On the Eradication of Imported Fire Ants': A Theoretical
Appraisar
By C. S.
D. E. VVEIDHAAS
Entomology Research Division, Agr. Res. Sen'., USDA, Gainesville, Florida 32601
LOFGREN AND
In 1957 the United States Congress established a cooperative Federal-State program which had as its objective the control and containment of the imported fire ant,
Solcllopsis sac1,issilllll riclltcri Forel, within the limits of
its distribution at that time. Long-lasting insecticides
(heptachlor and dieldrin) were used in the early stages
of the campaign, but the development of mirex bait (Lofgren et al. 1963, 1964; Stringer et al. 1964) and its adoption as the standard control in the early 1960's eliminated
the need for these insecticides in the large-scale control
program.
probability that a newly mated queen will establish a new
colony. We have considerable information about the
proper time of year for applications, but information on
the other two points is incomplete. However, we can
make good assumptions from the data that are available.
Timillg of ApPlicatiolls.-Bait
applications should obviously be made when ma..ximum control can be obtained.
However, they should not be made while new colonies
are being established by newly mated queens, because
mirex bait is effective for only a few days and these
queens alone in their brood chambers or cells and without
foraging workers would not be exposed to the bait. Also,
in heavily infested areas few new colonies are established
by queens because of competition with older colonies;
however, when old colonies are killed by the bait, the
probability that the queens in their brood chambers will
establish a colony is greatly enhanced. While these colonies would, of course, be susceptible to the next treatment, the chances for eventual eradication would be increased if their establishment were prevented. Therefore,'
we believe that treatments should be restricted when nuptial flight activity is greatest (late May until August)
even though they would be very effective against established colonies. On the other hand cold weather limits
the foraging activity of ants, so bait applicd from late
Nov~mber through February is not so effective as bait
applied in warmer weather. We therefore conclude that
the logical times for application of bait are the spring and
fall. In southern Florida, however, mating flights and
colony development are more or less continuous throughout the year, thus treatments in this area may have to be
applied every 3 or 4 months, depending upon the period
required for the development of new colonies. Another
possible exception to the restriction on applications during
summcr might be made if all old colonies were killed and
only incipient colonies remained. In such an area, applications might be needed to insure that the new colonies
did not exist long enough to develop alates.
~Iirex hait is very effective against the imported fire
ant and has been used extensively during the past 7 years.
However, data are needed to determine if it can be used
to eradicate this pest. This proof will, hopefully, be obtained as a result of the current large-scale tests now
heing made hy the Insects Affecting Man Laboratory in
Gainesville, Fla., in cooperation with the Plant Protection Division of the Agricultural Research Service and
with the cooperating States. Obviously, several years of
adual field trials are necessary to prove whether eradication is technically possible. However, with our present
knowledge we can estimate the chances for success with
mirex bait or with any other similar method of control.
The l1resent paper, therefore, presents theoretical calculations made to determine the number of applications of
bait that might be necessary to achieve eradication assuming different degrees of effectiveness of the bait, densities
of population of the ant, and rates of population increase.
It is hoped that the results of our calculations will serve
two principal functions. First, they can be used as a tentative guide for scheduling of bait treatments during eradication trials so that proper advance planning can be made;
and second, they can be used to guide research into those
areas where additional information is most urgently
needed. The computations are based on data obtained
from published research papers and from our personal
observations and from those of other investigators.
We recognize that the environmental impact of any
control or eradication program must be evaluated and
assessed before the program is undertaken.
Extensive
studies of the environmental effects of mirex bait are now
being conducted. However, our purpose in preparing this
paper is simply to assess the theoretical potential for
eradication, and we shall not review or discuss the environmental implications of such a program.
Production of Sexual Forms.-While
an ant colony may
develop alates within 4;~ months (Rhoades and Davis
1967), it takes at least 1-2 years for most new colonies
to produce any significant number, because colony development slows down or stops during the winter (NovemberMarch). Thus, in our subsequent calculations we have
assumed that no significant mating flights occur from
colonies less than 1 year old in south Florida or less than
2 years old in the remainder of the infested area.
Factors for COllsidcratioll.-If
eradication is to be
achieved, we must obtain high levels of control with each
of the several applications of bait. Obviously, many factors such as type of dispersal equipment, aircraft guidance
systems, weather, and competition for bait will have a
tremendous effect on whether good control will be
achieved with treatments made over millions of acres.
Other factors which also need to be considered include:
(1) the time of year applications are made, (2) the ability of ant colonies to produce sexual forms, and (3) the
1 Hymenoptera:
Formicidae.
• Mention of a pesticide does not constitute
by the USDA.
Established colonies of imported fire ants have mating
flights throughout the year over a large part of their distribution range if sufficiently warm weather occurs. However, the major nuptial flight activity occurs from May
to August, when as many as 6 queens per square yard of
ground surface have been found after a single flight (Entomology Research Division 1958). A conservative estimate of the total number of queens alighting in any area
during these months of great activity would be about 1
per square foot of ground, in round figures, about 45,000
per acre. Since the number of colonies capable of producing queens in the areas from wnich these queens came
recommendation
17
probably averaged about 10-20 per acre, the number of
queens produced by each colony was about 2300-4500.
This rate of production of alates agrees fairly well with
unpublished data obtained from G. P. Markin, Plant Protection Division, Agr. Res. Serv., USDA, who determined
queen production from the biomass of ant colonies. In our
calculations we have assumed the higher figure is more
accurate and that the average mature colony produces
about 4500 queens between May and August.
Table 1.-Reinfestation
ment with mirex bait.
Habitat
Pine woods
Improved pasture
Pecan groves
Idle land
Cultivated land
(gardens,
cornfields)
Totals
The number of queens making nuptial flights in late
winter or early spring is difficult to estimate. However,
these flights are made by queens produced during the
previous summer, and the numbers are generally low.
Glancey and Stringer of this laboratory
(unpublished
data) found that in Mississippi the number ranged from
31 to 1920 and averaged 400-500 per colony. On the other
hand, not all queens fly during any nuptial flight; also
many queens would be killed before leaving the nest by
spring bait treatments. Thus, we have estimated that the
number of queens participating in late winter or early
spring flights is 100 per colony.
Probability of Establishing New colonies (sllrvival).Incipient colonies from queens flying during the late
spring and early summer are generally not noticed (they
do not build mounds) before the first cool wet weather
in September. By then, most of the colonies have several
thousand or more workers, indicating that they are at
least 2-3 months old (thus are from queens flying prior
to August 1). As many as 1000 incipient colonies per
acre have been noted (Green 1962). However, queens
that do not have a large number of workers to care for
them probably do not survive cold weather or competition
from other colonies during the winter. This hypothesis
is evidenced by the fact that the number of incipient colonies present in any given area is invariably lower in the
spring than in the fall. Thus we conclude that in most
infested areas (excluding lower Florida, where the climate is moderate) queens from flights occurring after
August 1 do not establish colonies or, if they do, the number is insignificant. We therefore have assumed that all
colonies, mature and incipient, are susceptible to a fall
application of bait and that queens without foraging workers seldom survive the winter.
a Each plot was
was 960 acres.
rate on land 1 year after treat-
No.
plots·
Total no.
mature
colonies
before
treatment
Total no.
incipient
colonies
1 yr after
treatment
9
10
4
4
262
305
90
88
25
382
161
71
10: I
1: 1.25
1: 1.8
1:0.8
6
33
88
833
924
1563
1: 10.5
1: 1.9
of approximately
colonies in the area surrounding
1 acre.
Total
Ratio of
mature
to
incipient
colonies
area
treated
the treated area was very
similar to that in the test area. Then, since most queens
disperse over a distance of 1-2 miles during mating flights,
the treated area (maximum distance from edge to center
was
mile) must have been exposed to the same rate
of reinfestation as other nearby untreated land. Thus,
we can assume that the number of incipient colonies found
1 year later would approach the maximum number capable of becoming established in that area that year. If this
reasoning is correct the number of incipient colonies in
the area would also represent the maximum yearly potential for reproduction of the original colonies, and a comparison of the number of original colonies with the number of incipient colonies should give the potential rate of
increase for an imported fire ant colony.
*
The number of incipient colonies in any locality varies
enormously depending upon the type of terrain; therefore,
the data of Lofgren et al. (1964) is compiled according to
5 types of habitats. The results (Table 1) showed that
reinfestation (compared with the pretreatment population
of mature colonies) varied from a ratio of 10 old to I
new in the wooded habitat to 1 old to 10.5 new in the
cultivated habitat. Overall, the area was reinfested at a
ratio of 2 new colonies to 1 old colony. Thus each mature colony produced 2 incipient colonies or, in other
words, the biological rate of increase for an imported fire
ant colony is about 2-fold in 1 year.
The potential rate of increase of colonies must be
known before we can calculate the degree of control or
suppression of ant colonies that will be provided by successive treatments over a period of time. No data are
available on this subject, but estimates can be made from
information about the reinfestation of land treated with
mirex bait.
Another way of estimating the rate of increase was
also available. Previously we estimated that about 45,000
alate mated queens alight on an acre of ground during
each reproductive season. Then, since the average number
of incipient colonies for the plots listed in Table 1 was
about 50 per acre, it takes about 900 queens to produce
1 colony (a rate of survival of 0.11%). Since each colony
produces about 2300-4500 females per year, we can assume the rate of increase for a colony is about 2- to
5-fold. Our 2 estimates compare favorably, and a 2-fold
rate of increase is probably realistic. However, for our
calculations we have used the highest rate indicated (5X)
because eradication requires the elimination of ants responding with extremes of reproduction rather than with
average reproduction.
As noted, incipient colonies are rarely found on land
already heavily infested with imported fire ants, probably
because of competition or amalgamation of old and new
colonies. In contrast, small areas where the ants have
been eliminated become reinfested so readily that the
number of new colonies present 6-12 months later may
exceed the original number. This increase results directly
from lack of competition and therefore can be used to
calculate the reproductive potential.
From previous research (Lofgren et al. 1964) we have
accurate pretreatment counts of the number of mature
colonies in a large area north of Gulfport, Miss. (960
acres) which contained a variety of ecological habitats,
some favorable and some unfavorable for colony establishment. Almost all imported fire ant colonies present in
the area were subsequently killed by a mirex bait treatment during the fall of 1961. However, the density of
Because mating flights may also occur in the spring
before a spring bait application can be completed, it is
necessary to assign a rate of increase for mature colonies
present in the eradication area at this time. If only 100
females per colony take part in these flights, and if their
survival rate is the same as during the summer (0.11%),
18
treatment plus the number of potential colonies from mating flights occurring before the next scheduled treatment.
Table 2.-Theoretical
number of mirex bait treatments
required to eradicate the imported fire ant from 2,000,000
acres assuming 2 different population levels and seasonal
(April-August)
or continuous rates of increase.
Similarly, the number of colonies remaining after any
treatment in an area with a potentially continuous rate
of increase is given by the following equation:
No. bait applications required
to reduce population to <1
Population
density
(mounds/
acre)
10
0/0
Total no.
mounds
before
treatment
control
with
each
treatment
2,000,000
90
98
99
99.9
99.99
90
98
99
99.9
99.99
20,000,000
Seasonal
development
Cn + (R-n)kCn + nkCn_I
or [1+ (R-n)k]~ •• + nkCn_I
8
5
5
5
4
3
3
9
6
4
3
9
6
5
5
4
3
4
3
the rate of increase/colony would be about O.1X, or it
would take 10 old colonies to produce 1 incipient colony
from early spring mating flights.
Results of Calculations.-With
a 5-fold rate of increase,
it is necessary to impose a control method of greater than
(As indicated previously, the time
80% efficiency to achieve a suppression in any given iso-
for development of a mature colony is about 2 years if
alate production is seasonal and 1 year if it is continuous.)
lated population of the imported fire ant. Then, if the
degree of control of each generation exceeds the imported
fire ant's ability to reproduce, total population suppression should eventually occur. Our calculations show that
eradication over a 2,OOO,000-acre area could be achieved
with 3-9 applications if the levels of control were 9099.99%. Interestingly, if the level of control is 99.99%,
the number of treatments needed for eradication is the
same whether the population density is 1 or lOfacre and
whether the production of sexual forms is seasonal or
continuous.
If we assume that the survival rate from each bait
treatment is the same, then the reduction in the pretreatment population of ant colonies can be derived as follows:
AB"
=
(1)
C
where A = the number of colonies before treatment,
B = the survival rate expressed as a decimal fraction,
n
the number of bait treatments, and C
the number
of colonies surviving after "n" treatments.
=
=
While we have assumed a constant rate of control for
each series of treatments we do not mean to imply that
this will or has to occur to achieve eradication. A level
of control of 99.9% for 3 applications means only that the
survival rate of the imported fire ants after' the 3 treatments is 0.001", that is, 10-°. This same or approximate
rate of survival can be obtained with varying control
levels. For example, if the control achieved with each of
3 successive treatments were 98, 99.95, and 99.99%, the
survival rate would be 0.02XO.005XO.0001 or 10-9•
We can expand Eq. 1 to include rates of increase between applications in areas where reinfestation is seasonal
as follows:
C•• + 5SC •• + O.1SC"-I
or (1+5S)C •• + O.1SC"-I
=
=
D
D
(3)
Many variations of population density and levels of
control could be considered. However, our main purpose
is to determine the control levels needed for eradication
with a minimum number of bait treatments.
Thus, in
Table 2, we have considered only 2 population densities
(1 and 10 colonies per acre) and 5 levels of control (9099.99%). Also, we have arbitrarily chosen to consider
eradication blocks of 2,000,000 acres, an area corresponding to the size of the largest trial area now under study.
Methods of Calculations.-Based
on the data, assumptions, and conclusions in the previous sections, we have
developed 3 equations for calculating the number of ant
colonies surviving any treatments. These equations take
into account the rates of increase of the ant populations
between treatment. However, we are primarily concerned
with eradication with a minimum number of applications,
so we have not included a factor to compensate for new
colonies produced by alates from colonies developing after
the initial treatment.
D
D
In this instance, we have assumed the rate of increase
for each mature colony is 5X as in Eq. 2 and that the
mating flights occur uniformly throughout the year. Thus
the rate of increase (k) for any month would be 0/12 or
about 0.4. In addition, we have assumed that all colonies
are susceptible to the bait except those in the process of
being established by queens from mating flights occurring
1 month prior to bait application. With these assumptions the number of new colonies arising from flights
made 1 month before the bait application is represented
by (R-n)kCn, (R = the number of months since the
first treatment), and the number of new colonies arising
from flights from surviving colonies during the months
between treatments by IIkCn-I. The sum of these 2 values
plus the number of surviving pretreatment colonies, Cn,
gives the total number of colonies surviving any particular treatment and the number of potential colonies that
could arise from mating flights that occur before the next
scheduled treatment.
Continual
development
8
=
=
(2)
In this situation, we are concerned only with rates of increase resulting prior to or following the spring treatment, since queens from flights during the fall or winter
cannot establish colonies. Therefore, the rate of increase
of pretreatment colonies surviving the spring· treatment
is represented by 5C••, and the rate of increase from flights
prior to the spring treatments is represented by 0.1Cn-I.
Multiplication of the sum of these two terms by S, the
total number of spring treatments, and the addition of
the number of surviving pretreatment colonies, gives
D, the total number of colonies surviving any particular
Discussion and COllclusions.-The
total suppression of
any insect species is difficult to achieve. However, with
an alien pest such as the imported fire ant, eradication by
an effective, acceptable, and economical procedure is a
commendable goal. Obviously, attainment of this goal is
contingent on many factors, the primary one being a
highly effective method of control that is inexpensive reJa-
e..,
19
tive to the benefits gained by eradication. Mirex bait is
effective, and cost of application is relatively low (about
35¢ per acre including cost of bait, aerial application, and
electronic guidance system). However, even at this low
cost per treatment eradication would be expensive unless
it could be achieved with no more than 3-4 applications.
Our theoretical analysis demonstrates that 99.9% control
per application would be necessary to achieve eradication
with that number of treatments.
improving application systems and the one in which the
greatest monetary savings would be realized is in developing procedures to increase the width of the swaths
with a concurrent increase in swath overlap. Doubling
the width of the swath could reduce application costs to
half, greatly decrease the time spent in applying bait, and
permit applications when weather conditions are almost
favorable.
With respect to bio'.ogy and ecology, recent data collected by the Methods Improvement Laboratory of the
Plant Protection Division, several State universities, and
our laboratory have expanded immensely our knowledge
of the imported fire ant, but research should be continued
with particular emphasis on studies in thc extreme areas
of its range. Also, better techniques for detection of imported fire ant infestations are needed, particularly whcn
they are in the incipient stage. Early detection might
eliminate the need for one bait application in some
instances.
If we assume that eradication is possible, the logical
questions that fo1low are: (1) Is mirex bait effective
enough to give 99.9% control?
(2) Is our technology
capable of distributing the bait uniformly over vast areas?
(3) Do we know the biology and ecology of the ant
sufficiently we1l to manage an eradication program?
Small-scale tests have consistently shown that mirex
bait applied at rates of 1 Ib per acre or more will destroy
from 95 to 100% of the colonies (Lofgren et al. 1964,
Banks et al. 1970). Moreover, we have noted better control on large treatment blocks (1000-1,000,000 acres),
perhaps because migration of colonies onto large areas
is decreased. The final answer to the question of effectiveness of mirex bait must await results of the eradication trials now in progress, though we already know from
preliminary observations that 99.9% control is possible
with proper application.
REFERENCES CITED
Banks, W. A., C. E. Stringer, and N. W. Pierce. 1971.
Effect of toxicant concentration and rate of application of mirex bait on control of the imported fire
ant. J. Ga. Entomo!. Soc. 6 (4) : 205-7.
Entomology Research Division.
1958. Observations on
the biology of the imported fire ant. USDA ARS-
A factor to remember in bait distribution and ant control is that the bait is directed against a colony of insects
rather than individual insects. This makes control somewhat easier, because it is only necessary to direct the
bait into the foraging territory of the ant (a few hundred
to over 1000 square feet for the average colony 1 year
or older) rather than to contact each insect directly with
the control agent.
33-49.
Green, H. B. 1962. On the biology of the imported
fire ant: J. Econ. Entomol. 55: 1003-4.
Lofgren, C. S., F. J. Bartlett, and C. E. Stringer.
1963.
Imported fire ant toxic bait studies: evaluation of
carriers for oil baits. Ibid. 56: 62-66.
Lofgren, C. S., F. J. Bartlett, C. E. Stringer, and W. A.
Banks. 1964. Imported fire ant toxic bait studies:
further tests with granulated mirex-soybean oil bait.
Ibid. 57: 695-8.
Rhoades, W. c., and D. R. Davis. 1967. Effects of
meteorological factors on the biology and control of
the imported fire ant. Ibid. 60: 554-8.
Stringer, C. E., C. S. Lofgren, and F. J. Bartlett.
1964.
Imported fire ant toxic bait studies: evaluation of
toxicants. Ibid. 57: 941-5.
Techniques of applying bait have become much more
sophisticated with the use of electronic guidance systems
for the aircraft that carry the bait. With these systems,
planes can fly uniform flight paths for distances of 25
miles with swath spacings of less than 200 ft, which has
greatly enhanced the efficiency of the applications. However, little has been done to refine the bait distribution
systems on the aircraft. Thus, the most obvious area for
SENIOR FULBRIGHT-HAYS
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Exchange of Persons announces that applications for senior FulbrightHays awards for lecturing and research during 1973-74
in over 75 foreign countries will be accepted in the
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Registrants wi1l receive the detailed announcement of
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PROGRAM
FOR
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time to consider the possibilities and to apply before the
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Iiling for lectureships.
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lecturer's qualifications, salary, and other factors.
20