THE REPELLENCY OF FIVE ESSENTIAL OILS AGAINST THE ARGENTINE ANT (HYMENOPTERA: FORMICIDAE)

Abstract

The Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), is an invasive, cosmopolitan species that was introduced into the U.S. around 1891 via the port of New Orleans, LA (Foster 1908, Newell and Barber 1913, Suarez et al. 2001). In the U.S., the Argentine ant is a major pest in urban, agricultural and natural environments due to its unicolonial structure which allows the formation of supercolonies with multiple shared nests (Newell and Barber 1913, Suarez et al. 1999, Tsutsui et al. 2000, Vega and Rust 2001, Tsutsui and Suarez 2003). Because these ants were introduced and have lost genetic diversity, they lack intraspecific aggression, allowing populations to increase rapidly (Newell and Barber 1913, Holway et al. 1998, Holway 1999, Human and Gordon 1999, Tsutsui et al. 2001, Suarez et al. 2002), which provides a competitive advantage to L. humile when competing against native ants, other introduced ant species (Kabashima et al. 2007), and other insect and arthropod species (Holway et al. 1998, Holway 1999, Tsutsui and Suarez 2003). Other factors in the success of L. humile include sociotomy, polydomy, polygyny, lack of natural enemies, and human dispersal, mostly through commerce and various other business operations (Holway et al. 1998, Holway and Suarez 1999, Holway and Case 2000, Vega and Rust 2001, Tsutsui and Suarez 2003). Control of Argentine ants has typically relied on chemicals, more specifically, slow-acting baits and perimeter sprays (Vega and Rust 2001). Klotz et al. (2007) found that slow-acting fipronil sprays reduced ant activity by 90% in an 8-wk period; however, using a perimeter spray of fipronil and a perimeter-broadcast of bifenthrin granules achieved the greatest reduction of ant activity around structures. Rust et al. (2003) proposed that the broader the range of concentrations in insecticidal baits, the more effective the control because of delayed toxicity. However, they also noted that finding suitable bait bases and active ingredients that provide delayed toxicity are the most difficult obstacles to overcome when formulating effective Argentine ant baits. Klotz et al. (1995) suggested that the lack of information on Argentine ant biology has contributed to the failure of most traditional chemistries to successfully control Argentine ants. According to Curtis et al. (1990), a large number of plants that yield essential oils are known to be feeding deterrents to insects and other arthropods. Some of these plant-extracted oils have been the starting point for some commercially-produced repellents. The long-term repellent characteristics of essential oils have been challenged by some (Buescher et al. 1982, Rutledge et al. 1983, Nerio et al. 2010), who reported efficacy when freshly applied, but reduced effectiveness as the oils began to age. Thus, the objective of this study was to evaluate the repellent effects of freshly-applied and aged essential oils on Argentine ants, using a choice-based assay similar to that of Ebeling et al. (1966).

Full text

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PROCEEDINGS OF THE 2010 NCUE, PORTLAND, OR
THE REPELLENCY OF FIVE ESSENTIAL OILS AGAINST THE
ARGENTINE ANT (HYMENOPTERA: FORMICIDAE)
Christopher M. Scocco and Daniel R. Suiter
Department of Entomology, The University of Georgia, Griffin, GA
The Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), is
an invasive, cosmopolitan species that was introduced into the U.S. around 1891
via the port of New Orleans, LA (Foster 1908, Newell and Barber 1913, Suarez et
al. 2001). In the U.S., the Argentine ant is a major pest in urban, agricultural and
natural environments due to its unicolonial structure which allows the formation of
supercolonies with multiple shared nests (Newell and Barber 1913, Suarez et al.
1999, Tsutsui et al. 2000, Vega and Rust 2001, Tsutsui and Suarez 2003). Because
these ants were introduced and have lost genetic diversity, they lack intraspecific
aggression, allowing populations to increase rapidly (Newell and Barber 1913, Holway
et al. 1998, Holway 1999, Human and Gordon 1999, Tsutsui et al. 2001, Suarez et al.
2002), which provides a competitive advantage to L. humile when competing against
native ants, other introduced ant species (Kabashima et al. 2007), and other insect
and arthropod species (Holway et al. 1998, Holway 1999, Tsutsui and Suarez 2003).
Other factors in the success of L. humile include sociotomy, polydomy, polygyny, lack
of natural enemies, and human dispersal, mostly through commerce and various other
business operations (Holway et al. 1998, Holway and Suarez 1999, Holway and Case
2000, Vega and Rust 2001, Tsutsui and Suarez 2003).
Control of Argentine ants has typically relied on chemicals, more specifically, slow-
acting baits and perimeter sprays (Vega and Rust 2001). Klotz et al. (2007) found that
slow-acting fipronil sprays reduced ant activity by 90% in an 8-wk period; however,
using a perimeter spray of fipronil and a perimeter-broadcast of bifenthrin granules
achieved the greatest reduction of ant activity around structures. Rust et al. (2003)
proposed that the broader the range of concentrations in insecticidal baits, the more
effective the control because of delayed toxicity. However, they also noted that finding
suitable bait bases and active ingredients that provide delayed toxicity are the most
difficult obstacles to overcome when formulating effective Argentine ant baits. Klotz et al.
(1995) suggested that the lack of information on Argentine ant biology has contributed
to the failure of most traditional chemistries to successfully control Argentine ants.
According to Curtis et al. (1990), a large number of plants that yield essential oils are
known to be feeding deterrents to insects and other arthropods. Some of these plant-
extracted oils have been the starting point for some commercially-produced repellents.
The long-term repellent characteristics of essential oils have been challenged by some
(Buescher et al. 1982, Rutledge et al. 1983, Nerio et al. 2010), who reported efficacy
when freshly applied, but reduced effectiveness as the oils began to age. Thus, the
objective of this study was to evaluate the repellent effects of freshly-applied and aged
essential oils on Argentine ants, using a choice-based assay similar to that of Ebeling
et al. (1966).

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PROCEEDINGS OF THE 2010 NCUE, PORTLAND, OR
Seventeen treatment combinations (five oils at three concentrations each plus two
controls) were evaluated for their repellency to Argentine ants. Spearmint, wintergreen,
peppermint, cinnamon, and clove oils were acquired from Polarome International
(Jersey City, NJ). With the exception of spearmint (60% purity), all oils were technical
grade. Each oil was serially diluted in n-hexane to produce 5 ml of 10%, 1% and
0.1% (v/v) solutions. Two additional treatments (n-hexane alone [negative control] and
1% Cinnamite™ (cinnamaldehyde) suspension in water [positive control] [Mycotech
Corporation, Butte, MT]) were prepared and used as controls. Each of the 17 treatment
combinations were replicated 20 times with four replicates initiated each week over a
12-wk period.
In this bioassay, repellency was indicated by the number of Argentine worker ants
entering the treated harborages, with lower numbers indicating avoidance of the
harborage by the worker ants and, thus, relative repellency of the active ingredient.
The largest number of ants recovered (approx. 82%) was from the hexane-treated
harborages, which served as a negative control in these assays. There also was no
difference in the number of ants recovered from the hexane treatment after 2 h of
aging vs. 168 h of aging, thus, confirming complete hexane evaporation from the
Castone surface within the initial 2-h period. Cinnamite™, the positive control in these
assays, was highly repellent to Argentine ants when the workers were placed with the
harborages treated only 2 h earlier. However, significantly greater numbers of ants
entered the treated harborages 168 h after treatment (approx. 41%) in comparison
to only 4% entering the harborages only 2 h after treatment. Cinnamite™ clearly lost
repellency within 7 d of application.
Repellency and residual activity of the five essential oils were concentration dependent.
At 0.1%, the numbers of ants entering the harborages treated with the oils 2 h earlier
were significantly less than numbers of ants entering the hexane-treated harborages,
but these numbers were also greater than the numbers of ants recovered from
the Cinnamite™-treated harborages. The percentages of ants recovered from the
harborages were 82.4% for hexane (negative control), 58% for peppermint, 38.8% for
cinnamon, 35.2% for clove, 34.8% for wintergreen, 25.2% for spearmint, and 4% for
Cinnamite™ (positive control). At the higher concentrations, the percentages of total
ants recovered from the harborages treated with the essential oils ranged from 8.8%
to 14.4% at the 1% concentration and from 9.2% to 20% at the 10% concentration,
while the percentage recovered from the hexane-treated harborages was 82.4%.
At 0.1%, all essential oils tested exhibited significant declines in repellency within
7 d of application to the harborages. At 7 d after application, only the spearmint oil
treatment had significantly fewer ants entering the treated harborage than entered
the hexane-treated control. At 1%, only wintergreen oil showed a significant decline
in repellency within 7 d of application. However, the numbers of ants entering the
harborages treated 7 d earlier with the five oils, including wintergreen, was significantly
less than the numbers of ants entering the hexane-treated harborages. There were
no reductions in repellency over the 7-d period of any of the five essential oils when
applied at a 10% concentration to the harborages.

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PROCEEDINGS OF THE 2010 NCUE, PORTLAND, OR
These results, although only laboratory-based at this time, indicate the potential of
using plant-derived essential oils to repel Argentine ants from harborages. Essential
oils have been used since ancient times in many aspects, including pest control. They
have excellent repellent, toxic and/or fumigant effects toward insects that are of medical
(Buescher et al. 1982, Rutledge et al. 1983, Isman 2000, Omolo et al. 2004, Yang and
Ma 2005, Amer and Mehlhorn 2006, Jaenson et al. 2006), agricultural (Curtis et al.
1990, Zhang et al. 2004, Wang et al. 2006, Koschier et al. 2007), stored product (Hori
2003), and structural or urban importance (Appel et al. 2001, Meissner and Silverman
2001, Peterson et al. 2002, Meissner and Silverman 2003, Cheng et al. 2007, Wiltz et
al. 2007). Because of the recent ecologically friendly movement away from pesticides
to naturally-derived alternatives, essential oils are becoming increasingly popular
among consumers who want lower impact substitutes to traditional chemistries, with
the same reliability of control provided by chemical insecticides or repellents (Isman
2000).
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