- by A Sunjian and Li Hongmei
|Sunjian A and LI Hongmei (2006) A survey of Atta texana on the southern tip of Texas. Notes from Underground 11 (2)|
(revision date: 2006/12/15)
|Atta texana worker cutting grass blade in Mission, Texas.|
Addendum: A youtube video by Ms. B. Macdonald confirms Atta texana leafcutters in Alamo, TX as well (2008/4/12)
The authors conducted a 3-day survey of leafcutter ants in towns and parks on the southernmost tip of Texas, close to the Rio Grande River and the Mexican border, from September 13-16, 2006. The exact coordinates of all samples, including elevation and accuracy, were determined in the field using a Magellan eXplorist 100 GPS device, entered into a Nokia 9300 smartphone using DustyNotes Explorer software, which exported the data as a kml file for display and latter analysis in Google Earth.
Atta texana was found and samples were retrieved in the following areas (Google Map of the Lower Rio Grande Vallay towns):
Surveys were conducted but no A. texana were observed in Weslaco (Estero Llano Grande State Park), Harlingen (Harlingen Arroyo Colorado Park), and Brownsville.
It is notable that some of the samples were retrieved very close to the Rio Grande River. Colonies in Mission, Roma, and Falcon State Park were only tens of meters away from the Mexican border, thus lending some credence to the hypothesis that this species is present in significant numbers in the border towns on the Mexican side of the river.
It is also notable that the Atta leafcutters were not found in areas closer to the Gulf Coast and the mouth of the Rio Grande River, although the brevity of the survey might render this to be a mere artifact of inadequate sampling. There is some evidence that most Atta species with deep underground nests cannot tolerate areas with very high water tables and vertisol soils. For example, Wetterer (1991) found that Atta cephalotes and Atta colombica were not found in Palo Verde National Park in Costa Rica, while Sunjian and Li (2003) could not find these species as well on the tip of the nearby Nicoya peninsula, although the area was covered with Acromyrmex octospinosus. This might be attributed to the vertisol type of soil in these areas, which has poor drainage when wet and can contain large cracks when dry.
Another notable point is that a previous survey of Atta mexicana in Northern Mexico in 2001 found that this related species, although present in great numbers in the Mexican states of Tamaulipas, Coahuila, and Nuevo Leon, was not found closer than 100 km away from the Texas border (Sanchez-Pena 2005). This present survey confirms the absence of Atta mexicana in the Texas border towns in the Lower Rio Grande.
Finally, some notes on the ecology of Atta texana in the Lower Rio Grande Valley border areas. Like most Atta species, A. texana prefers to forage during the cool nights when the days are too hot, thus making surveys of this ant more difficult, and also limiting their exposure and contact with the native people in the area. Nevertheless, homeowners in some parts of the Lower Rio Grande border are usually aware of this species, as the ants can wreak occassional havoc on the plants and other garden vegetation of these luckless people. Reports of the ant as being pests varied from town to town, with the higher percentage of positive sightings being in Zapata and the other more western towns, while homeowners and park rangers in Brownsville, Harlingen, and Weslaco said that they had not seen A. texana in these more eastern areas.
The severity of Atta infestation also varied, and so did the response to their presence. One church in Sunjian had a medium sized nest on the front yard, and the maintenance people became agitated when asked about the colony, saying that they had tried to kill it several times by fumigating the nests, but all to no avail. Likewise in Zapata, a homeowner with a colony that had mounds emerging from all four corners of his home had tried every poison bait and chemical he could find, again with no results against the ants.
|A church yard in Sunjian had a medium sized nest on the front yard, which proved impossible to kill even with fumigation.|
|Exact locations of 11 sampled Atta texana colonies in the Lower Rio Grande. Note that some sample points cannot be seen due to compression of the map image (Google Earth Map generated from DustyNotes KML files).|
|Exact locations of Atta texana samples in Bentsen-Rio Grande Valley State Park in Mission, Texas. Note the sample retrieved close to the Mexican border (Google Earth Map generated from DustyNotes KML files)|
Sunjian A and May Li (2003). A Survey of Leafcutter Ants from the Southern Tip of the Nicoya Peninsula in Costa Rica. Notes from Underground 10 (1)
Sanchez-Pena, SR (2005). Essays on organismal aspects of the fungus-growing ant symbiosis: ecology, experimental symbiont switches and fitness of Atta, and a new theory on the origin of ant fungiculture. PhD. Dissertation.
Wetterer, J. K. (1991). Foraging ecology of the leaf-cutting ant, Acromyrmex octospinosus, in a Costa Rican rain forest. Psyche 98: 361-371.