Seed treatments and soybean aphids
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
included.
This issue of the Field Crop CAT Alert
includes an article by Terry Schulz and Kurt Thelen about seed
treatments and soybean yield response. From a soybean aphid standpoint, I
can confirm many of their findings.
Neonicotinoid seed treatments are recommended in soybean fields with a
history of economic damage from seed corn maggot, white grubs and other
soil insects. Bean leaf beetle is another early season pest that is
controlled by seed treatments. In Michigan, early-season populations of
bean leaf beetle are rarely high enough to merit treatment (Young beans
can tolerate considerable injury.), but in western states where beetle
populations are typically higher, seed treatments may be valuable.
However, there is no good evidence that bean pod mottle virus infection
(transmitted by bean leaf beetle) is less in seed-treated soybean.
Seed treatments for “insurance” or prophylactic control of soybean aphid are not
recommended as a standard practice. Laboratory and field studies
indicate that soybean aphid begins to survive on seed-treated plants 35
to 40 days after planting. In many parts of the United States and
Canada, this is just when aphids begin to colonize fields. In outbreak
years, populations in seed-treated fields often reach the 250 threshold,
and require foliar insecticide sprays, at the same time as in untreated
fields. Replicated university research trials across multiple states
and years do not show a significant yield increases from using seed
treatments under no or low aphid pressure. Many of these trials used
small plots, but others were done in 1-acre blocks (Michigan, Iowa and
Minnesota) or in replicated strips the length of a field (Nebraska).
During aphid outbreaks, using an IPM approach based on crop scouting and
thresholds to optimally-time a foliar spray results in significantly
greater yield compared to using a seed treatment. An IPM approach to
soybean aphid control also limits insecticide use to when and where it
is needed, reducing pesticide exposure and selection for resistance.
Formation of insecticide resistance is a growing concern, because
neonicotinoids are available as both seed treatments and foliar sprays
in many crops.
In Michigan, there is one small area where I believe past history may
justify a seed treatment for soybean aphid in predicted outbreak years.
Parts of Monroe County in southeast Michigan have some of the heaviest
infestations of buckthorn (soybean aphid’s overwintering host) in the
state. In the spring of 2005, colonization of soybean fields around
Dundee, Monroe and other towns near the River Raisin was early and
heavy, and aphids went over threshold by mid-June. Some fields were
sprayed early, while other fields had been seed treated. In essence, the
seed treatment replaced the first foliar spray. Later, fields were
reinfested and many were sprayed a second time in late July. Remember,
this is an exceptional area, based on the sheer amount of buckthorn. We
know egg numbers were very high in this area this past fall and aphids
may colonize fields early. Again, in this limited area of Michigan, the
assumption is that early colonization sets up a two-spray system, and a
seed treatment replaces the first foliar spray. From a time management
standpoint, I realize that the seed treatment has advantages on the
farm. From an IPM standpoint, though, two foliar sprays are equally as
effective, and the sprays can be timed if and when they are needed.