Richard J. W. Patrock

Developmental Rates and Host Specificity for Pseudacteon parasitoids (Diptera: Phoridae) of Fire Ants (Hymenoptera: Formicidae) in Argentina

Abstract This study extends our comparative knowledge of Pseudacteon interactions with Solenopsis fire ant workers. Reported in this work are development times for seven Argentinean parasitoid species reared on two hosts, Solenopsis richteri Forel and Solenopsis invicta Buren, under laboratory temperature regimes comparable with those of the climatic zones occupied by these host species. Developmental times spanned 31–66 d across phorid species, and in general did not differ between genders or host species, but were longer at lower temperatures. The size distribution of flies reared was bimodal, with a group of large (Pseudacteon borgmeieri, Pseudacteon nocens, Pseudacteon obtusus, and Pseudacteon tricuspis) and small (Pseudacteon cultellatus, Pseudacteon curvatus, and Pseudacteon nudicornis) species. P. borgmeieri was exceptional with respect to length of developmental time. Also reported are results of initial oviposition and developmental studies of some of these phorid species on other Argentinean Solenopsis ant species; P. curvatus was the only species able to complete its development on nonhost fire ants. These results support the concept of incorporating several complementary species of Pseudacteon in the biological control of pest fire ants.

Introducing Phorid Fly Parasitoids of Red Imported Fire Ant Workers from South America to Texas: Outcomes Vary by Region and by Pseudacteon Species Released

Abstract Since the first report on introductions to Texas of Pseudacteon decapitating flies, a variety of participants have released flies in a range of sites. The expansions of Pseudacteon populations have been systematically and widely monitored. Before 2002, the widely released initial species P. tricuspis Borgmeier did not become established. Severe drought in 1996–2001 and host-size-dependent sex ratio were proposed constraints in establishing this species. In recent years, however, these limitations have been lifted in some areas by favorable weather, irrigation of release sites, and/or by use of a smaller Pseudacteon species, P. curvatus Borgmeier, not reliant on larger fire ant workers to produce females. Beginning in 2002, the USDA-APHIS collaboration with USDA-ARS and Texas Cooperative Extension programs began to supplement release sites in Texas beyond those initiated by the University of Texas, Austin phorid fly project. In 2005, private citizens began to participate in the spread of Pseudacteon to new sites. By fall 2006, P. tricuspis, expanding from releases between 1999 and 2001, was found on more than 3 million hectares of Central and Coastal Texas, while P. curvatus, with its later start, is only now beginning to expand at some sites. Pseudacteon that established more easily in mesic and moderate climates has difficulty surviving unfavorable weather in South Texas. However, two sites where flies “failed” to become established were revealed to be false negatives after the record rains of summer 2007. Starting in late 2006, the first releases of P. obtusus Borgmeier in North America established, and three to five additional species are being released.

Development of Pseudacteon cultellatus (Diptera: Phoridae) on Solenopsis invicta and Solenopsis richteri Fire Ants (Hymenoptera: Formicidae)

Abstract Nothing was known about the life cycle of Pseudacteon cultellatus Borgmeier. Because this species seems to be a promising candidate for biological control of fire ants, we studied several aspects of its development. We measured selected life history traits of the fire ant parasitoid Pseudacteon cultellatus as a function of (1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), (2) temperature (22 versus 25°C), and (3) size distributions of available host ants (homogeneous small versus mixed sizes of workers). We found that larval, pupal, and total developmental periods of this phorid fly were between 12 and 18% longer on S. richteri than on S. invicta, and 11–19% longer at 22 than at 25°C. Although larval developmental times did not differ as a function of sizes of host offered, average time of development in the pupal stage was extended by 12% in the case of host size mixtures which included workers larger than the phorid’s preferred host size in comparison to homogeneously small ants. P. cultellatus exhibited a strong preference for small ants, especially when using S. invicta workers as hosts. We did not find a relationship between size of host and sex of emerging flies as previously documented for some other Pseudacteon species. P. culltelatus seems to be a promising species for controlling S. invicta as it develops faster in this smaller host, and because it prefers to attack small ants and does not need larger hosts to produce female flies. Thus, this P. cultellatus can attack the majority of ants in a colony without the risk of producing a male-biased sex ratio.

Distributional Patterns of Pseudacteon Associated with the Solenopsis saevissima Complex in South America

Abstract Classical biological control efforts against imported fire ants have largely involved the use of Pseudacteon parasitoids. To facilitate further exploration for species and population biotypes a database of collection records for Pseudacteon species was organized, including those from the literature and other sources. These data were then used to map the geographical ranges of species associated with the imported fire ants in their native range in South America. In addition, we found geographical range metrics for all species in the genus and related these metrics to latitude and host use. Approximately equal numbers of Pseudacteon species were found in temperate and tropical regions, though the majority of taxa found only in temperate areas were found in the Northern Hemisphere. No significant differences in sizes of geographical ranges were found between Pseudacteon associated with the different host complexes of fire ants despite the much larger and systemic collection effort associated with the S. saevissima host group. The geographical range of the flies was loosely associated with both the number of hosts and the geographical range of their hosts. Pseudacteon with the most extensive ranges had either multiple hosts or hosts with broad distributions. Mean species richnesses of Pseudacteon in locality species assemblages associated with S. saevissima complex ants was 2.8 species, but intensively sampled locations were usually much higher. Possible factors are discussed related to variation in the size of geographical range, and areas in southern South America are outlined that are likely to have been under-explored for Pseudacteon associated with imported fire ants.

Associations of Fire Ant Phorids and Microhabitats

Abstract We examined flight activity patterns for a guild of fire ant parasitoids in western Argentina in relationship to their host’s location (mound/foraging trail) and light condition (full sun/partial sun/full shade) at different scales, from the individually sampled mound to the full day’s summation for each species. We asked first whether taxa showed preferences among these conditions, and second, whether certain species and sexes might be found together more frequently than expected to by chance. All species, except the P. obtusus species complex, were significantly more likely to be found attacking ants at disturbed mounds than at paired foraging trails. The P. nocens complex and P. litoralis were more likely to be in the shade when temperatures were above the overall mean of the study (28.3°C), whereas others, such as the P. obtusus complex and P. tricuspis, were more likely to be in full sun under these same conditions. Our analyses indicated that a limited set of species, particularly P. nocens with P. litoralis, and males with female P. obtusus and P. tricuspis, were more likely to be found together than expected. We also found decreasing proportions of males with increasing time of analysis. We discuss the implications of host location, metereological conditions, and sex ratios in relationship to ongoing classical biological control efforts using species of these phorids.

Development of Pseudacteon nocens (Diptera: Phoridae) on Solenopsis invicta and Solenopsis richteri Fire Ants (Hymenoptera: Formicidae)

We studied the developmental performance of the large morph of Pseudacteon nocens Borgmeier (Diptera: Phoridae), a prospective biological control agent of imported fire ants (Hymenoptera: Formicidae). We measured selected life history traits of this parasitoid as a function of 1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), 2) temperature (22 versus 28 degrees C), 3) source population of the fly (Corrientes and Santiago del Estero, Argentina), and 4) varied size distributions of offered host ants. Developmental periods were influenced by host species, although the populations responded in opposing manners. Developmental times, however, were most strongly influenced by temperature with total developmental periods lengthened by 17-32% at 22 degrees C. Pupal mortality was also significantly lower at this temperature. Although numbers of progeny per female were significantly higher for the Corrientes population, we found no significant differences in progeny per female according to host species. Interestingly, we found that females were larger than males, and flies from Corrientes were larger than those from Santiago del Estero, even after statistical adjustments for host size. The modal frequency of host size elected in all treatment combinations tested was identical (0.6 mm), a size that represented the apparent threshold for producing female progeny. These laboratory and additional field observations demonstrate considerable interpopulational variation in P. nocens and lend further support to the applied approach focusing at the population, as opposed to the species level, with respect to both source and target areas for classical biological control introductions of Pseudacteon flies.

Host Specificity of Four Pseudacteon spp. (Diptera: Phoridae), Parasitoids of Fire Ants in Argentina (Hymenoptera: Formicidae)

Abstract Several South American species of Pseudacteon have been released for biocontrol of red imported fire ants Solenopsis invicta in the U.S. Here we provide additional data from host specificity tests on 4 additional candidate species, P. nocens, P. nudicornis, P. cultellatus, and P. obtusus, all of which are components of multi-species complexes that occur within Argentinean Solenopsis populations. All 4 species were tested with sequential, no choice exposures to the red imported fire ant S. invicta, and the tropical fire ant, S. geminata. Levels of intra-generic specificity ranged from moderate to high and all 4 species showed greater specificity than some Pseudacteon species already approved for release.

The Use By Red Imported Fire Ants, Solenopsis Invicta (Hymenopgefa: Formicidae), Of Potamogeton Nodosus (Potamogetonaceae) Leaves As Platforms Into The Littoral Zone In Texas, U.S.A

It has become increasingly clear that waterways facilitate the invasion process for many species, including terrestrial species, such as ants (Walker 2006). The underlying reason may be directly related to transportation, as in the case of the red imported fire ant, Solenopsis invicta Buren whose colonies can spread by rafting on moving water (Banks et al., 1981). In other instances, periodic flooding along riparian courses often creates disturbance regimes, which promote invasions (Ellis et al., 2001). Additional habitat considerations include elevated soil moisture (Holway et al., 2002) and high productivity associated with riparian ecosystems (Kennedy 1998, Paetzold et al., 2006). These conditions are likely to promote high colony growth rates and an associated higher propagule pressure, furthering the invasion process. Ants that are predisposed to be around water are more likely to be able to take opportunistic advantage of waterways for their dispersal (Suarez et al., 2005). Many Solenopsis species, especially the most widely distributed species, commonly forage around water (Wilson 1986). The black imported fire ant, for instance, has been found preying on tubeworms exposed during low tides (Palomo et al., 2003), while S. invicta is considered a threat to wildlife in wetland habitats (Freed and Neitman 1988). The latter study noted that the fire ants were able to extend their reach into the pond by utilizing unspecified vegetation as a walkway but gave no additional details. This note offers further information as to the use by fire ants of floating vegetation as natural roads. In June of 2002, I first observed S. invicta workers crossing long leaf pondweed, Potamotegon nodusus leaves lying on the water surface of a manmade lake in Lee County, TX (N30.14, W96.88). Potamotegon nodosus is a fast-growing aquatic plant that forms extensive platforms of networked leaves on the water surface of the littoral zone. In the situation observed, these platforms were growing in an emergent area of the lake with seasonal inundation. I was able to observe the ants closely out over the water because of the presence of a 15m long raised pier that paralleled the pondweed. Many of my observations were made using a Sony PCV DCR-TRV9. Volume 118, Number 5, November and December 2007 527

Solenopsis invicta: Evidence for Recent Internal Immigration across Provinces in Argentina

Ant tramp species refer to those with intercontinental distributions that coincide with a likely predilection for hitchhiking with human movements (Passera 1994). One such species, Solenopsis invicta Buren has been unintentionally transported across the globe from its native home in central South America over the last 70 years or more (Tschinkel 2006). Within South America it is found in a diverse set of biogeographic regions extending from its southern limit in the dry Chaco, across through the wet Mesopotamian region in Argentina and the Pampas of Uruguay and northwards through the Pantanal and other vegetational areas of Brazil (Pitts 2002). Its tramp status as well as that of others in the genus renders ‘original’ or natural distributional limits difficult to interpret, with disjunct populations of this species and others easily assigned to human transport (Trager 1991), rather than to relictual or other possible interpretations. Historical knowledge of the biogeography of the genus in many of these areas is lacking and suggests that in many cases, only molecular analyses are likely to offer resolution to competing distributional interpretations. Here we note the recent, presumably intracontinental immigration of Solenopsis invicta across provinces in Argentina. On 22 Aug 2006, while searching for Pseudacteon parasitoids of Solenopsis richteri in the Reserva Ecologica Costanera Sur (RECS) in Buenos Aires (S 34.6, W 58.3), we found 8 colonies of Solenopsis that we recognized as S. invicta . Voucher specimens were sent to James Trager who independently confirmed this diagnosis. Solenopsis invicta has never been recorded for the province (Pitts 2002) and the nearest source for the population is likely to be Rosario (ca. 250 km distant) in Santa Fe Province (Ross & Trager 1990). The fauna for the area (including that of the ants) is probably the best known for Argentina, as Buenos Aires is the demographic and academic center of the country. If a pest species such as S. invicta had been found previously it is very likely to have been recorded. RECS is built on reclaimed land from the Rio de la Plata based on a polder-system. Initial reclamation was begun in 1978 and the land was declared a natural reserve in 1985 (Faggi & Czgononi 1987). The first date suggests the earliest possible time that S. invicta could have been present in the locality. Our group was visiting the Reserva on a weekly to monthly basis from 1996 through 2001, including very thorough searches of potential hosts of Pseudacteon (Bruzzone 2004). We had failed to find S. invicta , however, which suggests an even more recent invasion. We considered the hypothesis that this population had been moved in with the large volumes of soil used in current reclamation work in the park but have determined that this material was extracted from local areas in Buenos Aires province. How S. invicta arrived here is an open question. The Reserva is a vegetated island (360 ha) abutting the Rio Plata in the middle of the hugely urbanized zone of Buenos Aires. Many plant species documented here are likely to have been swept in along the Parana, Uruguay Rivers or their tributaries (Faggi & Czgononi 1987). If the original colony(s) that founded this population had floated downstream such common vegetational flotsam might have provided necessary assistance. Buenos Aires is a major international shipping point and the ants might also have used boats as vectors. In either case, there are likely additional population fragments of S. invicta along riverfronts, though we have not yet found any. The observed population is likely to expand, given that red imported fire ant reproduction is predicted within the province by Morrison et al. (2004). Future studies examining competition between this population and surrounding S. richteri will be of interest since the dynamics might be used to reflect back on the initial confrontations between these 2 species in Alabama 70 years ago. We thank RECS for permits and UNQ for laboratory support. PJF thanks CONICET for oversight. Funding was generously supported by the Lee and Ramona Bass Foundation, the Helen C. Kleberg and Robert J. Kleberg Foundation and The State of Texas Fire Ant Project.