Citronella, diethyltoluamide, mosquito net: what people swear by to protect themselves against mosquitoes, frogs fail to do. And yet the wet room dwellers hardly suffer from bloodthirsty pests. Does her blood taste that bad? Or have they developed the ultimate mosquito repellent?
It's warm, humid and the air is buzzing with mosquitoes. It stings and itches everywhere. The hell? No, a paradise - at least for a frog fan, he could be granted a glimpse of one of these wonderful little, blue-legged creatures with a red back and black beady eyes right here. Or the pretty blue-black marbled relative who is dozing away quietly somewhere. But while he tries to scare away the buzzing insects as inconspicuously as possible, but not the colorful four-legged friends, even the greatest amphibian hunter may have a question: Why are his darlings actually spared from the annoying pests?
John Daly, who has been on the trail of dendrobates in the field for more than forty years, probably thought the same thing. Because in all the decades of his research work, he had never encountered a poison dart frog with a mosquito bite. It may well be that a mosquito lands, but the researcher from the National Institutes of He alth in Bethesda knew that it does not seem to be friendly to the blood supply of its intermediate station. Do they, like so many robbers, not like the cocktail of skin secretions that poison dart frogs secrete for their own protection? So maybe the body lotion from our own production can repel not only predators but also parasites and other pests that let mosquitoes live, but weaken them.
However, Daly and his colleagues were not able to fully study the several hundred alkaloids that frogs secrete in this way. They focused on one of the eighty or so pumiliotoxins called PTX 251D, which is found in varying amounts in Dendrobates, Epipedobates or Phyllobates aurotaenia, but also in Malagasy frogs such as Mantella or the South American toads of the genus Melanophryniscus. In studies with tobacco pests, the toxin had poisoned cutlet moth larvae even in the smallest concentrations - a very obvious candidate for a mosquito repellent.
The researchers chose Aedes aegypti, the transmitter of the yellow fever pathogen, as the test mosquito. If no mammal comes in front of her trunk, she accepts frog blood to fill her stomach. If their feeding place was contaminated with PTX-251D, the insects not only lost their appetite. The few who still dared to land and eat also showed clear signs of serious poisoning: they could hardly stand upright while staggering, some animals fell on their backs or amputated their legs themselves, and it was almost impossible to think of a rescue flight. "They looked dying or dead," sums up Daly and Co. To be on the safe side, the scientists didn't even give the animals the choice of whether they wanted to land or not in the next test. They each sucked five mosquitoes into a pipette with a slightly widened tip. Then they threaded a thin wire, which they had previously bathed in the pumiliotoxin, into the narrow glass prison. The mosquitoes couldn't get past the wire, they had to touch it. When they were later allowed to fly freely again, it took them significantly longer to get airborne, the researchers found.
But what about the dose? Were the conditions realistic at all? Definitely, if you take a closer look at the values: The first signs of poisoning appeared at concentrations of 0.1 micrograms of PTX 251D per square centimeter. The two centimeter tall Ecuadorian Epipedobates tricolor has an average of 37 micrograms of it on its skin. With an estimated body area of four to six square centimeters, that makes six to nine micrograms per square centimeter - more than enough to massively taint the meal for mosquitoes. Quite apart from the fact that other substances may also have a defensive effect.
It smells like more laboratory work – and the researchers have already found a new, suitable test participant: Uranotaenia mosquitoes, which routinely feast on frogs. Perhaps Daly and his staff will then find a secretion that could also protect humans from bites. Because a contact poison like PTX 251D, which accelerates the heartbeat and can cause fatal convulsions in mice, doesn't sound like a suitable ingredient. However, it remains to be seen whether frog fans would benefit from this on their tours of discovery: In the sweaty homeland of their darlings, every rubbed-in remedy disappeared within minutes. But what does it matter, face to face with Dendrobates and Co.