One mountain in Brazil is home to a surprising number of these parasitic wasps

Thought to thrive in milder climates, many species of Darwin wasps apparently love the tropics too

A pinned speciment of a Darwin wasp with a brilliant blue body.

The brilliantly blue Pimpla caerulea is one of the Darwin wasps that researchers found living on the slope of a Brazilian mountain.

Kari Kaunisto/Biodiversity Unit/University of Turku

The tropics are teeming with life, tending to hold far more species than milder environments closer to the poles. But one group of insects, the Darwin wasps, were thought to buck that trend.

Researchers who compared wasp diversity in the United Kingdom and the United States with tropical areas in the 1970s and ’80s concluded that these wasps were most diverse at mid-latitudes — say, Kentucky or England. But others thought that people just weren’t looking hard enough in the tropics.

It’s easy to look for wasps in a British garden, says Peter Mayhew, but “it’s very hard to do long-term work” in a tropical rainforest. Mayhew, a biologist at the University of York in England, was up to the challenge.

Now, after years of sifting through wasps collected from a single mountain in the Brazilian Atlantic rainforest a decade ago, Mayhew and colleagues have identified nearly 100 Darwin wasp species. The result, published November 7 in the journal Insects, suggests that the tropics are home to far more types of the wasp than was previously recognized.

Darwin wasps are one family of parasitic wasps, the Ichneumonidae, which lay their eggs on or inside other creepy-crawlies so that the hatched larvae have a ready-made meal (SN: 7/28/56; SN: 8/5/15). In this way, the Alien-esque wasps help control the populations of their prey, serving a vital ecological role similar to that of apex predators like wolves and sharks. With 25,000 described species, there are more kinds of Darwin wasp than there are known mammal and bird species combined.

A light-colored tentlike trap to catch insects is strung between plants in the forest of a mountain in Brazil.
Using an array of Malaise traps (one shown) placed up and down a Brazilian mountain’s slope, researchers determined how Darwin wasp diversity changes with elevation.Peter Mayhew

To understand where the wasps might live in the tropics, Mayhew and his Brazilian colleagues took a hike, several in fact. The team trekked up a mountain in Brazil’s Serra dos Órgãos National Park, placing pairs of traps known as Malaise traps at 15 sites along the way. The path started along a road, which made carting the traps — each consisting of a tent and a jar of alcohol weighing about a kilogram — relatively simple.

After that, though, the team had to carry them by hand up through the jungle. “The first hour is pure hell,” Mayhew recalls. “Very steep, very hot and very humid.” After laying the traps, members of the team returned every month for one year to swap the jars for fresh ones.

The rainforest is, unsurprisingly, full of critters, which led to each alcohol jar becoming what Mayhew calls “insect soup.” To simplify things for now, the researchers chose to focus on only half of the sample jars and look specifically for one subfamily of Darwin wasps called the pimplines.

With the help of an “army of undergraduates” and taxonomist Ilari Sääksjärvi of the University of Turku in Finland, the team identified 98 pimpline species, only 24 of which have been previously described and named. What’s more, there tended to be fewer pimpline species as researchers scaled the mountain — but the higher-elevation species weren’t found lower down. For comparison, the British Isles have 109 known pimpline species, and wasp diversity there has been sampled much more than in Brazil.

Based on this study, Mayhew says, middle to low elevations in the tropics could be targeted for protection in order to conserve the most Darwin wasp diversity there and preserve the insects’ key ecological role. “[At] 1,500 meters and below is where you get quite a lot,” he says.

The new research is “more evidence that, yes, this wonderful group of really interesting wasps has this amazing unexplored diversity in the tropics, and we need more people working on them,” says Laura Timms, a conservation biologist at Canada’s Credit Valley Conservation in Mississauga who was not involved with the research.

Mayhew next hopes to examine other types of Darwin wasp that were collected in the insect soup to see if they are as diverse as the pimplines.

The wealth of creatures collected in the traps could also be useful to researchers interested in other insects too, Mayhew says, such as leaf beetles and fireflies. “Quite a lot of what’s in those bottles is going to be new to science.”

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