Phenology and parasitism rates in introduced populations of <Emphasis Type="Italic">Pseudacteon tricuspis</Emphasis>, a parasitoid of <Emphasis Type="Italic">Solenopsis invicta</Emphasis>

We documented patterns of seasonal abundance and rates of parasitism in introduced populations of Pseudacteon tricuspis Borgmeier, a phorid parasitoid of the red imported fire ant, Solenopsis invicta Buren. Adult P. tricuspis populations were censused at monthly intervals for 1 year at three sites in northern Florida. Censuses were conducted by aspirating phorids attracted to disturbed S. invicta mounds. Pseudacteon tricuspis adults were present in every month at all sites, although abundances varied greatly among sites and over time. The highest densities of flies (up to 453 censused at 10 disturbed S. invicta mounds in 30 min) were observed in November, and changes in abundance over time were positively correlated among sites. Sex ratios were usually male biased. Parasitism rates were evaluated by collecting workers from field colonies and monitoring them in the laboratory for evidence of parasitism. Parasitism rates were very low – always less than 1%. The average parasitism rate per colony over 16 colonies and 2 years was 0.058%. No pupariation occurred within the first 8 days of collection, suggesting parasitism by P. tricuspis induced behavioral changes in parasitized workers that precluded such workers from our collections. If so, true field parasitism rates may be several times higher than measured here, yet still low in an absolute sense. These low parasitism rates can be reconciled with observed adult phorid densities by considering the large number of host ants present at the study sites.     

Post-Release Host-Specificity Testing of Pseudacteon tricuspis, a Phorid Parasitoid of Solenopsis Invicta Fire Ants

Inherent in any biological control program is the risk of nontarget effects. Pseudacteon tricuspisBorgmeier, a parasitoid phorid fly, has been introduced to the United States from South America as a potential biocontrol agent of the red imported fire ant, Solenopsis invictaBuren. We conducted tests of host specificity on introduced populations of P. tricuspis, which are attracted to alarm pheromones released by their hosts during events such as mound disturbances and interspecific interactions. We monitored disturbed mounds of S. invicta and its close congener, S. geminata(F.), during the expansion of P. tricuspis across north Florida and after populations had been established for ~3 years. We also tested host acceptance in established populations of P. tricuspis by offering trays containing S. invicta, S. geminata, and 14 additional ant species representing 12 different non-Solenopsis genera. Although P. tricuspiswas commonly observed to hover over and attempt to oviposit on S. invicta, we never observed any parasitization attempts on any other ant species. As predicted by laboratory tests, released populations of P. tricuspis appear to be highly host specific and pose no obvious threat to nontarget species.     

Zombie fire ant workers: behavior controlled by decapitating fly parasitoids

Laboratory observations were conducted on four separate red imported fire ant, Solenopsis invicta, colonies that contained workers parasitized by the decapitating fly, Pseudacteon tricuspis. Parasitized S. invicta workers remained inside the nest during parasitoid larval development and left the nest approximately 8 – 10 hours before decapitation by the parasitoid. When parasitized ants left the nest, they were highly mobile, were responsive to tactile stimuli, and showed minimal defensive behavior. Ants ultimately entered into a grass thatch layer, where they were decapitated and the fly maggots pupariated. This study reveals that parasitized ants exhibit behaviors that are consistent with host manipulation to benefit survival of the parasitoid. Received 9 November 2006; revised 26 January 2007; accepted 7 February.     

Evaluating biological control of fire ants using phorid flies: effects on competitive interactions

The effects of the parasitic phorid fly, Pseudacteon tricuspis Borgmeier, on the competitive interactions between the red imported fire ant, Solenopsis invicta Buren, and a native North American ant, Forelius pruinosus (Roger), were investigated in the laboratory. P. tricuspis is a highly host-specific endoparasitoid of S. invicta workers that is currently being reared and released as a biological control agent of S. invicta in the US. We tested the effect of P. tricuspis on the colony growth rate of S. invicta when S. invicta was forced to compete with F. pruinosus for a protein resource (freeze-killed crickets) in laboratory competition arenas. In addition to colony growth rate, we quantified the effect of the phorid flies on the foraging rate of S. invicta. Though S. invicta significantly reduced its foraging rate in the presence of the phorid flies, we did not detect an effect of the flies on colony growth rate. Possible explanations for these results include behavioral compensation by S. invicta for the presence of the flies. We present these laboratory results in light of a literature search indicating that laboratory tests of biological control agent efficacy are good predictors of field efficacy. We conclude that P. tricuspis alone is unlikely to suppress S. invicta populations in the field by reducing their competitive ability.     

Spatio‐temporal population sampling of a fire ant parasitoid

The spatial and temporal population structure of Pseudacteon tricuspis Borgmeier (Diptera: Phoridae), an introduced parasitoid of the invasive red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), was investigated at three locations in Louisiana, USA, during the fall of 2005. Georeferenced disturbed fire ant mounds that attracted adult P. tricuspis were analyzed using Spatial Analysis by Distance IndicEs to model the population structure of this parasitoid on a local spatial scale, and relate spatial abundance to host colony social form and infection with the fire ant pathogen, Thelohania solenopsae Knell, Allen and Hazard (Microsporidia: Thelohaniidae). Spatial associations between P. tricuspis counts and host S. invicta colonies infected with T. solenopsae were not detected. Overall, spatio‐temporal count patterns of P. tricuspis were largely random. However, high P. tricuspis counts were significantly associated with areas of polygyne host colonies. Occasional aggregations of P. tricuspis populations into discrete spatial patches and gaps were also found. The implications of this study are that population surveys of P. tricuspis should be conducted in several areas at a location to account for local patchiness in fly populations and/or varying densities and social form of host ant colonies.     

Phorid fly, Pseudacteon tricuspis, response to alkylpyrazine analogs of a fire ant, Solenopsis invicta, alarm pheromone

The phorid fly, Pseudacteon tricuspis Borgmeier, is a parasitoid of the red imported fire ant, Solenopsis invicta Buren. This fly has been reported to use fire ant chemicals, specifically venom alkaloids and possibly alarm pheromone to locate its host. A recent study identified 2-ethyl-3,6-dimethyl pyrazine as a component of the alarm pheromone of S. invicta. To determine the possible involvement of this fire ant alarm pheromone component in mediating fire ant-phorid fly interactions, we tested electroantennogram (EAG) and behavioral responses of P. tricuspis females to the commercially available mixture of 2-ethyl-3,6-dimethyl pyrazine and its 3,5-dimethyl isomer, as well as six structurally related alkylpyrazine analogs at varying doses. Pseudacteon tricuspis females showed significant EAG response to 2-ethyl-3,6(or 5)-dimethyl pyrazine (herein referred to as pheromone-isomer) at all doses, 0.001-10 μg. Among the tested alkylpyrazine analogs, 2,3-diethyl-5-methyl pyrazine showed significant EAG activity at 0.1 and 1 μg. 2,3-dimethyl pyrazine also showed significant EAG activity at 0.1 μg. Results of four-choice olfactometer bioassays demonstrated significant attraction of P. tricuspis females to the pheromone-isomer (2-ethyl-3,6(or 5)-dimethyl pyrazine) at all tested doses (0.01, 0.1, 1 and 10 μg). The analogs, 2,3-diethyl-5-methyl pyrazine and 2,3-dimethyl pyrazine were significantly better than the control at the higher doses (0.1, 1 and 10 μg). The pheromone-isomer was significantly better than both analogs at two doses, 0.1 and 1 μg. These results confirm that the reported fire ant alarm pheromone component plays a role in mediating attraction of phorid flies to host workers. Venom alkaloids were previously shown to attract P. tricuspis; therefore, we propose that fire ant alarm pheromones may act in tandem or synergistically with venom alkaloids to attract phorid fly parasitoids to fire ant workers.     

Host preference in Pseudacteon phorid flies: response of P. tricuspis and P. curvatus to black, red and hybrid imported Solenopsis fire ants in multiple choice bioassays

Host preferences in both sexes of Pseudacteon tricuspis Borgmeier (Jaguariuna biotype) and Pseudacteon curvatus Borgmeier (Formosa biotype) and their relative attraction to the imported fire ants (IFA), Solenopsis invicta Buren (red IFA), Solenopsis richteri Forel (black IFA) and S. invicta × S. richteri hybrids (hybrid IFA) were investigated in two separate experiments utilizing multiple choice flight bioassays. The results of both experiments clearly showed that both sexes of the Jaguariuna biotype of P. tricuspis could distinguish among the three IFA species and demonstrated greater preference for hybrid IFA and red IFA. This conclusion is supported by a variety of data collected on the number of fly visits, attack rate, and hovering duration (Experiment 1), and on the number of trapped flies (Experiment 2), which showed that black IFA is the least preferred of the three species. Similar results were recorded for the Formosan biotype of P. curvatus, although the data were not as strongly conclusive. Females of this biotype spent a significantly greater amount of time in hovering mode over red IFA and hybrid IFA compared to black IFA, but the other data were not significant. The red IFA is the natural host of both phorid fly biotypes and our results suggest that both biotypes may have evolved a specialized relationship with red IFA including an ability to discriminate it from related fire ants. These results are discussed in relation to the possible role of fire ant chemicals in mediating host preferences in phorid flies, contributions of male phorid flies to fire ant biocontrol, and the practical implications of the key findings.     

Fire ant alarm pheromone and venom alkaloids act in concert to attract parasitic phorid flies, Pseudacteon spp.

Pseudacteon tricuspis, Pseudacteon obtusus and Pseudacteon curvatus are three species of parasitic phorid flies (Diptera: Phoridae), which have been introduced as classical biological control agents of imported, Solenopsis fire ants (Hymenoptera: Formicidae) in the southern USA. Previous studies demonstrated the behavioral response of P. tricuspis to the venom alkaloids and alarm pheromone of the fire ant, S. invicta. In the present study, we compared the responses of P. tricuspis, P. obtusus and P. curvatus to Solenopsis invicta alarm pheromone, venom alkaloids, or a mixture of both chemicals in four-choice olfactometer bioassays. The main hypothesis tested was that the fire ant alarm pheromone and venom alkaloids act in concert to attract Pseudacteon phorid flies. Both sexes of all three Pseudacteon species were attracted to low doses of the fire ant alarm pheromone or venom alkaloids (i.e. 1 ant worker equivalent) alone. However, the flies were significantly more attracted to a mixture of both chemicals (i.e., 1:1 mixture of alarm pheromone + alkaloids) than to either chemical. The results suggest an additive rather than a synergistic effect of combining both chemicals. Comparing the fly species, P. tricuspis showed relatively greater attraction to cis alkaloids, whereas the alkaloid mixture (cis + trans) was preferred by P. obtusus and P. curvatus. In general, no key sexual differences were recorded, although females of P. tricuspis and P. obtusus showed slightly higher response than conspecific males to lower doses of the alarm pheromone. The ecological significance of these findings is discussed, and a host location model is proposed for parasitic phorid flies involving the use of fire ant alarm pheromone and venom alkaloids as long range and short range attractants, respectively.     

Sex ratio variation as a function of host size in Pseudacteon flies (Diptera: Phoridae), parasitoids of Solenopsis fire ants (Hymenoptera: Formicidae)

Some Pseudacteon (Diptera: Phoridae) flies are parasitoids of Solenopsis (Hymenoptera: Formicidae) ant workers in North and South America. Laboratory studies of sex allocation revealed a pattern of sex ratio variation as a function of host size, with more females arising from larger hosts. Environmental sex determination is a possible mechanism for the observed pattern, and examination of Pseudacteon life history reveals several traits assumed to be important in models predicting conditions under which environmental sex determination is favoured. Sex allocation patterns of Pseudacteon are compared with theoretical predictions and empirical data from better-studied hymenoptcran parasitoids that have haplodiploid sex determination. The pattern of sex ratio variation observed has important implications for biocontrol efforts of imported Solenopsis fire ants by the introduction of Pseudacteon parasitoids.     

Behavioral and Electroantennogram Responses of Phorid fly <Emphasis Type="Italic">Pseudacteon tricuspis</Emphasis> (Diptera: Phoridae) to Red Imported Fire Ant <Emphasis Type="Italic">Solenopsis invicta</Emphasis> Odor and Trail Pheromone

The phorid fly, Pseudacteon tricuspis is an introduced parasitoid of the red imported fire ant, Solenopsis invicta in the United States. Previous studies show that phorid flies are attracted to host ant workers at disturbed colonies, to colonies exhibiting aggressive interspecific interactions, and to fire ant mating flights. In a series of behavioral and electroantennogram (EAG) experiments, we confirm the possible use of fire ant odor as cues for host location by P. tricuspis. We tested the response of P. tricuspis of different sex and mating status to several host-related odor stimuli including live fire ant workers, extracts of worker whole body, head, thorax, and abdomen, and (E,E)-α-farnesene, a trail pheromone component of Solenopsis fire ants. Results from Y-tube olfactometer bioassays demonstrated the attraction of mated female P. tricuspis to live S. invicta workers. In addition, extracts of S. invicta worker whole body and thorax elicited strong olfactometer response in female flies (mated and unmated) and mated males, but not in unmated males. Pseudacteon tricuspis did not show significant attraction to extracts of S. invicta worker head and abdomen, or to (E,E)-α-farnesene, irrespective of sex and mating status. In EAG experiments, female and male P. tricuspis showed significant EAG response to extracts of worker whole body, head, and abdomen, and to a less extent, thorax extract, but not to (E,E)-α-farnesene. Females showed slightly greater EAG response than males, but EAG response was not affected by mating status. These results suggest that fire ant thorax is likely the source of kairomones used as host location cues by P. tricuspis, and support the hypothesis that fire ant worker trail pheromones are not likely used by P. tricuspis for host location.     

A newly-discovered mode of colony founding among fire ants

Summary Queen ants start new colonies either unassisted by workers (independent founding), assisted by workers from their natal nest (dependent founding), or assisted by the workers of other species (dependent, socially parasitic). The monogyne form of the fire ant,Solenopsis invicta, founds independently in summer, but in the fall it also produces a few sexuals some of which overwinter, then fly and mate in early spring. These overwintered queens lack the nutritional reserves and behaviors for independent colony founding. Rather, they seek out unrelated, mature, orphaned colonies, enter them and exploit the worker force to found their own colony through intraspecific social parasitism. Success in entering orphaned colonies is higher when these lack overwintered female alates of their own. When such alates are present, orphaning causes some to dealate and become uninseminated replacement queens, usually preventing entry of unrelated, inseminated replacement queens. Such colonies produce large, all-male broods. Successful entry of a parasitic queen robs the host colony of this last chance at reproductive success. Only overwintered sexuals take part in this mode of founding.     

Parasitoid–host relationships when host size varies: the case of Pseudacteon flies and Solenopsis fire ants

Abstract. 1. Phorid flies in the genus Pseudacteon are parasitoids of ants. Variation in host size preferences of four South American and two North American Pseudacteon species on monogyne and polygyne forms of their host Solenopsis species [ S. invicta Buren and S. geminata (F.), respectively] was documented.
2. Monogyne Solenopsis workers were, on average, significantly larger than polygyne workers, and the average size monogyne worker attacked was significantly larger than the average size polygyne worker attacked in four of the six Pseudacteon species.
3. Three South American Pseudacteon species attacked larger than average size workers, whereas one attacked smaller than average size workers, in both monogyne and polygyne forms. Both North American Pseudacteon species attacked larger than average size polygyne workers and smaller than average size monogyne workers.
4. Three Pseudacteon species were reared from eggs to adults in infected ants in the laboratory. The size of the emergent phorid fly was related positively to the size of the host worker ant, with females emerging from larger hosts. Similar patterns were documented for both monogyne and polygyne forms.
5. The mean size of worker host from which phorids emerged did not differ significantly between the monogyne and polygyne forms in the subsample of phorids reared to adults.
6. The observed patterns elucidate factors that may cause variation in Pseudacteon sex ratios, and have implications for biological control efforts of pest Solenopsis species.     

Assessing the effects of sodium on fire ant foraging in the field and colony growth in the laboratory

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Introduction of the fire ant decapitating fly <Emphasis Type="Italic">Pseudacteon obtusus</Emphasis> in the United States: factors influencing establishment in Texas

The introduction and establishment of Pseudacteon obtusus Borgmeier (Diptera: Phoridae) into the United States is an important milestone in the effort to introduce a suite of parasitoids and pathogens as prospective biocontrol agents of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae). P. obtusus is the fourth species of phorid fly to be established in the US and is now present in at least 11 sites in Texas. This species has a greater propensity for attacking foraging worker ants, and to a lesser degree it will attend mound disturbance events. The effectiveness of alternate field release strategies was evaluated across 28 sites in Texas, with establishment success 35 times more likely when no other phorid species were present at the outset of the releases. This result implies that competition between phorid species may interfere with establishment. Therefore release strategies should be directed to locations lacking other naturalized species or to times when abundances of other species are low.     

Diet-induced nonmelanized cuticle in workers of the imported fire ant Solenopsis invicta buren

Nonmelanized cuticle development was induced in workers of Solenopsis invicta by feeding them an insect-free diet. The nonmelanized workers weighed less and had smaller mean headwidths than workers from normal colonies. Although nonmelanized ant colonies appeared to function normally in the laboratory, their attempts at stinging were felt only as “pin pricks.” Chemical analysis of venom alkaloids and cuticular hydrocarbons indicated no qualitative differences between nonmelanized and normal workers. Tyrosine, an essential amino acid tanning precursor, was found in adequate quantities in the free amino acid pool of nonmelanized ants. The specific cause of the nonmelanized condition is not known.     

Foraging ecology and patterns of diversification in dipteran parasitoids of fire ants in south Brazil

1. At least sixteen species of parasitoid flies in the genus Pseudacteon (family Phoridae) attack fire ants in the Solenopsis saevissima subcomplex in South America. Little is known of behavioural or ecological differences among Pseudacteon parasitoids of fire ants, although their coexistence in multispecies communities would suggest that important differences exist. Seven Pseudacteon species in two separate communities were studied in south-east Brazil. The way in which hosts detect and respond to the presence of parasitoids, attack rates of the parasitoids, and host location behaviour of the parasitoids were examined.
2. Reductions in fire ant recruitment were more closely related to the number of ants attacked along a foraging trail than to the amount of time that a phorid was present.
3. Pseudacteon solenopsidis differed from other phorid species by flying backwards while pursuing ants, by attacking at lower rates than other phorids, and by spending longer around fire ant foraging trails than other phorids before departing. Fire ant recruitment to food often rebounded in the continued presence of P. solenopsidis.
4. In each of the two communities, certain Pseudacteon species appeared frequently at Solenopsis foraging trails, whereas others appeared predominantly at mound disturbances. Two distinct size classes of phorids were present in each community, and the community with the larger ant host species also had a third and larger phorid species. No phorid species from the same community had similar body sizes and similar host location behaviours, although numerous species from different communities shared both of these traits.
5. Heterogeneity in host size and in the ecological circumstances under which hosts are vulnerable to attack appears to have influenced the evolution and perhaps maintenance of diverse Pseudacteon communities.     

Experimental Conversion of Colony Social Organization in Fire Ants (<Emphasis Type="Italic">Solenopsis invicta</Emphasis>): Worker Genotype Manipulation in the Absence of Queen Effects

Colony social organization in the fire ant Solenopsis invicta appears to be under strong genetic control. In the invasive USA range, polygyny (multiple queens per colony) is marked by the presence of the Gp-9 ^b allele in most of a colony’s workers, whereas monogyny (single queen per colony) is associated with the exclusive occurrence of the Gp-9 ^B allele. Ross and Keller, Behav Ecol Sociobiol 51:287–295 (2002) experimentally manipulated social organization by cross-fostering queens into colonies of the alternate form, thereby changing adult worker Gp-9 genotype frequencies over time. Although these authors showed that social behavior switched predictably when the frequency of b-bearing adult workers crossed a threshold of 5–10%, the possibility that queen effects caused the conversions could not be excluded entirely. We addressed this problem by fostering polygyne brood into queenright monogyne colonies. All such treatment colonies switched social organization to become polygyne, coincident with their proportions of b-bearing workers exceeding 12%. Our results support the conclusion that polygyny in S. invicta is induced by a minimum frequency of colony workers carrying the b allele, and further confirm that its expression is independent of queen genotype or history, worker genotypes at genes not linked to Gp-9, and colony genetic diversity.     

Defensiveness of the fire ant, Solenopsis invicta, is increased during colony rafting

Colonies of the fire ant, Solenopsis invicta, can survive flood conditions by forming a raft of ants that floats on the water’s surface until the flood recedes or higher ground is found. Having been forced from the protection of their subterranean nests, rafting colonies are totally exposed and are without retreat. I tested the hypothesis that rafting S. invicta colonies would compensate for their elevated vulnerability by increasing their defensiveness. I measured defensiveness using the amount of venom workers delivered per sting (venom dose), since the repellent effects (i.e., pain and tissue damage) of fire-ant venom are dose-dependent. In the laboratory I assayed colony defensiveness before and after flooding colonies from their nests with water. Colonies were consistently and significantly more defensive while rafting (i.e., each colony’s workers delivered higher venom doses when their colony was rafting than they did when it was assayed pre-flood). The larger venom doses of rafting colonies may reduce their chances of being damaged by encounters with other animals by reducing the duration of such encounters through increased repellency. Encounters with S. invicta during flood conditions have the potential to be unusually dangerous; large concentrations of workers are exposed and available for defense, and they deliver significantly larger venom doses when they sting. Received 29 March 2005; revised 20 June 2005; accepted 24 June 2005.     

A modified rearing system for production of Pseudacteon curvatus (Diptera: Phoridae), a parasitoid of imported fire ants

A self-contained, climate-controlled box for exposing fire ants to attack by Pseudacteon parasitoids was developed. The initial system, which is being used to rear Pseudacteon tricuspis Borgmeier, consists of large (about 244 cm L × 97 cm W × 50 cm H), well-ventilated boxes (“attack boxes”) housed in a climate-controlled room (approx. 28 °C, 80% RH, 12:12 (L:D) h). Adult flies emerge into the boxes, which contain live host ants (Solenopsis invicta Buren). Host ants are confined in a series of trays within the box, each containing a pair of inverted cups under which the ants can hide. The cups alternate up and down, inducing the ants to trail back and forth within each tray. A modified system used for Pseudacteon curvatus Borgmeier consists of closed boxes, through which preconditioned air is pumped to maintain high relative humidity. Steam is used to generate humidity in a small, temperature-controlled room from which conditioned air is drawn, and infrared heating elements mounted above the attack box prevent condensation. This new system maintains high relative humidity (range 80–90%) crucial for activity and survival of P. curvatus. System performance was monitored in 4 ways: (1) environmental conditions within the system (RH, temperature, dew point, and light intensity); (2) production of P. curvatus; (3) successful development and emergence of P. curvatus; and (4) attack rates throughout the box. The modified system is capable of producing >1400 flies/day/attack box. This system will be useful for researchers with limited space in which rearing and research activities are conducted simultaneously.     

Group defense by colony-founding queens in the fire ant Solenopsis invicta

Mutualistic associations among nonkin can form when animalsin groups have a greater chance of overcoming challenges presentedby the environment than do solitary animals. Colony foundingby small groups of unrelated queens, a habit documented in severalspecies of ants, is often interpreted as a mutualistic interactionselected by intense competition among incipient colonies. However,many new colonies in these species are founded in areas wheretheir chief enemies are mature ant colonies, rather than othernewly founded colonies. In this study, we tested whether groupnest-founding in the fire ant Solenopsis invicta improved theability of queens to survive attacks by mature colonies. Inthe laboratory, queens in groups of three were more likely thansolitary queens to survive attacks by workers of the nativefire ant Solenopsis geminata. When newly mated queens were establishedexperimentally in the field, workers from mature S. invictacolonies caused the majority of queen deaths. Queens in groupsof two, but not in groups of four, had higher survival ratesthan did solitary queens during the period between colony establishmentand the appearance of the first workers. The advantage of cooperativedefense approximately counterbalanced the disadvantages causedby competition within foundress associations of two to threequeens. Previous studies have shown that colonies founded bymultiple queens produce larger worker populations than coloniesfounded by solitary queens; however, experimentally increasingworker number in incipient colonies had no effect on colonysurvival in the field.