Late-season weed issues and soybean aphid management

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I'm Ryan Miller, crop

extension Educator
earlier this morning.

We recorded in an episode of

the Strategic Farming
Field Notes program.

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We welcome you here
to the program.

These sessions today
are brought to you by

the University of
Minnesota Extension with

generous support from
Minnesota farm families,

and also from the Minnesota

Soybean Research and
Promotion Council,

as well as the Minnesota

Corn Research and
Promotion Council.

With that, my name
is Dave Nicolai.

I'm University of
Minnesota Extension

Educator in Field Crops.

We want to welcome our
guests this morning.

Dr. Tom Peters,

Tom's Extension Sugar
Beet Wheat Specialist,

working with both
the University of

Minnesota and North
Dakota State University.

And he's at his office
at Fargo this morning.

And Dr. Bob Koch,

extension tomologist
in soybeans at

the University of
Minnesota as well.

Let's take any more time.

I'm going to start off
here and turn the program

over with Tom and Tom.

It's certainly been
an interesting year.

We had, I think almost like

the month of August occurring
in the month of May.

This year we're extremely warm,

hot, at least for
some of us that

are used to a little bit
cooler temperatures.

Certainly I think
some of that might

have confounded weed biology,

Weed management and
that type of thing.

I know every year we talk

about the spread of
weeds and so forth.

But before we get into all
the herbicide business,

let's talk a little
bit about some of

our weed problems that are
typically with us every year.

And some of them are
more challenging

in situations with an
environment like this.

The two that come to my mind,

and you might have others and

talk a little bit about that,

of course, is water,
Hemp, and Kosha.

It doesn't really make any
difference whether you're

a sugar beet grower or
not. It affects all of us.

What are your observations
in looking at that in terms

of biology and some of

the problems that we ran
into this last year?

Yeah, let's Dave, thank you.

It's a privilege and an
honor to be on this morning.

One of the first things

about biology that
I like to think

about is when does a weed
germinate and emerge?

The textbook says
that Kosha is one of

the first weeds that emerge
in the spring. It did.

We saw a lot of Kosha,

especially in
northwest Minnesota,

but also in west central
Minnesota right after planting.

I would argue in
some of our fields

that Kosha was the first
thing to germinate.

It germinated ahead of the wet.

It germinated ahead
of the sugar beets.

But the other thing that we

observed this year about Kosha

is when it got hot in May
and June, it went dormant.

We didn't see as much of

the prolonged germination and

emergence that we sometimes see.

I attribute that to how warm it

got and the increase
in soil temperatures.

Water hemp is a little bit

of a different story
I think this year.

And I'm going to make a
bold statement, Dave.

I don't think water
hemp was as bad

across the state
in 2023 as 2022.

And the reason I say
that is, first of all,

there's some areas
of the state that

got good rain right
after planting.

The Pre worked.

The Pre were very
effective on water hemp.

Then the rapid
growth of our crops,

again attributed to the soil,

to the air temperatures,

we had tremendous early
season growth of our crops.

I think that to some degree,

helped to shield
away from water,

hemp germination and emergence.

Then also May and June
were pretty dry as well.

In a lot of areas,
especially with water,

a seed that grows off
the surface or 1 " deep,

usually in response to

rain events and
temperature fluctuations.

I don't think we had

the flushes of water
hemp that we had in.

Other years?

Well, I would agree, certainly.

I know in some of the
places I traverse

in Central and
southern Minnesota,

we had really dry conditions.

We had moisture early
to help the crop.

And that late April,
very early May,

and then we just turned
off the faucets,

so to speak, some places
for 3456 weeks almost.

And I think that
really did affect

the amount of water
hemp with that.

What's your recommendation?

Should we be complacent
and say, well,

we didn't have much of a
problem and we should be

good for the rest of

the year in the next
couple of years.

But not really that's
the case, is that no,

we need to actively manage
weeds year round again.

We have to worry about
the biology though.

One characteristic of pigweed

is the seed becomes viable
very quickly after flowering.

You only have two weeks.

I want our audience
to keep that in mind.

If you're out there
pulling weeds,

carry them out of the field,

because if you throw
them on the soil,

they're going to
finish making seed.

And you're going to
have to deal with

the seed for four to six years.

Carry them out if you're
going to mow them off,

feel good about mowing,

but realize that some
of the seed that's more

advanced is going to make seed

or some of the water hemp
is going to make seed.

Now Dave Kosha weed and lamb's
quarters are different.

Those weeds require more of

a full season to
make viable seed.

So going out and mowing those

in August is perfectly fine,

and I think you will
reduce the amount of

weed seed production
that you have.

So I would encourage

our listeners to actively
manage all weeds,

but feel good about managing

the second batch because
that seed isn't viable.

At this point, I
know we don't have

a tremendous amount of acres

of small grain and bit wheat,

but are there some things

about the stubble
and that crop coming

off and subsequent weed growth

in those areas that you've seen?

Well, I work with a lot

of small grains growers
in Northwest Minnesota.

Unfortunately, we have a lot

of weeds this year in our wheat.

That gets back to the
earlier comments I

made about Kosha coming
up as fast as the wheat.

We're already talking about

post harvest
management strategies.

Whenever we're talking
about post harvest,

we've also got to be thinking

about what next year's crop is

going to be and incorporate
those into our decisions.

But yes, we absolutely will.

We'll try to manage
weeds that are

in our residue combination.

I think you hinted to this,
even besides the dry weather,

you felt comfortable that
where we got some moisture,

that our Pres did a very good
job across the landscape.

Yeah, Pres are always an
interesting conversation,

so some people sometimes say
they didn't do a darn thing,

it was a waste of money.

And I think the performance of

Pre is usually attributed
to rainfall patterns.

If you got rain on your
Pre in a timely manner,

I think they work
very effectively.

But here's the thing, Dave.

The worst thing about
a pre herbicide is,

especially in 2023, is if
you made the application,

everything you did was correct.

But if you didn't get rain,

you might have
gotten a good rain

for ten or 14 days
after application.

But that may not
have been time wise

adequate where water help

may have germinated and
emerged before that.

Even though you did
eventually get rain,

the rain came after some of

the earlier weeds started
to germinate and emerge.

Really knowing when that event,

that rainfall event
occurred in relationship to

the weed biology is really

critical to truly evaluate
the performance of pre.

Let's just visit a little
bit about sugar beets.

I know it's very preliminary

obviously in a lot of
your observations,

but a lot of fields
that I looked at,

at least for the Southern
in look fairly clean.

Some not so much as others,

but where they made
a good attempt,

I think things work very well.

What are your comments about

the sugar beet crop and
coming in here for this year?

The same. I learned from
Alan Dexter many years ago,

that plant as early
as you can, people,

the farmers that were able to

plant the last week of April,

April 30, and then the first
few days of May early.

Planted fields look tremendous.

And oh, by the way, they
got rain at the end of

the week and that really helped
the weed control as well.

But I think we have a
very nice crop out there,

and it's not just sugar beets.

Dave? I think across the
board in my travels,

I'm seeing a pretty
nice crop or crops.

But one thing, about 20:23

we're only two weeks
away from a train wreck.

So we're going to
need to continue

to get rains because
we don't have

a lot of excess rain or

a lot of excess moisture
in the profile.

That's for sure.
I know that we're

down compared to a year ago,

probably 20 or 30% in terms
of qualifying as adequate.

Let's just re summarize

this recommendation
for this fall.

We indicated that

probably the hardest
weed to deal with is

water hemp because it's going

to go so fast
between what I call,

if you think about
like the dough stage,

to physiological maturity
of that water hemp seed.

That's very quick. I sometimes

I get questions
from growers say,

well in this crop or this area,

what if I apply a growth
regulator, herbicide or 24d?

Will that affect the
viability and so forth?

And I think that's
really problematic.

You might have more options
in some of these other weeds,

But when it comes to water hemp,

you can't wait too terribly
long is that correct?

That's exactly correct.

I think at this
time of the year,

and I'm talking about water
hemp that's in some of

our low growing crops like
sugar beet or soybeans.

Really the only good
option we have is

electricity or hand
pulling by electricity,

I mean using the weed zapper.

One of the things
that we've learned

about the zapper is we've

already experienced the yield
loss that's already there.

But one thing the
zapper does do is it

stops the physiological
maturity process.

In seed, it'll
reduce the amount of

water hemp seed that becomes
viable and mature as well.

That's one option that we have.

The other thing
that we can do is

manage water hemp after we
start taking the crops of

either using a fall
herbicide program

or possibly using
tillage or other means.

Let's think about 20:24
What lessons did we learn

about we control post
emergence in terms of timing,

what things worked, what are

some things we need to
keep in mind for that?

Certainly, if we're in
a sugar beet rotation,

we have a number of years here,

but we've got to leverage

that summations
that you observed.

Again, I know it's
very preliminarily,

but what are some things

we need to keep forward
and keep in mind for 2024?

Yeah, a couple of
things that I learned.

The first one is sometimes

we hear about non performance
where products don't work.

And I'm going to use an
example in Sugar beets,

Peters, I went out and used

Ultra Blazer and it
didn't do a thing.

My knee jerk reaction is, yeah,

the water hemp was too big,

but that's not fair.

I think if you see
an example like that

where you don't feel you

got the performance
that you needed,

you might consider saving

that seed and
sending it into Dr.

Serene or myself to
test that seed to

determine if there might be

a weed resistance
challenge going on.

I guess I've always viewed

weed resistance as
somebody else's problem.

Yeah, we've got
widespread resistance

with ALS inhibitors
and glyphosate,

but by and large the
other products all work.

I'm starting to doubt that.

I'm starting to wonder if

weed resistance is
becoming more widespread,

especially with the oxen,

especially with the
PPO inhibitors,

especially with the Group 27.

Dave, the other thing
that I learned,

I'm reminded all the
time that agriculture is

a continuous set of

activities that starts with

planting and ends with harvest.

Well, one of the things
that I didn't realize,

or I failed to realize is
how dry it was in 2023,

especially in August,
September, and October,

we didn't get a lot of breakdown
of the 2023 herbicides

and then add that to
the late spring that we

had heck in my front
yard in Fargo,

I had 2 ft of snow on 15 April.

We didn't get any herbicide
breakdown in the spring.

The combination of the events in

the fall combined
with the spring

I think reduce the breakdown

of some of the
products that we had.

And that resulted in
carryover in some places.

So those kinds of lessons
and learning about how

last year's products can impact

this year's crop or

this year's products
and next year's crop,

I think is a real
important message

for our listeners.
Well, thank you Tom.

I hope you can hang
on. We might have

a question or two
as we go forward.

So it's a good opportunity,

like we said, to get out,

take care of those field edges,

approaches, fence lines and

so forth as we go forward here.

And that we see situation
with that One last message,

Sugar beet, pre harvest
will start on Monday.

All right. So there'll be a
lot of trucks on the road.

Everybody be safe out

there when you're
out on the highways.

Sure. Sign of fall there

in terms of that summer
is coming to an end.

But Bob, we're not quite

done with this insect
situation here.

It just keeps coming
up and so forth.

And we think, well,
the month of July

is over. I guess we're all done.

But not necessarily
When we talk about

soybeans and other things,
it's weather confounding.

I know you've called
around a little bit,

I was at Farm Fest
this last week.

A number of growers,
we're talking about that.

It's not every field.

What are some words to
the why, so to speak,

that you might have a little
bit about where we are on

soybean aphids and what

should we be doing
across Minnesota.

Yeah, thanks, Dave. In terms
of what we should be doing,

we should definitely be
scouting our fields.

There's a lot of
variability out there.

I think a lot of
that depends on how

the rainfall patterns shook

out for different
parts of the state.

Areas that got some
decent earlier rains,

seems like that's
where we're seeing

a lot more soybean aphids
right now, especially.

And a Southwestern Minnesota.

Central Minnesota, even
into Northwest Minnesota.

I spoke with Aaron Lorenz,
our soybean breeder.

He has some trials
up in Crookston.

He had to get his plots
treated for aphids.

I was down in Lamberton

this week and setting up
an insecticide trial.

We had well over 1,000
aphids per plant.

It's a pretty big aphid
year in some spots.

But it's certainly not all
areas, not all fields,

areas that are a little drier
have continued to be dry,

are still looking at
issues with spider mites.

And we know those are
a very challenging

pest under dry conditions.

What are some of the concerns or

dangers here of pulling
the trigger too early?

We know that aphids

can develop an opportunity
to move from field to field.

But what's the downside on

beneficials and just lining
to be like from an insurance?

Well, I'll think because
my neighbors do it,

I'm going to do it.

Maybe talk a little
bit about that.

Yeah, you bring up a couple
of good points, Dave,

and I think they tie into
the biology of this insect.

The first is the movement.

Around this time of
year, we often see

these slab aphid populations
developing wings,

and then they can
move to new fields.

When they're moving
to new fields,

they're often
looking for some of

the later planted fields that

are not as far along

those are going to
be really attractive

and not only attractive,

but more suitable where

the aphid populations
can grow faster.

That movement can be triggered
by different things.

Can be triggered by the
quality of the slab plant.

Maybe there are too
many aphids there.

And then those aphid
mothers can detect

that they'll produce
a generation of

babies that will
develop wings and fly

away if you have

a field that's not
necessarily heavily infested.

Now, that can change in

a short amount of
time when you get

these winged aphids migrating

into a field and reproducing.

The other aspect
that you mentioned

are the natural enemies.

These are predatory insects
like lady beetles or

these tiny parasitic wasps
that attack the aphids.

These wasps lay their
eggs inside the aphids.

The wasp eggs hatch and then

the wasp larvae will feed
inside the aphid, killing it.

And then the wasp merges,

flies away, starts its
life cycle over again.

But what they do is they leave

behind what we call
an aphid mummy,

which are the dried,

rusty, puffy remains
of the aphid.

The fields we've been
sampling around the state,

we've got very high levels of

parasitism by these
parasitic wasps.

And it seems like in some areas,

these parasitic wasps and

the lady beetles might be

holding the aphid
populations in check.

We're reaching maybe 100,

200 aphids per plant,

but they're flat lining there.

They're not continuing
to increase.

But there are
clearly areas where

the aphids are outpacing
these natural enemies.

And sky rocketing to
much higher levels.

Again, you can't just

assume that you have

no aphids or you
have tons of aphids.

You got to get out there
and scout your fields and

really get a feel for what's

happening in each of
those particular fields.

We're backing off some of

these high temperatures
this week.

We're back in some
places up north '70s,

but typically in the '80s,

that's still pretty
prime temperature,

is it not, for aphid
survival at this point.

At least going forward, it
looks at the next two weeks.

Yeah. A lot of us like
to think about it,

that the temperatures where
we're feeling comfortable

outside in the '80s,

that's where aphids
are doing really well.

Once you get up
into the mid '90s

and higher aphid reproduction
starts slowing down,

they don't do as well.

It might have been those
high temperatures earlier

on this summer that we're
slowing down aphid populations.

Now, as you were saying, David,

we're getting some
somewhat cooler weather

and this could be really
good aphid growing weather.

Not only are we getting
that movement or

redistribution of the
aphids among fields,

but once they get
into those fields,

temperatures are looking pretty

good for them to
grow quite rapidly.

I've talked with Bruce Potter

over the years and so forth,

and a lot of things come

back to you keep going
out there in the field

and watching because things

can still be a problem
all the way up until

R five soybeans.
Any comments there?

Yeah, we recommend scouting
in using the threshold at

250 aphids per plant through
the R five growth stage.

In doing so, you're
going to protect

your yields from losses
caused by the aphids.

But what you're
also going to do by

scouting regularly and
using that threshold is

prevent some of the more

challenging later
season decision making.

We know that based on

some data from some of my
colleagues in Minnesota,

that infestations
into early R six can

sometimes cause yield loss
if they are much larger.

Infestations probably above,

well above the typical
economic injury level.

That's a really tricky
time to try to make

a decision to apply
that insecticide.

Hopefully, if we're scouting,

using the threshold before that,

we'll be putting ourselves
in a better situation.

Well, some of these
newer products

and pre mixes and

so forth that we have
to be careful about.

If you've made one application
ignoring that field,

things can come back and you've

already taken out
the beneficials.

There is such a
thing as movement

as a continuing
problem, is there not?

In terms of some
of these fields,

depending upon how
they're managed.

If they were applied much
earlier in the season.

Yeah. Dave, you can have
movement to the aphids and

recolonization after an
insecticide treatment.

But I think two things
I want to bring up.

One is insecticide resistance.

Keep in mind that we still

have pyrethroid resistance
soybean aids out there.

We've documented it now
over multiple years.

Pretty much across all of
the soybean growing regions

of Minnesota into the
neighboring states as well.

Manitoba, the Dakotas,
Iowa. Keep that in mind.

If you're applying a pyrethroid,

you want to get out
there and scout after

those applications to make

sure the insecticide did
what you wanted it to do.

You mentioned mixtures, Dave.

It seems like these mixtures

are still working quite well.

Even the mixtures that
contain a pyrethroid,

they're still doing pretty well,

even though the aphids might be

resistant to half that product.

Right? But one of the
challenges I think,

with the mixtures is if we go

in with a mixture on our
first aphid application,

if we need to come back in
for another application

against aphids or mites
or something else,

it limits our options, right?

Because you've
already gone in with

two different insecticide
groups probably.

And we don't really
have all that

many total insecticide
groups available to us.

So just keep that in
mind as you're thinking

about your arsenal insecticides

and how you might
rotate through them.

Well, I think that's oftentimes

the case and you might mention
a little bit about we can

aggravate or flare
that spider mite if

we don't get rain
here in some places.

No, it sounds like a
pretty good chance of

some rain Thursday
night in Friday.

And I don't know over a lot of

Minnesota that
situations with that.

But even a normal rain,

Bob isn't going to alleviate

and wash off the
soybeans, is it not?

I think we have
to make a comment

about a rainfall doesn't
negate the problem here.

Right, Dave? First, so you

mentioned aggravating
the pest populations.

One thing I want to mention,

if you've got a field
that has mites in it,

keep in mind what
insecticides you're using.

A lot of the pyrethroid

could make that problem worse.

They can flare the
mite populations.

If you're targeting spider
mites, things like birn,

there are some other mid,

remind me what the other
part of the question was.

Basically in terms
of monitoring there,

that it can be a lot of
variability with the rainfall.

That's right, Yeah.


In regards to the rainfall when

those Saban canopies are

closed and pretty
thick and lush,

they can provide a

pretty good amount
of protection there.

Right. So if you have

a pretty huge storm at this
year with a thick canopy,

it might knock the aphids
down a little bit,

but I wouldn't
necessarily count on it.

I think you'd want to get back
out there and scout again.

I think of that kind
of weather impact

a little earlier in the season

when the canopies are more open,

you get the hard driving
rain and some winds.

I think Ian Mccrae has shown

that that can knock the
aphid populations back.

In terms of spider mites.

Um, I also would not count

on a decent rainfall

necessarily knocking
their populations back.

I think what the more
important factor is there

is a good period of time with

high humidities to favor

the fungal diseases that knock
down the mite populations.

Not necessarily one or
two rainfalls, right?

And I know even in the area
and eastern part here,

they're only talking about
a quarter inch of rain,

possibly on Thursday night.

Really very negligible.

But just last thing,
on the soybean Aps,

keep in mind that you
mentioned some areas of

Western Minnesota that were

heavier with a populations but,

you know, more here on the
eastern side of the state.

We haven't seen that as much.

Now they're picking up
maybe in the Southeast,

but we had a lot of variability.

We had soybeans that
were planted and sat in

the soil for 345 or six
weeks before emergence.

So even in soybean crop,
it was up and down.

Up and down, and we were dry.

But interesting to see
how that comes out.

But the bottom line is,

we haven't seen the pressure.

You can't always paint
with a broad brush, right?

And I think that's certainly

been the case from what
you've seen from there,

and even on St. Paul campus.

I know we've looked around
and I haven't seen as much

either in terms of that.

Yeah, David, you know,
that all gets back

to getting out there
and scouting the field.

There's a lot of variability
out there this year.

If I can take just a
couple seconds here.

So want to mention
that in talking

with Angie Peltier
from Crookston,

she there's quite a bit

of a green clover worm
larvae in some fields,

so that might be something
people want to start

paying attention
to in some areas.

These are small
green caterpillars

sounded like they're in
their early in stars,

very young caterpillars yet.

But as they get bigger,

their consumption of leaf
area can really increase.

So these are
defoliating insects.

We've also got
grasshoppers out there in

pretty high numbers
in some areas.

Just another reminder,

something else to be
looking for when you're

scouting and keep the thresholds
in mind for this time of

year reproductive stage
soybean in Minnesota,

we're still using the
defoliation threshold of

20% Look at multiple
plants across the field.

Some leaves from
the top, middle,

and bottom of the plants

average all those
levels of defoliation.

Then if that field
wide average is above

20% that's where we would

recommend starting to line up
an insecticide application.

Some folks might be hearing
about some lower thresholds,

ten to 15% reproductive
growth stage is a soybean,

but in Minnesota, at

the University of Minnesota

recommending the 20% threshold.

Okay. Great. Well,
we've been through

a lot of communication

here this morning on
some important issues,

both weeds and insects.

Tom, you've had a little
bit of a break there.

Any last thoughts that you have

on from a weed science
perspective that

we did not address

that we should mention
here as we close?

No, I don't have anything
else but thanks for asking.

We certainly have that and we'll

have and so forth coming out.

And Bob, I know that we have

some other events coming
up here with Bruce Potter,

Lambert in the end of
August here on corn t Worm.

Our new insect specialist,

Fay is on staff now there.

And we're also going
to have another event,

probably at N Rosemont
area on September 7.

We watch for that
for a field days.

Dr. Dean Melk as well.

We'll have an opportunity
to address those as well.

I'll just put a plug
in for the crop news.

A couple of articles that
you've been involved with,

Bob and Bruce and other
people have talked

about soybeans, spider mites.

If you didn't pick up

everything that we're
talking about here,

I'll refer back to University
of Minnesota Crop News.

Finally, Tom, I'll put in

a shameless plug
for an article that

you and I were involved
with in the Crops and

Soils magazine on
herbicide resistance.

You can get it online.

If you get the magazine,

a Certified Crop Advisor,

or whatever, take a look at

the most recent edition
of Crops and soils.

And it gives a
good overview here

of what we're dealing with
in the upper Midwest.

With that, at this point,

I have another question here.

I'm going to segue
back over to Dean Lb,

because it's more
of a disease one,

and save Bob of being

a plant pathologist here
this morning with that.

But we want to thank both of

you for taking the time out of

your schedule to be a part
of the program again.

And so we'd like to also
just to capsulize here.

The sessions were brought to you

by University of
Minnesota Extension,

University of M Farm Families,

Minnesota Soybean Research
and Promotion Council,

along with the Minnesota

Corn Research and
Promotion Council.

Thank you again for

your time and thank
you for attending.

Late-season weed issues and soybean aphid management
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