By Ron Hammond, Andy Michel, Bruce Eisley
Knowing that Ohio growers will in all likelihood be planting more Bt
transgenic corn hybrids next spring, we continue to remind growers about
Insect Resistant Management (IRM) requirements. The purpose of employing
these practices is to help prevent insects from developing resistant to the
Bt gene. These are government regulations that must be followed. Growers who
fail to follow IRM requirements risk losing access to Bt corn technology.
There are two different Bt types that could be used, Bt for corn borer
and Bt for corn rootworms, with three different families of transgenic
hybrids available with the different Bt genes alone or in different stacked
configurations (YieldGard, Herculex, and Agrisure). There are some
similarities and some differences in the requirements for the two Bt types.


First, and foremost, is the need to plant a 20% refuge area to non-Bt
corn. A refuge is simply a block or strip of corn planted with a hybrid that
does not have the Bt gene for controlling the target insects. The primary
purpose is to maintain a pest population that is not exposed to the Bt
toxin, allowing susceptible insects to remain alive in the population to
mate with any resistant insects that survive in the transgenic area. This
allows any offspring to remain susceptible to the Bt.



In terms of the distance of the refuge from the Bt corn, there is a
major difference between transgenic hybrids for corn borer and rootworm. For
corn borer, the non-Bt refuge can be within, adjacent, or near the Bt field,
but has to be within 1/2 mile, preferably within 1/4 mile. For corn
rootworm, the non-Bt refuge has to be within the same field as the Bt corn,
or adjacent to it. When adjacent, it can be separated at most by a ditch or
a road, but not by another field.



When planting the refuge, there are various plantings options,
including a separate field refuge (mainly for corn borer), an adjacent
field, a separate but adjacent block next to the Bt, a block refuge within
the Bt field, the refuge planted along the perimeter, or a split-planter
refuge. If planting the refuge using a split planter, the strip width must
be at least 4 rows for corn borer (6 preferred) and corn rootworm.



There are other guidelines related to IRM, including management of the
Bt field and non-Bt refuge, and use of other insecticides. Both the
transgenic and non-transgenic areas should be managed in a similar manner.
Growers should plant both hybrids close to or at same time, and select Bt
and non-Bt hybrids that have similar growth and development characteristics.
If planting a refuge for rootworm, the cropping history must be the same
(i.e., if the Bt is planted following corn then the refuge must be planted
following corn).



Reducing inputs or placing the refuge on marginal land can reduce the
effectiveness of the refuge. In terms of insecticides, there are procedures
for controlling the target insect (corn borer or corn rootworm) and
secondary insects in both the Bt corn and in the refuge. The major ones are,
for European corn borer, the non-Bt corn refuge may be treated with
conventional insecticides only if the target pest reaches economic
threshold. However, a foliar Bt-based insecticide cannot be used within the
refuge.



For rootworms, a soil-, seed-, or foliar-applied insecticide for
control of rootworm larvae and other soil pests is allowed in refuge. If an
aerial insecticide is applied to the refuge for control of rootworm adults,
the same treatment must be applied at same time to Bt corn. Regarding the
use of rotations with Bt-rootworm hybrids, if the refuge is planted on
rotated grown, the Bt field must also be on rotated ground. However, if the
refuge is corn following corn, than the Bt field can be planted on either a
continuous or rotated field.



If growers are planting a stacked hybrid containing both types of Bt,
they have two approaches on how to manage their refuge. The first choice is
planting separate refuges for each target pest. However, we recommend the
second choice, the common refuge approach where corn without any Bt
technology is planted. The common refuge, also 20%, must be within or
adjacent to the transgenic field, as is the case with Bt-rootworm transgenic
corn.



A common refuge can be treated with soil insecticides or seed
treatments for rootworms, and also non-Bt foliar insecticides for control of
late season pests. If rootworm adults are present at that time, than the Bt
stacked hybrid must also be treated in a similar manner. Growers should seed
their seed dealer for any additional IRM requirements related to the use of
stacked traits.


It is of extreme importance that growers follow these guidelines. Not
only are we concerned about preventing resistance to the Bt technology, but
also to make sure growers do not loose the ability to use this technology
because of making wrong decisions.


There is another suggestion to mention to growers, especially those
planning on using transgenic rootworm corn in first year corn. If possible,
plan on leaving a strip in your corn without any protection from rootworm to
determine if the first year rootworm variant is REALLY a problem in your
field next year. Leaving an untreated strip (no soil insecticide nor seed
treatment) will be the only way to determine if the variant was actually
present AND an economic problem. Corn in these untreated strips should have
their roots dug and rated next July to determine the level of rootworm
injury.


[www.farms.com]