Thursday, November 3, 2016

Mosquitofish, the Wrong Answer for Mosquito Control, by Don Orth

Journalist and satirist, H. L. Mencken, once wrote that “For every complex problem, there is an answer that is clear, simple, and wrong.”  I have no doubt that he wrote something more complicated.   We simplified it.  That’s what we do – we search for a simple and painless solution.  Mosquito control is one of those complex problems.  Mosquitoes (family Culicidae) have the potential to transmit malaria, West Nile, Zika, dengue, chikungunya, yellow fever, and other pathogens. Mosquitoes are only one of several vectors involved and not all mosquitoes are alike.  The Asian Tiger Mosquito has spread to 36 states since appearing in the USA in 1985 (Rochlin et al. 2013).   Constant monitoring and control efforts are needed to prevent the spread of mosquito-borne diseases. Even your backyard water gardens get more controversial with rise of Zika virus and West Nile virus.       
Mosquito larvae.  Photo by Napat Polchoke via Getty Images. Source   
The simple control solution is to stock a fish that eats mosquito larvae.  It has been adopted time and time again since the early 1900s.  The first choice was Mosquitofish, Gambusia affinis.   Entomologist Leland O. Howard (1857-1950) was the first to advocate for the use of mosquitofish for mosquito control.  William Seal (1910) first transported mosquitofish from North Carolina to New Jersey for mosquito control.  At the same time, Gambusia affinis were transported to Hawaii, tested on mosquito eggs and larvae, and christened the “greatest mosquito killer in existence” by David Starr Jordan (1926).   From there it went all around the world to save lives threatened by mosquito-borne diseases.  Soon it had wider distribution than any other freshwater fish (Krumholz 1948; Pyke 2008).  The simple solution was widespread long before effectiveness trials were conclusive.

The mosquitofish solution to the problem was and continues to be “clear, simple, and wrong!” Yes, the mosquitofish are superbly adapted to eating mosquito larvae.  But the story is more complicated.   Gambusia are in the Family Poeciliidae, one of the most popular families of fishes used in scientific research.  Poeciliids are commonly referred to as guppies, swordtails, topminnows, and mosquitofish.   Gambusia occur over most of Central America, Mexico, the southeastern USA, and major islands of the Caribbean, where they are differentiated to adapt to desert rivers and springs, subtropical habitats, and marine islands.  Meffe and Snelson (2009) listed 45 extant species of Gambusia, yet only two are invasive.  These are the Eastern Mosquitofish Gambusia holbrooki and the Western Mosquitofish Gambusia affinis.    Click to view a photo of  Eastern Mosquitofish femaleWestern Mosquito fish femaleWestern Mosquitofish male.  The Genus name, Gambusia, was derived from the Cuban term, ‘Gambusino,’ which means "nothing", usually in the context of disappointment or scorn (Krumholz 1948). When an angler returns from fishing without any fish, Cubans reportedly say "been fishing for Gambusinos."
 Gambusia and mosquito larva in a simplified depiction. (Sholdt et al. 1972)
Two important life history traits of the Gambusia are viviparity (they bear live young) and the specialized body form designed for near-surface feeding. Louis Krumholz conducted the most thorough studies of Gambusia affinis in their native range, measuring and sexing 30,093 mosquitofish during the investigation. Young receive nutrition from the mother and incubation lasts 21-25 days.  Therefore, the young suffer low prenatal mortality. The young are precocious and live independent of the parents after birth. Number of offspring produced per female is influenced by female size; Krumholz (1948) counted 10 embryos from a 35 mm female and 315 from a 59 mm female.   Mosquitofish are reproductively active during the warm seasons of the year and females may produce live offspring up to 4 or 5 times per year (Krumholz 1948). Young mosquitofish may reach sexual maturity in under 4 weeks.  All these traits makes initial population growth after introduction explosive. 

Length (mm)  frequency of one sample of female mosquitofish, in July of 1939, from  Argonne Woods Pond, Cook County, Illinois.  Shaded bars represent gravid females. (Krumholz 1948).
Mosquitofish display aggressive behavior toward other fishes and rapidly colonize new habitats.  Males tend to be more aggressive than females and bite and nip at fish and amphibians. Mosquitofish are extreme habitat generalists and typically found along shallow margins of streams, ponds, and wetlands. The genetic variability in two common Gambusia species is among the highest found for vertebrates (Smith et al. 2009).   Furthermore, local populations are adapted to different conditions via drift and dispersal mechanisms.   Hence, widespread stocking is not a wise action.  

While they eat mosquitofish larvae, mosquitoes are not an exclusive food.  Hence, the failure of the clear and simple solution arises from the unfortunate side-effects.  The simple facts that they can consume 80% of their body weight each day or can eat between 71 and 463 mosquito larvae per day (Chipps and Wahl 2004) encourages continued interest in use of Gambusia for mosquito control. Under some circumstances mosquitofish control is moderate at best (Cech and Linden 1987; Bence 1988).   However, when mosquitoes are reduced, other mosquito predators will be eaten. If there is a heavy, matted growth of algae, larvae will be protected from mosquitofish. If the pond has zooplankton (and most do), consumption of mosquito larvae drops substantially as the mosquitofish feed on the cladocerans, ostracods, copepods (Bence 1988).  As a consequence of the flexible feeding of the mosquitofish, mosquito populations rebound after an initial reduction.   Further, the high consumptive food demand of the mosquitofish at warm temperatures leads to many problems with effects on non-target animals.  Marshall Laird, from the Memorial University of Newfoundland, in 1977, wrote: "Time has proved that mosquitofish eventually became harmful in some areas to which they were introduced half a century ago --- the harm ranged from eating the eggs of economically desirable fish, to endangering rare indigenous species.” (Laird 1977, p 336).  
Distribution maps of Gambusia affinis (left) and Gambusia holbrooki (right). Source USGS 
Eating mosquito larvae is one thing.  Controlling mosquito populations is quite another.   Ineffectiveness of mosquitofish for mosquito control has been cited by numerous authors.  In fact, in many cases the introduction of mosquitofish reduced or eliminated populations of native mosquito predators.  They may be named “mosquitofish” but their aggressive behavior means they will eat eggs and larvae of fish, insects, and amphibians.   Numerous trials have been done and many introductions of mosquitofish have occurred outside their native range.   The evidence continues to support Marshall Laird’s contention that mosquitofish eventually prove more harmful than good on all continents (Bence 1989; Arthington 1991; Rupp 1996; Arthington and Lloyd 2009; Courtney and Meffe 2009).  Numerous fish farms continue to sell live mosquitofish, even through Amazon.com.   Why are mosquitofish in use today?   Some states do not restrict them because they are native species. Elsewhere, mosquitofish still have public relations value.   

Do you have nuisance mosquitos to control?  Look to native fishes; in many trials native fish have proven more effective at reducing mosquito by eating their larvae.   Small sunfish and catfishes are natural mosquito predators. Locally, you can try Fathead Minnows Pimephales promelas (Irwin and Paskewitz 2009).  The name does not suggest it will be the greatest mosquito killer in existence.  But, the Fathead Minnow is widely distributed from the Rocky Mountains to the Appalachian Mountains.  You may use mosquitofish in closed container water gardens, but if there are other fish in your small ponds, you don’t want to add mosquitofish.

The search for a feasible and rational control method continues. Bacterial larvicides, such as Bacillus sphaericus Neide (VectoLex), are expensive alternatives. but Culex mosquitoes develop resistance after repeated applications.  Currently Miami Beach is asking the Food and Drug Administration for emergency permission to release genetically modified mosquitoes in response to emergence of the Zika virus.  Another innovation is the use of nanoparticles, e.g. silver nanoparticles, to reduce mosquito populations without detrimental effects of fishes (Subramaniam et al. 2015).   

My message is clear and simple. Do not introduce mosquitofish outside their native range. Use native fish for mosquito control.  For more information on mosquitos and mosquito control, consult the American Mosquito Control Association, or your county extension agent. 

References
Arthington, A. H. 1991. Ecological and genetic impacts of introduced and translocated freshwater fishes in Australia. Canadian Journal of Fisheries and Aquatic Sciences 48 (Suppl. 1): 33-43.
Arthington, A.H., and L.N. Lloyd. 2009.  Introduced Poeciliids in Australia and New Zealand.  Pages 333-348 in G.K. Meffe and F.F. Snelson, Jr., Editors. Ecology and evolution of livebearing fishes.  Prentice-Hall, Inc., Engelwood Cliffs, New Jersey.
Bence, J. R. 1988. Indirect effects and biological control of mosquitoes by mosquitofish. Journal of Applied Ecology 25:505-521.
Cech, J.J., Jr., and A.L. Linden. 1987.  Comparative larvivorous performance of mosquitofish, Gambusia affinis, and juvenile Sacramento blackfish, Orthodon microlepidotus, in experimental paddies.  Journal of the American Mosquito Control Association 3:35-41.
Chipps, S.R., and D.H. Wahl. 2004. Development and evaluation of a western mosquitofish bioenergetics model. Transactions of the American Fisheries Society 133 (5):1150-1162.
Congdon, B. C. 1994. Characteristics of dispersal in the eastern mosquitofish, Gambusia affinis. Journal of Fish Biology 45: 943-952.
Courtenay, W. R. & G. K. Meffe. 1989. Small fishes in strange places: a review of introduced poeciliids. Pp. 319-331, in: G. K. Meffe & F. F. Snelson (eds.), Ecology and evolution of livebearing fishes (Poeciliidae). Prentice Hall, New Jersey, 453 pp.
Irwin, P., and S. Paskewitz.  2009. Investigation of fathead minnows (Pimephales promelas) as a biological control agent of Culex mosquitoes under laboratory and field conditions.   Journal of the American Mosquito Control Association 3:301-309.
Jordan, D.S. 1926. Malaria and the mosquitofish.  Scientific American 1926:296-297.
Krumholz, L. A. 1948.  Reproduction in the western mosquitofish, Gambusia affinis affinis (Baird & Girard) and its use in mosquito control.  Ecological Monographs 18:1-43.
Laird, M. 1977. Enemies and diseases of mosquitoes. Their natural regulatory significance in relation to pesticide use, and their future as marketable components of integrated control. Mosquito News 37:331-339.
Nico, L., P. Fuller, G. Jacobs, M. Cannister, J. Larson, A. Fusaro, T.H. Makled and M. Neilson. 2016. Gambusia affinis. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
Nico, L., and P. Fuller. 2016. Gambusia holbrooki. USGS Nonindigenous Aquatic Species Database, Gainesville, FL.
Pyke, G.H. 2008. Plague minnow or mosquito fish? A review of the biology and impacts of introduced Gambusia species.   Annual Review of Ecology, Evolution, and Systematics 39:171-191.
Rochlin, I. D.V. Ninivaggi, M.L. Hutchinson, and A. Farajollahi. 2013.  Climate change and range expansion of the Asian Tiger Mosquito (Aedes albopictus) in northeastern USA: implications for public health practitioners.  PLOS ONE       
Rupp, H.R. 1996.  Adverse effects of Gambusia affinis.   Journal of the American Mosquito Control Association 12(2):155-166. 1996.
Seal, W.P. 1910. Fishes in their relation to the mosquito problem. Bulletin of the U. S. Bureau of Fisheries 28: 831-838.
Sholdt, L.L., D.A. Ehrhardt, and A.G. Michael. 1972. Guide to the Use of Mosquito Fish, Gambusia, for mosquito control.   Navy Environmental and Preventative Medicine Unit No. 2, Norfolk, Virginia.   18 pp.
Smith, M.H., K.T. Scribner, J.D. Hernandez, and M.C. Wooten.  2009.  Demographic, spatial, and temporal genetic variation in Gambusia.  Pages 235-257 in G.K. Meffe and F.F. Snelson, Jr., Editors. Ecology and evolution of livebearing fishes.  Prentice-Hall, Inc., Engelwood Cliffs, New Jersey.
Subramaniam, J., and 16 coauthors.  20q5.  Eco-friendly control of malaria and arbovirus vectors using the mosquitofish Gambusia affinis and ultra-low dosages of Mimusops elengi-synthesized silver nanoparticles: towards an integrative approach?  Environmental Science Pollution Research 22:20067-20083.

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