Healthy frog populations may help keep disease-carrying mosquitos at bay, but not in the way you might expect.
The recent outbreak of the Zika virus has apparently sparked a demand for live frogs and toads as pets. Online advertisements in Zika virus affected countries are popping up, making claims that the advertised frogs and toads are “anti-dengue [and] zika” pets. But just how valid are these claims? While most adult frogs and toads are indeed voracious invertebrate consumers and may eat an occasional mosquito, it’s their tadpoles that are most likely to reduce populations of disease-carrying mosquitos.
Mosquitos are major vectors of human disease including malaria, dengue, and Zika fever. Hundreds of millions of people are infected by a mosquito borne illness each year resulting in over one million deaths. Because of this, methods of reducing mosquito numbers, particularly around human populations, are in high demand. Pesticides that target adult or larval mosquitos, and biocontrol methods such as predatory fish introductions have been used around the world in an attempt to reduce mosquito numbers, but often have serious negative impacts. Pesticides may cause mortality in an array other wildlife (take DDT for example), and the introduction of the small but aggressive Mosquitofish (Gambusia holbrooki) is linked to amphibian declines. Despite recent advertisements selling frogs “to combat dengue and zika", the mosquito-controlling ability of frogs has received relatively little attention.
Adult frogs eat a variety of things but mosquitoes don’t appear to be an important part of the adult diet of any adult frog or toad. A three-year study in Germany looked at the diet of over two thousand frogs and found that mosquitos made up less than 1% of their diet. A single frog is therefore unlikely to have much of an impact on mosquito numbers, but as healthy frog populations can reach tens of thousands of individuals per hectare in the wild, they might have a substantial impact overall.
Despite the potential impact of frogs and toads in consuming mosquitos, it’s actually their tadpoles that are likely to have the biggest impact on mosquito populations. Mosquitos and frogs breed in many of the same places (pools, puddles, ditches, swamps, and water-filled containers) and often at the same time (the breeding of mosquitos and most frogs peaks during and after heavy rain). It’s here that the real battleground in the fight between frogs and mosquitos occurs- a battle of larvae.
Tadpoles eat a wide variety of items such as detritus, unicellular organisms, algae pollen, fungi, copepods, and even other tadpoles, and while many species probably don’t eat mosquito eggs and larvae, quite a few do.
There’s actually not that much research on the impact of tadpoles on mosquitos, but what does exist indicates that they can have quite an impact. Several Asian frog species that are commonly associated with humans are voracious predators of mosquito eggs, and mosquito larvae are consumed by a diverse array of frog and toad tadpoles from around the world. Some species are really good at it, with Spadefoot Toad tadpoles reportedly “comb[ing] the water industriously at the surface and beneath” for mosquito larvae (Barber & King, 1927). For countries with native salamanders, potential for mosquito control is likely to be even higher, with salamander larvae being such efficient predators of mosquito larvae that they may actually limit the production of mosquitoes in wetlands.
Tadpoles may also compete with mosquito larvae so much that they reduce mosquito larval survival. This competition seems to be largely over food, but may also be via chemical warfare, with tadpoles possibly producing chemicals to slow the growth of mosquito larvae. Tadpoles of one of the most common backyard frog in Australia, the Striped Marsh Frog (Limnodynastes peronii), appear to be doing battle with mosquito larvae in this way.
Having frogs in your neighbourhood may also cause mosquitos to decide to breed elsewhere. Mosquitos have apparently cottoned on to the fact that amphibian larvae are bad news for their offspring and may actively avoid laying eggs in water with tadpoles or salamander larvae.
So, are a few pet frogs likely to help keep potentially disease-carrying mosquito numbers down inside your house? Based upon our current knowledge, this seems unlikely. However, having healthy frog populations breeding in your neighbourhood may. Through a combination of predation and competition, tadpoles are capable of killing mosquito eggs and larvae, and simply by being there, may cause mosquitos to avoid laying eggs in a pond. Because many frog species do live in close association with humans and can breed in bodies of water that fish can’t reach, frogs and other amphibians may play an important yet overlooked role in mosquito control.
Global amphibian declines have had dramatic consequences for ecosystem structure and functioning, but may also have implications for human health. For this reason, ensuring healthy amphibian populations is likely to benefit not just our environment, but also our health.
Dr Jodi Rowley
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