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Out-‘slugging’ soybeans an emerging issue

No-till playing a key role.


November 12, 2007
By Ralph Pearce


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22aThe spring of 2004 brought a relatively new host of concerns and conditions
to the collective attention of growers and researchers. Slugs in soybeans was
one of those concerns. It was not as though they are a new threat to soybeans,
just that their numbers and frequency seemed to be on the rise.

Charlie Dunsmore, a grower from St. Pauls, Ontario, can attest to the idea
that slugs are becoming a problem. He has seen them in his own fields and in
those of his neighbours, yet there is no single solution. "As more growers
go to no-till, there's more trash in the field, so there's a situation where
it's a good spot for slugs," says Dunsmore, adding that he had considerable
slug damage in his soybeans in 2004. "The other thing is it was really
wet in the spring, so a lot of the fields were flooded, and the slugs seemed
to do quite well in those damp, wet fields."

Slug damage is evident during the early stages of development of the soybean
crop. According to Horst Bohner, soybean specialist with the Ontario Ministry
of Agriculture and Food, slugs can feed on the germinating seed in the ground
or the seedling, affecting overall vigour at the very least. "What more
typically happens is that they feed on the foliage as the beans are just coming
up at the unifoliate or during the first trifoliate, and at that stage, they
can completely defoliate the plant," says Bohner, noting the feeding usually
takes place in field sections that are lower, wetter or have more residue, like
the headlands.

Damages and defense
One thing is certain: when slugs do damage, it is often severe, with whole patches
of fields being defoliated or chewed right down to within an inch of the ground.
"Certainly it's one of these pests that can be devastating for a specific
field, and within parts of the field, you can have 100 percent yield loss,"
says Bohner. "But how much of an overall provincial impact are they having?
Usually it's relatively small."

Bohner also agrees with Dunsmore in his assessment of more no-till farming
leading to high slug populations. No-till soybeans following forages or cover
crops are at very high risk. But the weather continues to play a significant
role on so many fronts from year to year. Bohner also knows of some no-till
growers in Ohio who have 'put away' their no-till drills for the time being,
opting for some conventional tillage in an effort to battle their slug problems.

Solving the problem is no simple solution
Conditions for 2004 were an obvious boon to slugs, but Bohner cautions the idea
that cooler weather can cause a reduction in slug populations. Actually, slugs
prefer cool wet conditions throughout the growing season. "In Ontario,
both eggs and adults overwinter, so we have one generation with two populations,
with some maturing in the spring and some in the fall," he explains.

Dunsmore has seen some success with two treatments: one is using 28 percent
in successive nighttime applications and the other is introducing limited tillage.
The 28 percent is somewhat of a hit and miss notion, concedes Dunsmore. "But
the theory is, if you spray 28 percent in the middle of the night, because that's
their feeding time, the salt in the 28 percent will make them curl up and die,"
he says. "The biggest problem is that only a third of the slugs will come
out into the field every night, so you have do it almost three or four times
in a row to get a good kill, and that does get expensive."

The use of 28 percent is in a dilute solution of roughly 10 percent, so as
not to burn the existing crop in the field. Bohner recommends a ratio of nine
litres (two gallons) of 28 percent mixed in with 82L (18gal) of water. "I
have not heard of anyone who's had burn problems at that rate, but of course,
if you went straight 28 percent, you'd get burn," says Bohner, acknowledging
the limited success Dunsmore has had using 28 percent while noting it is not
foolproof.

As for cultivation, Dunsmore agrees that some die-hard no-till growers may
shy away from conventional tillage or even some light discing in the spring,
but it is one of the best ways to get rid of slugs. "The black ground gets
hot and they don't want to stick around," he says. Bohner notes that a
good fall burndown is recommended to expose the eggs and adults that are getting
ready to overwinter.

Bohner cautions that in a year like 2004 when weather and field conditions
remained wet and cool through May and June, replanting soybeans did not necessarily
solve the problem. "The slugs haven't disappeared, so you can run into
the problem the second time," says Bohner, citing the same tillage rationale
that Dunsmore uses. "It's all about that residue, and with the black soil
on top, the slugs are less likely to survive."