Nests: Building, Structure, and Maintenance

Home, home building, and home improvement. Pogonomyrmex ants and their nests , at various locations in North America.
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Pogonomyrmex salinus

● NW of Mattawa [Grant Co], WA USA [el. 152 m / 500 ft] view location in Google Maps » ● 9/4/08 7:52AM sunny, air temp: 21°C / 69°F

This Pogonomyrmex salinus nest is in the process of transitioning from a crater, to a dome. The wide crater opening can still be seen on the left (filled with workers), while the right side of the nest is expanding and rising into a dome-like structure. Much of the mound surface is covered with chaff, and chewed-down stalks of grass can be seen as well. An immature black widow spider's (Latrodectus hesperus) web is suspended above the left side of the nest - apparently the ants were not diligent enough in their plant removal operations...[scroll down for additional notes/references]

ADDITIONAL NOTES/REFERENCES: Based on our observations, juvenile western black widow spiders (Latrodectus hesperus) are frequent predators of P.salinus ants in central Washington state. We have not observed any adult L. hesperus preying on Pogonomyrmex salinus, nor have we seen any near the ants, or their nests. This is consistent with the findings of MacKay, in his study of L. hesperus and Pogonomyrmex rugosus, where it was observed that females apparently stop feeding on ants when they become adults (according to MacKay, it is possible that males continue to prey on the ants throughout their lives, though as noted above, we have not yet observed this). Latrodectus hesperus spiders often position themselves in a web just outside the entrance to a Pogo nest. Pogos have been known to close a nest entrance, and open a new one a short distance away, to avoid these predators. Some species, like Pogonomyrmex rugosus, may stop foraging altogether (living on stored seeds), until a spider (or spiders) leave the vicinity of the nest (L. hesperus will move to an active nest nearby, if this occurs). We have seen evidence of these behaviors in Pogonomyrmex salinus (in central WA) as well, with web-covered entrances apparently blocked with debris by the ants, and no foraging activity occurring. Despite their powerful mandibles and formidable stings, Pogos do not appear to be able (or willing) to attack, kill, or repel these predators directly - relying instead on the more passive methods of avoidance mentioned above. We have also observed P. salinus workers attempting to remove webs (most likely webs of L. hesperus) from the vicinity of the nest entrance, however in these cases, no spiders were present. In areas where we observe western black widows preying on P. salinus, it is usually evident that the spiders are not specializing on ants to the exclusion of all other arthropods, as we sopmetimes see the remains of beetles, and other insects in their webs.

·MacKay, W. 1982. The Effect of Predation of Western Widow Spiders (Aranea; Theridiidae) on Harvester Ants (Hymenoptera: Formicidae). Oecologia (Berl) (1982) 53:406-411