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.”