‘Croak down’ on mosquitoes, say ecologists: Drop in frog and odonate populations helps these disease-causing insects

A Lily Squatter. Photo: M Yuvan   

We can do without the background music they score for dripping-wet, monsoonal nights. But we cannot do without them. Frogs croaking from overnight puddles and nearby ponds may intrude on our sleep, but they are a pre-eminent ally against another intruder, one that upsets not only sleep but health.

Frogs have a taste for mosquitoes, and make short work of them, and that includes mosquito larvae and eggs. Along with the odonates — the dragonflies and damselflies — these amphibians are a pest control agency that offers a pro bono service working effectively from home. They are helpful neighbours, but we are “cementing them out”, note ecologists and naturalists against the backdrop of disturbing dengue outbreaks in parts of the county.

Around naturalist M Yuvan’s homestead on Perungudi Station Road in Velachery, the bullfrogs should have put in an appearance by now “serenading” him at night, but instead, their silence is keeping him awake and worried.

“Until three years ago, I used to hear bullfrogs — both the Indian bullfrog (Hoplobatrachus tigerinus) and Jerdon’s bullfrog (Hoplobatrachus crassus) — in my locality in Velachery. From my observation, the bullfrogs are the last to emerge, doing so after the rains have picked up momentum. In other places, some friends have seen and heard them; but in Velachery where I live, I have not.”

A Painted Frog. Photo: M Yuvan

A Painted Frog. Photo: M Yuvan  

Yuvan remarks that on this section, development is moving at a speed equivalent to an equine’s four-beat-gait, carpeting anything in its path with concrete.

Invariably, in almost all cases, the juggernaut cannot be stopped. However, with some thoughtfulness, a mid-way house between development and ecology can certainly be reached, assert ecologists.

Conservation biologist at Society of Odonate Studies (SOS) and consultant at WWF-Kerala, Sujith V Gopalan has done his doctorate on amphibians at western ghats, and he notes how frogs are unlikely to disappoint us if we left a small patch alone for the cause of urban biodiversity.

“Not only do frogs take to ponds and lakes, but are also drawn to microhabitats. Each frog species has its own microhabitat preferences,” says Thiruvananthapuram-based Sujith and goes on to pluck out a couple of examples from his coordinates. “There are even frogs that live on trees — Polypedates maculates, commonly called the Indian tree frog, is a ready example. There are tiny bush frogs such as the Wayanad bush frog, which are very common and if you have some shrubs around the house, they will display an ability to survive in there. That is not happening. We put out those tiles all around us and do not even let a tiny patch of grass to grow around us. There is no space for these organisms to survive.”

The symphony of the widely distributed common Indian toad continues, but with subdued intensity. Though there are no numbers in India to back the belief that frogs and toads are dwindling in urban spaces, Yuvan says ecologists see it as a certainty, guided by their qualitative experience.

While concretisation is certainly making the amphibians and odonates feel unwelcome, that is hardly ploughing a lone furrow. It is working in cahoots with at least one another cold-blooded assassin — polluted waters.

“Sewage will also affect frog population. The frog is such a clean creature that at the slightest hint of pollution, it will vanish, and that makes it one of the best biological indicators. Frogs do cutaneous respiration. Some of their respiration is done through their nose, but much of it through their skin, and they are hugely sensitive to polluted water. In Patashala (run by the Krishnamurti Foundation of India at a location near Chennai) there used to be farmer — Kaniappan — who would say, ‘If a lake is silent, do not drink from it.’ What he would be implying by that is: Pesticide would have ended up there, and it would be totally devoid of frogs,” says Yuvan.

“There are a whole lot of aquatic creatures that need very healthy wetlands, which are important to their and our health. You take water scorpions, diving beetles, bat spinners, water skaters and whirligig beetles — all these are predatory insects feeding right from sludge worms to mosquito larvae. They need very clean water.”

Odonates are also a nemesis to mosquitoes and they win their battles in water, as their larvae get the better of the latter. “Dragonflies and damselflies lay their eggs in water and they hatch out in water; and the larvae feed voraciously on mosquito larvae,” says Sujith.

As with many other beneficial insects, a good number of odonate species come with an easily bruised sensitivity to polluted water.

“There are a few dragonflies — for instance, ditch jewel (Brachythemis contaminata), scarlet skimmer (Crocothemis servilia) and Senegal golden dartlet (Ischnura senegalensis) — that may be seen in slightly polluted waters. But there are dragonflies — for instance, blue river damsel (Pseudagrion microcephalum), Coromandel marsh dart (Ceriagrion coromandelianum), cloudwing (Tholymis tillarga) and wandering glider (Pantala flavescens) — that need clean waters,” observes Yuvan, adding that all these odonate species are found in Chennai.

“The catch here is that the problem-causing insects like mosquitoes and midges can survive in any water. Put them in sewage also and they will survive. There is no competition or predation. The beneficial creatures need clean water.”

The dreaded mosquito species Aedes aegypti is known to breed in water that has collected in containers, tyres and any cistern that can keep water. On account of that understanding, there is an inclination to identify this species entirely with clean water, but observations and studies have shown that they do tolerate polluted waters.

Sujith notes that in pastoral settings marked by farming, pesticides in water can prove deleterious to amphibian and odonate populations. “In rural areas, frogs may be thriving, but in water there is no control. When large amounts of pesticide enter water, these creatures are more affected than are mosquitoes.”

Sujith in fact strongly believes that odonates and amphibians have reduced in both urban and rural settings, and “there is little natural control against mosquitoes.” He underlines the implication with a calculated guesstimate: “To give a picture, if a mosquito lays 100 eggs, there is now a chance that 98 of them hatch and the adults also survive, because there is no natural predators for them.”

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Printable version | Jan 20, 2022 2:05:14 PM | https://www.thehindu.com/sci-tech/energy-and-environment/croak-down-on-mosquitoes-say-ecologists-drop-in-frog-and-odonate-populations-helps-these-disease-causing-insects/article37732920.ece

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