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Pheidole megacephala (Bigheaded Ant)

California Pest Rating for
Pheidole megacephala (Bigheaded Ant)
Hymenoptera: Formicidae
Pest Rating: A

Initiating Event:

On April 18, 2014, Dr. Rosser Garrison identified samples of ants collected at a residence in Costa Mesa, CA as Pheidole megacephala, bigheaded ant. The species is currently Q-rated, so a pest rating proposal is needed to determine future direction.

History & Status:

Background: Pheidole megacephala is a “tramp ant” invasive species that has spread around much of the tropical, subtropical, and temperate world where it is typically associated with the disturbed environments created by human habitations. Bigheaded ant primarily nests in soil and is often found nesting in disturbed soil, lawns, flowerbeds, under objects like bricks or cement slabs, flower pots, around trees and water pipes, and along the bases of structures and walkways2. Unlike other ants in the genus Pheidole, bigheaded ant is a major nuisance pest which frequently invades homes in search of food. Like subterranean termites, the ants sometimes build covered foraging tubes on building foundations or shrubs2. Bigheaded ants are omnivorous. Outdoors, they typically feed on honeydew, insects, seeds, and small vertebrates such as bird hatchlings. Indoors they are often found feeding on meat, pet food, oily foods such as peanut butter, and grease on stoves, counters, walls, or dish cloths3. Bigheaded ant invades homes so frequently in southern Florida that it is now considered to be the most common ant that triggers residents to call pest control companies2. Colonies of bigheaded ant have multiple queens and often form “supercolonies” where groups of queens and workers move off to expand the colony2. These large interconnected colonies make control of the ants difficult, as colonies can extend between and across properties. Other ant species are excluded from these areas. The most likely pathway for the long distance spread of bigheaded ant is when colonies in potted plants are moved1,2.

Worldwide Distribution: Pheidole megacephala is believed to be native to Africa1. From there it has spread to parts of Asia, Oceania, Central America, South America, Europe, Hawaii, and the Caribbean1. In the continental United States, bigheaded ant has been present in Florida since before 19332.

Official Control: Pheidole megacephala is listed as a quarantine pest by French Polynesia, Japan, and the Republic of Korea14.

California Distribution: In April 2014, Pheidole megacephala was found at a residential property in Costa Mesa. CDFA conducted a visual and SPAM-bait survey of a 5-mile radius around this site. At this time the ants were found to be confined within a 400m radius within the single residential neighborhood and an adjacent golf course. As of December, 2014, bigheaded ants have not been found in the environment of California outside of this neighborhood.

California Interceptions: Pheidole megacephala is commonly intercepted by CDFA. There have been 1,514 interceptions since January 1, 2000.

The risk Pheidole megacephala (bigheaded ant) would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Pheidole megacephala tends to favor cool areas with high humidity3. The ants are likely to do well in coastal California and in irrigated areas elsewhere. Bigheaded ant receives a Medium (2) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Pheidole megacephala is omnivorous and receives a High (3) in this category.

Evaluate the host range of the pest. Score:

– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.

3) Pest Dispersal Potential: Pheidole megacephala reproduces rapidly. Colonies have multiple queens and each queen lays up to 292 eggs each month2. Bigheaded ant spreads relatively slowly naturally, but colonies in potted plants can be transported long distances rapidly. Bigheaded ant receives a High (3) in this category.

Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Pheidole megacephala is likely to injure agriculturally important animals when it tends honeydew producing insects in agricultural systems. The ants are known to tend economically important insects such as Asian citrus psyllid (ACP) (Diaphorina citri)9, green scale (Coccus viridis)4, grey pineapple mealybug (Dysmicoccus neobrevipes), and many others. While the ants are tending honeydew producers they consume predatory insects such as lady bugs and parasitic wasps including Tamarixia radiata9. This can disrupt the biological control component of existing IPM programs4 and allow honeydew producing pest insects to flourish, increasing crop damage and production costs. In southern Florida bigheaded ants are the primary ants that tend ACP9. In experiments they have been shown to greatly reduce the success of biological control with Tamarixia radiata9. For example, on Murraya paniculata bigheaded ants reduced the parasitism of ACP from 20.36% to 0.39%9. Furthermore, in their native range in Africa, bigheaded ants are known to use detritus and soil to build protective shelters over the psyllid Diaphorina enderleini10. These structures offer psyllids protection from environmental threats such as predators and contact insecticides. Bigheaded ants are listed as a quarantine pest by Japan, Korea, and French Polynesia14. The presence of these ants as hitchhikers on a wide variety of commodities may trigger disruptions to California exports. Bigheaded ants have also been documented chewing into drip irrigation systems, which may interfere with the delivery of water for agricultural uses5. Pheidole megacephala receives a High(3) in this category.

There may also be some positive economic impact from the entry of bigheaded ant to California. Bigheaded ants have been shown to help control pest insects in some circumstances by consuming damaging pest insects that do not produce honeydew and replacing them with less damaging honeydew-producing insects7. They also consume termites8.

Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.
B. The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.

5) Environmental Impact: Pheidole megacephala has the potential to cause massive, long-term alterations to natural communities and large-scale changes ecosystem processes. Although they are limited by the absence of water in dry areas, they can be expected to gradually invade most ecosystems and severely affect all native invertebrates. For example, they are well known to displace native ant fauna13. In an Australian rainforest, all insect larvae were found be absent from areas colonized by bigheaded ant12. Bigheaded ants can be expected to consume any threatened or endangered invertebrates that they encounter. The ants are also likely to facilitate the spread of noxious weeds through the environment by feeding on beneficial insects introduced for biological control6. Furthermore, bigheaded ant is also likely to trigger new treatments by residents as it invades homes in search of food and water. Many residents of Florida contact pest control companies to arrange chemical treatments due to infestations of homes by these ants2. Bigheaded ant receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
D. The pest could trigger additional official or private treatment programs.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Pheidole megacephala (bigheaded ant): High (14)

Add up the total score and include it here.

– Low = 5-8 points
– Medium = 9-12 points
– High = 13-15 points

6) Post Entry Distribution and Survey Information: Pheidole megacephala is only known from a single incursion into a neighborhood in Costa Mesa. It receives a Not established (0) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.

– Not established (0) Pest never detected in California, or known only from incursions.
– Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
– Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
– High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (14)


There is much uncertainty with the introduction of exotic ants to California. New species of ants may play a role in slow, long-term changes to our ecosystems that are difficult to observe on a short time scale. Alternatively, there are other species of ants already present in California and new species may have lesser effects. Argentine ant (Linepithema humile) occupies a similar niche to bigheaded ant and is already widely distributed in the state. It is possible that argentine ant may be able to outcompete bigheaded ant under some environmental conditions.

Bigheaded ant has not been observed building dirt shelters to protect ACP like it does closely related psyllids in its native range. However, most places where both ACP and bigheaded ant occur have frequent rainfall that may destroy any shelters. It is possible that the drier climate of California may be more favorable for this shelter-building behavior. If this occurs, it may complicate control of ACP by contact insecticides. If bigheaded ant were to both disrupt biological control of ACP by Tamarixia radiata and build shelters that protect ACP from contact insecticides, this may trigger additional management changes to organic citrus production in California.

Conclusion and Rating Justification:

Pheidole megacephala is likely to have significant economic impacts in California by disrupting biological control components of IPM programs and disrupting exports. This can be expected to increase production costs by triggering additional pest management. Bigheaded ant can also be expected to have significant long-term environmental impacts on the state and is under consideration for official control. An “A”-rating is justified.


1CABI Invasive Species Compendium. http://www.cabi.org/isc/datasheet/40133

2Warner, John and Rudolph H. Scheffrahn. 2013. Featured creatures: big headed ant. http://entnemdept.ufl.edu/creatures/urban/ants/bigheaded_ant.htm

3Northern Territory Government. Big headed (or coastal brown) ant fact sheet. http://www.lrm.nt.gov.au/__data/assets/pdf_file/0018/11268/BigHAnt_Fact-Sheet.pdf

4Reimer, Neil J., Mei-li Cope, and George Yasuda. 1993. Interference of Pheidole megacephala (Hymenoptera: Formicidae) with biological control of Coccus viridis (Homoptera: Coccidae) in coffee. Environmental Entomology 22(2): 483-488. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Interference+of+Pheidole+megacephala&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=1

5Chang, Vincent C.S., Asher K. Ota, and Deborra Sanders. 1980. Parallel ridge barrier to control ant damage to orifices of drip irrigation tubes. Journal of Economic Entomology 73: 403-406. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Parallel+ridge+barrier+to+control&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=1

6Reimer, N.J. 1988. Predation on Liothrips urichi Karny (Thysanoptera: Phlaeothripidae): A case of biotic interference. Environmental Entomology 17(1): 132-134. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Predation+on+Liothrips+urichi&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=1

7Jones, Vincent P., Daphne M. Westcott, Naomi N. Finson, and Roy K. Nishimoto. 2001. Relationship between community structure and southern green stink bug (Heteroptera: Pentatomidae) damage in macadamia nuts. Environmental Entomology 30(6): 1028-1035. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Relationship+between+community+structure&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=4

8Cornelius, Mary L. and J.Kenneth Grace. 1996. Effect of two ant species (Hymeoptera: Formicidae) on the foraging and survival of the Formosan subterranean termite (Isoptera: Rhinotermitidae). Environmental Entomology 25(1): 85-89. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Effect+of+two+ant+species&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=4

9Navarrete, Bernardo, Heather McAuslane , Mark Deyrup and Jorge E. Peña. 2013. Ants (Hymenoptera: Formicidae) Associated with Diaphorina citri (Hemiptera: Liviidae) and their Role in its Biological Control. Florida Entomologist, 96(2):590-597. http://swfrec.ifas.ufl.edu/hlb/database/pdf/21_Navarrete_13.pdf

10Alene, Desiree Chantal, Champlain Djieto-Lordon, and Daniel Burckhardt. 2011. Unusual behavior—unusual morphology: mutualistic relationships between ants (Hymenoptera: Formicidae) and Diaphorina enderleini (Hemiptera: Psylloidea), associated with Vernonia amygdalina (Asteraceae). African Invertebrates 52(2):353-361. http://www.bioone.org/doi/abs/10.5733/afin.052.0210?journalCode=afin

11Jahn, Gary C. and John W. Beardsley. 1996. Effects of Pheidole megacephala (Hymenoptera: Formicidae) on survival and dispersal of Dysmicoccus neobrevipes (Homoptera: Pseudococcidae). Journal of Economic Entomology 89(5): 1124-1129. http://esa.publisher.ingentaconnect.com/search/article?option1=tka&value1=Effects+of+Pheidole+megacephala+%28+Hymenoptera%3a+Formicidae+%29+on+survival+and+dispersal+of+Dysmicoccus+neobrevipes&operator9=AND&option9=publications&value9=esa&sortDescending=true&sortField=default&pageSize=10&index=1

12Hoffmann, Benjamin D., Alan N. Andersen, and Greg J.E. Hill. 1999. Impact of an introduced ant on native rainforest invertebrates: Pheidole megacephala in monsoonal Australia. Oecologia 120: 595-604. http://www.issg.org/database/species/reference_files/phemeg/hoffmann1999.pdf

13Vanderwoude, C., L.A. Lobry De Bruyn, and A.P.N. House. 2001. Response of an open-forest ant community to invasion by the introduced ant, Pheidole megacephala. Austral Ecology 25(3): 253-259. http://onlinelibrary.wiley.com/doi/10.1046/j.1442-9993.2000.01021.x/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false

14USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD). https://pcit.aphis.usda.gov/pcit/

Responsible Party:

Jason Leathers, 1220 N Street, Sacramento, CA, 95814, (916) 654-1211, plant.health[@]cdfa.ca.gov.

 Pest Rating: A

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