Summary. Elongate twig ant populations have increased in Louisiana over the past few years. Many people are curious about these strange ants and become fearful of them when they experience their voracious sting. The status and habits of the elongate twig ant (Pseudomyrmex gracilis) are reviewed for Louisiana. Identification tips are given to distinguish it from other ants in Louisiana.
Introduction/Identification. The elongate twig ant (Pseudomyrmex gracilis, formerly known as Pseudomyrmex mexicanus) is a large, peculiar ant that has recently invaded Louisiana. It is one of the most easily recognized ants in the state because of its size, coloration, shape and solitary behavior. It is strikingly marked with orange and black (see photo above) and the petiole (the connection between the thorax and abdomen) is elongated, thus giving it the appearance of a wingless wasp.
Distribution. The native range of the ant extends from southern Texas to Central America, but it was introduced to South Florida sometime before 1960 and to Hawaii in the 1970s. Since then, it has spread not only northward through Florida and along the Gulf Coast but also northward and eastward in Texas, steadily encroaching on Louisiana. It can be locally abundant and it is known with certainty from around Baton Rouge, New Orleans and Alexandria and probably occurs throughout South Louisiana.
Biology/Behavior. Little is known about its habits in Louisiana, and most of the following observations come from other states. The elongate twig ant forms small colonies that nest in plant cavities -- most often in hollow stems, vines, twigs or thorns -- and has been known to nest in the tunnels of the sugarcane borer (Diatraea saccharalis). Apparently, the ant relies almost entirely on the excavation activities of other insects for nest sites. They are known to be associated with the so-called “ant-acacia” trees, which have been introduced into Florida. Both the queen and the workers are highly active, agile ants. They have been known to feed on both fungal and vegetable material, but insects, and moth larvae in particular, make up the bulk of the diet. The ant is often seen in association with aphids and mealybugs, presumably attracted by the honeydew produced by these sap-feeding insects. The elongate twig ant’s effect on native flora and fauna is currently unknown, but there are not yet any indications of serious problems associated with the introduction of this enigmatic ant.
Pest Status. While not known to be a significant pest by any criteria, the elongate twig ant is fairly aggressive and does have a rather potent sting that can been painful. It forages predominantly in trees and shrubs, and stings often result after walking through dense vegetation and attempting to brush the ant from the body. Another way people get stung is if the ant is crawling on them and is pinched under a watch band or in the crook of an elbow. However, because of the ant’s solitary foraging behavior, it is unlikely to cause major problems. It has even been suggested that the ant could potentially become a beneficial predator of larval moth pests in agricultural systems, though much investigation is still required before any conclusions can be drawn. This ant has been increasing in number in South Louisiana, and it is being seen more often in backyards. They are often seen on patio furniture, especially those placed under trees. People sitting in their gardens or trimming plants are most often those that report sightings of this ant.
Additional Reading:
Beardsley, J.W. 1979. Notes and exhibitions: Pseudomyrmex gracilis mexicanus (Roger). Proceedings of the Hawaiian Entomological Society 23:23.
Dash, S.T. 2004. Species diversity and biogeography of ants (Hymenoptera: Formicidae) in Louisiana, with notes on their ecology (unpublished master’s thesis, Louisiana State University).
Ward, P.S. 1985. The Nearctic species of the genus Pseudomyrmex (Hymenoptera: Formicidae). Quaestiones Entomologicae 21:209-246.
Wetterer, J.K. and A.L. Wetterer. 2003. Ants (Hymenoptera: Formicidae) on non-native Neotropical ant-acacias (Fabales: Fabaceae) in Florida. The Florida Entomologist 86(4):460-463.
Whitcomb, W.H., H.A. Denmark, W.F. Burnen, and J.F. Carroll. 1972. Habits and present distribution in Florida of the exotic ant, Pseudomyrmex mexicanus (Hymenoptera: Formicidae). The Florida Entomologist 55(1):31-33.