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Corn rootworm pressure difficult to predict for 2006

Experts say growers who plant corn after corn should evaluate the risk and consider rootworm protection.


November 13, 2007
By Top Crop Manager

Topics

22aPlanting corn after corn is something Innerkip, Ontario's Vollmershausen Farms
tries to avoid, but Larry Vollmershausen says that is not always possible when
managing a rotation.

In 2005, they planted 300 acres of second year corn and were very aware of
the potential impact corn rootworm could have on yield. "You can get corn
rootworm in those back-to-back fields because the bugs tend to stay in the soil,
especially after a good year where you have a lot of root matter," Vollmershausen
explains. To protect their corn against yield robbing corn rootworm, they planted
DKC46-24, a Dekalb hybrid that provides in-plant corn rootworm protection. "Any
time we don't have to handle insecticides, it's a big plus. It's great technology
when they put it in the seed. It really works."

Vollmershausen's second year corn had to endure some tough drought conditions
in July, but it met the challenge. "We didn't have it on real good soil,
it's on sand farms, but it did the job," he says. "You could tell
by the way the corn remained standing. If it didn't have rootworm protection,
it would have been all flopped over."

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According to researchers and agronomists, 2005 corn rootworm pressure was likely
the heaviest in recent memory and certainly reinforced the need for growers
to consider rootworm protection when planting corn after corn. Weather and soil
conditions were the main reason for increased corn rootworm numbers, says Dekalb
agronomist Derek Freitag. "In a year like 2005 you're really going to see
higher impact. The warm spring and the good soil moisture really helped larvae
survival, which led to a high level of feeding.

"We had quite a few calls from growers who didn't believe they had rootworm,"
says Freitag. "They never had lodged plants and they didn't have a problem
the previous year during 2004's cooler, wet spring. But the combination of 2005's
warm spring and July rains softened the soil and exposed a problem for many
growers as plants started leaning over and goose necking."

Ridgetown College, University of Guelph professor Art Schaafsma also observed
heavier rootworm activity. "I think on corn-after-corn fields, the number
of beetles this year was higher overall than I've seen in a couple of years."

Typically, adult corn rootworms can be found feeding on ears later in the growing
season, but larvae feeding underground cause most rootworm damage. In June,
larvae are attracted to roots by the carbon dioxide they produce. The worms
then feed on tiny root hairs and tunnel into larger roots. In severe cases of
injury, both main and brace roots may be destroyed.

Plants with damaged root systems lack the ability to extract enough nutrients
and moisture from the soil to maintain plant vigour. As a result, weakened plants
tend to bend and lodge. As the plant attempts to correct its growth, it tends
to grow in a 'goose neck' shape. With severe lodging, harvest losses can also
contribute to yield reduction.

Schaafsma's simple advice to growers is avoid growing corn after corn, but
when growers find themselves growing corn on the same ground in consecutive
years, they should consider rootworm protection. He adds that typically, protection
is not required on sandy soils, "but as you get into finer soils such as
the clays, you have higher risk in second to third year corn."

Schaafsma advises growers to evaluate their risk and then determine which option,
traditional insecticides or the higher rate of Poncho or rootworm Bt hybrids,
delivers the protection they need.

Freitag notes that in 29 farm scale trials conducted over 2003 and 2004 on
multi-year corn, Dekalb corn rootworm hybrids provided a 10.9bu/ac more yield
than the isoline (or same hybrid) without in-plant protection. "My rule
of thumb is that growers should have some form of rootworm control in second
year corn," says Freitag. "It's good insurance and in a year like
2005 it really pays off."

Both Freitag and Schaafsma admit it is difficult for growers to try to predict
whether they will see heavy rootworm pressure from one year to the next because
of winter mortality of eggs and other factors that are hard to assess. However,
they do suggest that field scouting during the month of August can provide some
clues. Schaafsma advises growers to scout fields once a week during the month
to assess beetle numbers. "If beetles are noticeable and you can count
one beetle per plant, you have a higher risk. If you are hard pressed to find
beetles, your risk is way down. The magic number is one," he says. -30-

Triple stacked hybrids fit well in multi-year
corn
Planting 'triple stacked' corn hybrids in second year corn ground
makes a lot of sense to this grower.

Ross Townsend Jr., who farms with his father, Ross Sr., and brother Keith near
Tavistock, Ontario, says a Roundup Ready hybrid that also provides in-plant
corn borer and corn rootworm protection is a good fit for their operation. "The
number one reason a triple stacked hybrid makes sense is because we've switched
to Roundup Ready corn. We also have a problem with corn borer and on a rotation
basis we do grow some corn on corn," explains Townsend.

With the triple stacked, "you're getting one bag of corn seed and getting
all the genes you want in it. When it comes to rootworm, we used to use a lot
of insecticides growing corn on corn. Having to dump all those bags of insecticide
into the planter slows down planting. The insecticides also pose more risk to
the equipment operator."

The Townsends planted Dekalb's DKC46-22 triple stacked hybrid for the first
time in 2005. When it comes to managing rootworm, Townsend likes what he sees
from the hybrid's in-plant rootworm protection.

"About three years ago when they were first testing the new hybrids, we
ran a test in a second year corn field that compared the rootworm hybrids to
non-rootworm hybrids treated with a normal insecticide," explains Townsend.
"When we dug up the roots and compared them, the rootworm hybrids definitely
had a lot bigger root mass. The rootworm hybrids had healthier stalks and roots
and a healthier crop means more yield."

Jamie Rickard, Dekalb brand marketing manager, says "Stacked hybrids cost
more than a conventional bag of seed, but when you do the math, you can certainly
see the value they provide.

"Since the release of Bt corn in 1996, the trend has been towards putting
more value into the seed and less in cultural or chemical controls. Triple stacked
hybrids are just the latest example," says Rickard. "When evaluating
triple stacks you have to consider that you are getting corn borer and corn
rootworm control, early-season insect protection from Poncho 250 and the convenience
and flexibility of Roundup Ready. That all adds up to yield and performance,"
he says. "All you have to do is plant the seed and spray Roundup."

Rickard says growers can still opt to plant conventional hybrids, "but
when you add up the costs of conventional weed control, rootworm insecticide
and potential loss of yield due to early season insects as well as corn borer,
stacked trait hybrids are strong options that deliver excellent value."

Dekalb will have three stacked trait options available for 2006 and Rickard
expects that number will likely double the following year.

No economic impact yet from new rootworm
variant

Researchers have yet to find any evidence that a rotation-resistant variant
of the western corn rootworm has had any impact in Ontario fields.

"It is across the border in Michigan and it's likely here as well,
but so far we haven't seen any economic damage from it," reports
Ridgetown College, University of Guelph professor Art Schaafsma.

Researchers have been monitoring the movement of the variant since it
was first found in 1987 in a field in Ford County, Illinois. The variant
has since spread northeast through Illinois, Indiana and into Michigan
and Ohio.

Corn rootworms traditionally lay their eggs in cornfields in anticipation
of a corn crop the following year, but when growers rotate to soybeans,
the offspring perish because they do not have anything to feed on. The
new variant of the western rootworm, however, lays its eggs in soybean
fields rather than corn fields. These eggs survive crop rotation efforts
and feed on corn plants the following year.

In 2005, Schaafsma and his research team picked 70 first year corn fields
at random and found that 12 to 15 fields had some damage, but none of
them had sustained an economic impact. "We had one field with high
damage, but after talking with the grower we found that the field was
actually a second or third year corn field."