Chemical Ecology In The Red Imported Fire Ant (Solenopsis invicta Buren) (Hymenoptera: Formicidae)

Abstract:
Chemical signals play a central role in the complex organization of ant 
societies.  The red imported fire ant Solenopsis invicta Buren is one of the 
most thoroughly studied ant species in this respect.  Fire ants, moreover ants 
are extraordinary chemists.  They produce a wide variety of glandular secretions 
employed for chemical communication.  Many of these glandular secretions are 
produced in copious amounts and are stored in special glandular reservoirs.  
Glandular reservoirs found in most species of ants are the Dufour's gland, the 
metaplural glands, the mandibular glands, the pygidal gland and in certain 
groups a poison gland apparatus.  A short list of some of  the chemically 
elicited responses in ants include alarm, recruitment to new food or nest sites, 
grooming and eclosion assistance, kin and nestmate recognition, caste 
determination and control of competing reproductives.  These types of chemically 
mediated responses are important in the maintenance or establishment of social 
structure found in many ant species.  Research indicates that a typical fire ant 
colony may operate with some 10 to 20 kinds of signals, with all but one or two 
being chemical in nature.  In this paper I discuss and provide examples of the 
patterns of chemical ecology in the red imported fire ant Solenopsis invicta 
Buren. Four key types of chemical communication are discussed.  These types are 
1) worker to worker, 2) queen pheromones, 3) brood pheromones, and 4) venom 
ecology.

INTRODUCTION
	Chemical communication is the thread from which the complex organization 
of ant societies are bound.  Within the last 30 years, tremendous progress has 
been achieved in the understanding of insect pheromones, their modes of 
perception, biosynthetic regulation and release (Vargo and Laurel 1994).  In 
these respects, the imported fire ant Solenopsis invicta (Buren) is perhaps the 
most thoroughly studied ant species.  Fire ants produce a wide variety of 
glandular secretions employed for chemical communication.  Several of these 
glandular secretions are produced in copious amounts and are stored in special 
glandular reservoirs.  Glandular reservoirs found in most species of ants are 
the Dufour's gland, the metaplural glands, the mandibular glands, the pygidal 
gland and in certain groups a poison gland apparatus.  Some of the general 
behavioral responses elicited by chemical signals among ants range from simple 
alarm or attraction signals to caste determination or control of oogenesis among 
competing reproductives (Hölldobler and Wilson 1990)(Table 1).
	Chemicals involved in interactions between organisms are called 
semiochemicals (Vander Meer 1983).  When individuals of the same species are 
involved, the behavior-mediating chemicals are called pheromones.  
Allelochemicals are broadly classified as chemical mediators between organisms 
of different species.  Pheromones that elicit immediate responses are called 
releaser pheromones.  Primer pheromones are responsible when the response is 
delayed, such as in caste determination or control of competing reproductive 
oogenesis.  Ants use semiochemicals as response and primer mediators that are 
critical in the maintenance or establishment of social structure.  Research 
indicates that a typical fire ant colony may operate with some 10 to 20 types of 
signals, with all but one or two being chemical in nature (Hölldobler and Wilson 
1990).  This review covers some of the general patterns of chemical ecology in 
the red imported fire ant Solenopsis invicta (Buren).  Four key types of 
chemical communication are discussed.  These types are 1) worker to worker, 2) 
queen pheromones, 3) brood pheromones, and 4) venom ecology.
WORKER TO WORKER
	Worker to worker chemical communication in ants requires recognition cues, 
such as orientation, recruitment, nestmate recognition, attraction and alarm 
signals (Vander Meer 1983).  The first fire ant pheromone system to be 
investigated was the trail pheromone (Wilson 1959 in Vander Meer 1983).  
Wilson's (1959) investigation into the trailing responses of red imported fire 
ants lead to the discovery that Dufour's gland secretions function as both a 
releaser and orienter in trail following.  The Dufour's gland and poison sac 
open into the poison bulb where their secretions may be emitted singularly or in 
concert.
	Foraging workers provide food (solid or liquid) for the colony.  When a 
foraging worker discovers a novel food source it is faced with a potential 
problem, can the ant secure and single handily retrieve the food item or should 
the ant abandon the valuable food source and report back in the direction of the 
nest?  When an ant decides it needs help in securing a food source it lays a 
chemical trail down in the direction of the home nest.  The chemical trail 
markers are laid on the ground through the sting.  Ant pheromone trails consist 
of a series of smears made by periodic extension, contact and withdrawal of the 
sting with the ground substrate (Wilson 1962a).  Secretions of the Dufour's 
gland in S. invicta works as an attractant that recruits other workers over 
short distances to the trail and ultimately to the food source.  Food quality is 
conveyed by the strength of the chemical trial.  The strength of a trail varies 
depending upon the level of trail reinforcement and recruitment.  After a food 
source has been exhausted the pheromone trail is no longer reinforced and 
dissipates through evaporation or decomposition.  Vander Meer et al. (1981) and 
Williams et al. (1981) collectively have identified six trail pheromone 
components from the Dufour's gland; I) Z, E -(- Farnesene, II) E,E -(- 
Farnesene, III) Z, E -Homofarnesene, IV) Z, Z -Homofarnesene, V) N -Heptadecane, 
VI) Z, Z, Z -Allofarnesene.  There appears to be a great deal of diversity in 
the source and type of trail pheromones (Vander Meer 1983).  One subcategory of 
the recruitment pheromone complex involves orientation induction.  An 
orientation inducer pheromone exerts its effects by changing the physiological 
state of a recipient rather than releasing a quantifiable behavior.  The 
orientation inducer pheromone has been interpreted as a chemical analogue of the 
physical "waggle" dance inducing effects (Vander Meer et al 1990).  Activity-
concentration thresholds have indicated that the inducer portion of the 
recruitment pheromone requires approximately 250 times more worker equivalents 
for a response than the orientation pheromone.  This suggests the recruitment 
subcategories are differentially activated by the varying amount of Dufour's 
gland material released (Vander Meer et al 1990).  Additionally, there have been 
at least three other behavioral categories associated with this single glandular 
secretion.  These categories are mass foraging, colony emigration and alarm.  
	Key to the stability of ant colonies is the ability to discriminate colony 
members or nestmates from non-nestmates.  Nestmate recognition in most social 
animals involves chemical olfactory cues.  These chemical cues (e.g. cuticular 
hydrocarbons, colony odor) can be environmentally as well as genetically 
correlated (Obin and Vander Meer 1988). Cuticular hydrocarbons used in fire ant 
nestmate recognition may also be used by other organisms to gain entry into the 
colony.  The myrmecophilous beetle Myrmecaphodius excavaticollis (Blanchard) was 
found to "take-up" species-specific cuticular hydrocarbons from host Solenopsis 
species (Vander Meer and Wojcik 1982).  Combined with its armored exterior and 
passive hydrocarbon integration mechanism these beetles can withstand multiple 
aggressive fire ant hosts.
	Workers in single queen colonies of S. invicta readily attack intruders 
from neighboring conspecific colonies.  Obin and Vander Meer (1988) designed an 
aggression bioassay to investigate nestmate (worker to worker) recognition in 
monogyne colonies of S. invicta.  They determined that environmentally 
correlated cues dominated a recognition cue hierarchy in field and laboratory 
colonies.  Further, they established that diet alone can significantly modify 
recognition labels and templates of laboratory-reared workers.  As with other 
ant species (Jutsum et al. 1979) group recognition cues used by S. invicta 
workers can be directly or indirectly derived from diet.  Obin and Vander Meer 
(1988) demonstrated that aggression between non-nestmates was reduced by rearing 
laboratory colonies on the same diet, while aggression between colonies 
maintained on different diets did not differ significantly from that observed 
when the colonies were initially collected.  Polygyne (multiple queens) colonies 
operate on a slightly more plastic nestmate recognition system than monogyne 
colonies.  The polygyne form of S. invicta are polydomus, with individual nest 
mounds housing from several to several hundred functional queens (Obin et al. 
1993).  In field situations, workers from polygyne colonies may become merged 
with relatively little aggression.  Unlike monogyne workers, freshly collected 
workers form polygyne colonies are not aggressive toward conspecifics of either 
social form (Morel et al. 1990). Worker genotype influences on "Discriminators" 
transferred from queens also affected recognition between workers.  However, 
Obin and Vander Meer (1988) suggest that the motivation underlying nestmate 
recognition and nest defense activities may be independent of direct queen 
influences.
	Ants are efficient housekeepers.  Whereby, workers are responsible for the 
removal of dead ants from the colony.  The term "necrophoric behavior" has been 
used to distinguish corpse removal form other colony cleaning tasks (Wilson et 
al. 1958, in Howard and Tschinkel 1975).  Howard and Tschinkel (1975) 
demonstrated that the removal of dead ants from the nest is mediated entirely by 
contact chemical cues in the fire ant S. invicta.  Their bioassays indicated 
that the necrophoric releaser is absent at the onset of death but appears 
rapidly and plateaus within about an hour.  The rate of signal appearance was 
unaffected in heat or freeze killed workers indicating a non-enzymatic origin.  
A large number of corpses were subjected to serial "Soxhlet" extractions with 
the solvents, methanol, tetrahydrofuran, and ether.  Corpses that had been 
exhaustively extracted do not release necrophoric behavior, but the extracts do 
when applied to bits of filter paper.  Further, they suggest that contact 
chemical stimuli issuing from the feces in refuse piles bring about the end of 
necrophoric behavior and thereby maintain refuse piles.
	Mating flights of S. invicta are marked by excited workers on and around 
the mound.  When environmental conditions are right, workers busily open holes 
in the mound (which is usually sealed), for alate males and females to emerge.  
When sexually mature alates move out of the nest they are accompanied by 
attentive workers.  This type of behavior indicates that some potential alate 
semiochemicals are present that stimulate the formation of worker retinues to 
accompany alates during preflight staging.  In fact, Obin and Vander Meer (1994) 
designed a bioassay to investigate these aspects in induced laboratory mating 
flights of S. invicta.  They demonstrated that workers preferentially entered, 
searched and recruited nestmates to vials containing either an alate corpse or 
alate residue.  Also, they were able to elucidate a previously unreported 
behavior for fire ants, i.e., "back-and-forth jerking" and "group recruitment."  
Olfactory cues of both male and female gynes (but not workers) attracted 
workers, induced alarm - recruitment, and promoted alate retrieval.
	Alarm pheromones are present in some fire ants and elicit frenzied 
behavior among workers (Wilson 1962b).  When several workers of  Solenopsis 
saevissima are crushed between two pieces of clean glass and held one to three 
centimeters above resting workers, these workers exhibit increased excitement 
and tend to cluster beneath the crushed bodies and lift their gastors in a 
defensive posture.  This alarm pheromone has been linked to the cephalic region 
of the ant and may be stored in a mandibular gland as in other myrmicine species 
(Wilson 1962b).  Yet, Vinson (1997) points out that an alarm pheromone has not 
been identified in S. invicta.
QUEEN PHEROMONES
	At the center of colony existence lies the queen ant.  The queen controls 
the colony through the production of eggs, the type of eggs she produces, and by 
releasing pheromones that influence the behavior and physiology of workers and 
potential reproductives.  The first specific fire ant queen pheromone 
investigated was an attractant to workers produced by colony queens (Jouvenaz et 
al. 1974 in Vander Meer 1983).  Vander Meer et al. 1980 discovered that the 
attractants and queen recognition pheromones were stored in the poison sac and 
dispensed by the sting apparatus.  Apparently, this was a novel storage site for 
a queen pheromone.  The poison sac was a novel storage site because piperdine 
alkaloids normally associated with the venom of imported fire ants are not 
responsible for the behavior elicited by the queen pheromone.  Tests aimed at 
eliminating the alkaloids from bioassays indicated that the pheromone consists 
of relatively minor non-alkaloid components of the poison sac contents.  Queen 
ants have a small degenerated Dufour's gland and do not lay trails, but they do 
have fully developed poison sacs.  Queens are not aggressive and do not sting, 
indicating alternate functions for their poison sac contents than defense.  
There appears to be marked divergence of alkaloid compositions among workers and 
queens and evidence suggests multiple use functions of queen pheromones.  In a 
similar vein, the sting apparatus in queens is involved in the egg laying 
process and in the application of queen attractant - recognition pheromones and 
antimicrobial agents on eggs (Vander Meer and Morel 1995).  This pheromone has 
some distinct advantages for the eggs such as increased initial worker 
attraction and attention after oviposition, and the bactericidal activity of 
venom increases egg survivorship (Vander Meer 1983).  Several lines of evidence 
argue against a "significant" role for queen discriminators in fire ant colonies 
(Obin and Vander Meer 1988).  However, research indicates that chemical cues 
facilitate caste-recognition within fire ants.  Queens use subcaste specific 
labels and queen templates that reflect relative concentrations of queen 
pheromones and changes in sensitivity to these compounds associated with 
temporal polyethism are important recognition cues (Obin and Vander Meer 1989).
	Other effects of queen pheromones that have been documented in S. invicta 
include control of the development of male and female sexuals (Vargo and 
Fletcher 1986), dealation (wing shedding) and ovary development in winged virgin 
queens (Fletcher and Blum 1981), and ovary development among cohabiting egg-
laying queens in polygyne colonies (Vargo 1992).  Still, there remains little 
specific information on the mode of action of queen pheromones, except that 
phermonal regulation of ovary development involves juvenile hormone (JH) (Vargo 
and Laurel 1994).  Queen pheromones are typically placed in the class of primer 
pheromones.  Primer pheromones have less rapid but more profound effects on 
target animals than releaser pheromones.  Currently, there is little information 
on the chemical nature of social insect queen primer pheromones, modes of 
perception, or physiological mode of action (Vargo and Laurel 1994).  Vargo and 
Laurel 1994 investigated the mode of action and perception of a queen pheromone 
that inhibits dealation and ovary development in virgin queens of the fire ant 
S. invicta.  After removal from the pheromone signal (the queen) winged virgin 
queens went through ovary development and dealation.  Bioassays indicated that 
individuals from monogyne colonies were more responsive than individuals from 
polygyne colonies.  It has been suggested that the mode of perception of the 
inhibitory queen pheromone is by stimulating sensory cells in the antennae of 
winged virgin queens.
	Quantitative pheromone effects have been discovered in fire ant colonies.  
Queen pheromones may act quantitatively in the regulation of queen number within 
colonies (Fletcher and Blum 1983).  Some specific mechanisms include recognition 
by workers of unique quantitative blends of pheromones produced by queens and 
quantitative effects of pheromones acting at the colony level on workers and at 
the individual level on other queens.  This phenomena varies in its degree of 
queen regulation between polygyne and monogyne colonies.
BROOD PHEROMONES
	In S. invicta there exist brood pheromones that serve for the 
identification of brood by adult workers (Lamon and Topoff 1985).  Brood 
pheromone studies have shown that fire ant workers will retrieve brood or 
inanimate objects with the appropriate pheromone applied and place them in their 
nest's brood chamber.  Bigley and Vinson (1975) isolated a brood recognition 
pheromone and determined it to be a triglyceride lipid compound, triolein.  
Further, it has been demonstrated that chemical stimuli are utilized for the 
social facilitation of eclosion by S. invicta (Lamon and Topoff 1985).  Adult 
workers are required to strip away pupal cuticle for proper eclosion.  Adult 
workers then consume the pupal cuticle.  Chemical stimuli involved in this 
stage-specific social interaction are present at the onset of eclosion.  In a 
bioassay inanimate objects (bits of wood, paper, or metal shavings) treated with 
an extract of eclosing pupae were retrieved and placed in colony brood chambers 
with the appropriate age group, where they were attended by other workers.  The 
extracts for these behavioral tests were prepared by soaking >50 eclosing worker 
pupae in .5ml of methylene chloride for 24 hrs (Lamon and Topoff 1985).
VENOM ECOLOGY
	Ants use their stings for not only defense, as in many social bees and 
wasps, but also for prey capture, and pheromone dispersal (Obin and Vander Meer 
1985). In fire ant workers the ability to sting is an absolute necessity for 
survival.  The poison sac contents of fire ants are unique consisting largely of 
piperidine alkaloids.  They also have an interesting venom dispersal mechanism 
and some key behavioral correlates associated with venom alkaloid activity.  
Behavioral and chemical studies have indicated that fire ants dispense venom 
through the air by raising and vibrating their gastors ("gastor flagging").  
Foraging workers utilize air-dispensed venom (ca. 500 ng) to repel potential 
competitors encountered in their foraging areas.  Nurse ants while brood tending 
dispense smaller quantities (ca. 1 ng) to the brood surface, presumably as an 
antibiotic.  Solenopsis invicta venom is primarily (> 95%) composed of 
piperidine alkaloids (2-alkyl or alkenyl 6-methyl piperidines).  In vitro and 
antibiotic properties of the piperidine alkaloids found in fire ant venom has 
been established in laboratory assays (Obin and Vander Meer 1985).  Worker 
derived extracts on the brood were removed from 8000 immature ants by rinsing 
the sample for 1 min in a 2:1 chloroform-methanol (5 ml) solvent.  Venom 
alkaloids were isolated and purified by (1) washing three times with 0.5 N 
sulfuric acid (200 ul) (2) separation of the aqueous phase followed by 
basification with 1.5 N potassium hydroxide (250ul), and (3) extraction of the 
alkaloids from the basified aqueous phase three times with hexane (200ul).  An 
internal standard was added to the hexane extract prior to GLC analysis (Obin 
and Vander Meer 1985).
	It has been suggested that the selection pressures contributing to the 
evolution and maintenance of insecticidal repellents and antibiotic secretions 
in ants stem from predation and/or competition due to other invertebrates 
(especially other ants), and conditions in which bacterial and fungal growth are 
favored (Obin and Vander Meer 1985). This is in contrast to the evolutionary 
development of stings in social bees or wasps, whose purpose seems to be solely 
defensive. Fire ant venom is unique because it contains aliphatic substituted 
alkaloids that are cytotoxic and a small amount of protein (Vinson 1997).  The 
protein in fire ant venom causes an allergic response in less than 1% of the 
people stung.  For most people, being stung by a fire ant is not a life 
threatening event but is a memorable event no less.  Individual fire ant stings 
are not as severe as those caused by some bees and wasps (Vinson 1997).  However 
confrontations with fire ants may involve hundreds of ants.   Once bitten there 
is an initial burn which subsides for a brief calm before the more severe 
symptoms occur.  Local swelling occurs and a red wheal appears within a hour 
marked by itching and a burn, hence the name "fire" ant.  Pustules may form 10-
12 hours which are easily broke and infected.  If left alone the pustules dry up 
in several weeks and may even leave a darkened or lightened skin blemish for 
months.

SUMMARY
Fire ant semiochemicals are extremely diverse in their sources of origin, modes 
of action and modes of perception.  This versatility in pheromone source and 
function bodes well for the conservative approach of nature.  As in worker to 
worker communication, recognition cues emitted through the Dufour's gland 
function as both releasers and orienters.  Queen pheromones such as attractants 
and queen recognition cues (non-alkaloid components) are stored in the poison 
sac and are dispensed by the sting apparatus.  Workers use venom as both a 
defensive and antibiotic agent.  Fire ants produce a number of pheromones and 
many interact with each other.  This makes it necessary to understand their 
integrative functions and develop bioassays to account for these interactions.  
As improvements in bioassay designs and organic microanalysis are realized so 
will new lines of evidence in the chemo-reception of ant behavior.

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Table 1. Some categories of chemical communication in the red imported fire ant 
Solenopsis invicta (Buren) (Hymenoptera: Formicidae) including stimulus, 
response and relevant references.
Stimulus
Response
Reference
Nest odor
none if odor is undisturbed

Hölldobler and Wilson 1990

Body surface attractants
oral grooming, clustering

Regurgitation
feeding

Dufour's secretions/attack
attraction to disturbed worker

Dufour's secretions/trail
attraction and trial following

Cephalic substances
alarm behavior
Hölldobler and Wilson 1990

Onset of ecolsion
facilitation of eclosion
Lamon and Topoff 1985

Queen presence
control, attractant and arrest
Vander Meer et al. 1980

Recognition cues
nestmate recognition
Obin and Vander Meer 1989

Brood care
attraction/antimicrobial agents
Vander Meer and Morel 1995

Alate mandibular pheromones
alate attraction/retrieval/alarm
Obin and Vander Meer 1994

Blends of Dufour's gland
recruitment/orientation
Vander Meer et al. 1990