The tawny crazy ant, Nylanderia fulva, is a highly invasive pest from South America that has invaded the southeastern United States. This ant’s exact point and time of introduction into the U.S. is unclear. In 2002, a pest management professional discovered a large population of unknown ants in Pasadena, Texas. This unknown species looked identical to another pest ant found in Florida, the Caribbean crazy ant (N. pubens), but colony size and foraging behavior were notably different. It wasn’t until 2012 that a combination of phylogenetic and morphological tools were used to identify this ant as N. fulva. Given the visual similarities between tawny crazy ant and Caribbean crazy ant workers, it is believed that some early records of the Caribbean crazy ant in Florida that predate 2002 may have in fact been the tawny crazy ant. However, this cannot be confirmed due to a lack of reference specimens.
Throughout various historical texts, the tawny crazy ant has been referred to by other names including the hairy crazy ant, the Rasberry crazy ant, the Caribbean crazy ant (primarily in Florida) and hormiga loca. The common name, tawny crazy ant, was officially accepted by the Entomological Society of America in 2013.
In both their native and introduced ranges, tawny crazy ants pose a serious ecological threat and can become a major structural pest. They do not sting, but workers can produce formic acid, which may cause irritation. Populations can reach incredible densities that completely overwhelm an area, injuring ground-nesting animals, displacing other ants and infesting structures. Workers also cause severe damage to electrical equipment when foraging ants infest electrical boxes and circuit breakers.
Tawny crazy ants are monomorphic (same-sized) ants that get their name from their tawny (reddish-brown) color and quick, erratic movement. Workers measure between 0.07-0.09 inches (2.0-2.4 mm) in length, are covered in many hairs, and have a one-segmented petiole (one node). They also have a 12-segmented clubless antenna and an acidopore (opening with a circle of hairs) at the tip of their gaster. Males are slightly larger than workers, measuring approximately 0.1 inches (2.7 mm) in length. Queens are the largest caste members, measuring up to 0.2 inches (5.3 mm) in length.
The tawny crazy ant looks nearly identical to a closely related species, the Caribbean crazy ant. Only males can be used to morphologically differentiate the two species. The parameres (external reproductive organs) of tawny crazy ant males are triangular and less sclerotized with fewer macrosetae (hairs) while the parameres of Caribbean crazy ant males are more rounded, well sclerotized and have dense macrosetae. Workers are only distinguishable using molecular phylogenetic analyses.
At the time of this publication (2020), the tawny crazy ant was confirmed in Georgia, Florida, Alabama, Mississippi, Louisiana and Texas. States with the most widespread infestations are Texas and Florida. Given this ant’s propensity to spread through infested landscape goods and other similar items, the tawny crazy ant has the potential to continue expanding its invasive range into other southeastern states. To reduce its expansion, especially in bordering states, pest management professionals should familiarize themselves with this species and be prepared to report any suspected encounters to your state department of agriculture.
BIOLOGY AND BEHAVIOR
Like all ants, tawny crazy ants are social insects. Their colonies are polygynous (multiple queens) and polydomous (multiple nests), capable of producing extremely dense populations that can overwhelm urban and rural environments. Colonies reproduce through budding, in which a mated queen leaves a nest with several workers and brood to establish a new nest. Colony growth is seasonal and is largely influenced by temperature. Generally, reproduction increases as temperatures begin to warm in early spring (>59°F, >15°C). Colony growth peaks during the summer months when temperatures and rainfall are highest and decreases as temperatures fall leading into winter.
Nesting preference for the tawny crazy ant is variable and opportunistic, with high humidity being a primary factor when choosing a location. Nests have been observed in a range of man-made and natural materials including electrical or utility boxes, leaf litter, mulch, bark, potted plants and various other landscape goods. Nests are relocated frequently, especially when disturbed, which contributes to difficulties when managing this invasive pest.
Tawny crazy ants do not form mounds but can tunnel in loose soil, landscape materials, or under objects such as pavers. They are scavengers that quickly dominate and aggressively defend resources against competing ant species, but they show little to no aggression to neighboring tawny crazy ant colonies. This low intraspecific (within species) aggression is one factor that allows this ant to establish large and dense colonies that cover expansive territories. Workers are omnivorous scavengers, but generally display a stronger preference for carbohydrate-rich (sugar-rich) resources. This is especially true during the summer months when worker numbers are highest in the colony. Foraging workers are often seen feeding on and defending natural carbohydrate sources that include nectar-producing plants and honeydew-producing hemipterans like aphids and scales.
Long-term management of this ant has proven to be challenging. Control efforts that implement a single control tactic such as contact insecticides or localized bait application may provide a quick knockdown of workers but will not ensure long-term success. Colonies can rebound in as few as 2-3 days after a single insecticide application if no other control measures are taken. Effective control can only be achieved through an integrated pest management approach that combines exterior and interior sanitation, exclusion and chemical control methods.
Sanitation should focus on eliminating any harborage, food and moisture sources. Outdoor efforts that focus on eliminating harborage are extremely important and can have the biggest effect on population reduction around a structure. First, keep landscape well maintained by trimming bushes and shrubs and eliminating overgrown brush. This also helps to eliminate nectar-producing plants and honeydew-producing hemipterans. Additionally, remove accumulated yard waste such as excess leaf litter or fallen branches to eliminate preferred nesting sites. Ensure that trash receptacles are kept clean and stored away from the structure. Lastly, address any excess moisture conditions such as poor drainage on the property. Indoors, clean up food spills immediately and remove trash regularly. Store food in sealed containers and keep pet food picked up when pets are not eating.
Preventing ants from entering a structure through exclusion is an effective way of minimizing the likelihood of infestation indoors. Inspect the exterior of the structure regularly for any gaps, cracks or crevices along the foundation, around windows and doors, and around any utility access points. If access points are found, be sure they are filled with an appropriate sealant that is labeled for the application site. Lastly, inspect and repair any worn or damaged screens and door thresholds.
Chemical control efforts for tawny crazy ants should begin with insecticidal baits applied to the exterior of the structure. Ant baits are generally formulated as carbohydrate, protein or oil-based to appeal to the different food preferences of ants. Therefore, before applying bait, you should assess the foraging preferences of the population by “pre-baiting” ants with a carbohydrate (honey or jelly) and a protein (peanut butter or Vienna sausage) to determine which food they prefer. Once the bait is selected, be sure to follow all label directions for application and safety instructions. Depending on the size of the infestation, large-area baiting may be necessary. The palatability of some liquid and granule baits can be extended by using protective bait stations to shelter baits from environmental factors such as rain and UV exposure.Opens in modal lightbox
In addition to insecticidal baits, contact liquid or granular insecticides are also effective management tools when applied correctly and according to label instruction. Slower acting, non-repellant insecticides are ideal for perimeter applications because ants will unknowingly pick up and transfer the active ingredient throughout the colony. Closer to the structure, repellant insecticides can be effective along exterior thresholds where they may serve to deter ants from entering the structure.
Bentley, M.T.; Oi, F.M.; Gezan, S.A.; Hahn, D.A. (2015) Tunneling Performance Increases at Lower Temperatures for Solenopsis invicta (Buren) but not for Nylanderia fulva (Mayr). Insects. 6: 686-695.
Gotzek, D., S. G. Brady, and R. J. L. Kallal, John S. (2012) The importance of using multiple approaches for identifying emerging invasive species: the case of the Rasberry crazy ant in the United States. PLoS ONE 7: 1-10.
Eyer, P., McDowell, B., Johnson, L.N.L. et al. (2018) Supercolonial structure of invasive populations of the tawny crazy ant Nylanderia fulva in the US. BMC Evol. Biol. 18: 209. https://doi.org/10.1186/s12862-018-1336-5
Oi, F, D. Calibeo, J. Paige III, and M. Bentley (2011) Integrated Pest Management (IPM) of the Tawny Crazy Ant, Nylanderia fulva (Mayr). ENY-2006. Gainesville: University of Florida Institute of Food and Agricultural Sciences, https://edis.ifas.ufl.edu/pdffiles/IN/IN88900.pdf. Accessed 2, 21, 2020.
Wang Z, Moshman L, Kraus EC, Wilson BE, Acharya N, Diaz R. (2016) A Review of the Tawny Crazy Ant, Nylanderia fulva, an Emergent Ant Invader in the Southern United States: Is Biological Control a Feasible Management Option? Insects, 7(4):77.
BY MIKE BENTLEY, PH.D., NPMA DIRECTOR OF TRAINING AND EDUCATION