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April, 2007
Source:
Integrated Crop
Management IC 498 (7), - April 23, 2007 issue
Iowa
State University
Seed treatments in soybean:
managing bean leaf beetles
by Jeffrey D. Bradshaw and Marlin E. Rice, Department of
Entomology, and John H. Hill, Department of Plant Pathology
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Bean leaf beetle.
(Marlin E. Rice) |
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To date, our recommendation for
the chemical control of bean leaf beetles and bean pod mottle
virus has been for an early and a mid-season application of a
pyrethroid insecticide (e.g., Asana®, Mustang®, or Warrior®).
These insecticide applications should be timed such that fields
are treated as soon as bean leaf beetles are first detected in
the field (the early-season application) and again when the
first generation emerges in early July (the mid-season
application). These applications have been shown to improve
yield and seed quality under high disease and beetle pressure.
However, the hope of greater yield and convenience has driven
earlier soybean planting dates and a number of growers will
likely use seed-applied insecticides this year. We have recently
completed a three-year seed treatment study and here we report
the results on beetle suppression. Next week we will address the
effects of seed treatments on virus disease control.
Our study
During 2002-2004, at three Iowa locations, we studied the
effects of seed- and foliar-applied insecticides to manage bean
leaf beetles and bean pod mottle virus in soybean. Bean leaf
beetle populations reached a record high in 2002 and declined
thereafter. However, these changes in beetle populations gave us
the opportunity to test our recommended management program for
this pest complex under differing levels of beetle infestation.
Overall, we wanted to know if a seed-applied insecticide could
replace an early-season foliar application within the framework
of our current recommendation.
We compared systemic seed-applied insecticides and
foliar-applied pyrethroids with untreated checks. Note that we
did not apply any insecticides according to the recommended
threshold for late-season beetle injury to pods.
Our results
In our study, an early-season insecticide gave the greatest
reduction in beetle abundance with the reduction being most
consistent for seed-applied insecticides. Often the addition of
a mid-season foliar application did give a greater suppression
in overall beetle abundance than the seed-treated insecticide
application alone.
Soybean yield can be improved if a seed treatment is used in
place of an early-season foliar insecticide applied for bean
leaf beetle. In contrast, seed quality was most consistently
improved with an early- plus mid-season foliar application
(targeting the over-wintered population and first generation).
Compared to seed treatments alone, the early-season foliar
application resulted in the greatest improvement in seed quality
(~5% improvement). The addition of a second insecticide
application (to target the first generation) improved seed
quality further still (~10% improvement). However, soybeans that
received a seed treatment alone often had reduced seed quality
even if a mid-season foliar application was added. This
relationship between seed quality and yield was most pronounced
in 2002 (our outbreak bean leaf beetle year). However, in 2003
and 2004 (years when bean leaf beetle populations were smaller),
the use of either a seed treatment or an early-season
foliage-applied insecticide produced similar levels of seed
quality.
The economic gain threshold as determined in this study was ~2.0
bushels per acre for an early- plus mid-season insecticide
application (assuming a crop value of $6.33). The use of a
seed-applied insecticide for the early-season application
improved yield ~2.4 bu/acre beyond the gain threshold, while
using a foliar application for both timings gave ~2.1 bu/acre
beyond the gain threshold. Look in next week's Integrated Crop
Management newsletter to learn about the impact of these
insecticides and timings on virus disease control.
Jeffrey D. Bradshaw is an entomology research associate studying
the beetle/virus interaction and their management. Marlin E.
Rice is a professor of entomology with extension and research
responsibilities. John H. Hill is a professor of plant pathology
with research and teaching responsibilities in virology.
This article originally appeared on pages 123-124 of the IC-498
(7) -- April 23, 2007 issue.
http://www.ipm.iastate.edu/ipm/icm/2007/4-23/beanleafbeetle.html
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