The crop-protecting power of parasitoids
Gruesome bugs employ their natural wiles to keep crop pests in check
By: Kevin Ma
| Posted: Wednesday, Feb 27, 2013 06:00 am
So you’re a cutworm munching on some canola in a farmer’s field when a wasp stings you on the bum.
You think nothing of it at first. A few weeks later, though, you notice your insides churning, almost as if hordes of tiny worms were wriggling about your guts.
After 20 days, you realize the truth – there really ARE hordes of tiny worms wriggling about your guts, and they’re eating you alive. In a fatal twist, they burst from your body en-masse, transforming you into a writhing mass of segmented tentacles for a few brief moments before you die.
That was the scenario that played out live on camera at the Enjoy Centre this week during the CanoLAB agriculture conference, where about 240 agriculture specialists gathered to learn about crop diseases and pests.
Entomologist Vincent Hervet’s session on beneficial insects examined parasitoid wasps and cutworms. Not only did he have tubs full of dirt and cutworms for guests to dig through, he also had cutworms infested with wasp larvae, many of which burst forth during his presentation.
Those larvae will soon spin cocoons and emerge as new wasps in about 10 days, Hervet says, as the screen behind him showed a larval explosion in progress.
“For those who have seen the movie Alien, it’s kind of the same thing,” he said.
Chest-bursting wasps like these are actually parasitoids, not parasites, notes Alberta Agriculture insect management specialist Scott Meers.
“A parasite lives on its host but doesn’t kill it,” he explains.
A parasitoid, in the process of completing its life cycle, kills its host and lives on to infest others. That makes them the stars of the bio-control world, Meers continues. By promoting the growth of parasitoids, farmers can take out the pests that plague their fields.
Most parasitoids are species-specific, Meers says, which means they won’t attack other beneficial bugs if introduced. Still, some are generalists, Hervet notes, so they must be introduced with caution.
Cutworms are caterpillar-like creatures with three pairs of legs near their head capsule and five pairs near the back, says Hervet, who studies them at the University of Lethbridge. They cut holes in the leaves and stems of canola, sometimes killing the plants.
“If you have an outbreak at the seedling stage, they can wipe out a good portion of the field,” he said.
Such outbreaks have become increasingly common in Alberta in recent years.
When a canola plant is under worm attack, he continues, it emits a chemical “scream” that draws parasitoid wasps and flies to its location. Those bugs, which range from a millimetre to two centimetres long, home in on the cutworms and lay eggs on or in them (using a pointy stinger in the latter case).
Those eggs hatch into larvae that eat the worm inside out, eventually killing it as they burst through the skin. Within minutes, as he shows his audience, they spin a large white or yellow cocoon around themselves for protection as they grow into adults. These cocoons can often be seen on the tips of canola plants, he notes.
There’s been a renewed interest in pest-fighting bugs in the last decade or so as farmers look to reduce pesticide use, says Keith Gabert, an agronomist with the Canola Council of Canada.
Parasitoids are poorly understood and there’s scant research into their actual effect on pest populations, Meers says.
“It’s a very subtle control, and very unappreciated, because they just do their work and therefore we don’t have the pest,” he said.
Parasitoids appear to be keeping the wheat midge in check in central Alberta, Meers says.
“If we sprayed all those areas every year for wheat midge, it’d be in the tens of millions, maybe hundreds of millions of dollars.”
Parasitoids have also all but eliminated the need to spray for the cereal leaf beetle in the U.S., he continues. That bug was spotted in the Edmonton area for the first time last year, he notes, and the province plans to import a parasitoid to combat it.
It’s tough to introduce beneficial bugs to your crop unless you’re in a greenhouse, Gabert says, as they skitter off as soon as they’re released. That means it’s important to encourage the growth of any that are already in your fields.
Farmers should therefore not spray unless it’s economically necessary, Meers says.
“If you’re at the (economic) threshold, yes, you should spray,” he says. Otherwise, “we’re probably working against ourselves, because we’re taking out our beneficial (bugs) instead.”