(Past issues can be found in the archives section)

SDSU Soybean Defoliation Research--
How Tough is the Soybean Plant?

   
   
                     (Click on thumbnail for a bigger picture)

Thistle caterpillars, bean leaf beetles, grasshoppers, alfalfa webworms,
and white-lined sphinx moth larvae are all insects that can consume
huge amounts of soybean leaves.

However, based on a research conducted by Dave Mills (entomology graduate student)
last season, we know that the soybean plant can tolerate a lot of leaf destruction and
still produce good yields.

To verify the tolerance of soybeans to defoliation, Dave and our entomology crew mowed
(using a shrub trimmer) soybean leaves to simulate 90 percent defoliation at full bloom,
45 percent defoliation at full bloom, and 38 percent defoliation at pod-fill.

The yield reductions at harvest were 18 percent, 8 percent, and 27 percent, respectively.

These results indicate that soybeans can indeed sustain heavy defoliations and still
produce decent yields.  Soybeans with 90 percent of their leaves destroyed at full-bloom,
for example, still produced 82 percent of their potential seed yield at harvest.

However, our research also showed that soybeans are very sensitive to defoliation during the
pod fill stages of development. A  38 percent leaf loss resulted in 27 percent loss in seed yield.

Soybean defoliation data generated by Iowa State University (Ritchie et al. 1997; How a
soybean plant develops) has stated the following relationships between defoliation and yield:

(a) Fifty percent defoliation at V6 or sixth node stage will reduce yield by  3 percent.
(b) Fifty percent defoliation at R2 or full bloom stage will reduce yield by 6 percent.
(c) One hundred percent defoliation during the pod fill stages will reduce yield by 75 percent.

With these information in mind, we could then calculate the economic thresholds of any
insect soybean defoliators from thistle caterpillars to bean leaf beetles to alfalfa webworms.

  Economic Thresholds of Insect Defoliators in Soybeans

The procedures and mathematical formula for calculating economic thresholds of insect defoliators
in soybeans can be found in FS 905: Grasshopper and bean leaf beetle economic thresholds in
soybeans.  Control cost, market value, yield potential, and stage of development are taken into
account in calculating the economic thresholds.

Notice that the economic thresholds are lower for the reproductive stages.  Again, this is because
soybeans are more sensitive to defoliation once they start forming and filling pods.

The thresholds below were calculated assuming a soybean market value of $5 per bushel, and
a yield potential of 40 bushels per acre.

A simple procedure for estimating percent defoliation using a graphing paper copied onto
a transparency can be found in the bean leaf beetle Web site.

White-lined Sphinx Moth Larvae Join the Defoliators

     

Another insect has joined the already long list of defoliators this year.  It is called the white-lined
sphinx moth larva.  The name comes from the fact that the adult sphinx moth (a.k.a., hummingbird
moth or hawk moth) has white lines on its wings.  The larva or hornworm is a defoliator so the
economic thresholds above also apply to them if found on soybeans.  Refer to the July 2 Issue
for thresholds on sunflowers.

Agronomy Educator Justin Keyser (Gregory County) reported damage on sunflower seedlings
due to this insect about 3 weeks ago.  Yankton, Charles Mix, and Tripp counties have also
reported large infestations of white-lined sphinx moth larvae near urban areas.

Craig Anthony (Yankton County Livestock Educator) sent me the "can of worms" used for
the pictures above.  The adult moths came from Bob Fanning's (Lyman County Agronomy
Educator) corn borer light trap in Kennebec.

 Web posted on July 10, 2001 by Mike Catangui

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