common name: elongate twig ant, Mexican twig ant (suggested common names)
scientific name: Pseudomyrmex gracilis (Fabricius) (Insecta: Hymenoptera: Formicidae: Pseudomyrmecinae)

Introduction - Synonymy - Distribution - Description - Life Cycle - Nests - Hosts - Damage - Management - Selected References

Introduction

Pseudomyrmex gracilis (Fabricius), often called the elongate or Mexican twig ant, is a neotropical, arboreal ant best known for its associations with plants and its terrible sting (Wheeler and Wheeler 1956, Ward 1990). This ant belongs to the subfamily Pseudomyrmecinae and is described as a long, solitary, wasp-like ant (Ward 1990, Ferster et al. 2000). It feeds on live insects (especially lepidopteran larvae), fungus spores, and tends aphids for honeydew (Whitcomb et al. 1972, Ferster et al. 2000).

Synonymy

Pseudomyrma gracilis var. longinoda Enzmann 1945
Pseudomyrma gracilis var. velifera Stitz 1933
Pseudomyrma gracilis var. glabriventris Santschi 1922
Pseudomyrma gracilis var. guayaquilensis Forel 1907
Pseudomyrma pilosula F. Smith 1877
Pseudomyrma variabilis F. Smith 1877
Pseudomyrma canescens F. Smith 1877
Pseudomyrmex mexicanus Roger
Pseudomyrma mexicana Roger 1863
Pseudomyrma dimidiata Roger 1863
Pseudomyrma sericata F. Smith 1855
Pseudomyrma bicolor Guerin 1844
Formica gracilis Fabricius 1804

(From Ward 1993 and Deyrup 2003)

Distribution

Originally from Mexico, P. gracilis is found from the southern United States to Argentina and Brazil (Ward 1993). In the U.S., P. gracilis is only found in Texas, Florida, and Hawaii, but could possibly be established in Louisiana. (Whitcomb et al. 1972, Ward 1985, Starr et al. 2004). Pseudomyrmex gracilis is also found in Jamaica (Ward 1985, 1993).

Interactive distribution map for P. gracilis at Discover Life

Pseudomyrmex gracilis was first discovered in Florida at Dade Co. in the 1960s (Whitcomb et al. 1972, Deyrup et al. 1988, Ferster et al. 2000). Currently, P. gracilis can be found throughout most of the Florida peninsula and some locations in the panhandle (Johnson 1986, Deyrup et al. 1989, Klotz et al. 1995, Ferster et al. 2000).

Description

Adult: The adult is best described as a large (8 to 10 mm), slender, wasp-like ant (Whitcomb et al. 1972, Ward 1985, 1990, Forster et al. 2000). It has large eyes, a two-segmented petiole, and a well-developed sting (Whitcomb et al. 1972, Ward 1990). Its head is broad and has a twelve-segmented antenna (Ward 1985, 1993, Ferster et al. 2000). Erect hairs cover its body (Ferster et al. 2000).

head

head

head

The color of P. gracilis specimens differs greatly due to large geographical variations (Ward 1985, 1993). The color can range from dark brown-black, to orange-brown, or a mixture of orange and brown [bicolored] (Ward 1985, 1993). Bicolored ants usually have a dark head and abdomen while the thoracic region is light orange in color, but variations occur (Whitcomb et al. 1972, Ward 1993, Ferster et al. 2000). The variation found in Florida is mostly described as bicolored with a black head, black abdomen, and an orange middle region. This bright color pattern may serve as a warning to predators, especially vertebrates, of the ant's painful sting (Holldobler and Wilson 1990).

adult - lateral view

adult - lateral view

adult - dorsal view

adult - dorsal view

Larva: A worker larva is long (6 mm), slim, subcylindrical, and covered with short hairs (Wheeler and Wheeler 1956). Like the adult, it is arboreal and can be found inhabiting plant cavities (Wheeler and Wheeler 1956, Holldobler and Wilson 1990). The most distinguishing characteristic is the trophothylax (Wheeler and Bailey 1920, Wheeler and Wheeler 1956). The trophothylax is an enlarged pocket just behind the mouth, but located on the 1st thoracic segment. The larva is fed by a worker adult, which places a firm, dry pellet into the larvae's trophothylax (Wheeler and Wheeler 1956).

Life Cycle

There is no specific information relating to the colony life cycle of P. gracilis, but the following is common to most ant species. Ant species are perennial and colony growth can be achieved through three stages of development: founding, ergonomic, and reproduction (Holldobler and Wilson 1990). During the founding stage, virgin queens leaves the nest to mate with a male reproductive. The queen mates and then begins a new nest by laying eggs and tending the brood. Once the first group of workers develop, the workers take over colony maintenance and the queen's only responsibility is to lay eggs. The colony grows by producing more and more sterile workers; this is the ergonomic stage. Finally, the colony is large enough to produce reproductives (alates) that will disperse generating new colonies, which is the reproduction stage. (Holldobler and Wilson 1990).

alate - dorsal view

alate - lateral view

Nests

Nests are small, monogyne (single queen) colonies that can be found in a wide range of habitats including rainforests, mangroves, thorn scrub, secondary growth fields, and hardwood hammocks (Ward 1985, 1993). There is only one small entrance into the nest (Ferster et al. 2000).

There are no known pheromone trails associated with P. gracilis to aid nest relocation. Instead, adult workers carry other aged workers, queens, and males to the new nest site. This is done by the worker seizing its nestmate by the petiole or head and then curling the nestmate over the worker's body (Holldobler and Wilson 1990).

Pseudomyrmex gracilis is most likely to be found nesting in naturally occurring hallow cavities such as dead twigs, small branches, or large plant stalks (Wheeler and Wheeler 1956, Whitcomb et al. 1972, Deyrup et al. 1988, Holldobler and Wilson 1990, Ward 1993). This species is able to excavate their own nesting cavity if the material is pliable, but it is not unusual for nests to be found in tunnels made by cerambycids (long-horned beetles) or other insects (Whitcomb et al. 1972, Holldobler and Wilson 1990). However, Pseudomyrmex spp. are best known to be associated with bull horn acacias (Wheeler and Wheeler 1956, Whitcomb et al. 1972, Deyrup et al. 1988, Ward 1993).

Hosts

Unlike other members of Pseudomyrmecines, P. gracilis is not an acacia-specialist, but rather a generalist, nesting in a wide range of different vegetation (Ward 1993). Its nest can be high up in trees, in blades of grass or herbs, and in shoulder high shrubs (Whitcomb et al. 1972, Ward 1985, Ferster et al. 2000). Known nests of P. gracilis have been found in exotic ant-acacia trees in Florida such as Acacia cornigera (Wetterer and Wetterer 2003). Nests also have been found in Brazilian peppertree, Schinus terebinthifolius Raddi, that have died due to freezing temperatures (Whitcomb et al. 1972, Cassani 1986).

Pseudomyrmex gracilis will defend its host plant by swarming and stinging intruders (Wheeler and Wheeler 1956).

Damage

There is little or no ornamental plant or structural damage caused by P. gracilis. However, a survey by Klotz et al. (1995) reported P. gracilis nesting in wooden doors inside several homes.

Management

Pseudomyrmex gracilis can have a relatively painful sting, but because they are usually only encountered outside in small numbers, management is not necessary (Ferster et al. 2000).

Selected References

Authors: Patricia L. Toth, University of Florida
Photographs: April Nobile, California Academy of Sciences. www.antweb.org
Interactive Distribution Map: Courtesy of Discover Life. Designed by: The Polistes Corporation
Project Coordinator: Thomas R. Fasulo, University of Florida
Publication Number: EENY-418
Publication Date: September 2007
Copyright 2007 University of Florida

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