An Inordinate Fondness for Wasps
There are probably more species of them than any other animal group.
When talking about whether theology has anything to learn from science, the British biologist J. B. S. Haldane used to quip that God must have “an inordinate fondness for beetles.”
He had a point. Around 380,000 species of beetle have been described, which accounts for a quarter of all known animal species. There are more species of ladybugs than mammals, of longhorn beetles than birds, of weevils than fish. Textbooks and scientific papers regularly state that beetles are the most speciose group of animals; that is, there are more of them than there are of anything else.
But Andrew Forbes, from the University of Iowa, thinks that this factoid cannot possibly be right.
In a new paper, published online as a pre-print, Forbes and his colleagues argue that nature’s apparent beetlemania is more a reflection of historical bias than biological reality. Beetles are often conspicuous, shiny, beautiful, and varied—qualities which meant that 19th-century naturalists like Charles Darwin collected them for sport, and eagerly compared the size of their collections. Thanks to their inordinate fondness for beetles, we have a disproportionately thorough picture of the group’s diversity. The same can’t be said of other groups of insects that are smaller on average, harder to study, and less charismatic.
Forbes studies parasitoid wasps. These creatures use their stingers to lay eggs in (or on) the bodies of insects and other hosts. The grubs, upon hatching, devour their hosts alive, sometimes commandeering their minds and changing their behavior, and sometimes bursting out of their desiccated carcasses. There’s a wasp that takes cockroaches for walks after turning them into docile zombies, a wasp that forces spiders to spin a protective cocoon all while sucking them dry, a wasp that turns caterpillars into half-dead, head-banging bodyguards, a wasp that conscripts ladybirds into acting as babysitters.
Their lives are grisly and sinister, but their abilities are incredible.
No one really knows how many of them there are. There aren’t many scientists who specialize in studying them. They can spend much of their lives hidden inside the bodies of other insects. And since they specialize in body-snatching, they can be incredibly small. The smallest of them, the fairy wasps, parasitize millimeter-long insects, and are themselves no bigger than a single-celled amoeba. This means that when scientists try to catalog the number of insects in a given area, they often ignore all but the biggest and most conspicuous parasitic wasps.
Other insects, however, do not ignore them. It seems that every species of insect is targeted by at least one species of parasitic wasp—if not several. There are even parasitic wasps that exclusively target other parasitic wasps—they’re called hyperparasites. (They include the crypt-keeper wasp that was newly identified last year.) To Forbes’s knowledge, no insects have escaped these parasites. Even those that live under water have their own particular wasp nemeses.
The beetles certainly aren’t immune. “When we collect a species of beetle in large numbers, we’ll rear out 10, 20, maybe 30 different wasp species,” Forbes says. Surely then, he argues, parasitic wasps must outnumber beetles in terms of species?
The only way that wouldn’t be true is if these parasites were mostly generalists, and the same wasps were targeting many species of beetle. In their paper, Forbes and colleagues argue that this is unlikely. They focused on four different genera of North American insects that have been very well studied: Rhagolettis fruit flies, Malacosoma tent caterpillars, Dendroctonus bark beetles, and Neodiprion sawflies. They then tallied the parasitic wasps that are known to specifically target these groups and these alone.
By extrapolating this conservative figure out to other insect groups, they estimated that the parasitic wasps probably outnumber the beetles by somewhere between 2.5 and 3.2 times. It’s hard to say for sure without a full count of either group, but Forbes makes a compelling case.
“It’s certainly preaching to the choir,” says Josephine Rodriguez, an entomologist from the University of Virginia’s College at Wise. “It doesn’t surprise me at all. I really think the only people that would disagree would be the fly people.”
“Look at it this way,” says Lynn Kimsey, from UC Davis. “Beetles have never been very good at parasitism. Wasps, on the other hand, are all about parasitism. So for each beetle species there are probably at least two wasp parasitoids: One [that targets] the eggs and one [that targets] the larvae.” Kimsey recently surveyed the insects that live in Southern California’s sand dunes, and found that wasps—and other hymenopterans, like ants and bees—accounted for 42 percent of all the insects present, and outnumbered the beetles by two to three times.
There are a few other animal groups that could potentially be more speciose than the wasps. For example, insects also harbor bloodsucking mites and parasitic nematode worms. The same argument that Forbes makes for the wasps could also be made for these other groups, which are studied even less. “We don’t have the knowledge,” Forbes says. “The best thing that can happen is that other folks try to prove us wrong and we learn a lot more about mites and nematodes.”
In the meantime, the wasps are the ones to beat, and the implications of their supremacy are unsettling. It’s easy to see parasites as niche and unsavory creatures, but they’re actually practicing the most successful lifestyles around. The defining innovation of the animal kingdom is not the stone tool of the ape nor the flight-capable feather of the bird, nor the hive mind of the ant, but the egg-laying stinger of the parasitic wasp.