Parasitism may enhance rather than reduce the predatory impact of an invader

Invasive species can have profound impacts on communities and it is increasingly recognized that such effects may be mediated by parasitism. The ‘enemy release’ hypothesis posits that invaders may be successful and have high impacts owing to escape from parasitism. Alternatively, we hypothesize that parasites may increase host feeding rates and hence parasitized invaders may have increased community impacts. Here, we investigate the influence of parasitism on the predatory impact of the invasive freshwater amphipod Gammarus pulex. Up to 70 per cent of individuals are infected with the acanthocephalan parasite Echinorhynchus truttae, but parasitized individuals were no different in body condition to those unparasitized. Parasitized individuals consumed significantly more prey (Asellus aquaticus; Isopoda) than did unparasitized individuals. Both parasitized and unparasitized individuals displayed Type-II functional responses (FRs), with the FR for parasitized individuals rising more steeply, with a higher asymptote, compared with unparasitized individuals. While the parasite reduced the fitness of individual females, we predict a minor effect on population recruitment because of low parasite prevalence in the peak reproductive period. The parasite thus has a large per capita effect on predatory rate but a low population fitness effect, and thus may enhance rather than reduce the impact of this invader.     

The first record among Dolichoderinae (Formicidae) of parasitism by Strepsiptera

Summary. We present the first record of parasitism of Dolichoderus bispinosus nests by Strepsiptera belonging to the family Myrmecolacidae. This becomes only the fourteenth species of ant and the fifth subfamily to be identified as a host to Strepsiptera. Of the three colonies examined all were parasitized. Prevalence of parasitism among adult ants was less than 2% in each case. However, among alate males of one colony, nearly 24% were parasitized. In conjunction with a reanalysis of previously published data we discuss the possibility that ant castes are differentially parasitized by Strepsiptera. We review the natural history of strepsipteran parasitism in ants, effects on host behaviour and incidences of parasitism in the hope of enabling detection of this parasite by myrmecologists.     

Rediscovery of the workerless inquiline antPogonomyrmex colei and additional notes on natural history (Hymenoptera: Formicidae)

Summary Pogonomyrmex colei is a workerless inquiline ant known only from nests ofP. rugosus, its closest relative. Ten of 776 (1.3%) host nests were parasitized at a site in central Arizona, while none of 1499 potential host colonies were parasitized at two other locales. Colonies ofP. colei are perennial, and host alate females in 9 of 10 colonies demonstrates that host queens survive parasitism. Three of 10 colonies died over 19 colony years of observation, while only 1 of 601 colonies became newly parasitized. Mating occurs in morning for up to 2–3 days following summer and fall rains and in afternoon during cool fall days. Mating is intranidal just outside the nest entrance, with males returning to the natal nest. MaleP. colei may be flightless because their wing area is reduced compared to host males. Females fly from the nest and locate potential host colonies by following trunk trails. Workers are the largest barrier to nest establishment, as they removed over 90% ofP. colei females placed in trunk trails or that entered host nests. Males and females ofP. colei andP. anergismus, the only other congeneric inquiline species, are diminutive compared to their hosts, with females 30% lighter than host workers. Fat content is lower and water content is higher inP. colei andP. anergismus females than in their hosts.     

Some aspects of Artemia biology affected by cestode parasitism

Artemia individuals from a wild population, bearing cestode cysticercoids (Hymenolepididae) were kept in the laboratory during four months to observe influences of the parasite on the host biology. No differences were found between parasitized and unparasitized shrimps regarding final adult size. The higher survival of parasitized animals suggests that parasitism is not pathogenic. The most dramatic effect was host castration. Parasitized individuals showed higher total lipid levels, probably linked to carotenoid pigments conveying a red color to parasitized shrimps.     

Species and Colony Components in the Recognition Odor of Young Social Wasps: Their Expression and Learning (Polistes biglumis and P. atrimandibularis; Hymenoptera: Vespidae)

In Polistes, nestmate recognition relies on the learning of recognition cues from the nest. When wasps recognize nestmates, they match the template learned with the odor of the encountered wasp. The social wasp Polistes biglumis use the homogeneous odor of their colony to recognize nestmates. When these colonies become host colonies of the social parasite P. atrimandibularis, colony odor is no longer homogeneous, as the parasite offspring have an odor that differs from that of their hosts. In trying to understand how the mechanism of nestmate recognition works in parasitized colonies and why parasite offspring are accepted by hosts, we tested the responses of resident Polistes biglumis wasps from parasitized and unparasitized colonies to newly emerged parasites and to nestmate and non-nestmate conspecifics. The experiments indicate that immediately upon eclosion both young parasites and young hosts lack a colony odor and that colony odor can be soon acquired from the accepting colony. In addition, while residents of nonparasitized colonies recognize only the odor of their species, resident hosts of parasitized colonies have learned a template that fits the odors of two species.     

Physiologische wechselbeziehungen zwischenPieris brassicae und dem endoparasitenApanteles glomeratus II. Der einfluss der parasitierung auf die ern?hrung des wirtes

Zusammenfassung Der Parasitismus vonApanteles glomeratus (L.) bewirkt bei den Larven seines Wirtes,Pieris brassicae (L.), quantitative Veränderungen des Futterkonsums, die mit der Wachstumshemmung des Wirt-Parasit-Systems eng korrelieren. In Abhängigkeit vom Alter der Parasitenlarven und von deren Individuenzahl variiert die im 5. Wirtsstadium abgegebene Kotmenge zwischen 12% und 144% der durchschnittlichen Kotmenge unparasitierter Raupen. Unternormale Fraßleistungen beruhen auf der Reduktion der täglichen Fraßmenge, übernormale auf einer Verlängerung der Fraßperiode. Die Ergebnisse deuten darauf hin, daß in parasitierten Wirten nicht nur die Futterausnutzung verbessert wird, sondern daß die Parasitenlarven auch die Nahrungsaufnahme des Wirtes steuern.

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Summary Food consumption of unparasitisedPieris larvae (A) and larvae parasitised when one day (B), 6 days (C) or 11 days old (D), respectively, was indirectly measured by dry weight of faeces produced during the 5th larval instar of the host. The general inhibition of growth of the host by parasitism, and the dependence upon parasite load of the growth of the host-parasite system has been previously ascertained. Food consumption of the host larva is also affected by parasitization, parasite age and parasite load. Thus the daily rate of faeces production is similar in A-larvae and in heavily parasitized B- and C-larvae, but is reduced in slightly to moderately parasitized hosts, making no difference between B- and C-larvae. This means that the increased food consumption of heavily parasitized C-larvae is the result of a prolonged feeding period, while the reduced uptake in slightly parasitized hosts is caused by a drop in the daily feeding rate.The increment of fresh weight of unparasitized hosts and the host-parasite system, respectively. during the 5th instar is poorly correlated with faeces dry weight in A-larvae, but the correlation is good for B- and C-larvae. Regressions of parasitized hosts, which are quite similar in B- and C-larvae, differ significantly from that of A-larvae. Although the comparison of these parameters may only permit a rather tentative interpretation, the results suggest that utilization and conversion of food follow different rules in unparasitized and parasitized hosts. The conditions in D-larvae, in which parasitization was mostly quite unsuccessful and in which parasites could only pass through embryonic development and the first days of their L₁-stage, are totally unclear.The relationship between the fresh weight gain and the faeces dry weight during the 5th instar is expressed by the preliminary conversion coefficient Qu. Its differences between the types of host larvae and its dependence upon the parasite load are demonstrated. There is no doubt thatApanteles parasitism in general and the parasite load in particular are controlling essential processes of nutrition in the host.

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Gefördert durch die Deutsche Forschungsgemeinschaft     

The effect of mermithid parasitism on predation of nymphal Baetis bicaudatus (Ephemeroptera) by invertebrates

We investigated how infection by the mermithid nematode Gasteromermis sp. affected predation on its nymphal mayfly host, Baetisbicaudatus, by two invertebrate predators – the stonefly nymphs of Kogotusmodestus and the caddisfly larvae of Rhyacophilahyalinata. Predation trials and behavioral observations were conducted in stream-side, flow-through experimental chambers. When parasitized and unparasitized prey were offered in equal numbers, K. modestus consumed significantly more parasitized than unparasitized nymphs. R. hyalinata consumed equal numbers of both prey types. Behavioral observations of foraging K.␣modestus on parasitized and unparasitized prey suggested that the increased consumption of parasitized nymphs was due to differences in the behavior of infected mayflies in response to the predator. Specifically, parasitized nymphs drifted less often to escape an approaching predator (non-contact encounters) compared to unparasitized nymphs, which increased the number of contact encounters and attacks that occurred between K.␣modestus and parasitized prey. Because all hosts are castrated, these behavioral alterations affect only the fitness of the parasite, which is killed along with its host by invertebrate predation. We present a number of hypotheses to explain why the parasite causes increased predation on its host. These include the large size of the parasite affecting the sensory abilities of the host, the larger energetic costs of escape behavior for parasitized individuals, and natural selection from fish predation against drifting behavior by parasitized individuals. Received: 27 May 1996 / Accepted: 30 September 1996     

Variation in life-history traits among Urtica dioica populations with different history in parasitism by the holoparasitic plant Cuscuta europaea

Several studies demonstrate that natural enemies (e.g. parasites) have profound negative effects on the life-history traits of their hosts. If the host can compensate for the negative effects of parasitic infection by altering its life history, these modifications may partly form the basis of resistance or tolerance against parasites. Thus, parasites may be of considerable importance in shaping the evolution of life-history traits of their hosts. To examine if previous parasitism is associated with differences in life-history traits of the host, I conducted a common garden experiment with Urtica dioica plants originating from eight populations of which four were unparasitized, and four parasitized by the holoparasitic plant, Cuscuta europaea. A field survey indicated no differences between unparasitized and parasitized populations in, for example, the number of plant species and nutrient levels in the soil. Thus, it seems reasonable to assume that differences in life-history traits between the two population types in the common garden would reflect the effects of previous selection by the parasite. In the common garden, plants from parasitized populations started to flower later and allocated less biomass to asexual reproduction (measured as the production of stolons, i.e. clonal propagation) compared to plants from unparasitized populations. These results thus indicate that selection by the parasite may have favoured later onset of flowering, and may have selected against asexual reproduction.     

Interaction of a host plant and its holoparasite: effects of previous selection by the parasite

If parasites decrease the fitness of their hosts one could expect selection for host traits (e.g. resistance and tolerance) that decrease the negative effects of parasitic infection. To study selection caused by parasitism, we used a novel study system: we grew host plants (Urtica dioica) that originated from previously parasitized and unparasitized natural populations (four of each) with or without a holoparasitic plant (Cuscuta europaea). Infectivity of the parasite (i.e. qualitative resistance of the host) did not differ between the two host types. Parasites grown with hosts from parasitized populations had lower performance than parasites grown with hosts from unparasitized populations, indicating host resistance in terms of parasite’s performance (i.e. quantitative resistance). However, our results suggest that the tolerance of parasitic infection was lower in hosts from parasitized populations compared with hosts from unparasitized populations as indicated by the lower above‐ground vegetative biomass of the infected host plants from previously parasitized populations.     

Phorid fly parasitism rates vary between individual colonies of the leaf-cutting ant Atta laevigata

1. Phorid flies are important natural enemies of leaf-cutting ants. Female phorids lay eggs in the worker ants' bodies and the parasitoid larvae feed on the host's internal tissues.
2. Here we investigated the parasitism rates by four species of phorids when attacking individual Atta laevigata colonies at three different field sites in order to further understand the host-parasitoid relationship.
3. There were distinct variations in parasitism rates when comparing phorid species attacking individual colonies. In 13 of the colonies sampled, parasitism by Apocephalus attophilus or Eibesfeldtphora erthali was predominant. However, in four of the colonies, Myrmosicarius grandicornis and Apocephalus vicosae were the predominant species.
4. Variations in parasitism rates by the four phorid species were investigated in relation to the size of the potential hosts that had been collected from the foraging trails of each colony.
5. When correlating the size of potential hosts to phorid species, ants with head capsule widths of >2 mm were predominantly parasitized by A. vicosae and E. erthali. Smaller ants, with head capsule widths of 2 mm or less, were predominantly parasitized by A. attophilus and M. grandicornis.
6. The highest parasitism rates by E. erthali were observed in individual colonies that had the highest proportions of large workers on the trails.
7. Although almost half the size of E. erthali, A. attophilus parasitized both large (head capsule width > 2 mm) and small workers (<2 mm). This was related to the fact that between 1 and 16 larvae can develop within an individual host, depending on host size.
8. M. grandicornis parasitized smaller workers (mean head capsule width = 2 mm) as these phorids are themselves small and only one larva develops within each host.
9. This study demonstrated that the parasitism profiles of individual A. laevigata colonies were related to the size of foragers on the trails.

     

斑痣悬茧蜂寄生对甜菜夜蛾幼虫取食和食物利用的影响

为研究甜菜夜蛾Spodoptera exigua幼虫被斑痣悬茧蜂Meteorus pulchricornis寄生后的取食以及食物利用情况,在室内采用重量法测定了甜菜夜蛾4龄幼虫被寄生后取食量、体重增加量、营养指标的变化。结果表明：被寄生甜菜夜蛾幼虫的取食量、生长率和食物利用效率等明显受到抑制,幼虫被寄生后第3-6 d 取食量显著小于未被寄生幼虫,寄生后第4 d 的幼虫取食量只有正常幼虫的29.89%,第5 d只有48.69%。幼虫在寄生后的第3-5 d体重增加量显著小于未被寄生幼虫,分别为正常幼虫的21.51%,38.87%和14.42%,相对生长率则也显著低于后者。被寄生甜菜夜蛾幼虫的营养利用表现也明显不同于未被寄生幼虫,反映生长和代谢效率的食物利用率（ECI）和食物转化率（ECD）均显著降低,而反映吸收效率的近似消化率（AD）则提高。虽然在寄生后第4 d出现了相反的现象,其原因可能在于第4 d取食量明显减少,而体重仍在增加。本研究表明,斑痣悬茧蜂寄生明显抑制寄主甜菜夜蛾幼虫的取食、食物利用效率和生长。     

Larval amphibian growth and development under varying density: are parasitized individuals poor competitors?

Population density and infection with parasites often are important factors affecting the growth and development of individuals. How these factors co-occur and interact in nature should have important consequences for individual fitness and higher-order phenomena, such as population dynamics of hosts and their interactions with other species. However, few studies have examined the joint effects of density and parasitism on host growth and development. We examined the co-influences of rearing density and parasitism, by the trematode Echinostoma trivolvis, on the growth and development of larval frogs, Rana (=Lithobates) pipiens. We also examined the potential role of parasite-mediated intraspecific competition by observing how unparasitized individuals performed when housed with other unparasitized tadpoles, versus housing with a combination of unparasitized and parasitized hosts. Mean mass and mean developmental stage were reduced under high rearing densities. The presence of parasitized conspecifics had no significant effect, but there was a significant interaction of density and parasitism presence on host mass, due to the fact that parasitized conspecifics grew poorly at high densities. Unparasitized individuals reared with parasitized and unparasitized conspecifics fared no better than unparasitized individuals reared only with one another. This result indicates that infected hosts compete as much as uninfected hosts for resources, even though infected individuals have reduced mass under high-density conditions. Resource acquisition and resource allocation are different processes, and parasitism, if it only affects the latter, might not have a discernible impact on competitive interactions.     

Resource density regulates the foraging investment in higher termite species

1. Resource density can regulate the area that animals use. At low resource density, there is a conflict in terms of balance between costs of foraging and benefits acquired. The foraging of the higher termite Nasutitermes aff. coxipoensis consists of searching throughout trails and a building galleries phase. 2. In this study, a manipulative field experiment was used to test the hypothesis that colonies of N. aff. coxipoensis forage towards a more profitable balance between the establishment of trails and gallery construction at low resource density. 3. The experiment was conducted in north‐eastern Brazil. Seven experimental plots were established with a continuous increase in resource density (sugarcane baits). Entire colonies of N. aff. coxipoensis were transplanted from their original sites to the experimental plot, totalling 35 nests. The number, branches and total length of trails and galleries were quantified. 4. The results show that N. aff. coxipoensis optimises its foraging output, intensifying the establishment of trails at the cost of gallery construction when resource density is low. The number of trails, the number of trail branches and the total length of trails decreased with increasing resource density. Interestingly, at low resource density, the search effort was concentrated on forming longer and a greater number of trails, a small proportion of which were converted into galleries. The opposite relationship was observed at high resource density. 5. These results suggest an optimisation of search efforts during foraging depending on resource density, a mechanism that may help researchers to understand the use of space by higher termite species.     

<Emphasis Type="Italic">Toxoneuron nigriceps</Emphasis> parasitization delays midgut replacement in fifth-instar <Emphasis Type="Italic">Heliothis virescens</Emphasis> larvae

We have analyzed the effects of Toxoneuron nigriceps parasitization on the midgut development of its host Heliothis virescens. In parasitized H. virescens larvae, the midgut epithelium undergoes a complete replacement, which is qualitatively not different to that observed in synchronous unparasitized larvae, with similar temporal profiles of cell death and metabolic activity. However, the whole gut replacement process is significantly delayed in parasitized larvae, with complete differentiation of the new gut epithelium being observed 4 days later than in unparasitized controls. The administration of juvenile hormone before commitment and of 20-hydroxyecdysone (20E) after commitment delays and fosters, respectively, the replacement process of the midgut epithelium; moreover, the injection of 20E into developmentally arrested and 20E-deficient host last-instar larvae parasitized by T. nigriceps immediately triggers regular gut development. These hormone-based experiments suggest that endocrine alterations in the larval host, induced by T. nigriceps parasitism, are responsible for the temporal alterations in the gut replacement process. The role of this parasitoid-induced developmental change in the host regulation process is discussed. This work was partially supported by FAR 2006–2007 (University of Insubria) to G.T., by MIUR-FIRB-COFIN (grant no. RBNE01YXA8/2004077251), and by the Centro Grandi Attrezzature (University of Insubria).     

Oviposition avoidance of parasitized aphid colonies by the syrphid predator Episyrphus balteatus mediated by different cues

Oviposition decisions made by members of a guild of natural enemies can have evolved to avoid intraguild predation, potentially avoiding the disruption of the extraguild prey control. We have studied the oviposition preference of the aphidophagous predator Episyrphus balteatus De Geer (Diptera: Syrphidae) within colonies of Myzus persicae Sulzer (Hemiptera: Aphididae) in the presence of two developmental stages of the aphid parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae). Results from a greenhouse choice experiment showed that E. balteatus females lay significantly fewer eggs in colonies with mummified aphids than in unparasitized colonies. Colonies of parasitized, but not yet mummified did not contain significantly fewer eggs than colonies with unparasitized aphids. In three no-choice experiments, we assessed stimuli coming from aphid honeydew, from the aphids themselves and also from extracts of the aphid bodies, and all of these stimuli mediate the discrimination of mummified aphids from healthy aphids. To a lesser extent these stimuli also contribute to the discrimination against aphids that are parasitized but not yet mummified. These results suggest that the effects of these two species could be complementary for the control of M. persicae, since the species that acts as an intraguild predator, E. balteatus, avoids ovipositing on aphid colonies parasitized by the intraguild prey, A. colemani.     

Density-dependent habitat selection by brown-headed cowbirds (<Emphasis Type="Italic">Molothrus ater</Emphasis>) in tallgrass prairie

Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds (Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments were established along a geographical gradient in cowbird abundance within the Flint Hills region of Eastern Kansas and Oklahoma, USA. The focal host species of our study, the dickcissel (Spiza americana), is the most abundant and preferred cowbird host in the prairie of this region. Cowbird relative abundance and cowbird:host abundance ratios were used as estimates of female cowbird density, whereas cowbird egg density was measured as parasitism frequency (percent of dickcissel nests parasitized), and parasitism intensity (number of cowbird eggs per parasitized nest). Geographical variation in cowbird abundance was independent of host abundance. Within study sites, host abundance was highest in wooded edge plots, intermediate in the experimental perch plots, and lowest in prairie interior. Cowbirds exhibited a pattern of density-dependent selection of prairie edge versus experimental perch and interior habitats. On sites where measures of cowbird density were lowest, all cowbird density estimates (female cowbirds and their eggs) were highest near (100 m) wooded edges, where host and perch availability are highest. However, as overall cowbird density increased geographically, these density estimates increased more rapidly in experimental perch plots and prairie interiors. Variation in cowbird abundance and cowbird:host ratios suggested density-dependent cowbird selection of experimental perch over prairie interior habitat, but parasitism levels on dickcissel nests were similar among these two habitats at all levels of local cowbird parasitism. The density-dependent pattern of cowbird distribution among prairie edge and interior suggested that density effects on perceived cowbird fitness are greatest at wooded edges. A positive relationship between daily nest mortality rates of parasitized nests during the nestling period with parasitism intensity levels per nest suggested a density-dependent effect on cowbird reproductive success. However, this relationship was similar among habitats, such that all habitats should have been perceived as being equally suitable to cowbirds at all densities. Other unmeasured effects on cowbird habitat suitability (e.g., reduced cowbird success in edge-dwelling host nests, cowbird despotism at edges) might have affected cowbird habitat selection. Managers attempting to minimize cowbird parasitism on sensitive cowbird hosts should consider that hosts in otherwise less-preferred cowbird habitats (e.g., habitat interiors) are at greater risk of being parasitized where cowbirds become particularly abundant.     

The effect of the acanthocephalan parasite (Pomphorhynchus laevis) on the drift of its intermediate host (Gammarus pulex)

SUMMARY 1. The drift of Gammarus pulex in a population in which approximately 20% of adults are infected with the acanthocephalan parasite Pomphorhynchus laevis was monitored at margin and mid-river sites in the River Teme, England over a 24-h period in mid-summer.
2. Drift densities showed a diurnal pattern with a large increase at night, independent of parasite burden.
3. There was no significant effect of site (margin or mid-river) on the proportion of parasitized and unparasitized G. pulex found in the drift or the benthos.
4. The drift of parasitized G. pulex was significantly greater than unparasitized animals. Gammarids harbouring only one parasite were found in significantly higher proportions in the drift than those with two or more parasites.
5. At both sites (margin and mid-river) the proportion of unparasitized adult G. pulex in the drift was significantly lower than that in the benthos. However, there was a significantly higher proportion of parasitized animals in the drift than in the benthos.     

Influence of Parasitism on Shoal Choice in the Banded Killifish (Fundulus diaphanus,Teleostei, Cyprinodontidae)

Banded killifish, Fundulus diaphanus, were presented individually with a choice of ‘shoaling’ with either of two conspecific stimulus shoals, one consisting of fish with externally visible black spots (indicating the presence of a parasitic trematode worm, Crassiphiala bulboglossa), the other consisting of fish without such spots. Both parasitized and unparasitized test fish significantly preferred to shoal with unparasitized stimulus shoals over parasitized ones. The relative preference of unparasitized killifish for unparasitized shoals increased with increasing degree of parasite load in parasitized stimulus fish. However, no significant preference by unparasitized fish for unparasitized shoals was observed if unparasitized shoals were presented versus mixed shoals consisting of parasitized and unparasitized fish. We conclude that killifish preferred unparasitized shoals over parasitized shoals only if all fish in the respective stimulus shoals were phenotypically uniform (either with or without black spots). A final experiment showed that killifish used black spots as an indicator of parasite presence to make their shoal choice. The significance of these results for the formation of parasite-assorted shoals is discussed.     

The evolution of social parasitism in <Emphasis Type="Italic">Acromyrmex</Emphasis> leaf-cutting ants: a test of Emery’s rule

Summary Emerys rule predicts that social parasites and their hosts share common ancestry and are therefore likely to be close relatives. Within the leaf-cutting ant genus Acromyrmex, two taxa of social parasites have been found, which are thought to occupy opposite grades of permanent social parasitism, based on their contrasting morphologies: Acromyrmex insinuator differs little in morphology from its free-living congeneric host species and produces a worker caste, and is thus thought to represent an early grade of social parasitism. At the other extreme, Pseudoatta spp. exhibit a very specialised morphology and lack a worker caste, both of which are characteristics of an evolutionarily derived grade of social parasitism. Here we present a molecular phylogeny using partial sequences of cytochrome oxidase I and II of about half of the known Acromyrmex species including two social parasites, their hosts and all congeneric species occurring sympatrically. We show that the two inquiline parasites represent two separate origins of social parasitism in the genus Acromyrmex. The early-grade social parasite A. insinuator is highly likely to be the sister species of its host Acromyrmex echinator, but the derived social parasite Pseudoatta sp. is not the sister species of its extant host Acromyrmex rugosus.Received 18 November 2002; revised 16 July 2003; accepted 24 July 2003.