The diseases transmitted by Aedes mosquitoes, such as dengue, chikungunya, yellow fever, and Zika, are ever-increasing. Rapid and unplanned urbanization adversely impacts various endemic species such as ants and facilitates the breeding of Aedes mosquitoes. We have observed the predatory potential of ants over Aedes eggs in urban breeding habitats, and their impact on Aedes mosquito breeding was determined by a field experiment that mimicked the natural breeding habitats. It was found that 99.4% of eggs were removed from the experimental containers by foraging ants in 4 days. The present study demonstrates the role of ants as a natural regulator, limiting Aedes mosquito breeding.

In the past few decades, the incidence of diseases such as dengue, chikungunya, yellow fever, and Zika transmitted by Aedes mosquitoes has increased several-fold around the globe (Wilder-Smith 2017) despite intensive mosquito control efforts. Out of 400 million arboviral infections, dengue alone contributes to 390 million infections annually with 96 million clinical cases (WHO 2021). An estimated 3.9 billion people in 129 countries are at the risk of acquiring dengue, with 70% of its burden falling in Asia (Brady et al. 2012). The spread of arboviral infections is linked with anthropogenic alterations of the environment such as deforestation, urbanization, and climate change, which favor the breeding of vector mosquito species, e.g., Aedes aegypti (L.) and Ae. albopictus (Skuse) (Hales et al. 2002). One of the main reasons for the increasing incidence of dengue is rapid and unplanned urbanization that offers newer breeding sites for Ae. aegypti and Ae. albopictus among clustered human populations (Kolimenakis et al. 2021). The disturbance in the ecosystem caused by urbanization alters the habitat of various endemic ant species and creates newer habitats and breeding opportunities for newer species such as Aedes mosquitoes (McKinney and Lockwood 1999). Because of their high abundance, their organization as populations, and their feeding habits, ants have a major influence on the biosphere (Way and Khoo 1992). Generally, the mosquito eggs, laid on water or upon the soil or stuck to the container habitat, would appear to be vulnerable to predacious arthropods, even though only a few such predators have been recorded (James 1966). The desiccation resistance and long survivability of Ae. aegypti eggs, up to 62 months and 23 days (Muniaraj 2019), pose additional threat. In the absence of natural predators of Aedes eggs, if all the eggs were allowed to hatch, once the container is filled with water, the resultant adult populations could be devastating for public health. Although a few studies have reported the presence of red fire ants, Solenopsis invicta Buren, around tree holes (Dunn 1926, Buxton and Hopkins 1927, James 1966, Summerlin and Welch 1984, Burnham et al. 1994, Lee et al. 1994), none have detailed their predatory potential on the eggs of Aedes mosquitoes in domestic and peri-domestic discarded containers/tires in an urban environment, where dengue is a major health problem.

In this context, the 1st observational study was conducted as a part of the dengue outbreak investigation in 2017 at Warangal City, Rayaparthy, Mogulapally, Hansaparthy of Warangal District; Hyderabad city of Hyderabad district, and Kasipet, Mandamarry, Hazipur, Mancharial city of Adilabad district of Telangana State, India, during 2017. A total of 1,768 containers were examined for the presence of Aedes larvae and in a radius of up to 1 m around the containers for the presence and movement of ants. If no ants were observed in and around the container, the same was considered negative for ants. The presence of a single ant in the vicinity of 1 m around the container was considered positive for ants. Representative ant specimens were collected from each site and brought to the laboratory for taxonomic identification, using standard keys (Tiwari 1999).

Based on the outcome of the observational study, a field experiment (artificial setup mimicking the conditions of Aedes breeding) was conducted in Aedes-prone selected urban areas of Madurai city, Tamil Nadu State, India, in August 2020 to determine the predatory potential of ants in eliminating Ae. aegypti eggs from the breeding places. In this experiment, 2 sets (A and B) of 10 enamel bowls (Turtle King, Bangalore, India; 7.5 cm high × 14 cm wide, capacity 600 ml) were used in each of the 3 selected sites (Goripalayam-I, Sellur-II, and Anupanadi-III of Madurai city [lat 9°56′10.97″N, long 78°08′09.33″E; elevation 140 m; room temperature of 24–38°C; RH 55–60%], Tamil Nadu, India). A paper strip with approximately 100 eggs of Ae. aegypti was placed in each bowl. In set A (experiment), after placing the eggs, the bowls were kept at 10 different selected locations of site I at a distance of 25 feet. In set B (control), after placing the eggs, each bowl was placed in the center of an enamel tray with water around the bowl and kept nearby the bowls of set A at a distance of 2 feet (60 cm) between the bowls (Fig. 1A, 1B). While the eggs in set A bowls were accessible to predatory ants, the eggs of set B (control) were protected by water in the tray. Although each bowl was examined every 1 h from the start of the experiment to the end for the movement of ants or any other insect predators, the paper strip with eggs from each bowl was examined from 9 to 11 a.m. with a magnifying lens, and the number of eggs was recorded. This daily observation was continued until the complete disappearance of eggs in set A bowls. The continued observation of bowls for the movement of ants did not show any preference in foraging over day or night. The movement of ants around the bowl was noted, photo- and video-graphed. The same experiment was repeated in sites II and III. The day-wise mean number of eggs missing was calculated from the data of sites I, II, and III. In total, in both sets A and B, there were 10 containers each with 100 eggs kept in sites I, II, and III, numbering 6,000 eggs altogether (set A and B = 100 eggs × 10 containers × 2 sets [A and B] × 3 sites = 6,000 eggs). The percentage of eggs missing every day was calculated from the total number of eggs kept on day 1 in sets A and B and a line graph was drawn (Fig. 2); statistical analysis was done to compare the mean data of set A and set B.

Fig. 1.

(A) A ceramic bowl with paper strips containing approximately 100 eggs of Aedes aegypti. (B) Control. (C, D, F) Ants in the process of dislodging the eggs from the paper. (E) On the rim of the bowl, a dislodged egg held between the mandibles being taken away by an ant. (G) Foraging ants in the container with water. (H) The ants searching for Aedes eggs along the rim of water. The black circle, foraging ant; yellow circle, floating ant; red circles, drowned ants. (I) The ant carrying one dislodged egg (red arrow) by holding in its mouth. (J) A dislodged egg gets attached to the hind leg of an ant (black arrow), carried along with the eggs held in its mouth (red arrow).

Fig. 1.

(A) A ceramic bowl with paper strips containing approximately 100 eggs of Aedes aegypti. (B) Control. (C, D, F) Ants in the process of dislodging the eggs from the paper. (E) On the rim of the bowl, a dislodged egg held between the mandibles being taken away by an ant. (G) Foraging ants in the container with water. (H) The ants searching for Aedes eggs along the rim of water. The black circle, foraging ant; yellow circle, floating ant; red circles, drowned ants. (I) The ant carrying one dislodged egg (red arrow) by holding in its mouth. (J) A dislodged egg gets attached to the hind leg of an ant (black arrow), carried along with the eggs held in its mouth (red arrow).

Close modal
Fig. 1.

Continued.

Fig. 2.

Removal of Aedes aegypti eggs by ants in the field experiment.

Fig. 2.

Removal of Aedes aegypti eggs by ants in the field experiment.

Close modal

In the 1st observational study, of 1,768 containers examined, 129 (73%) containers were found positive for ants but free from Aedes breeding, which indicates the possible active elimination of Aedes eggs by foraging ants. Another 40 (22.5%) containers were positive for both ants and Aedes breeding, which indicates that despite the active foraging of ants, some of the eggs remained without being taken away by ants at the time of observation. Only 8 (4.5%) containers had Aedes eggs without the presence of any ants. This indicates that the ants were unable to approach or find the eggs and hence there was a risk of Aedes breeding. The collected ants were identified as Solenopsis invicta, Myrmicaria brunnea Saunders, Diacamma rugosum Le Guillou, and Monomorium minimum Buckley. A few interesting observations were made, such as despite having chances of falling into the breeding water, ants such as S. invicta were constantly trying to dislodge the eggs from the wall of the containers, such as refrigerator trays, in the surveyed houses. In the event of active foraging and dislodging eggs from the walls of containers (Fig. 1G), a few ants fell into the breeding water and drowned (Fig. 1H). A few others had successfully taken the eggs and carried them to their nests (Fig. 1I). In a few cases, the sticky chorion of the egg had stuck to the leg or other body parts of the ants and was carried along with the eggs held between mandibles to the ant hole or -hill (Fig. 1J).

In the 2nd field experiment, the 1st-day observation showed 40.3% of eggs missing from the artificially kept bowls, leaving the containers with 59.7% eggs. The 2nd day witnessed the missing of 39.5% of eggs and only 20.2% of eggs remained. On the 3rd and 4th days, 17.27% and 2.33% eggs were found missing by leaving behind 2.93% and 0.6% eggs, respectively (Fig. 2). It shows that 99.4% of eggs were removed by foraging ants in 4 days. In the control bowls, the eggs were not disturbed by ants due to the water barrier, and no eggs were found missing from days 0 to 4. The Pearson correlation showed that the number of eggs and days were negatively correlated and the difference was statistically significant (r = −0.949, N = 5, df = 4, P = 0.014 < 0.05). The data analyzed through independent sample t-test showed that the difference between the eggs in experimental and control groups was statistically significant (P = 0.003 < 0.05). Observation of the bowls revealed that S. invicta and Myrmicaria brunnea were actively foraging for Aedes eggs, and the contents of the eggs were either consumed directly or the eggs were taken to the anthill (Fig. 1CF). The other 2 species, D. rugosum and Monomorium minimum, which were observed in the dengue outbreak study in Telengana State, were not found in the study sites of Madurai. While the eggs kept in the control bowl could not be accessed by any foraging ants, the eggs kept in the test bowls were actively foraged by the ants, showing that the ants are playing a major role in eliminating the eggs laid by Aedes mosquitoes in domestic/discarded containers. It was also observed that eggs were collected individually without any assistance from other ants to dislodge the eggs from the paper sheet. Some of the bowls with Aedes eggs were not at all visited by any ants during the 1st and 2nd days of the experiment, thus showing that they were not attracted towards Aedes eggs; rather, they found the eggs during random searching. After finding the eggs, the 1st ant returns without taking the eggs, followed by the arrival of many ants to the site of Aedes eggs, showing that the 1st ant could communicate the message to other ants that brought many of them to the site of Aedes eggs. The foraging ants seem to be acting as natural regulator for Aedes breeding, which may subsequently reduce its related health consequences. The urban area with booming housing developments provides more places not only for humans, but also for the Aedes mosquitoes with reduced predator population (Wilke et al. 2019) such as ants. Although ants are considered scavengers, understanding their role in regulating mosquito populations offers a new perspective to design novel vector management and Aedes control strategies. As urbanization grows rapidly across the globe, the basic challenge is to understand how it affects biodiversity and its possible consequences on human health to mitigate the overall outcome.

The technical assistance of V. Asifa, K. M. Agathiyar, and K. Manimaran is greatly acknowledged.

Brady
OJ,
Gething
PW,
Bhatt
S,
Messina
JP,
Brownstein
JS,
Hoen
AG,
Moyes
CL,
Farlow
AW,
Scott
TW,
Hay
SI.
2012
.
Refining the global spatial limits of dengue virus transmission by evidence-based consensus
.
PLoS Negl Trop Dis
6
:
e1760
.
Burnham
KD,
Baldridge
RS,
Duhrkopf
RE,
Vodopich
DS.
1994
.
Laboratory study of predation by Solenopsis invicta (Hymenoptera: Formicidae) on eggs of Aedes albopictus (Diptera: Culicidae)
.
J Med Entomol
31
:
770
771
.
Buxton
PA,
Hopkins
GHF.
1927
.
Researches in Polynesia and Melanesia
.
Lond School Hyg Trop Med
I–IV:1–260.
Dunn
LH.
1926
.
Mosquito bred from dry material taken from holes in trees
.
Bull Entomol Res
17
:
183
187
.
Hales
S,
Wet N de, Maindonald J, Woodward A.
2002
.
Potential effect of population and climate changes on global distribution of dengue fever: an empirical model
.
Lancet
360
:
830
834
.
James
HG.
1966
.
Location of univoltine Aedes eggs in woodland pool areas and experimental exposure to predators
.
Mosq News
26
:
59
63
.
Kolimenakis
A,
Heinz
S,
Wilson
ML,
Winkler
V,
Yakob
L,
Michaelakis
A,
Papachristos
D,
Richardson
C,
Horstick
O.
2021
.
The role of urbanisation in the spread of Aedes mosquitoes and the diseases they transmit—a systematic review
.
PLoS Negl Trop Dis
15
:
e0009631
.
Lee
DK,
Bhatkar
AP,
Vinson
SB,
Olson
JK.
1994
.
Impact of foraging red imported fire ants (Solenopsis invicta) (Hymenoptera, Formicidae) on Psorophora columbiae eggs
.
J Am Mosq Control Assoc
10
:
163
173
.
McKinney
ML,
Lockwood
JL.
1999
.
Biotic homogenization: a few winners replacing many losers in the next mass extinction
.
Trends Ecol Evol
14
:
450
453
.
Muniaraj
M.
2019
.
Extremely long viability of Aedes aegypti (Diptera, Culicidae) eggs stored under normal room condition
.
J Med Entomol
56
:
878
880
.
Summerlin
JW,
Welch
JB.
1984
.
Observations on the red imported fire ant, Solenopsis invicta, (Hymenoptera: Formicidae) in tree hole mosquito breeding sites
.
Mosq News
44
:
589
590
.
Tiwari
RN.
1999
.
Taxonomic studies on ants of southern India (Insecta: Hymenoptera: Formicidae)
.
Mem Zool Surv India
18
:
1
96
.
Way
MJ,
Khoo
KC.
1992
.
Role of ants in pest management
.
Annu Rev Entomol
37:1, 479–503.
WHO [World Health Organization].
2021
.
Dengue and severe dengue
[Internet].
Geneva, Switzerland
:
World Health Organization
Wilder-Smith
A,
Gubler
DJ,
Weaver
SC,
Monath
TP,
Heymann
DL,
Scott
TW.
2017
.
Epidemic arboviral diseases: priorities for research and public health
.
Lancet Infect Dis
17
:
e101
e106
.
Wilke
ABB,
Chase
C,
Vasquez
C,
Carvajal
A,
Medina
J,
Petrie
WD,
Beier
JC.
2019
.
Urbanization creates diverse aquatic habitats for immature mosquitoes in urban areas
.
Sci Rep
9
:
15335
.

Author notes

1

ICMR-Vector Control Research Centre Field Station, No. 4, Sarojini Street, Chinna Chokkikulam, Madurai - 625 002, Tamil Nadu, India.

2

ICMR-Vector Control Research Centre, Medical Complex, Indira Nagar, Puducherry - 605006, India.