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HOST-SPECIFICITY TESTS, FIELD RELEASES, AND ESTABLISHMENT OF THE
SMALL DECAPITATING FLY, Pseudacteon curvatus BORGMEIER (DIPTERA:
PHORIDAE), IN FLORIDA
RICARDO JOSE VAZQUEZ
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
This thesis is dedicated to someone who has been very supportive of my endeavors, my
mother, Nancy Elena Vazquez-Fuertes and to my father, the late Rodolfo Elias Vazquez-
I am deeply grateful to my maj or advisor, Dr. Sanford D. Porter, for his guidance,
support, and encouragement while obtaining this degree. I also thank the members of my
committee, Dr. Philip Koehler and Dr. Richard S. Patterson, for their valuable advice,
expertise, and review of this thesis.
I thank the USDA-ARS Center for Medical, Agricultural & Veterinary Entomology
in Gainesville, Florida, for support of my research efforts. I acknowledge Dr. David Oi
and Eileen Carroll for providing laboratory colonies of ants used in this thesis. Dr.
Roberto Pereira is thanked for providing descriptive data on some of the research sites
used. The Whitehurst Cattle Co., Shelley Mickle, and Morrill farm are thanked for
providing their pastures in this study.
This research could not have been completed without the assistance of several
individuals. I am grateful to Cynthia Vann, Lloyd Davis, Stacey Knue, and Alvaro
Romero for their help in rearing and collection methods of some of the insects used in
this thesis. Special thanks go to Dr. Juan A. Briano, Dr. Fudd Graham, Dr. Paul Pratt,
Josh King and Lloyd Morrison for their editorial comments on the chapters of this thesis.
Special thanks also go to Debbie Hall for her patience, support and helpful insights into
procedural matters related to the university. Dr. Don Hall and Dr. Winnie Cooke are
thanked for having the confidence in my abilities to tutor at risk athletes at the University
of Florida. Many thanks go to the faculty, students, and staff of the Entomology and
Nematology Department for their advice, guidance, and friendship. Special thanks are
given to my family and friends for all of their love and support. I am especially thankful
to my flance, Cara Congdon, for her editing of this thesis and her confidence that I would
succeed in obtaining this degree as well as anything I set my mind to.
TABLE OF CONTENTS
ACKNOWLEDGMENT S ................. ................. iii...__ ....
LIST OF TABLES ................. ..............vii .....___.....
LIST OF FIGURES ................. ..............viii____ .....
AB STRACT ................ .............. ix
1 GENERAL INTRODUCTION .............. ...............1.....
2 HOST SPECIFICITY OF A BIOTYPE OF THE FIRE ANT DECAPITATING FLY
Pseudacteon curvatus (DIPTERA: PHORIDAE) FROM NORTHERN
ARGENTINA ................. ...............5.................
Introducti on ................. ...............5.................
Materials and Methods ................ .... ...............7.
No-choice tests with native fire ants. ............. ...............8.....
Paired preference tests ................. ...............10........... ....
Statistical Analysis. ............. ...............12.....
R e sults.................. .... ......... .. ......... .............1
No-Choice tests with native fire ants ................. ...............12...............
Paired preference tests ................. ...............13........... ....
Discussion ................ ...............14.................
3 FIELD RELEASE AND ESTABLISHMENT OF THE DECAPITATING FLY
Pseudacteon curvatus ON RED IMPORTED FIRE ANTS IN FLORIDA ...............20
Introducti on ................ ...............20.................
Release M ethods....... ...... ...............23........
Release sites ............... .. .......... ........... .............2
Monitoring fly establishment and dispersal. ............. ...............24.....
Re sults .............. ...............25.................
Discussion ................ ...............26.................
4 RE-CONFIRMING HOST SPECIFICITY OF THE FIRE ANT DECAPITATING
FLY Pseudacteon curvatus AFTER FIELD RELEASE IN FLORIDA ....................30
Introducti on ................. ...............30........ ......
Materials and Methods .............. ...............32....
Re sults .... .... ....... ...............34.......... ......
Discussion ................ ...............35.................
LI ST OF REFERENCE S ................. ...............37................
BIOGRAPHICAL SKETCH ................. ...............45.......... .....
LIST OF TABLES
2-1. Comparison of percent host specificity to red imported fire ants (Solenopsis invicta)
for two biotypes of the decapitating fly Pseudacteon curvatus (Formosa, Las
Flores) when exposed to native fire ants (either Solenopsis xyloni or Solenopsis
geminate, see footnotes for calculation details). ................... ...............1
3-1. Number of adult Pseudacteon curvatus flies found at Whitehurst Ranch, Morrill
Pasture, and Mickle Pasture release sites in Florida from April 2003 to May 2004.29
4-1. Number ofPseudacteon curvatus flies collected hovering in attack mode over non-
host ant species, native fire ants (Solenopsis geminata), and red imported fire ants
(Solenopsis invicta) during sequential series of field trials (see methods). .............36
LIST OF FIGURES
2-1. No-choice trials of the Formosa decapitating fly Pseudacteon curvatus with the
imported fire ant Solenopsis invicta and the two native fire ants Solenopsis
geminata and Solenopsis xyloni. ............. ...............17.....
2-2. Paired preference trials of the Formosa decapitating fly Pseudacteon~ddd~~~ddd~~~dd curvatus.......18
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
HOST-SPECIFICITY TESTS, FIELD RELEASES, AND ESTABLISHMENT OF THE
SMALL DECAPITATING FLY, Pseudacteon curvatus BORGMEIER (DIPTERA:
PHORIDAE), IN FLORIDA
Ricardo Jose Vazquez
Chair: Sanford D. Porter
Major Department: Entomology and Nematology
In recent years, decapitating flies in the genus Pseudacteon Coquillett have been
studied extensively as potential biological control agents because many are common
parasitoids of red imported fire ants, Solenopsis invicta Buren. The purpose of this study
was to evaluate host-specificity of a biotype ofPseudacteon curvatus Borgmeier
collected from red fire ants in Formosa, Argentina, and determine if it is sufficiently host
specific to be released in the field. I also established three field populations and
monitored post-release nontarget effects of P. curvatus.
I tested the host specificity of Pseudacteon curvatus Borgmeier from Formosa,
Argentina, on North American colonies of the red imported fire ant, Solenopsis invicta,
and the native fire ants, Solenopsis geminate (Fabricius) and Solenopsis xyloni McCook.
In no-choice tests, rates of pupal production were 87% lower in S. xyloni compared to S.
invicta and zero in S. geminate. In choice tests, the Formosa biotype preferred to hover
77-87% of the time, attack at a ratio of 10 to 1, and produce higher numbers of pupae on
S. invicta workers when compared to results observed with the native fire ants. These
results indicate that the Formosa biotype displays a high degree of specificity towards red
imported fire ants when compared to native fire ants.
The Formosa biotype of P. curvatus was released after host-specifieity tests
demonstrated that this biotype was specific to imported fire ants. Field releases were
conducted at 3 sites in Florida in the spring and summer of 2003 and monthly monitoring
followed. Field releases were successful in that field reared flies were collected and
identified within 5 weeks at the first site and then monthly thereafter. As of May 2004,
field populations ofP. curvatus dispersed one mile North and South from the first release
site. Flies from the summer releases were found in April 2004. This was the first
successful establishment of P. curvatus on red imported fire ants in the United States.
Post-release monitoring confirmed that the Formosa biotype ofP. curvatus was
not attracted to non-Solenopsis ants. Flies were attracted to the native fire ant, S.
geminata, at very low rates (<5% of that with S. invicta) but virtually no oviposition
attempts were observed. Overall results were consistent with laboratory predictions
except attraction rates to nontarget fire ants in the field were much lower than in the small
laboratory test chambers.
Biological control may be defined as the use ofparasitoid, predator, pathogen, or
competitor populations to suppress a pest population, making it less abundant and
damaging than it would otherwise be. All living species are attacked by natural enemies-
parasites, predators, or pathogens- which feed on them in one way or another and in
many cases regulate their population densities (Van Driesche and Bellows 2001). Many
potentially injurious pests are kept at very low levels and never reach economic pest
proportions due to the effective action of naturally-occurring natural enemies, without
deliberate intervention by man.
Natural enemies can be utilized in three maj or ways: (1) importation of exotic
species and their establishment in a new habitat, (2) augmentation of established species
through direct manipulation of their populations, as by insect mass production and
periodic colonization, and (3) their conservation through manipulation of the
environment. When successful, the utilization of natural enemies is an inexpensive, non-
hazardous means of reducing pest populations and maintaining them below economic
injury levels (Van Driesche and Bellows 2001). Success in biological control is often
dependent on a thorough understanding of the organisms involved, both injurious and
beneficial, and their intricate interactions.
The modern history of biological control can be dated from the control of the
cottony cushion scale, Icerya purcha~si Maskell, by introduced natural enemies on citrus
in California in 1888 (Van Driesche and Bellows 2001). Ever since then, hundreds of
biological control proj ects have been successfully carried out in many parts of the world.
Although biological control was first practiced against insect pests, it is by no means
restricted to any particular group of noxious organisms. It is applicable, and has indeed
been successfully attempted, against insects and other arthropod pests, other animals as
diverse as snails and rabbits, weeds and plant pathogens.
A maj or ant pest in the United States is the red imported fire ant, Solenopsis invicta
Buren, which was introduced at Mobile, Alabama, in the 1930s and has over the years
spread throughout the southeastern states (Buren et al. 1974, Lofgren et al. 1975, Vinson
and Greenberg 1986). Since its introduction, S. invicta has spread to more than 316
million acres in several states and Puerto Rico (Callcott 2002) and recently has reached
Arizona, California, Caribbean Islands (Davis et al. 2001), New Zealand (Harris 2001),
and Australia (Solley et al. 2002). Solenopsis invicta has had a substantial impact on
humans, wildlife, livestock, and agricultural crops (Adams and Lofgren 1981, Allen et al.
1994, Barr and Drees 1996). It is usually the dominant ant species in areas it infests due
to its high reproductive capacity, aggressive foraging behavior, and lack of natural
enemies (Porter et al. 1997). Attempts to control S. invicta have relied on pesticides and
these efforts have been effective but temporary and too expensive for larger area
application such as pastures (Brown 1961, Allen et al. 1994, Davidson and Stone 1989).
Even though chemical applications temporarily reduce fire ant densities in small areas,
fire ants still outcompete native ants and other arthropods, while still expanding their
global range (Callcott and Collins 1996, Morrison et al. 2004, Porter and Savignano
In recent years, research efforts on controlling imported fire ants have focused on
natural enemies such as the parasitic ant Solenopsis daguerrei (Santschi) (Calcaterra et al.
1999, 2000, Briano et. al. 2002), the microsporidium pathogen Jhelohania solenopsae
Knell, Allen & Hazard (Oi et al. 2001, Oi and Williams 2002), and dipteran parasitoids in
the genus Pseudacteon~ddd~~~ddd~~~dd Coquillett (Feener 2000, Porter 1998a, Morrison 2000). Earlier
work noted that Pseudacteon species that attack fire ants appear to be specific to fire ants
(Borgmeier and Prado 1975; Disney 1994). Field tests in both South America (Porter et
al. 1995) and in the United States (Vazquez and Porter 2004) along with laboratory tests
in South America (Folgarait et al. 2002) and the United States (Gilbert and Morrison
1997, Porter and Alonso 1999, Porter 2000, Vazquez et al. 2004a) have demonstrated
Pseudacteon species to be host specific to Solenopsis fire ants.
The small decapitating fly, Pseudacteon curvatus Borgmeier, normally parasitizes
fire ant workers in the saevissima complex in South America (Borgmeier 1925, Porter
1998a). Pseudacteon curvatus is distributed over a large geographical area from Sho
Paulo, Brazil to Buenos Aires province, Argentina (Folgarait et al. 2004, Porter and
Pesquero 2001). Phorid flies have a unique characteristic of decapitating their host during
pupation and affecting fire ant behaviour during oviposition attempts (Morrison 1999).
The life cycle of a Pseudacteon fly begins with a torpedo-shaped egg oviposited into the
thorax of a worker ant (Porter 1998a). The egg dramatically increases in size and
completes development in about 4 days (Consoli et al. 2001). The first instar spends a
short time (about a day) in the thorax and molts into the second instar before it moves
into the ant' s head (Consoli et al. 2001). During most of the third instar, the maggot
probably relies on ant hemolymph for nutrition before pupation (Porter 1998a).
When the maggot is ready to pupate, it releases an enzyme that causes the
intercuticular membranes of its host to degenerate which loosens the head and sometimes
the legs. The maggot then proceeds to consume the entire contents of the ant head leading
to the decapitation of its living host. The ant's mandibles and tongue apparatus are
pushed aside by a series of hydraulic extensions by the maggot (Porter et al. 1995b,
Porter 1998a). The maggot orients itself under the tentorial arms inside the head capsule
where the first three segments compress and harden forming a distinctive sclerotized cap
that fills the oral cavity (Porter 1998a). Depending on temperature, pupal development
takes 2-6 weeks (Porter 1998a). The sclerotized cap pops open and the adult fly slips out
of the ant head capsule within a few seconds. Newly emerged flies mate and lay eggs
within several hours of eclosion (Porter 1998a). Female flies contain 100-200 sexually
mature eggs in their ovaries upon emergence (Zacaro and Porter 2003) and adult flies can
live 3-7 days under laboratory conditions (Porter 1998a).
Classical biological control can be used to control red imported fire ants without
the heavy reliance on pesticides. Overall obj ective of my thesis proj ect was to evaluate
the natural enemy P. curvatus collected from red fire ants in Formosa, Argentina, for the
suppression of S. invicta populations in the United States by asking the following
questions: 1) is a biotype of P. curvatus from Formosa, Argentina, sufficiently host
specific to be released in the Hield, 2) can this biotype successfully establish on red
imported fire ants in the field, and 3) do post-released populations ofP. curvatus confirm
quarantine specificity predictions of minimal to non-existent nontarget effects?
HOST SPECIFICITY OF A BIOTYPE OF THE FIRE ANT DECAPITATING FLY
Pseudacteon curvatus (DIPTERA: PHORIDAE) FROM NORTHERN ARGENTINA1
Before an exotic biological control agent is released in the field, the degree of host
specificity must be assessed to determine if that agent will likely parasitize non-target
species (Marohasy 1998, van Klinken 2000, van Klinken and Heard 2000, Browne and
Withers 2002). Phorid flies of the genus Pseudacteon Coquillett are being studied as
potential biological control agents because many are common parasitoids of imported fire
ants. Pseudacteon~ddd~~~ddd~~~dd species that parasitize fire ants appear to be specific to fire ants
(Borgmeier and Prado 1975, Disney 1994). Field tests in South America have
demonstrated that Pseudacteon flies are not attracted to ants in other genera (Porter et al.
1995a). Furthermore, most Pseudacteon~ddd~~~ddd~~~dd flies in Brazil and Argentina appear to be
specific to fire ants in the saevissima complex of the genus Solenopsis (Gilbert and
Morrison 1997, Morrison and Gilbert 1999, Porter and Alonso 1999). However, a few
flies in some of these species (Pseudacteon curvatus Borgmeier and Pseudacteon
tricuspis Borgmeier) will parasitize fire ants in the geminata complex (Porter et al. 1995a,
Gilbert and Morrison 1997, Porter 2000).
Pseudacteon curvatus is a small decapitating fly from South America that normally
parasitizes fire ant workers in the saevissima complex (Borgmeier 1925, Williams and
Whitcomb 1974, Porter et al. 1995a). In its native habitat, P. curvatus is distributed over
SIn Press, Environmental Entomology 2004
a large geographical area from Sho Paulo, Brazil westward into Mato Grosso do Sul,
Brazil and Southward to Buenos Aires Province, Argentina (Porter and Pesquero 2001,
Folgarait et al. 2004). Pseudacteon curvatus flies reach peak abundances during the
summer seasons in South America (January through March; Fowler et al. 1995, Folgarait
et al. 2003) and North America (July through September; Fudd Graham, personal
communication). Mating occurs on the ground in the morning hours (Wuellner et al.
2002). Studies on oviposition behavior have shown that female P. curvatus flies attack
workers that are significantly smaller than the colony mean (Morrison et al. 1997).
Oviposition behavior consists of flies hovering in attack-mode 3-5 mm above their host,
orienting themselves to workers, and diving in to strike the thorax of workers inj ecting
eggs via an ovipositor.
In a previous series of host specificity tests, a P. curvatus biotype that was
collected from the black fire ant, Solenopsis richteri Forel, in Las Flores, Argentina was
able to attack and develop successfully in two native fire ants: Solenopsis geminata
(Fabricius) and Solenopsis xyloni McCook (Gilbert and Morrison 1997, Porter 2000).
Parasitism rates were very low in S. geminate, indicating that this ant would not be a
good host (Porter 2000). However, parasitism rates on S. xyloni in the laboratory reached
minimum levels at which P. curvatus might be able to sustain a population in the field
(Porter 2000). Since imported fire ants are the number one enemy of native fire ants and
since P. curvatus is a much greater threat to imported fire ants than native fire ants, Porter
(2000) argued that releasing these flies would most likely benefit native fire ants rather
than hurt them. Subsequently, the Las Flores biotype was approved for field release in
Field releases of this biotype successfully established populations in Alabama and
Mississippi on hybrid fire ants (S. richteri x S. invicta) and black imported fire ants, but
failed in Florida on red imported fire ants (Graham et al. 2003, Vogt and Street 2003).
Field releases of the Las Flores biotype appears to have failed on red imported fire ants
because this fly was too host specific. Although no-choice laboratory tests showed that
the Las Flores biotype parasitized red and black imported fire ants equally, host
preference tests revealed that the Las Flores biotype strongly preferred S. richteri and
hybrid fire ants when tested against Solenopsis invicta Buren (Porter and Briano 2000,
Folgarait et al. 2002). The preference for black imported fire ants was not unexpected
because the Las Flores biotype was originally collected from black fire ants in South
Because the Las Flores biotype failed to establish on S. invicta populations in the
U.S., a new biotype of P. curvatus was collected from S. invicta fire ants in Formosa,
Argentina. The obj ective of this study was to determine if this new biotype of P. curvatus
was sufficiently host specific to be released in the field with existing permits obtained
from previous tests using the Las Flores biotype. I also compared the Formosa results
with the previous Las Flores biotype study.
Materials and Methods
The new biotype ofP. curvatus flies were collected attacking &. invicta fire ants 35
km NW of Formosa, Argentina by Sanford D. Porter and Juan A. Briano (October 2001).
Flies were collected by setting up several trays (42 x 28 x 15 cm; Panel Control Corp.,
Detroit, MI) containing several thousand fire ants. Pseudacteon curvatus flies were
allowed to attack the fire ant workers for 4-5 h while the workers ran from one side of the
tray to the other as previously done by Porter (2000). These workers were airfreighted to
the quarantine facility in Gainesville, Florida. Flies were reared in a large self-contained,
climate controlled attack box exposing fire ants to attacks similar to the one described by
Vogt et al. (2003).
No-choice tests with native fire ants
To determine whether the Formosa biotype of P. curvatus will actively attack and
develop in native Solenopsis fire ants, no-choice trials were conducted with S. xyloni and
S. geminata. Ten plastic trays (42 by 28 by 15 cm; Panel Control Corp., Detroit, MI)
were used in the no-choice tests, each with screened vents and tight-fitting glass lids
similar to those described by Porter and Alonso (1999). The trays contained a single solid
bottom covered with a 2-3 cm layer of moistened plaster to maintain high humidity.
Plaster was made by using a 1:1 mixture of pottery plaster (US Gypsum Co., Chicago,
IL) and plaster wall patch (DAP Inc., Baltimore, MD). Plaster was moistened before each
test run. Prior to moistening, the plaster bottoms were scraped to remove residues left
behind from previous use. When test trays were reused, native fire ants were not used in
trays that had been used by imported fire ants (and vice versa) unless the plaster bottoms
were replaced. This procedure avoided confounding results with odors previously
deposited on the plaster bottoms.
Timer motors were used to automatically raise an inverted cup in one end of each
tray while lowering a cup at the other end of each tray. This caused the test ants to
continuously trail back and forth between the two cups. Timer motors were set to run for
8 h per d (10:00 to 18:00 h). A small piece of laboratory tissue (Kimwipes, Kimberly
Clark, Roswell, GA) moistened in IM sugar water solution served as a food source for
the flies and a bunch of artificial flowers in the center of each tray provided a perching
location. The laboratory was maintained at 27-280C.
Trials were conducted in a complete randomized design (CRD) with ant species as
treatments and trials serving as experimental replication. Seven trials were conducted
with S. geminate, six trials with S. xyloni, and eight control trials with S. invicta. The
laboratory colonies of S xyloni were collected from California (July 2001) and S. invicta
and S. geminate colonies were collected near Gainesville, Florida (August-September
2002). Workers from test colonies were sieved through a U.S. standard 20-mesh sieve to
separate out the smaller workers that are preferred by P. curvatus females. Each test
group contained 0.5 g of small workers and 1.0 g of brood. Different colonies were used
for each trial to assure that results were not due solely to differences in the attractiveness
of individual colonies.
Flies were aspirated with an Allen-type double chamber aspirator from a holding
box, retained in vials, knocked down with CO2, and separated by sex within 20 sec on a
cold table at which point they were placed into vials according to sex. Fifteen to twenty
newly emerged female flies and an equivalent number of males (for mating) were added
to all no-choice trials over a period of 2 d (treatments and controls always received equal
numbers). Flies were added on 2 consecutive days to increase fly numbers and reduce
effects of temporal variation. Trials lasted 4 d to cover the complete lifespan of
ovipositing flies (1-2 days). Flies were introduced into the trays via an inj section port as
described by Porter and Alonso (1999). On each trial date, the number of female flies
hovering in attack-mode over test ant species were recorded every 10 min over a
continuous period of 2-3 h between 1 100-1700 EST, the time period that flies are most
active. An average from the observations of female flies in attack-mode was taken for
each tray and used in a one-way analysis of variance. Pseudacteon curvatus males do not
hover over ants (Wuellner et al. 2002). All flies were dead by the end of the 4 d trials.
After tests, ants were removed from the trays, retained in small boxes (20 by 12 by 5 cm)
with tight-fitting vented (2 by 3 cm) lids, and inspected for pupating flies every other day
for a period of 3 5 d. Inside each retainment box, I placed a small 3 cm block of moist
plaster and a nest tube with water held in the end by a cotton ball (16 by 125 mm). Ants
were fed fresh sugar water every 2 d. I removed dead workers from the small boxes and
placed them inside condiment cups (4 oz) with moist plaster bottoms where the larvae
could pupate. Determination of pupating flies in ant head capsules was made by looking
for a sclerotized cap flanked by two respiratory horns as described by Porter (1998a).
The total number of pupae produced in each ant species was divided by the fifteen to
twenty female flies used in the trials to produce an average pupae production rate per
female fly. Pupae were held for a total of 25 d to determine rates of adult emergence.
Paired preference tests
Host preferences of Formosa P. curvatus flies were examined as paired difference
tests consisting of seven paired trials with S. invicta and S. xyloni (August-September
2002) and six paired trials with S. invicta and S. geminata (September 2002). Trials were
conducted in three white plastic trays (42 by 28 by 15 cm; Panel Control Corp., Detroit,
MI) with screened vents and tight-fitting glass lids. In the bottom of each tray, two long
side-by-side holes were cut and two smaller trays were glued (30 by 7 by 5 cm) as
described by Porter (2000). This configuration produced two parallel chambers in the
bottom of the big tray that allowed the testing of two species of ants at the same time.
Ants were contained in the two bottom trays by coating their sides with Fluon (AGC
Chemicals Americas Inc., Bayonne, NJ). To maintain high humidity, four moistened 3 by
3 by 4 cm sponges were placed in the corners of the test trays and a 1 cm thick layer of
hard plaster (Castone; Dentsply, York, PA) was poured into the bottom of both bottom
trays. The plaster and sponges were moistened before each test run. Small 20 cm desk
fans were directed toward the vents of the test boxes so that high humidity did not cause
condensation on the glass lid or the sides of the trays.
Flies were introduced into the trays as described previously. A small opaque
inverted cup (4 cm diameter) with a large wire loop glued to the top was placed on the
plaster in each of the two bottom trays. These cups were moved back and forth from one
end of a tray to the other with a plastic rod each time most of the ants had crawled under
a cup to hide. This procedure kept the ants trailing continuously from one end of a bottom
tray to the other so that the flies always had an opportunity to attack the ants. Smaller
workers were obtained by sieving as described above. We used 0.3 g of workers and 0.6
g of brood for each test group.
Each test used ants from a different colony and received 10-15 female flies and an
equivalent number of males. Trials lasted about 3 h during which time we recorded the
number of females hovering in attack-mode over each species every 10 min during 1 100-
1500 EST to produce an average number of females in attack-mode. When possible, I
recorded the number of oviposition attempts in 20 sec intervals for individual flies
hovering in attack-mode over each group of ants to produce an average rate of attack per
minute per attacking female. When the test boxes were reused, workers from one species
of fire ant were not placed in a side that had been used by another species. At the end of
each trial, worker ants were transferred to small boxes and checked for pupating flies as
described in no-choice tests. The head capsules of dead workers were inspected for fly
larvae or pupae every 1-2 d for a period of 25 d so that most larvae had time to complete
development in their host. The total number of pupae produced per ant trial was divided
by number of females used to produce an average production rate. Voucher specimens of
flies have been deposited in the Florida Collection of Arthropods, Gainesville, Florida.
A one-way analysis of variance was used to evaluate differences between treatment
means in attacking activity during the no-choice tests. Pupal production, from all three
species of ants in the no-choice tests, were compared across experiments using a Kruskal-
Wallis test. A X test was used to evaluate the percentage of pupae that completed
development in the no-choice tests. Data did not receive transformation. In the preference
tests, a two-tailed, paired t-test was used to compare fly activity (hovering in attack-mode
and attack rates) and pupal production, however, a Wilcoxon signed-rank test was used to
compare pupal production in one paired trial of the preference tests. All analyses were
conducted using Minitab 13 (2003).
No-Choice tests with native fire ants
The mean number of flies hovering in attack-mode over S. invicta workers was not
significantly different than the mean number over either S. xyloni or S. geminate workers
(Fig. 2-1A; 1.40 + 0.28 (mean & SE) versus 1.08 & 0.25 and 0.90 + 0.42, F = 0.60; df = 2,
12; P = 0.56). In the no-choice trials, P. curvatus flies successfully developed in S. xyloni
workers but failed to develop successfully in S. geminate (Fig. 2-1B). The mean number
of pupae produced per female fly in S. invicta workers was 7 times higher than the mean
number for pupae produced from S. xyloni workers (Fig. 2-1B; 5.03 A 1.55 (mean & SE)
versus 0.66 & 0.24, H = 13.31, df = 2, P < 0.001, Kruskal-Wallis test). The percentage of
pupae that successfully developed to adult flies was 65% in S. invicta (349/535) and 18%
in S. xyloni (10/56) (likelihood ratio X2 = 47.72, df = 1, P < 0.001).
Paired preference tests
Pseudacteon curvatus strongly preferred S. invicta over either species of native fire
ant in the preference tests (Fig. 2-2). Female flies that were hovering in attack-mode
preferred to hover over S. invicta 77% of the time rather than S. xyloni (Fig. 2-2A; 1.66 &
2.6 (mean & SE) versus 0.54 & 2.0 flies/observation, t = 7.58, df = 6, P < 0.001, paired t-
test). Similarly, hovering female flies in attack-mode preferred to hover over S. invicta
87% of the time rather than S. gentinata (Fig. 2-2A; 3.00 & 4 (mean & SE) versus 0.52 & 4
flies/observation, t = 3.74, df = 4, P = 0.02, paired t-test). The attack rate was 2.8 times
higher for female flies in attack-mode over S. invicta than for female flies in attack-mode
over S. xyloni (Fig. 2-2B; 6.03 & 0.82 (mean & SE) versus 2.27 & 0.68 attacks/min, t =
5.83, df = 6, P < 0.001, paired t-test). The attack rate was 16 times higher for female flies
in attack-mode over S. invicta than for flies in attack-mode over S. gentinata (Fig. 2-2B;
7.02 & 1.41 (mean & SE) versus 0.44 & 0.28 attacks/min, t = 4.73, df = 4, P = 0.009,
In the paired tests, the mean number of pupae produced per female fly was higher
in S. invicta than for either native species (Fig. 2-2C). In the S. invicta 5. xyloni tests, 4
times more pupae were found in S. invicta workers than in S. xyloni workers (Fig. 2-2C;
1.98 & 0.71 (mean & SE) versus 0.48 & 0.23 pupae per female fly, t = 2.63. df = 6, P =
0.039, paired t-test). In the S. invicta 5. gentinata tests, normal numbers of pupae were
found in S. invicta workers, but no pupae were found in S. gentinata workers (Fig. 2-2C;
1.71 f 0.59 (mean f SE) versus 0 pupae per female fly, T= 0, N1 = N2 = 6, P < 0.05,
Wilcoxon signed-rank test).
The results of this study indicate that the Formosa biotype displays a high degree of
specificity towards red imported fire ants when compared with native fire ants. Formosa
biotype flies were observed hovering in attack mode over all fire ant species at similar
rates during no-choice tests (Fig. 2-1A). However, rates of pupal production were much
lower in S. xyloni compared with S. invicta and zero in S. geminate workers used in the
no-choice tests (Fig. 2-1B). Paired preference tests demonstrated that the Formosa
biotype ofP. curvatus actively prefers to hover over red imported fire ants at
significantly higher rates rather than native fire ants (Fig. 2-2A). Of those flies that chose
to actively hover, attack rates were also higher with red imported fire ants when
compared with native fire ants (Fig. 2-2B). As in the no-choice tests, rates of pupal
production were much lower in S. xyloni compared to S. invicta and zero in S. geminata
workers used in the paired preference tests (Fig. 2-2C).
Results from the no-choice and paired preference tests demonstrate that the
Formosa biotype ofP. curvatus is more host specific to red imported fire ants than the
results from a previous study with the Las Flores biotype (Porter 2000). When
considering attacking flies in the no-choice tests with S. xyloni (Table 2-1), the Formosa
biotype had about the same percent specificity to S. invicta as the Las Flores biotype
(23% vs. 28%). However, host specificity to S. invicta as measured by pupal production
was much higher in the Formosa biotype than the Las Flores biotype (87% vs. 65%;
Table 2-1). Similarly, percent preference for S. invicta in the paired preference tests, with
S. invicta and S. xyloni, was similar in both biotypes (77% vs. 74%) but host specificity
as calculated by attack rates was much higher in the Formosa biotype than the Las Flores
biotype (62% vs. 2%; Table 2-1). These data indicate that much of the increased host
specifieity for S. invicta exhibited by the Formosa biotype is the result of a higher
proclivity to attack or attempt oviposition on S. invicta than on S. xyloni.
Comparisons between the Formosa and Las Flores biotypes with S. gentinata,
demonstrate that hovering flies in the no-choice tests (Table 2-1) were more host specific
to S. invicta in the Las Flores biotype than the Formosa biotype (89% vs. 36%).
However, in regard to pupal production, the Formosa biotype was 100% host specific to
S. invicta while the Las Flores biotype was 94% host specific (Table 2-1); in other words,
a few of the Las Flores flies were able to develop on S. gentinata but none of the Formosa
flies were able to develop. In the paired preference tests, percent preference for S. invicta
over S. gentinata was higher in the Formosa biotype than the Las Flores biotype (87% vs.
78%; Table 2-1). Host specifieity as calculated by attack rates was also higher in the
Formosa biotype than the Las Flores biotype (94% vs. 86%; Table 2-1). Since the rate of
pupal production was zero in both the no-choice and paired preference tests for S.
gentinata (Figs. 2-1 and 2-2), I conclude that the Formosa biotype will not be a threat to
S. gentinata. These trials demonstrate that the Formosa and Las Flores biotypes differ
substantially in host specificity.
Vink et al. (2003) also observed variability in host specificity between two biotypes
of2~icroctonZus acrlib~'iopside (Hymenoptera: Braconidae). Other studies on the host
specificity of parasitoid biotypes have shown that geographic variation in host specificity
between biotypes was due to the presence of cryptic species (Heimpel et al. 1997,
Alvarez and Hoy 2002). Although my results demonstrate that there is geographic
variation between the Formosa and Las Flores flies, I cannot rule out the possibility that
the variability seen is due to the presence of a cryptic species.
Because the Formosa flies were more host specific than the Las Flores biotype,
they were released from quarantine in the spring of 2003 under a previous permit from
the State of Florida and the Finding of No Signifieant Impact (FONSI) issued by the
USDA-ARS. Trial Hield releases are underway and initial results look promising.
Fig. 2-1. No-choice trials of the Formosa decapitating fly Pseudacteon curvatus with the
imported fire ant Solenopsis invicta and the two native fire ants Solenopsis
geminate and Solenopsis xyloni. (A) Mean number of hovering flies in attack
mode per observation. (B) Mean lifetime number of pupae produced per female
fly in each trial. The dashed line indicates a conservative estimate of the minimum
number of offspring per female necessary to produce a self-sustaining population
as calculated by Porter (2000). Error bars show SE calculated from trial means.
The number of replicates (n) is indicated below each bar.
S. intvicta/S. xylonti S. inlvicta/S. geminlata
Fig. 2-2. Paired preference trials of the Formosa decapitating fly Pseudacteon curvatus.
(A) Percentage of hovering flies in attack mode over either Solenopsis invicta or
one of the two native fire ant species. (B) The average rate of attack per min per
attacking female fly. (C) Mean number of pupae produced per female fly as a
result of the 4 hr trials. Error bars show SE calculated from trial means. The
number of replicated pairs (n) are shown below bars.
Paired Preference Tests
Table 2-1. Comparison of percent host specifieity to red imported fire ants (Solenopsis
invicta) for two biotypes of the decapitating fly Pseudacteon curvatus
(Formosa, Las Flores) when exposed to native fire ants (either Solenopsis
xyloni or Solenopsis geminate, see footnotes for calculation details).
Measures of host
specify city S. invicta 5S. xylonia S. invicta 5S. geminataa
Formosa Las Floresd Formosa Las Floresd
No Choice Tests
Hovering Fliesb 23% 28% 36% 89%
Pupae/Female Flyb 87% 65% 100% 94%
Paired Preference Tests
% Preference" 77% 74% 87% 78%
Attacks/Min/Femaleb 62% 2% 94% 86%
a A value of 100% indicates complete host specifieity to red imported fire ants while 0%
indicates no host specificity to imported fire ants when compared to one of the native fire
b Values were calculated by subtracting native fire ant value from imported fire ant value
and then dividing by imported fire ant value.
" Percentages were directly taken from host preference tests.
d Data for Las Flores biotype comes from a previous study conducted by Porter (2000).
FIELD RELEASE AND ESTABLISHMENT OF THE DECAPITATING FLY
Pseudacteon curvatus ON RED IMPORTED FIRE ANTS IN FLORIDA2
Solenopsis invicta Buren and Solenopsis richteri Forel are two invasive species that
have been able to thrive without their natural enemies. Both species of Gire ants were
accidentally introduced into the United States through Mobile, Alabama in 1918 (S.
richteri) and in the 1930's (S. invicta), leaving behind most of their natural enemies in
South America (Jouvenaz 1990). It is speculated that the absence of natural enemies is
the reason fire ant densities are 5-10 times higher in the United States than they are in
South America (Porter et al. 1992, Porter et al. 1997).
Since introduction, S. invicta has spread throughout the entire southeastern United
States (Callcott and Collins 1996) while S. richteri along with a hybrid species (S.
richteri x S. invicta) have been found in Alabama, Mississippi, and Tennessee
(Shoemaker et al. 1994). Solenopsis invicta causes several billion US dollars in damages
annually to agricultural crops, electrical equipment, livestock, and human health risks in
the United States (Drees et al. 2002). Chemical baits have been used as a means to
control imported fire ants in high traffic areas such as playgrounds and residential lawns
(Drees et al. 2002). Unfortunately, chemical treatments tend are costly and generally need
to be used several times a year for adequate levels of control (Collins et al. 1992). A
2 In Press, BioControl 2004
possible alternative to chemicals that is more environmentally friendly is classical
In recent years, decapitating phorid flies in the genus Pseudacteon~ddd~~~ddd~~~dd Coquillett have
been studied extensively as potential biological control agents because many are common
parasitoids of imported fire ants (Gilbert and Patrock 2002, Morrison et al. 1997, Porter
1998a, 2000, Vazquez et al. 2004a). Pseudacteon flies were first reported to be attracted
to Solenopsis fire ants by Borgmeier (1921) in Brazil with further studies by Williams
(1980). About 20 species ofPseudacteon are found in South America that attack fire ants
(Porter and Pesquero 2001). Extensive specificity tests in both South America (Folgarait
et al. 2002, Porter et al. 1995a) and in the United States (Gilbert and Morrison 1997,
Porter and Alonso 1999, Porter 2000, Vazquez and Porter 2004, Vazquez et al. 2004a)
have demonstrated high levels of host specificity in Pseudacteon~ddd~~~ddd~~~dd species to Solenopsis
Pseudacteon tricuspis Borgmeier was the first species to be successfully released
(Porter et al. 1999). These flies were initially released in Texas (Gilbert 1996) in early
1995, but attempts failed probably because low numbers of flies were used and weather
conditions were hot and dry. The first successful field release occurred in the late summer
of 1997 (Porter et al. 1999) in North Florida. Additional releases had been done
throughout various southeastern states (Gilbert and Patrock 2002, Porter et al. 2004). A
long-term impact study with P. tricuspis found that parasitism pressure from this single
phorid species was not a significant regulating factor in fire ant populations indicating
that additional species of phorid flies or other natural enemies will be needed (L.
Morrison, personal communication).
A second species, Pseudacteon curvatus Borgmeier, was collected from black fire
ants in Las Flores, Argentina (Porter 2000). Pseudacteon curvatus is smaller than P.
tricuspis and studies on oviposition behavior have shown that female P. curvatus flies
attack fire ant workers that are significantly smaller than the colony mean (Morrison et al.
1997). Field releases of this species (summer 2000 and fall 2001) were successfully
established in Alabama and Mississippi on hybrid fire ants and black imported fire ants,
but failed in Florida on red imported fire ants (Graham et al. 2003, Vogt and Street 2003).
A new biotype ofP. curvatus was collected from S. invicta fire ants in Formosa,
Argentina (October 2001). A series of quarantine host-specificity tests demonstrated that
this new biotype was sufficiently host specific to be released in the field (Vazquez et al.
2004a). The obj ective of this study is to document the release, establishment, and
dispersal of P. curvatus around Gainesville, Florida.
Materials and Methods
The P. curvatus flies released in this study were collected by SDP and JAB in
October 2001 from a roadside site about 3 5 km NW of Formosa, Argentina on route 81
(km 1219; 250 56. 139' S, 58o 30.723' W). The collection procedures were similar to
those described by Porter (2000). These flies were then imported to quarantine facilities
in Gainesville, FL under a permit from USDA-APHIS. We released these flies in the field
after conducting host specificity evaluations on nontarget organisms in our quarantine
facility. The flies were released under a previous permit from the Florida Department of
Agriculture and the Finding of No Significant Impact (FONSI) issued by the USDA-
Immature flies were released in parasitized workers. Fire ant workers were
collected from medium to large sized fire ant mounds by shoveling dirt with 5-15 g of
ants and brood into a bucket. Mounds were individually marked with numbered flags
and/or wooden stakes. Numbers were also painted on the ground near the mounds.
Workers were collected from mounds over a 2-3 week period. These workers were then
separated from the soil in the laboratory by drip flotation (Banks et al. 1981). Workers
were separated from brood using sorting sheets and sieved with a U.S. standard 20-mesh
sieve to remove sexual, queens, and excess large workers (>0.9 mm head widths) not
normally parasitized by P. curvatus. Groups of 1.0-1.5 g of sieved ants from a single
colony together with about 1 g of brood were placed in large attack boxes (244 x 96 x 56
cm) similar to those described by Vogt et al. (2003). Flies were allowed to parasitize the
ants for 2-3 days. Parasitized workers were then removed from attack boxes and retained
in small containers (20 x 12 x 5 cm) with tight-fitting vented (2 by 3 cm) lids. Brood was
removed with sorting sheets if it did not originate from the same colony to avoid potential
aggressive interaction as they emerged. To release the ants, the mounds were first
disturbed and small containers with 3 small holes in one end were placed near the
disturbed area so parasitized workers could recruit back into their mother colony,
generally within 5-30 min. The ants were returned to their mother colonies 3-4 days after
they were collected. Weather conditions during the releases were sunny and dry. On
sunny days, ant mounds were drenched with 200 mL of water and shaded with paper
plates to prevent desiccation until ants rej oined their nestmates.
The first release was conducted at Whitehurst Ranch, on the border of Levy and
Marion Counties near Williston, FL. The site is a 220 ha well-managed cow pasture,
approximately 15 mi SW of Gainesville, FL with a mixture of monogyne and polygyne
imported fire ant colonies. The release site was an area of about 2.5 ha bordering a small
pond shaded with pine and oak trees. The number of flies released per group of
parasitized workers was approximately 300 per day for 15 days for a total of about 4500
parasitized workers. Releases at this site were conducted in March 2003 using workers
from 42 mounds.
A second release was done at Morrill Farm and a third at Mickle pasture. Morrill
farm was one of the release sites used for Pseudacteon tricuspis (Porter et al. 2004). The
site is a 16-ha cow pasture with a mixture of trees, bushes, and two small ponds. The
Mickle pasture is a private residential home with a small 3 acre cow pasture. There is one
small pond and a mixture of trees where flies were released. I released about 260 flies per
day for 3 weeks simultaneously at both the Morrill and Mickle sites for a total of about
5600 parasitized workers at each site. Releases were conducted from May-June 2003.
Monitoring fly establishment and dispersal
Monitoring for P. curvatus establishment was done by disturbing 4-5 mounds in the
release area. Disturbed areas were closely inspected for hovering flies. Flies were easily
aspirated with an Allen-type double chamber aspirator and identified with a hand lens.
Generally, 4-5 mounds were monitored every 5-10 min over a period of up to 30 min.
After 30 min, another set of 4-5 mounds was disturbed and the observation cycle was
repeated. After each observation, the ants were stirred periodically to keep them active. If
no flies were observed, several pinches of ants in each mound were usually macerated
between the fingers to release pheromones that attract the flies (Morrison and King
2004). Monitoring for flies was generally done between 1130-1630 EST, on days with air
temperatures greater than 24 oC when adult flies are active. Sun shades were placed over
mounds on hot sunny days so that the ants could remain active on the surface during the
monitoring period. On severely hot days, mounds were sprinkled with several liters of
water to reduce the heat stress experienced by the ants.
I monitored dispersal from release sites by observing disturbed fire ant mounds at
0.5 mi intervals from the release site. As described above, 4-5 mounds were closely
inspected for hovering flies over a period of up to 30 min. After 30 min, another set of 4-
5 mounds were disturbed 0.5 mi in either a North, West, or South direction from last
observation site. Monitoring for dispersal was conducted up to a period of 2 h per d.
Flies were aspirated with an Allen-type double chamber aspirator, retained in small vials,
and knocked down with CO2 for identification with a hand lens.
The Hield releases at Whitehurst Ranch were successful. First generation flies were
found 5 weeks (April 2003) after the initial release (Table 3-1). There was a period of
intense rainfall following initial Hield releases. Initial counts (April-June 2003) of flies per
5 mounds were between 7-18 flies (Table 3-1). In the month of October, large fly
population numbers were recorded (Table 3-1). During 2003, I collected many more P.
curvatus flies than P. tricuspis flies (71 versus 9) even though P. tricuspis flies had been
at the site for several more years (Porter et al. 2004). Fly presence was not monitored
during the November and December months due to cold temperatures. Pseudacteon
curvatus flies successfully over-wintered and I was able to Eind flies at Whitehurst Ranch
from January to May of 2004. Fly abundances in 2004 have ranged from 27-55 flies per 5
mounds (median = 40).
Flies were also successful at establishing at both Morrill and Mickle sites. In April
and May 2004, 5-10 flies were found at the Morrill Pasture and 15-25 flies found at the
Mickle site (Table 3-1). Earlier attempts to monitor P. curvatus presence at these two
sites failed to yield positive results from observations made twice weekly for several
months (June-October 2003, Table 3-1). In the summer of 2003, I found large numbers of
P. tricuspis at the Mickle site (80-110 flies) but no P. tricuspis flies were found at the
Morrill site in spite of the fact that they had been very abundant in previous years. In
April and May 2004, I found a total of 75 P. curvatus flies at the Mickle site compared to
12 P. tricuspis; at the Morrill site it was 15 to 0 respectively.
In August 2003, flies had dispersed about 200 yd from the original release site at
Whitehurst Ranch. In April-May 2004, the flies had expanded 1.6 km both North and
South and about 0.8 km in a westward direction (May 2004). To the East of the site is a
heavily wooded area that is not accessible to motor vehicles. I found a total of 15 flies on
these outer boundaries as well as the presence of a few P. tricuspis flies. Dispersal rates
for the Morrill and Mickle pastures will be conducted in late summer 2004.
This study documents the first successful release and establishment of the
decapitating fly P. curvatus on red imported fire ants in the United States. A post release
specificity test in the fall of 2003 (Vazquez and Porter 2004) with non-Solenopsis ants
and the native fire ant, Solenopsis geminate (Fabricius), confirmed laboratory predictions
(Vazquez et al. 2004a) that this biotype from Formosa would be specific to red imported
fire ants. First generation flies were found within 5 weeks after release at the Whitehurst
Ranch while it took 8 months for flies to be found at both Morrill and Mickle sites (Table
3-1). Monthly monitoring at the Whitehurst site consistently yielded positive results until
cold weather conditions hindered monitoring (Table 3-1). Flies have successfully
overwintered at the Whitehurst site by the abundance of many P. curvatus flies with
small numbers of P. tricuspis flies during the months of January through May 2004
(Table 3-1). The confirmation of establishment of P. curvatus flies at the Morrill and
Mickle sites in late spring 2004 are also reassuring (Table 3-1). Another 2003 release of
P. curvatus on red imported fire ants appears to have been successful at a site near
Columbia, SC (T. Davis and M. Horton, personal communication). Monitoring at the
Morrill pasture in April 2004 yielded positive results for P. curvatus flies. Pseudacteon
curvatus flies have been found at larger abundances than P. tricuspis at all release sites
indicating that P. curvatus flies may be a more effective biocontrol agent at least in some
Weather conditions at Morrill and Mickle were very dry when flies were first
released (May-June 2003); followed by an extended winter season into the month of
February (2004). The combination of hot and cold temperature extremes at both Morrill
and Mickle could be the attributing factor why flies were not found until 8 months after
release. Studies on Pseudacteon parasitoids have shown considerable seasonal variability
in population abundances according to species throughout the year (Folgarait et al. 2003,
Fowler et al. 1995, Morrison et al. 1999, 2000, Wuellner and Saunders 2003).
Dispersal of flies outside of the Whitehurst site is encouraging in that this confirms
these flies are reproducing well and expanding its range. The 1.6 km expansion range, in
the first year observed for P. curvatus at the Whitehurst site, is similar to initial Hield
release expansion rates for that of P. tricuspis (1.5 km in the first year; Porter et al. 2004).
This fly is another species in our arsenal in controlling red imported fire ant
populations. It is unlikely that a single new fly will have a substantial impact on imported
fire ant densities in the short term. Nevertheless, it is hoped that several species of
decapitating flies combined with other natural enemies from South America will have
substantial impacts on imported fire ant populations in North America as they appear to
have on fire ants in South America.
Table 3-1. Number of adult Pseudacteon~ddd~~~ddd~~~dd curvatus flies found at Whitehurst Ranch,
Morrill Pasture, and Mickle Pasture release sites in Florida from April 2003 to May 2004.
Number of Flies (Number of Mounds Checked)
Months Whitehurst Ranch Morrill Pasture Mickle Pasture
April 2003 7 (15)
May 2003 7 (26)
June 2003 18 (11) 0 (8) 0 (7)
July 2003 15 (10) 0 (10) 0 (10)
August 2003 14 (7) 0 (9) 0 (7)
September 2003 10 (21) 0 (10) 0 (8)
October 2003 305 (26) 0 (8) 0 (8)
January 2004 27 (6)
February 2004 45 (4)
Niarch 2004 16 (8)
April 20004 26 (10) 10(8) 20 (3)
May 2004 55 (20) 5 (4) 60 (6)
a Flies were not checked on November and December of 2003 due to cold temperatures.
RE-CONFIRMING HOST SPECIFICITY OF THE FIRE ANT DECAPITATING FLY
Pseudacteon curvatus AFTER FIELD RELEASE IN FLORIDA3
Critics of biological control programs have argued that there is a lack of post-
release monitoring on nontarget effects arising from released exotic insects. Howarth
(1991) stated that negative environmental impacts of biological control introductions
have not been well documented. Similarly, others have complained that releases of
nonindigenous species on target organisms have led to reduction in populations of
nontarget species due to inappropriate protocols on host specificity of these
nonindigenous species (Barron et al. 2003, Civeyrel and Simberloff 1996, Hopper 2001,
Howarth 1991, Secord and Kareiva 1996, Simberloff and Stiling 1996a, b). However, in
spite of these criticisms the biocontrol community appears to have a good record of
environmental safety (Lindgren 2003, McEvoy et al. 1991). Similarly, Pemberton (2000)
analyzed works dealing with 117 natural enemies of 55 weed species and found that only
1 natural enemy completes development in a nontarget plant. A significant problem
appears to be that biocontrol practitioners have not always done an adequate j ob of
documenting the post establishment host specificity of organisms that they release.
However, this problem is beginning to be rectified. For example, post-release
monitoring has been done for releases of the chrysomelid beetle Galerucella calmariensis
on purple loosestrife Lythrum salicaria (in Michigan; Landis et al. 2003, in Canada;
3 In Press, Florida Entomologist 2004
Lindgren 2003, in Oregon; Schooler et al. 2003), the fungal pathogen Neozygites
floridanadd~~~~~ddddd~~~~ on the cassava green mite M~ononychelhts tanajoa in West Africa (Hountondj i
et al. 2002), the parasitoid wasp Trichogramnma brassicae on the European corn borer
Ostrinia nubilalis in Switzerland (Kuske et al. 2003), a South American mirid
Eccritotarsus catarinensis on the waterhyacinth Eichhornia cra~ssipes in South Africa
(Coetzee et al. 2003), the rubber vine moth Eucla~sta whalleyi on the rubber vine
Oryptostegia grandiflora in Australia (Cruttwell McFadyen et al. 2002), the tephritid fly
Acinia picturata on the exotic weed Phechea odorata in Hawaii (Alyokhin et al. 2001),
and the melaleuca weevil Oxyops vitiosa on M~elaleuca quinquenervia in Florida (Paul
Pratt, personal communication). All of these studies have found minimal or no non-target
The host range of phorid decapitating flies in the genus Pseudacteon~ddd~~~ddd~~~dd have been
studied extensively prior to field releases as self sustaining biocontrol agents of imported
fire ants (Folgarait et al. 2002, Gilbert and Morrison 1997, Morrison and Gilbert 1999,
Porter 1998, Porter 2000, Porter & Alonsol999, Vazquez et al. 2004a). Pseudacteon
tricuspis Borgmeier flies were successfully established on red imported fire ant
populations at eight sites in North Florida (1997-1999; Porter et al. 2004). In the fall of
2003, host specificity ofP. tricuspis was tested in the field and results demonstrated that
these phorid flies had no attraction to non-host organisms including native fire ants
(Lloyd Morrison, personal communication). These results are consistent with predictions
from quarantine laboratory tests (Gilbert and Morrison 1997, Porter and Alonso 1999)
and field tests in South America (Porter 1998) prior to its release in the United States.
A second phorid fly species, Pseudacteon~ddd~~~ddd~~~dd curvatus Borgmeier from Formosa,
Argentina, was released in Florida to control populations of red imported fire ants,
Solenopsis invicta Buren (Vazquez et al. 2004b). Pseudacteon curvatus is a small
decapitating fly that normally parasitizes small red imported fire ant workers. Quarantine-
based host specificity testing predicted that this Formosa biotype was highly host-specific
to S. invicta and that nontarget effects to the native fire ants, Solenopsis geminata
(Fabricius) and Solenopsis xyloni McCook would be minimal to non-existent (Vazquez et
al. 2004a). The obj ective of this paper is to document the host specificity of established
field populations of the Formosa biotype of P. curvatus.
Materials and Methods
The P. curvatus flies were collected attacking S. invicta fire ants 35 km NW of
Formosa, Argentina by Sanford D. Porter and Juan A. Briano (October 2001).
Pseudacteon curvatus was first successfully released and established in Florida at
Whitehurst Farm, 15 mi SW of Gainesville, FL in the spring of 2003 (Vazquez et al.
2004b). Field observations of host specificity were made in October 2003 between 1300
and 1530 EST, when the temperatures were > 24oC. I tested the attraction of established
P. curvatus flies to 15 species of non-Solenopsis ants: Aphaenoga~ster miamniana Wheeler
(0.8-0.9 mm head width, 0.2 g of workers used), Aphaenoga~ster c.f. carolinensis Wheeler
(0.7 mm, 0.7 g), Camponotus floridd~~dd~~ddanus (Buckley)(2.2 mm, 4 g), Camponotus impressus
(Roger)(0.7-0.8 mm, 0.6 g), Crematogaster minutissima Mayr (0.6 mm, 2 g),
Crematoga;ster pilosa Emery (0.7-0.9 mm, 2 g), Cyphomyrmex rimosus (Spinola)(0.6
mm, 0.2 g), Dorymyrmex bureni (Trager)(0.7-0.9 mm, 0.3 g), Forelius pruinosus
(Roger)(0.5 mm, 0.3 g), Linepithema humile Mayr (0.6 mm, 2 g), Odontomachus
brunneus (Patton)(1.8 mm, 0.4 g), Pheidole dentat Mayr (0.6 mm minors, 1.2 mm
maj ors, 0.6 g), Pogonomyrmex badius (Latreille)(2. 1-2.4 mm, 1.4 g), Pseudomyrmex
pallidus (F. Smith)(0.6 mm, 0.1 g), Trachymyrmex septentrionalis (McCook)(0.8-1.0
mm, 0.2 g), and 6 colonies of S. invicta (0.6-1.4 mm, 1.5 g) workers. In the laboratory,
P. curvatus successfully parasitizes Solenopsis ants with head widths of 0.6-1.1 mm
(median of 0.74 mm; Morrison et al. 1997 and SDP unpublished data). All ant species
used in these tests were collected near Gainesville, Florida (September 2003).
Trays with the 15 non-Solenopsis ants were set out first. Trays were 40 x 26 x 8 cm
in size and contained only one species of ant. The non-Solenopsis ants were then
removed after 30 min and replaced with the 6 trays of S. invicta. At the conclusion of 30
min, the S. invicta trays were replaced with the 15 trays of non-Solenopsis ants to
determine if the flies originally attracted from the S. invicta trials would exploit the other
genera in the absence of its primary host (no-choice). Established Pseudacteon curvatus
flies observed hovering in attack mode over each tray were collected at 5 min intervals
for 30 min. All flies were aspirated with an Allen-type double chamber aspirator and
retained in vials until the conclusion of each 30 min trial when they were identified to
species using a hand lens. Aspiration of flies normally does not change attack behavior
once flies are released (Morrison et al. 1997). Collection and identification for presence
of P. curvatus flies was necessary since P. tricuspis flies were present at the study site
from a release in Gainesville, Florida, in the summer and fall of 1997 (Porter et al. 2004).
Sampled flies were then released prior to setting up additional trays. These methods were
replicated on two consecutive days.
Further tests ofP. curvatus host specificity were conducted with five trays ofS.
invicta and five trays of the native fire ant, S. geminate. Each tray contained 2 g of
workers and 2 g of brood. As described above, the five trays of S. gentinata were set out
first for 30 min. Solenopsis gentinata trays were then removed and replaced with the S.
invicta trays and these trays were observed for 30 min. At the conclusion of 30 min, the
five trays ofS. invicta were replaced again with the five trays ofS. gentinata for an
additional 30 min. Attacking flies were collected at 5 min intervals as described above.
These methods were replicated on two days (five days apart) at the same site mentioned
The P. curvatus flies were not attracted to any of the 15 non-Solenopsis genera
during the sequential series trials over the two days (Table 4-1). However, the flies were
readily attracted to S. invicta (99 on day 1 and 38 on day 2, Table 4-1). As is normal,
these flies hovered above their host, oriented themselves to workers, and readily struck
the thorax of workers during oviposition. When the six S. invicta trays were removed and
replaced again with the 15 trays of non-Solenopsis ants, P. curvatus flies were not
observed hovering over any of the non-Solenopsis trays. Pseudacteon~ddd~~~ddd~~~dd curvatus flies were
present at all six S. invicta trays during the trials.
In the S. invicta versus S. gentinata trials, P. curvatus flies were not observed
hovering or attacking over S. gentinata during the first day and only 2-4 flies were
observed hovering on the second day (Table 4-1). Flies collected above the native fire
ants generally hovered briefly without attacking. Only one fly attempted to oviposit, but
it flew away immediately after without returning. In quarantine tests, this biotype would
occasionally attack S. gentinata workers but attacks were never successful (Vazquez et al.
2004). Pseudacteon curvatus flies were present at all five S. invicta trays during the first
day and present at four of five trays on the second day. Pseudacteon curvatus flies were
present at none of the five S. geminate trays during the first day and at 1 of 5 and 3 of 5
trays on the second day (Table 4-1).
Established P. curvatus individuals were attracted to S. invicta over S. geminata by
a ratio of about 30 to 1 (1 19 to 4 total flies, Table 4-1). These results were better than
results predicted from quarantine tests where P. curvatus hovered over S. invicta versus
S. geminata at a ratio of 1.3 to 1 in no-choice tests (Vazquez et al. 2004). Perhaps this
difference was because P. curvatus flies in the laboratory tests were confined in small test
containers leading to higher rates of hovering. Furthermore, attacks on S. geminata were
very rare to non-existent in the field confirming laboratory choice tests where attack rates
were 16 times higher for females hovering over S. invicta than for flies hovering over S.
geminata (7.02 & 1.41 (mean & SE) versus 0.44 & 0.28 attacks/min, respectively; Vazquez
et al. 2004). I demonstrated in quarantine tests (no-choice and choice) that the Formosa
biotype of P. curvatus does not complete development in S. geminate (Vazquez et al.
Post-release populations of P. curvatus were not attracted to any of the 15 non-host
ant genera. In host-specificity tests with a biotype from Las Flores, Argentina, P.
curvatus hovered over most of 19 non-host genera in quarantine conditions (Porter 2000);
however, they generally hovered without attacking and no parasitism occurred in any of
the 19 non-host genera (Porter 2000). Results from this study demonstrate that host
specificity ofP. curvatus is restricted to S. invicta and poses no realistic threat to the
congener S. geminata or ants in other genera.
Table 4-1. Number ofPseudacteon curvatus flies collected hovering in attack mode over
non-host ant species, native fire ants (Solenopsis gentinata), and red imported
fire ants (Solenopsis invicta) during sequential series of field trials
Ant Species Flies Collected Trays
0-10 min 11-20 min 21-30 min Total Attacked
S. invicta vs 15 non-host genera (day 1)
All 15 genera 0 0 0 0 0/15
S. invicta 14 56 29 99 6/6
All 15 genera 0 0 0 0 0/15
S. invicta vs 15 non-host genera (day 2)
All 15 genera 0 0 0 0 0/15
S. invicta 7 14 17 38 6/6
All 15 genera 0 0 0 0 0/15
S. invicta vs S. gentinata (day 1)
S. gentinata 0 0 0 0 0/5
S. invicta 28 20 18 66 5/5
S. gentinata 0 0 0 0 0/5
S. invicta vs S. gentinata (day 2)
S. gentinatae 0 1 1 2 1/5
S. invicta 14 16 23 53 4/5
& etntb 1 3 0 4 3/5
a No oviposition attempts were observed.
b Only one oviposition attempt was observed.
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Ricardo Jose Vazquez was born on January 14, 1971, in Rio Piedras, Puerto Rico.
In 1981, he moved with his family to St. Augustine, Florida. He attended St. Augustine
High School and enjoyed being a member of the jr. ROTC department. When he
graduated high school in 1989, he enrolled in the United States Marine Corps as an
enlisted communications center operator. Through the military, Ricky has traveled to
distant lands such as Japan, Korea, and Somalia. After 4 years of honorable service,
Ricky enrolled at the St. Johns River Community College in St. Augustine, FL, were he
earned an A.A. degree with honors. While at SJRCC, Ricky was president of the Phi
Theta Kappa Honor Society. He transferred to the University of Florida to pursue an
undergraduate degree in entomology. While at the University of Florida, Ricky gained
practical experience in research by working for the University of Florida' s Entomology
and Nematology Department (urban entomology laboratory) and at the Center for
Medical, Agricultural & Veterinary Entomology (USDA-ARS) in Gainesville, Florida.
During graduate work, Ricky tutored at risk athletes at the University of Florida and did
volunteer work for the Alachua Country Fire Rescue Reserves as a first responder. He is
a member of the Gamma Sigma Delta Honor Society of Agriculture, Blue Chips and
Salsa investment club, Entomological Society of America, Florida Entomological
Society, Entomology and Nematology Student Organization (ENSO) and the Urban
Entomological Society (UES).