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Twospotted Spider Mites Spotted in SD Soybeans

       

                                                                         (Click to enlarge)

Significant infestations of twospotted spider mites have been observed near Beresford, Brookings,
Bruce, and Herrick.

Lowell Feucht (Agronomist with the Beresford Farmers Coop), reported the spider mite infestation
near Beresford on August 1.  Lowell and I visited the field last Friday (August 2) and took the pictures
shown above.

According to Lowell, what alerted him to the infested field was the yellowish color of the soybeans
when viewed from the road.  Upon closer examination, Lowell and the crew at the coop accurately
diagnosed the problem as one caused by spider mites.

About two weeks earlier, Dave Mack (Agronomist with Ramsdell's of Brookings) observed
severe infestations of the twospotted spider mite near Bruce and Brookings.  Dave mentioned
that some soybean plants in the Brookings field were already dropping leaves.

It was actually Professor Jim Smolik who first reported spider mites on soybeans this year
on his experimental plots near Herrick.  Professor Smolik sprayed for the mites in early July.

Spider mite infestation of soybean is not a common occurrence in South Dakota.  The relatively
hot and dry weather of this season may have contributed to spider mite proliferation on our
soybeans.

Twospotted spider mites are known to overwinter as adults on alfalfa and broadleaf weeds.
They resume activity in the spring and are spread around by the wind.  Spider mites are not
insects and are devoid of wings.

Spider mites have syringe-like mouthparts that can pierce and suck the juice out of plant cells.
Injured leaves may look stippled and discolored.  Heavily infested leaves may eventually drop
to the ground due to severe water loss.

Early pod shattering and wrinkled beans may also result from spider mite infestations later
in the season. 

A magnifying lens or microscope will be needed to confirm spider mite infestations because
of their small size.  

Spider mites can be a devastating pest of soybeans.  However, because of their relatively
rare occurrence, no economic thresholds are currently available.

What we know, however, is that a 20% leaf loss during the pod- through seed-stages of
soybean growth is economically significant and must be avoided.  Reproductive stages
soybeans are quite sensitive to leaf loss.

Only two insecticides are labeled for use against spider mites on soybeans:

Lorsban 4E (0.5-1.0 pint per acre, 28-day PHI) and Dimethoate 400 (1 pint per acre, 21-day PHI).

Always read and follow label directions.

Western Bean Cutworms Still a Threat to the North

Although the western bean cutworm flights in the southern counties are past, the threat to the
north may have just started.

Our light traps in Brookings and Howard both indicated surges in western bean cutworm moths over
the past few days.  Corn fields from Brookings to Aberdeen must be scouted accordingly.

Western bean cutworm is a relatively new pest of corn in SD.  We still do not know when they
infest corn in the northern counties.

Both Bt and non-Bt corn are susceptible to western bean cutworm infestations.

 Last Updated on Monday, August 5, 2002 by Mike Catangui