. The whole morning scene was mesmerizing and enchanting.

Eventually, Sharon called me over to ask "what about this one here?" and pointed out a butterfly that had me perplexed initially. At first I thought it was a Variegated Fritillary, which would be expected at this time of year. The wing shape was a little odd, though, and when I glimpsed the underside, it was studded with large, silver spots. Wow, a Gulf Fritillary. That is not a very common butterfly in these parts. I got several images of the insect and we went our merry way. Later, I posted what I thought was our somewhat significant find to the Facebook group "Arthropods Colorado" for more folks to enjoy.

Enter Robb Hannawacker. He raised the stakes substantially higher by proposing that the butterfly was in fact a Mexican Silverspot, Dione moneta. I was embarrassed to admit that until he mentioned it I was not even aware of the species at all, let alone where it is supposed to occur. Robb mentioned the identifying markings, and how our specimen matched them, and it is now pretty obvious that is what we have.

According to my copy of the Kaufman Field Guide to Butterflies of North America, the Mexican Silverspot "Strays to south Texas, rarely to southwest and Big Bend region. More at home in mid-elevation tropical woodlands but reported to breed sporadically in lower Rio Grande Valley....Recorded late spring to late fall (multiple broods in Mexico." The butterfly['s normal range extends from Mexico through Central America to Brazil. The larval foodplants are passion vines, which do not occur in Colorado.

The most recent northern record for the species in the online database of Butterflies and Moths of North America was on May 8, 2005 in Roosevelt County, New Mexico, by Christopher Rustay. As near as I can tell, this is the first record for Colorado, at least of recent vintage. Please alert me to additional historical records. Thank you. Now, go forth and see what amazing species you can find.