Sticky traps for monitoring Pseudacteon parasitoids of Solenopsis fire ants

Effectiveness comparison of multiple sticky-trap configurations for sampling Pseudacteon spp. phorid flies (Diptera: Phoridae)

A variety of traps have been developed for monitoring introduced populations of Pseudacteon spp. phorid flies (Diptera: Phoridae) across their established range in the United States. Such traps typically exploit common aspects of phorid fly biology and behavior, such as their attraction to live or dead red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae), as well as the perching behavior of these parasitoids. However, populations of multiple species of phorid flies have been established in the United States to serve as biological control agents against S. invicta, and it is unclear if all trap designs are equally effective in sampling this variety of phorid species. This study investigated the effectiveness of six trap designs simultaneously during three sampling events in south-central Texas. Interactions between two species of phorid flies (Pseudacteon tricuspis Borgmeier and P. curvatus B.) and their hosts have been intensively studied at this location for over eight years. When analyzed independently, there were no significant differences in the mean number of P. curvatus or P. tricuspis phorids collected by any of the trap designs during any of the sampling events. However, when the total number of phorids collected were combined, significant trap performance differentials were observed during the October 2010 sampling event. Furthermore, there were significant differences among male flies during the September 2012 observation. Additionally, a trap component cost comparison is provided. The consistent and relatively equivalent performance of the phorid traps investigated in these trials suggests that all are appropriate for phorid surveillance, and cost and ease-of-use considerations may be the most important criteria when selecting a trap design.

Parasite Lost: Chemical and Visual Cues Used by Pseudacteon in Search of Azteca instabilis

An undescribed species of phorid fly (genus: Pseudacteon) parasitizes the ant Azteca instabilis F Smith, by first locating these ants through the use of both chemical and visual cues. Experiments were performed in Chiapas, Mexico to examine a) the anatomical source of phorid attractants, b) the specific chemicals produced that attract phorids, and c) the nature of the visual cues used by phorids to locate the ants. We determined that phorid-attracting chemicals were present within the dorsal section of the abdomen, the location of the pygidial gland. Further experiments indicate that a pygidial gland compound, 1-acetyl-2-methylcyclopentane, is at least partially responsible for attracting phorid flies to their host. Finally, although visual cues such as movement were important for host location, size and color of objects did not influence the frequency with which phorids attacked moving targets.

Short-term population redistribution of Pseudacteon tricuspis (Diptera: Phoridae) from point source releases

There is a need for quantitative data on patterns and rates of movement of organisms to understand their movement behavior and predict their rates of spread. Opportunities for studying movement of biological control agents are presented during release programs. However, despite these opportunities, patterns and range of dispersal are often not considered. For example, information about effects of wind on dispersal patterns and heterogeneities in rates of movement is critical to predicting future range expansion of biological control agents and determining proximity of multiple releases. Here, the pattern and range of movement of a fire ant parasitoid, Pseudacteon tricuspis Borgmeier, was investigated by performing a series of mass-release-resighting experiments. Flies were released at a central location surrounded by radial transects containing trays of host ants at variable distances along four axes. Resighted flies were censused at these trays at 30 min intervals, up to 2 h postrelease. The dispersal pattern of P. tricuspis in the short term was consistent with a simple diffusion model. On average, 50% of P. tricuspis dispersed ≤10 m, and 95% dispersed ≤29 m. Diffusion rates were variable, depending on release densities, but tended to decline over time after release. Drift of dispersing flies was detected in several trials, and was attributed to prevailing wind dynamics. Data from this assessment of the short term redistribution pattern of P. tricuspis could be useful in determining proximity of releases of this, and other fire ant parasitoids.

Fire ant decapitating fly cooperative release programs (1994-2008): two Pseudacteon species, P. tricuspis and P. curvatus, rapidly expand across imported fire ant populations in the southeastern United States

Natural enemies of the imported fire ants, Solenopsis invicta Buren S. richteri Forel (Hymenoptera: Formicidae), and their hybrid, include a suite of more than 20 fire ant decapitating phorid flies from South America in the genus Pseudacteon. Over the past 12 years, many researchers and associates have cooperated in introducing several species as classical or self-sustaining biological control agents in the United States. As a result, two species of flies, Pseudacteon tricuspis Borgmeier and P. curvatus Borgmeier (Diptera: Phoridae), are well established across large areas of the southeastern United States. Whereas many researchers have published local and state information about the establishment and spread of these flies, here distribution data from both published and unpublished sources has been compiled for the entire United States with the goal of presenting confirmed and probable distributions as of the fall of 2008. Documented rates of expansion were also used to predict the distribution of these flies three years later in the fall of 2011. In the fall of 2008, eleven years after the first successful release, we estimate that P. tricuspis covered about 50% of the fire ant quarantined area and that it will occur in almost 65% of the quarantine area by 2011. Complete coverage of the fire ant quarantined area will be delayed or limited by this species' slow rate of spread and frequent failure to establish in more northerly portions of the fire ant range and also, perhaps, by its preference for red imported fire ants (S. invicta). Eight years after the first successful release of P. curvatus, two biotypes of this species (one biotype occurring predominantly in the black and hybrid imported fire ants and the other occurring in red imported fire ants) covered almost 60% of the fire ant quarantined area. We estimate these two biotypes will cover almost 90% of the quarantine area by 2011 and 100% by 2012 or 2013. Strategic selection of several distributional gaps for future releases will accelerate complete coverage of quarantine areas. However, some gaps may be best used for the release of additional species of decapitating flies because establishment rates may be higher in areas without competing species.

Phorid flies, Pseudacteon spp. (Diptera: Phoridae), affect forager size ratios of red imported fire ants Solenopsis invicta (Hymenoptera: Formicidae) in Texas

Multiple species of Pseudacteon phorid flies (Diptera: Phoridae) are currently being released throughout the southern United States to aid biological control of red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae). It is anticipated that these flies will interfere with S. invicta foraging, allowing native ant assemblages to outcompete S. invicta for available resources. Numerous studies have shown a decrease in S. invicta foraging intensity when exposed to phorids. This study documents a behavioral change in phorid-exposed S. invicta colonies at a phorid release site in central Texas. Significant differences in forager size ratios were detected between phorid-exposed and phorid-absent colonies. A similar phenomenon was recently documented in the native range of these insects in South America as well. Experimental manipulation of ratios of S. invicta worker sizes has been shown to have important effects on colony success. This newly documented phorid-mediated S. invicta colony-level effect represents a significant shift in S. invicta foraging dynamics and may provide an additional mechanism by which phorids can influence S. invicta populations in their United States range.

Sampling and dynamics of Pseudacteon tricuspis (Diptera: Phoridae) in Louisiana

The population dynamics of a fire ant (Solenopsis invicta Buren) parasitoid (Pseudacteon tricuspis Borgmeier) was studied in Louisiana. The objectives of this study were to understand the daily and seasonal population dynamics of P. tricuspis in the context of environmental variables, determine whether P. tricuspis populations were synchronized over local and regional scales, determine the sex ratios and frequency distributions of P. tricuspis at disturbed S. invicta mounds, and determine the minimum sample size and sampling methodology that would provide an estimate of the true relative abundance of P. tricuspis at any location. Daily patterns of relative abundance of P. tricuspis followed a quadratic pattern, with peak fly activity during the afternoon. Seasonally, P. tricuspis relative abundance was variable but tended to positively fluctuate with soil moisture levels. Peak seasonal population abundance occurred during the late summer and fall, whereas population abundance was lowest during the late winter and early spring. Population dynamics of P. tricuspis were found to be synchronized at local and regional scales. Sampling protocols were derived for sampling P. tricuspis populations in Louisiana to account for local spatial heterogeneity in relative population abundance. To obtain an estimate of the true relative population abundance of P. tricuspis, at least 15 S. invicta mounds should be sampled, preferably allocated in groups to different areas at a location to account for spatial population heterogeneity. Phorid fly sampling should be conducted during the afternoons during the summer and fall, when fly abundance is highest.

Distributional patterns of Pseudacteon associated with the Solenopsis saevissima complex in South America

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.