Dead Ant Walking

Understanding of the behavior and biology of ants will help pest management professionals develop effective pest control strategies while minimizing negative environmental impact. One of the important goals of pest management is to maximize the impact against pest species with the minimal use of insecticide. With the development of non-repellent, slow-acting insecticides, it is possible to exploit ant behavior to maximize the transfer of insecticide for control of ants.

INSECTICIDE TRANSFER. Insecticide transfer occurs when an ant ingests or contacts an insecticide and the insecticide moves to another ant. The insecticide will be transported to another location where it was not originally applied. If it is translocated in sufficient quantities, recipients will be killed. This transfer of insecticides is facilitated by the social interactions of ants.
These interactions include 1) trophallaxis or the mouth-to-mouth or anus-to-mouth exchange of food and chemicals, 2) mutual grooming and contact, 3) necrophoresis or the carrying of dead ants, 4) feeding on dead ants or excretions, and 5) contacting secondarily contaminated surfaces. Ant baits typically exploit trophallaxis or social feeding in ant colonies. Feeding on dead nestmates and contacting secondarily contaminated surfaces probably plays a minor role in ant control. However, the removal of dead ants from nests provides a unique opportunity to control ants.
One might ask why should ants remove and carry dead nestmates from the colony? Let’s imagine you are a worker ant. You are living in an underground nest with thousands — or maybe hundreds of thousands — of sisters. In such a crowded society and confined nest, hygiene is very important. Dead nestmates, or the remains of insect prey in the nest, should be removed promptly to prevent the nest from being cluttered with wastes and possible disease organisms. Thus, social insects, such as ants and honeybees, have developed conspicuous behaviors and corpse removal activities referred to as necrophoresis.
The term “necrophoresis” (from Greek words, nekro, which means “dead,” and pherein, which means “carry” or “bear”) was originally used by Edward O. Wilson in 1958, to distinguish it from other sanitation tasks in the ant colony. Dead workers are transported out of the nest by their nestmates and deposited in piles near the nest entrance. These piles are referred to as “graveyards,” “refuse piles” or “bone piles.”
Field and laboratory observations show that Argentine ants frequently remove their dead nestmates and deposit them on graveyards or refuse piles. We located ant graveyards in the field by placing several dead Argentine ants coated with a red fluorescent pigment on an ant trail. We returned at night with a portable UV light (365 nm). The dead ants with fluorescent pigment glowed, making the graveyards visible. The graveyards were located about 2 to 3 m away from the nest entrance. The observed pile contained at least 3,000 Argentine ant corpses (head capsules were counted), other ant species including Pyramica spp. (Myrmicinae), Hypoponera sp. (Ponerinae) and other insect parts (for example, honeybee head capsules and beetles). When these piles contain other insects in addition to their own dead ants, it is more appropriate to refer to them as refuse piles.
Most studies suggest that chemical cues on the dead ant releases necrophoric behavior. Wilson et al. (1958) hypothesized that decomposition compounds accumulate in the ant’s corpse after death. However, Howard and Tschinkel (1975) reported that the necrophoric signal appeared very quickly (within an hour) after ants are killed. Our recent studies suggest that dead Argentine ants are recognized by chemical changes of two characteristic ant-produced compounds. The two compounds have been identified with the help of Dr. Jocelyn Millar at the University of California, Riverside, and our study will be published in the Proceedings of the National Academy of Sciences (PNAS) soon.

NECROPHORESIS & TRANSFER. Necrophoresis is one of the primary ways by which healthy ants contact insecticide-contaminated corpses in or around their nests. Of the eight insecticides tested, however, not all are equally transferred via necrophoresis. Our laboratory studies have shown that deposits of Termidor (fipronil) were most effectively transferred. Only fipronil was horizontally transferred among nestmates to provide significant secondary kill in Argentine ant colonies. Its high-contact toxicity, non-repellency and affinity for lipids (for example, wax layer of insect cuticle) contribute to its transfer (Choe and Rust 2008).
Furthermore, 10 dead ants, killed by a one-minute exposure on 40-ppm fipronil deposit, killed more than 200 nestmates in 10 days when they were placed in the nest box. Many foragers inspected and eventually moved the insecticide-contaminated dead nestmates to refuse piles, and subsequently died (Choe and Rust 2008). The excellent control reported in field studies with Termidor is in part to this transfer effect. In a field study, a 0.06 percent Termidor spot treatment on active ant trails reduced Argentine ant activity by 90 percent around houses with only 25 percent the amount of insecticide normally used in a conventional perimeter treatment (Klotz et al. 2007).
More ants picked up lethal doses of insecticide when the insecticide-contaminated dead ants were placed closer to the nest (Choe and Rust 2008). The crowded nesting conditions are probably responsible for more frequent contacts with the insecticide-contaminated dead nestmates. If a larger percentage of the ants are engaged in nest maintenance activities, they are more likely to contact and carry dead ants.
It is possible that other insecticides besides fipronil could have potential for horizontal transfer in ants via necrophoresis. For example, Faye (1996) reported high necrophoric activity and active formation of dead pile in mound vicinity when insecticidal rotenone was applied in the red imported fire ant nest as a drench formulation.

MAXIMIZING TRANSFER. If we maximize the amount of insecticide transfer by exploiting ant behavior, we can achieve control with smaller amounts of insecticide. Ants will spread insecticide and maximize the transfer effect. It is essential that the insecticides are non-repellent. Insecticides that repel ants result in minimal physical contact and transfer. Secondly, it is necessary to use slow-acting insecticides. Fast-acting insecticides such as pyrethroids kill ants so quickly that the ants cannot recruit nestmates or establish trails. Speed of action is also dependent upon the concentration of insecticide applied. If you knock down or kill the ant on contact, then the toxicant will not be transferred to other individual, especially if the ant is away from the nest. Third, if you can find ant nests or large foraging trails, apply the insecticide to the nest or on the trail. The workers will carry the toxicant to the nest and die within the nest, resulting in increased necrophoresis by healthy nestmates.

FUTURE APPLICATIONS. We have developed a new strategy to exploit necrophoresis referred to as “virtual baiting” (see figures on page 55). This strategy would be appropriate for environmentally sensitive sites where broadcast insecticide applications might be of concern (for example, schools, hospitals, endangered species sites and commercial buildings).
Virtual baiting exploits the foraging/recruitment behavior and necrophoresis of worker ants. It doesn’t require formulating the toxicant into special food baits. The ants are attracted to a sugar water feeding station. In the process, they are exposed to extremely low doses of fipronil. The ants then die in 24 to 36 hours. This is long enough to permit ants to recruit additional workers to the site and for many of them to die in the nest, and thus facilitate necrophoresis. This is an experimental study and Termidor is currently not registered for this use.
A recent preliminary study indicated that dead ants accumulated in the virtual bait station over two to four weeks made the sucrose solution less attractive to foragers. We are currently investigating the chemicals responsible for the avoidance. We are developing removable sucrose solution cartridges that would enhance the performance of the virtual bait station by minimizing the number of dead ants accumulating in the bait dispenser.
Our investigation of chemical signals associated with ant necrophoresis may also help develop novel bait strategies. For example, “artificial ants” that chemically mimic dead ants and release necrophoric responses might be used to deliver non-repellent insecticides or insect growth regulator (IGR) analogs to pestiferous ant colonies. As Dr. Knipling (1970) wrote in a forward for the book “Chemicals Controlling Insect Behavior,” “An understanding of the nature of the chemicals that regulate or influence insect behavior is a prerequisite for detailed biological investigations on their potential value in the management of insect population and in their use for practical insect control.”

References
Choe, D.-H., and M. K. Rust. 2008. Horizontal transfer of insecticides in laboratory colonies of the Argentine ant (Hymenoptera: Formicidae). Journal Economic Entomology 101: 1397-1405.
Faye, D. D. 1996. Effects of tannic acid, benzoic acid, and Azatin ECTM on True StopTM and True GrowthTM efficacy on the red imported fire ant Solenopsis invicta Buren control, pp. 124-130. In Proceedings, 1996 Imported Fire Ant Research Conference, 16-18 April 1996, New Orleans, LA. Howard, D. F., and W. R. Tschinkel. 1976. Aspects of necrophoric behavior in the red imported fire ant, Solenopsis invicta. Behaviour 56: 157-180. Klotz, J. H., M. K. Rust, L. Greenberg, H. C. Field, and K. Kupfer. 2007. An evaluation of several urban pest management strategies to control Argentine ants (Hymenoptera: Formicidae). Sociobiology 50: 391-398.
Knipling, E. F. 1970. Foreword. In M. Beroza (ed.), Chemicals Controlling Insect Behavior, Academic Press, New York, USA.
Wilson, E. O., N. I. Durlach, and L. M. Roth. 1958. Chemical releasers of necrophoric behavior in ants. Psyche 65: 108-114.

The authors are XXX and XXX at the Department of Entomology, University of California, Riverside. Choe can be reached at dchoe@giemedia.com.