Why Fly into a Forest Fire?

The things a man will do for a wasp. Especially for a wasp rarely encountered and known to have odd habits. Nathan Schiff decided he wanted to collect a wasp that lays eggs on wood–freshly burned wood. To find it, Schiff was going to have to run into a forest fire.

Firefighters turned out to have concerns about someone darting among them as they battled big, uncontrolled blazes. Schiff, an entomologist at the U.S. Forest Service Southern Research Station in Stoneville, Miss., had to settle therefore for official permission to enter fire scenes only after the peak of the action, during the firefighters’ mop-up phase. At this point, smoke still billowed across the scene and occasionally a small branch exploded into flame, but Smokey clearly had already won.

Starting in 1996, Schiff spent two weeks each August in northern California and Oregon, scouring fire sites for the wasp, Syntexis libocedrii. “It took me more than a year and 11 fires before I found my first two,” he says. “At my 13th fire, I found hundreds and hundreds.”

The species turns out to be not so much of a rarity as just a real pain to collect, he says. An entomologist has to be at the right fire at the right time, but so far, no one can know ahead of time what will be right.

Despite Schiff’s success in finding the allegedly rare wasp, he continues to chase fires. He’s found more than a dozen species of fire-loving, or pyrophilic, insects that rush into the smoke in a red-hot hurry to start families. “I was hooked on pyrophiles,” he says.

That’s how he joined the international community of researchers who follow the insects that follow the fires. Their flaming passions encompass both finding new members of the fire-bugs club and understanding the insects’ infrared-sensing organs, which are so unusual that the U.S. Air Force is trying to mimic them. And Schiff and his colleagues are also investigating why insects flock to fire zones in the first place.

Pyrphilic insects

Studying pyrophilic insects has never been easy. Naturalists had long observed the bean-size, black Melanophila beetle converging on fires from miles away. Not until the mid-1960s did anyone figure out how the beetles located the fire. Physiologist William G. Evans, now an emeritus professor at the University of Alberta in Edmonton, finally published the answer to the puzzle: a pair of infrared sensors in little pits on each side of the beetles thorax.

Details have continued to emerge, and scientists now have a good picture of the beetles fire-finding technique. Each pit bristles with some 70 sensory fingers, called sensilla. Infrared radiation strikes a sensillum and heats a little sphere inside it. The sphere expands and presses against a nerve cell, which then yells, “Fire!”

The beauty of this system comes from its superb performance at room temperature, explains bioengineer Ashley J. Welch at the University of Texas in Austin. People have invented some pretty decent infrared sensors, but the really sensitive ones need to be cooled well below room temperature.

The Air Force is funding a network of research labs, included Welch’s, to study infrared sensing in animals. The consortium focuses on two groups of animals–Melanophila beetles and pit vipers–that have notable infrared sensitivities at room temperature. The snakes use their sensors to locate warm-blooded prey.

Welch and his students have been refining measurements of the beetles’ feats, and he sounds skeptical about the folklore that Melanophila sense infrared radiation from 50 kilometers away. Beetles may be flying that far to reach a fire, but it’s not clear what sense they use in the early stage of their quest. For infrared distance-sensing, “our estimates are more conservative,” Welch says, without getting specific.

These beetles also carry fine smoke detectors on their antennae, a team of European scientists reported in 1999. Stefan Schütz of the Justus Liebig University in Giessen, Germany, and his colleagues monitored nerve responses from a beetle antenna hooked to a gas chromatograph. The researchers puffed smoke from burning pines through the chromatograph and watched to see which individual components of the smoke kicked up a response in the beetle antenna. In the most dramatic response, just a few parts per billion of a substance called guaiacol set off the signal, the researchers found.

Just this year, a lab with Air Force funding reported another marvel of beetle sensitivity. The hefty black Australian beetles called Merimna atrata, which are about 2 centimeters long, sport infrared sensors, too. But they’re a different kind, Helmut Schmitz and his colleagues at Friedrich Wilhelms University in Bonn, Germany, report in the December 2000 Naturwissenschaften.

These beetles fill roughly the same ecological niche in Australia as Melanophila do in the Northern Hemisphere. At dawn and dusk, the beetles swarm to fires, where the insects mate, and females tuck the next generation under the bark of seared trees.

The receptors on these Australian beetles dont look like the hair-filled pits of Melanophila. Instead, the researchers found a pair of small, sensitive spots on each of two abdominal sections. On the surface, the patch wrinkles into a miniaturized honeycomb and contains some 800 endings of a branching nerve.

This nerve is of the type that other insects without the honeycomb patches use to sense heat. In contrast, Melanophila‘s heat sensors come from a different group of insect nerves that often detect mechanical disturbances in bristles.

Schmitz and his colleagues wired the spots to monitor nerve activity. Neither hands clapping, a gentle tickle with a bristle, nor a puff of air evoked any response. Temperature changes did, however. The researchers saw the nerve fire after they warmed the spot with a laser, light from a light bulb, or infrared radiation.

The newly discovered mechanism more closely resembles infrared-sensing pits in snakes, such as rattlers, than the Melanophila sensors, Schmitz says. The two beetle species must have derived from different evolutionary paths, he concludes.

Wood-boring beetles

Although the Air Force is intrigued by the special infrared sensors, insects may not need them to find a fire, says taxonomist Richard L. Westcott, who specializes in the beetle order that includes Melanophila and Merimna. These wood-boring beetles focus on stressed plants, which might be recognizable by chemical signatures that they release into the air. Getting burned alive in a forest fire is pretty stressful, Westcott points out.

Schiff notes that a burning tree sends up a rich smoke of gases, several of which could easily serve as come-hither signals. “Insects work with smells, and fires are smelly,” Schiff says.

For example, no one’s yet found infrared sensors on the pyrophilic insect Xenomelanophila miranda. Primarily for that reason, Westcott and other taxonomists have banished the species from its former place in the Melanophila genus. Yet both Westcott and Schiff are intrigued by the beetles interest in fire.

Only a few specimens of this species had been collected before 1999, and, in Westcotts words, “collectors go insane over them.” He was ecstatic to discover that at an Oregon fire in 1999, Schiff found 57 specimens. There had been hundreds.

So far, Schiff has documented some 20 species of insects in the Pacific Northwest that seem especially attracted to fires. “There’s a guild of insects that follow fires,” he says.

Some, like Merimna and Melanophila, rush to a fire, mate, and then lay eggs in the smoking trees. Others seem to be camp followers, such as a species of a robber fly that preys on the Melanophila beetles.

Plenty of questions come to mind about all these various species’ sensory systems, life cycles, and even their numbers and distributions. Yet there aren’t plenty of answers. “All of this work is obscenely time-consuming,” Schiff points out. One weekend, and not an unusual one at that, he commuted to three fires and put 1,300 miles on his car. In 5 years, he’s attended 28 fires.

Besides being dirty and dangerous, it’s hard work. Schiff grumbles, “If I were setting fires myself, they’d be a lot closer to the road.” The species he studies may be flying in, but the scientist has to hike.

Despite these difficulties, Schiff’s starting to address the basic question of just why pyrophilic insects seek fires. In a poster presentation last December at the joint annual meeting of two entomological societies, Schiff speculated that fires could provide a great way for widely dispersed bugs to meet a mate.

A typical siricid wasp, one of the species that Schiff monitors, lays perhaps 100 eggs in a single tree. The young emerge en masse several years later in need of mates. A great way to find some that aren’t siblings is to follow some smoke. “I think of forest fires as nightclubs for these bugs,” Schiff says.

Another advantage to pyrophilia, Schiff speculates, could be access to fire-sale bargains that provide resources for the family. The wasps he studies look for trees in trouble, and a fire means a lot of fresh material available to those who act fast.

Schiff has documented such eager action. “Ive seen wasps land on wood that’s still so hot it burns their feet off,” he says.

The payoff for getting to vulnerable wood quickly may be very high. Schiff is following up on some older research showing that siricid-wasp larvae cant digest wood fiber. Their mothers get around this problem by squirting a fiber-digesting fungus into the wood when laying eggs. To ensure that the youngsters will get food, the mom must find a weak tree that won’t fight off the fungus.

Could wasps and their fungus have evolved a mutually beneficial partnership? Schiff’s been collecting the fungus at fires, and he’s noticed that it doesnt fruit readily. Maybe it doesnt have to, he muses, because the wasps carry it from tree to tree and thus do the work of dispersing it.

Should anyone come across any of these insects at the scene of a fire, Schiff wants to correct some misconceptions. Contrary to firefighter lore, siricid wasps don’t sting. However, Melanophila beetles gathering at a fire do nip people, and yellow jackets, perhaps disoriented in their search for their nests, often sting firefighters. Veterans’ warnings to rookies about the siricid wasps stinging or laying eggs in human legs belongs in the category with snipe-hunting tips and other ostensible advice for newcomers.

In a sense, Schiff has joined the guild of fire-followers himself, with all its thrills and frustrations. He says hes learned a lot from Smokey’s crews, who, in spite of their pranks on rookies, are very aware of the insects they meet.

Of course, the most common thing Schiff hears from the firefighters is that, yes, a moment ago they saw one of the bugs that he’s looking for, but doggone it, they just killed it with a shovel.