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Predators line up for grasshoppers

Grasshoppers might be voracious — but so are the many things that consume them

Farmers fear grasshoppers because, according to legend, they eat everything.

There’s a flip side to this and farmers can use it to their advantage. In the grand scheme of Prairie ecology everything eats grasshoppers.

“They do have a positive side,” Dan Johnson of the University of Lethbridge told the Manitoba Agronomists Conference earlier this winter.

“Native grasshoppers feed native birds like owls and grouse as well as songbirds that are served by a large guild of completely non-pest species out on the grasslands.”

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For our purposes, grasshoppers may be seen as the insect version of bison. On a native grassland they’re Mother Nature’s way of converting rough forage into high-quality protein for vast numbers of Prairie animals. From the frogs in the slough, to the burrowing owls right up to coyotes and foxes, the plump Prairie grasshopper provides an important addition to the menu.

That means farmers have a lot of friends out there hunting and eating grasshoppers and this helps keep their numbers in check. A variety of predators means a variety of techniques such as straight predation, eating their eggs, parasitizing them or a grisly form or death by fungus. As grasshopper numbers increase it’s natural for predators to increase with them.

A lot of them are caught by various animals and many are picked off on the wing by birds. This can lead to amazing adaptations by the grasshoppers to avoid this. The Carolina grasshopper actually flies in a way to discourage birds.

“It flies back and forth like a butterfly,” Johnson says. “When birds see a butterfly, many species don’t chase it because they know they can’t catch it. A grasshopper that flies like a butterfly fools some of those birds.”

It may not fool other predators. Hiding among the plants they eat are a number of spiders. Some of them, like the banded garden spider use the web that we often see when we find spiders at work.

“They eat a lot of grasshoppers and they’re all across the Prairies,” Johnson says. “The problem is, they’re not here every year but they’re on a peak so they’re here now.”

Down on the ground the wolf spider practises ambush hunting and does the stalk and pounce. They’ll eat quite a few of the instars earlier in the season as well as adults later on. The ground is also hazardous for the eggs. The larva of blister beetles, ground beetles and bee flies are important predators of grasshopper eggs. Field crickets may even eat up to 50 per cent of them in some areas.

There are also a number of flies such as the robber fly, a large and powerful insect that catches them and pierces them with a sharp beak. They inject an enzyme that dissolves the internal organs, sucks them out and then discards the husk.

One truly ghoulish way of dealing with a grasshopper is practised by parasitoid flies or wasps. They don’t actually eat the grasshopper themselves. They lay an egg on or in the hopper and, once the larva hatches, it goes to work eating it from the inside. When it matures it pupates then breaks out and leaves the husk.

What really takes a population down is disease because, as the ’hoppers become more plentiful they start spreading it among themselves. One really interesting twist on this are certain types of fungus such as Entomophaga grylli. It starts with a spore infection that gets inside and spreads throughout the animal. It can be an effective biological control agent.

“That is a naturally occurring agent that will kill quite a few. I mean five or 10 per cent can be knocked down and it’s a bit of suppression,” Johnson says. “In the early ’60s it killed an outbreak of the clear-winged grasshoppers.”

Biological controls

Photo: Getty Images

Predators and parasitoids:

Insects that eat grasshopper eggs include ground beetles, field crickets, and larvae of some species of blister beetles and bee flies. Spiders, robber flies, some wasps, and many species of birds feed on grasshopper nymphs and adults. Parasites of grasshoppers include mites, insects and nematodes.

Pathogens:

The protozoan Nosema locustea can significantly reduce grasshopper populations.

The fungus Entomophaga grylli can help control grasshoppers under warm, humid conditions. This fungal disease leaves corpses of its victims clinging to the stems of plants.

Preserving natural enemies by only using insecticides based on threshold guidelines is an important component of managing grasshoppers. When only low levels of grasshoppers are present, broad-spectrum insecticide applications can do more harm than good by reducing the number of natural enemies, which leaves grasshopper populations free to increase without natural population checks.

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