nicholasparlmer's Identifications

Consider me dumbfounded, but Atta mexicana are officially established in Santa Cruz Co., AZ. With all of the recent alate observations north of the border (including a few of my own) it has become certain that established populations of this species have pushed further north than Imuris, Sonora. This site had previously been believed to be the northernmost extent of the species circa ~2000, but within the last 25ish years it seems as though the higher elevation habitat north of Imuris has become habitable for this species. Whether or not that is due to climate change, an urban heat island effect in Nogales, or this species adapting to a cooler climate, I am not sure.

As much as I would love to, I cannot take all of the credit for this observation. @ameeds recently made me aware of a BugGuide post by entomologist Salvador Vitanza, where he thoroughly imaged and documented Atta mexicana workers at this exact locality in Nogales, AZ. I was so dumbfounded that within 2 days I was at the locality to verify for myself, and sure enough I found almost exactly what was detailed in the BugGuide Post.
https://bugguide.net/node/view/2306508

Once I confirmed that there were, in fact, Atta workers here, my next task was to locate the colony's "center", the large pile of waste fungus created by these ants above their nesting site. The "nest entrance" detailed by Salvador Vitanza was clearly just a foraging entrance; this species creates underground tunnels from their nest to more fruitful foraging areas - up to 150 meters in any given direction. However, due to the foraging entrance in question having a foraging trail over 100 meters long (BugGuide post says 140m, but I observed the trail fading out at around 105m - trail length likely varies day-by-day). This long trail most likely means that the colony's center was nearby to the foraging entrance. It is worth noting that all foraging activity was on the south side of the street - not a single worker was seen on the north side of the street.

I began my search by exploring the undeveloped land around the property. Unfortunately this area is quite steep, which made it somewhat difficult to survey, but upon searching thoroughly, all I was able to find was an additional foraging entrance underneath a tree located against a backyard wall of the property the original foraging entrance was in front of. I did peer over the wall, but was unable to see any obvious signs of Atta within the backyard of the property.

In the front yard of the property, very close to the original foraging entrance, I noticed 2 additional foraging entrances. One was located in a crack in the walkway , with workers foraging along the side of a concrete barrier for a garden. The second was located in the yard next to a large stone, and had a relatively large and pronounced entrance similar to the one located next to the street. From this entrance a short trail traveled to a Juniper tree, where workers were thoroughly removing plant material and bringing it inside. To see Atta harvesting Juniper so enthusiastically was astonishing to me - I never expected conifers to be a potential food item to Atta.

Given that all four of the foraging entrances I found were centered around this property, and no additional foraging entrances were found away from the property, I suspect the colony's nest is located on the property. I have included a map (last image) of the four foraging entrances and the location of the main long trail.

As a last note on this colony, I do suspect that it is a mature colony. Given the long foraging trails, wide array of plant materials being accepted, and presence of alates in the area, it is likely this colony has been in the area for several years now, and has had the opportunity to spread offspring to other parts of Nogales.

After thoroughly documenting this Atta colony, I explored the rest of the street and a bit of the surrounding area to search for more colonies, but could not find any. I definitely do not expect that this is the only colony in Nogales, AZ, but I was unable to find any additional signs of the species in my limited search. More extensive and widespread searching will definitely be required to find additional signs of this species in Nogales, AZ.

The last note has to do with the male alates found in this observation: https://www.inaturalist.org/observations/179394123
Given that Green Valley is around 35 miles north of Nogales - almost the same distance between Imuris and Nogales, it seems there are only 2 possible explanations. Either those male alates hitchhiked on a truck from Nogales to Green Valley (not impossible considering I-19), or this species extends much further north into AZ than anyone expected. The latter has some merit - previously the higher elevation environment around Nogales was the main barrier preventing Atta from spreading northwards. With this habitat now suitable, I believe there is effectively nothing but time preventing this species from spreading even further north. This may seem like a long-shot, but I suspect that if this population in Nogales is able to persist, we will be seeing Atta in Tucson in the next 30 years.

A single dealate queen was found in a small wash, probably a straggler from a daytime/morning flight. It appears to be the same species as the first phoretica queen found in Arizona, but it is difficult to tell without proper specimen photos for both of them, as a lot of specific features seem to be very dependent on the angle. There are some minor (although probably not notable) differences. Specifically the hair length on the head, mesoscutellar disc, and gaster (maybe). Possibly also the antennal scape length.

Didn't believe my eyes when I first saw it, I never expected that they would actually go this far north. This opens up their range a whole lot, although I do think that they'd still be more common in the south.

Not associated with any other ants, but hyatti, xerophila, and micula are the most common (or at least most conspicuous) Pheidole species in the area. P. rhea may also be present, but I haven't seen them here yet.