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Armyworm another of many pests to watch

Impact can vary year to year.


November 12, 2007
By Ralph Pearce


Topics

20aAs growers continue to deal with a relatively new range of insect pests and
diseases, there is a need to understand just how damaging one condition may
be compared to other, better-known afflictions in the field.

In June of 2004, agronomists, consultants, seed dealers and extension staff
with Ontario's agriculture ministry used their weekly meetings to express concerns
regarding armyworm. One notation stated that armyworm activity was quite high,
particularly in wheat fields in Kent County. Of some concern was that armyworms
could move out of wheat into adjacent corn fields.

Armyworms are like many emerging insect pests and diseases; on their own, and
in general terms, their impact is limited. But when their effects are compounded
with a variety of diseases and stresses that seem to be growing in frequency
from year to year, growers must recognize them as a threat, no matter how small.
When they do become a problem, the damage can be significant.

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True armyworms can be identified by black bands that run along the
top of each of its proleg

Dr. Art Schaafsma, a professor at Ridgetown College-University of Guelph, specializes
in corn and wheat diseases and insect pests, and cautions that in the race to
identify and target various insect pests, there is some confusion concerning
armyworms. “First, you must understand that we're dealing with two armyworm
species,” he says, noting much of the discussion in North America has recently
focussed on fall armyworm, as opposed to true or common armyworm. However, fall
armyworms do not overwinter in Ontario and are therefore not as great a concern
to growers in the province. This is where the confusion can arise.

True armyworms do overwinter in Ontario, and are therefore a greater threat
to early season wheat and corn fields and pastures. True armyworms can be identified
by black bands that run along the top of each of its prolegs. Their heads are
also a lighter brown colour.

Fall armyworms tend to feed on maturing stands of corn, including kernels.
Their heads are black in colour, with an inverted 'Y' towards the front.

Bt corn a definite asset
When it comes to controlling true or common armyworms, Schaafsma says growers
can see a fringe benefit from using various Bt hybrids. “Interestingly,
the YieldGard and MON810 events designed for corn borer control seem to protect
against common armyworm in corn,” explains Schaafsma, who generated data
from some bioassays he carried out in recent growing seasons. “Common armyworm
infestations are sporadic in Ontario and are normally controlled through a complex
of natural parasitic insects and a virus.”

In fact, one thing to look for in scouting for armyworms is the presence of
small, white or yellowish eggs on the back of the larva's head. These eggs indicate
the armyworm has been parasitized by a fly and will die in a matter of days,
negating the need for foliar spraying.

From time to time, however, more severe outbreaks can occur, but as Schaafsma
points out, the usual victims are cereal crops. In 2004, wheat was the best
looking crop at the time when the adult moths emerged and mated, leaving females
to fly off in search of green vegetation on which to lay their eggs. “Corn
is damaged when the armyworms move from the grassy fence rows and cereal crops,”
says Schaafsma. In a corn field, armyworm feeding can be so severe that only
the midrib and stalk remain. “In the case where armyworms move into a corn
field, it's important to monitor the fields and fence rows for the presence
of armyworm from the end of May until the second week of June.”

In wheat fields, armyworms can feed on the awns, the kernels and the heads.
As the wheat field dries down, armyworm larvae will move on to neighbouring
fields.

Good news in corn

Since armyworms generally attack corn plants later in the growing season, and
because they feed on the leaves as opposed to the growing point, it is possible
for corn plants to survive moderate feeding. Thresholds for corn are two or
more larvae per seedling with at least 10 percent feeding. If feeding takes
place after the whorl stage, the plant can sustain up to 50 percent loss before
applying a foliar spray. In wheat, the threshold is four larvae per square foot,
with emphasis on head clipping. It is recommended that spraying take place at
night since it is the time armyworms are most active in their feeding. Schaafsma
adds that if the infestation is caught early enough, spraying an entire corn
field may not be necessary.

In corn, Dylox 420, Sevin XLR Plus and Matador 120E are effective, with Dylox
and Matador carrying a caution against more than three applications per year.
In wheat, Dylox 420 LC, Sevin XLR Plus and Lannate Toss-N-Go are the recommended
products. Dylox recommends use 21 days before harvest, Sevin XLR Plus 14 days
to harvest for wheat with Lannate Toss-N-Go recommended 20 days before harvest.

The Bottom Line

We're never sure what new pests are coming, so it's good to know there
are two different types that we will need to distinguish between. 
Lennie Aarts, Wainfleet, Ontario.

Armyworms are not a new pest to Ontario; they have been around for years.
Their frequency of infestation has been sporadic across the province.
As an industry, we have the means and the experience to deal with this
pest. The key to controlling armyworms is scout your fields, identify
the problem and use the proper solution to manage the situation.
Leo Guilbeault, Belle River, Ontario.