Top Crop Manager

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Bad ’06 heightens rootworm risk for ’07

If you can't rotate, resistant hybrids get nod for rootworm control.


November 13, 2007
By Top Crop Manager

Topics

38aRootworm numbers spiked in many Ontario corn fields in 2006, boosting the threat
that growers could see as much or even more root pruning, goosenecking and yield
losses when they plant corn on corn in medium to heavier soils in 2007.

The timing is tough. It comes just when more Ontario growers are looking to
put corn back into their corn stubble for reasons ranging from new ethanol markets
to lackluster soybean pricing and rotation plans that were confounded because
growers could not get their wheat planted in the fall, as well as the perennial
need on many livestock farms for continuous corn.

"We did see more rootworm in 2006," says Tracey Baute, Ridgetown
based field crops insect specialist for the Ontario Ministry of Agriculture,
Food and Rural Affairs (OMAFRA). "We saw more hot spots over a wider geographic
area too, and it's making us a bit apprehensive about whats in store for 2007."

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For Dekalb agronomist Derek Freitag, the proof was in the combine tanks at
harvest. Freitag works with growers' on-farm strip trials in continuous corn
fields, comparing the new rootworm resistant hybrids versus the same hybrids
minus the rootworm protection. "Multi-year, we figure on a 10 bushel per
acre yield advantage," Freitag says. "Last year, we had fields where
eliminating rootworm injury gave growers an extra 30 bushels and more.

"It's a wake-up call," Freitag adds. Typically, the crops he sees
with 10 bushel losses show little if any above-ground damage. "When you
start seeing goosenecking and lodged corn, you can be pretty sure the losses
are significant."

For 2007, growers' first job is to rate their farm-by-farm risk. In hindsight,
Baute says, it would have been better to have started early last August, scouting
potential corn-on-corn fields to check how many beetles were there to mate and
lay their eggs. The rule of thumb is that on fields with medium to heavy soils,
defense measures are recommended when the count hits an average of one adult
per plant during the month of August.

Otherwise, think back to rootworm damage observed at harvest, she advises,
and keep your eyes and ears open for reports of damage in your region. "As
always, the best solution is rotation," Baute says. "Where you can't
rotate, and where the soil is a heavier loam or a clay, my first choice would
be resistant hybrids."

In research, Baute points out, transgenic rootworm Bt corn is more consistent
and out-performs the high rate of Poncho seed treatment when rootworm pressures
are at mid to high levels. Transgenics also perform as well or better than traditional
insecticides, are simpler, have no need for planter-mounted insecticide hoses,
and eliminate concerns about grower exposure to pesticides.

"Resistance is an excellent line of defense," agrees Gilles Quesnel,
Kemptville based field crops specialist for the OMAFRA. "Rotation is always
best, but if I couldn't rotate, I'd be looking at resistant hybrids, providing
they're a good agronomic and yield choice for your field, especially where I
was concerned that the rootworm pressure might be fairly high."

In addition to August scouting for adults, Quesnel says growers can gauge rootworm
intensity by digging plants in the fall, even after harvest. Use a shovel to
prevent root tearing, then soak the root ball in a pail overnight and wash the
soil away with a hose the next day. If the problem is rootworm, you will find
a smaller root mass. Some rootlets may be severely pruned, right back to the
nub. Larger roots may be hollowed out.

Above-ground symptoms are less reliable. Growers are used to associating goosenecking
with rootworm. However, says Quesnel, heavy June winds combined with wet soils
helped to lay pockets of some fields on their sides last year, causing goosenecking
even where rootworm was not present. As well, plants that lean at harvest but
show no sign of goosenecking or bent stalks are usually blamed on wind. "That's
often right, but not always," Quesnel says. "You should be checking
their roots too."

Freitag agrees. "A lot of rootworm damage gets mis-diagnosed," he
says. "When you see them from the combine seat, you say they just blew
over, but if you get out and pull a few plants, you may find that despite the
straight stalk, the problem is rootworm. There just are not enough roots to
hold the plant up, especially in wet soils."

Also misunderstood is the way rootworm pressures can vary from region to region
and year to year. "We didn't have as much rootworm injury as growers did
west of Toronto," Quesnel reports. "I believe it's because the heavy
rains we got in June saturated our soils and helped to drown a lot of the larvae.
But there is still injury and where you're planting second year and especially
third year corn, you need to think about protection."

In the south and west, Baute puts at least part of the blame for last year's
rootworm spike on the mild winter, when fewer eggs were killed by severe cold.
Also helping were good soil conditions through June that meant abundant roots
for the larvae to feed on.

A bad rootworm year like 2006 can prove doubly bad, Baute says. Not only does
it produce yield losses in the first year, there is also a potential hangover
for the second year, if growers are going to plant corn again this spring in
fields flush with eggs. "I can't say it too often," Baute says.

"If you can, rotate. If you plant soybeans it doesn't matter how many
eggs are in the field. The larvae are going to starve." Where growers cannot
rotate, however, it is best to evaluate risks and plan to protect fields that
are likely to need protection, Baute says.

"Years like 2006 help show us what we've been losing in other years,"
Freitag agrees, "Hopefully, it's the kind of year that will make us even
better going forward."