For several years running – basically, since we had the landscaper plant the big ocotillo out front – we have seen what appear to be ant trails where leaves or flowers from the ocotillo and other sources running across the driveway (see left panel in the clickable composite image above). We knew that these trails were left by nocturnal ants since we were out one night to watch the ISS flyover and saw the ants actively going back and forth along the trail. The destination is their anthill located about fifteen feet beyond the pavement which is shown in the right panel above.
These are desert leafcutter ants (Acromyrmex versicolor), the common fungus-farming species in the Sonoran Desert of Arizona.
In summer (especially the hottest months like August), their foraging is almost entirely nocturnal because daytime surface temperatures are too extreme. They form clear trails while carrying plant material back to the nest, exactly as in your driveway photo, and often deposit piles of leaf fragments around the entrance crater.
Ocotillo (Fouquieria splendens) is one of the plants they regularly harvest. Studies of Arizona colonies show they collect both fresh young leaves and dry vegetation from ocotillo along with other desert perennials such as paloverde, mesquite, ironwood, creosote, and catclaw acacia. Fresh leaves make up a notable portion of the forage when available (especially after rains when ocotillos leaf out).
They do not eat the leaves. Workers cut and carry pieces underground to cultivate a specialized symbiotic fungus in garden chambers; the ants and their larvae feed on the fungus. Colonies can be sizable, with crater-like nest entrances (often multiple) in sandy or gravelly soil, and they clear or accumulate plant debris around the openings.
A related but less widespread species, Atta mexicana (Mexican leafcutter), also occurs in southern Arizona and shows similar nocturnal summer foraging and leaf-carrying, but Acromyrmex versicolor is the typical one across much of the Sonoran Desert, including areas like Wickenburg. Both are native and generally harmless to people (they can bite if handled but do not sting like harvester ants).
The trail of fresh green leaves leading ~15 feet to the entrance, plus the accumulation right at the hole, is classic behavior for these ants during a summer night’s work. They will likely continue as long as temperatures stay high and suitable plant material is available.
That is some interesting information about the ants. We don’t consider them a nuisance and expect to just leave them to do their thing. They are part of God’s wonderful desert nature at work in our own front yard.
