Features Agronomy Insect Pests
Wasps may control lygus bugs

Lygus bugs may be candidates for biocontrol by a peristenus wasp imported from
Europe. But even better, in looking at the ecology of lygus bugs on the prairies,
scientists are finding lygus have native peristenus parasitoids, which may be
able to reduce lygus numbers if they have a suitable environment.

In some areas of the northeastern US, peristenus wasps imported from Europe
have reduced lygus bug populations by as much as 75 percent. In about 20 years,
the wasps have established themselves in several states and have migrated across
the Canadian border. The wasps have also been imported into central Canada.

Now, a team of entomologists with Agriculture and Agri-Food Canada (AAFC),
including Hector Cárcamo, of the Lethbridge Research Centre, is looking
into the potential for establishing the parasitic wasps on the prairies.

"Wasps will eventually spread from the east," says Cárcamo.
"But, if the imported species of peristenus can parasitize the lygus species
that are common pests here, and survive our winters, we can consider speeding
up the process. First, we have to be sure the wasps won't harm native peristenus
species."

The peristenus wasps attack lygus nymphs while they are quite small, in the
first or second instar stages. At this point, lygus look very much like aphids,
but they are hard-bodied. Older nymphs are bigger and fight off the tiny wasps,
which are only about 3mm long.

The female peristenus stings the lygus bug and lays an egg into it. The egg
hatches after five to seven days and the larva feeds on the body of the lygus
for another seven to 10 days, then falls to the ground and pupates just under
the soil surface.

The lygus bugs can remain active, and continue feeding and growing for part
of the time when the wasp larva is consuming its body. Although this is something
of a disadvantage, the bug is destroyed before it reaches its most damaging
adult stage. With biocontrol, beneficial insects are not harmed and lygus cannot
develop resistance to insecticide as has happened in parts of the US.

Cotton producers in California buy peristenus wasps each year and release them
to control lygus bugs. This is called inundative biocontrol, but it is costly,
particularly for peristenus since they are very expensive to rear.

Classical biocontrol has been widely used to control weeds accidentally imported
without their natural enemies, especially rangeland weeds, for which chemical
control is impractical. Importing a natural enemy of the weed, usually an insect,
can bring the weed into balance with its new environment.

Lygus bugs are native insects, but they have thrived in farm environments feeding
on a wide range of crops – they are pests in alfalfa, canola, strawberries,
tree fruit and cotton – and they can move easily from one host to another.
Canadian entomologists are studying the hosts used by lygus bugs before they
arrive in canola. They need to understand host-pest interactions to develop
successful biocontrol.

Species must match geographical location
For
a natural enemy to control a pest, its life cycle must match that of the pest.
For lygus, this may mean choosing a different species of peristenus for each
region as the lygus have different life cycles. North of about 53 degrees latitude,
lygus have only one generation per year, but further south, they may have three
generations. All four species of lygus bug that are common in western Canada
can be parasitized by peristenus wasps.

At first, scientists thought native peristenus wasps were rare here, as it
took them 200 sweeps to find each wasp. But, when AAFC technician, Carolyn Herle
examined lygus nymphs, she found up to 30 percent parasitized by a native peristenus.

Scientists in Saskatoon have found parasitism levels of 70 percent. The 75
percent drop in lygus numbers in parts of the US was linked to 55 percent parasitism.

A survey in the US Pacific Northwest found parasitism, by a different species
of peristenus from that in the east or on the prairies, was around 10 percent
or less, except in a few sites. At one site, scientists found 32 percent of
lygus were parasitized.

Encouraging survival of native peristenus species may offer more effective
control of lygus bugs than importing exotic species.

"We need a species that can sustain itself here, so it can fit into the
ecological niche the lygus bugs provide," says Cárcamo. "It
may be better to provide an environment that's more supportive of native peristenus,
which are adapted to the species and life cycles of lygus here."

Scientists have linked healthy peristenus populations to undisturbed and direct
seeded sites. Imported populations established well in alfalfa fields, but not
in annually cropped fields. Adult peristenus feed on nectar of fleabane, goldenrod
and cruciferous weeds, so weeds may also be important to their survival.

Lygus bugs are attacked by other insects as well as peristenus. A fairy wasp
parasitizes lygus eggs and a fly called alophorella parasitizes adult lygus.
Spiders and predatory insects such as ladybugs, damsel bugs and big-eyed bugs
feed on lygus nymphs.

While lygus bug infestations were not wide-spread in 2004, the research that
Cárcamo and his colleagues are conducting may eventually help to reduce
the need for large-scale insecticide applications. -30-