SEE Mail Issue 070809 (August 9, 2007)

	Extension Entomology Mail Mike Catangui, Ph.D. Associate Professor & Extension Entomologist
August 8, 2007 (Issue 070809)

Photos added 8-24-07:

Today's Update (Friday 8-24-07): I was on the field this afternoon and saw some nice looking spider mites
here in Brookings on R6 (full seed) soybean; see above photos that I took with my camera and microscope.
I am of the opinion that some growers who ignored the spider mites may get a nasty surprise at harvest. The
spider mite populations appear to be multiplying unabatedly despite the rain. I did recommend that spider mites
be controlled at 11 mites per square inch; today, I counted up to 70 mites per square inch in the untreated
plots. Time permitting, I might spray the mites even beyond R6 and see what happens.

BROOKING, SD – Significant numbers of twospotted spider mites are again being reported to me this year.

Timely SDSU Extension Entomology research results from 2006 may help growers decide whether to spray
or not for these arachnids.

Gary Erickson, Brown County Agronomy Educator, referred a soybean grower to me two weeks ago who
had spider mites on his soybean. He "only" had 2,000 acres to spray (they think big in Aberdeen). I again
checked with Gary today and confirmed that the infested field was sprayed last week with an insecticide
containing the active ingredient chlorpyrifos.

While spraying our precision-planted soybean plots in Beresford and Brookings, I also observed some spider
mites on the plants this week and last. I took some photos for you (more to come):

Spider mites are arachnids and not insects. Scouting for spider mites may be difficult as they cannot ordinarily
be seen with the naked eye. However, mite injury on soybean can be diagnosed easily by looking for white
“stipples” on the upper surface of infested leaves. The mites can actually be found on the underside of the
leaves and may produce webbings and a “bronzed” surface appearance. The twospotted spider mite is the
species found on soybean. Mites damage crops by piercing plant cells with their mouthparts and sucking the
plant juices.

There are currently no established economic thresholds for spider mites on soybeans in SD; SDSU Extension
Entomology research in 2006 indicated that about 11 spider mites per square inch of leaf area may reduce yield.
I therefore recommend that spider mites be controlled if you have this level of infestation. Collect about 10 tri-
foliates at random from 10 plants on at least 10 sites per 40-acre field. Include the center of the field.

Freeze the collected trifoliates to immobilize the mites. You will need a microscope to count the mites properly.
A transparency with a 1 x 1 inch grid will greatly help in counting. You can download a graph from my old
publication (http://plantsci.sdstate.edu/ent/entpubs/blb_defol.htm).

Population density of 11 mites per square inch translates to about 3,500 mites per plant. Details of our study
can be found at the Extension Entomology Web site.

We knew the average number of square inches per leaf and the average number of leaves per plant; the number
of mites per plant can therefore be estimated using simple math. We used a LICOR LI-3000A portable leaf
area meter to figure out the average leaf area per leaf.

Pesticides labeled for use against spider mites on soybean are as follows: Dimate (1 pint per acre, 21 day PHI);
Lorsban 4E (0.5-1.0 pint per acre, 28 day PHI); and Proaxis (3.84 fluid ounces per acre, 45 day PHI). Always
read and follow label directions.

Why have the spider mites been abundant this year and last? Some experts will point out drought; I hypothesize
something else and it is related to our over-reliance on one group of active ingredient in our insecticides.

Luckily, we are currently testing brand new compounds that will work on mites, and, did I hear somebody
reporting whiteflies? Buford, did you say whiteflies?!! Whiteflies?!!

Web Posted on Friday, February 29, 2008 by Mike Catangui. Last Updated on Friday, August 24, 2007.