AGRICULTURE

Hot and dry conditions are the story across the region

The weather conditions last weekend allowed anyone who was outside last Saturday evening to view  spectacular light shows to our north and east, and then eventually south, compliments of Mother Nature.  Unfortunately, this thunderstorm activity left southwest Michigan without rain and with very high temperatures.  Both corn and soybean plants on lighter ground were showing the effects of heat and moisture stress.  Evapotranspiration rates are approaching 0.3"- 0.35" per day for corn and 0.2"-0.25" in rapidly growing soybeans at this stage of their development.  We really could use a rain in most areas to help insure that the crops, which were off to an good early start, continue their development and reach their excellent potential yields.  The double hit of low yield combined with the near record low prices would really hurt a lot of field crop producers this summer.   

Corn insect levels remain low, little damage from WCR has been seen in first year corn following soybeans during 1999.

Western corn rootworm beetle activity was very light in the corn fields that I walked through in Cass and Van Buren counties while collecting samples for root analysis.   Japanese beetles were more prevalent, but their numbers also seemed to be a bit lower than usual as well.  The above normal temperatures that we have received this season may have pushed the crop development a little ahead of the pests, which may explain the reduced numbers of these silk clipping beetles at this point.  Late planted fields that are just now reaching pollination may bear watching for insect feeding activity in the silks.  Remember, pollination can occur normally with as little as 1/2 inch of viable silk remaining on the ear.  Second generation European Corn Borers (ECB) may still pose a threat, especially to the latest planted corn in a particular area.   Moth flight may be on the upswing again.  Damage from first generation ECB has generally been quite low in commercial corn fields this year. 

Root samples were collected from several first year corn fields in Cass and Van Buren Counties that were monitored for WCR beetle activity in soybeans last season.  While feeding damage was observed in virtually all of the samples that were analyzed (confirming the presence of the rotation resistant WCR variant), damage was very limited, with only one of the fields sampled showing a root injury rating of 3 or above.  An Iowa scale root rating of 3 or above is generally considered to be the minimal level of economic injury.  The field that had the highest injury rating did have the highest numbers of beetles per 100 sweeps last season (in those two counties).  WCR activity in soybean fields was a lot lower during 1998 than in 1997, and observed injury to first year corn in the area has been reduced this year compared to 1998.  I would urge producers in Southwest Michigan to monitor their soybean fields that will be rotated to corn to determine if it will be necessary to apply a soil insecticide to protect next years crop.  

Dry weather raises concern over spider mites in soybeans in Southwest Michigan.

  Photograph of the Two Spotted Spider Mite taken by Dr. Marlin E. Rice, Iowa State University Department of Entomology.  Click here to visit the excellent ISU Entomology Image Gallery.

As we move into August, dry conditions often can lead to population explosions in two spotted spider mites.  This year is no exception.  A few producers in Van Buren county have begun to spot spray for the pest.  Spider mites, which are a member of the arachnid family (8 legs instead of 6), can cause damage to soybeans by feeding on the the plant sap on the underside of soybean leaves.  Spider mites are often a translucent greenish white to yellow in appearance, but can also be orange to reddish brown in coloration.  Early spider mite injury to soybeans can often look like herbicide injury.  Observe several plants, look carefully for small yellow spots on the leaves.  As injury symptoms worsen, the leaf can take on a bronze coloration and the surface can almost appear to be sandblasted.  Look for live mites on the underside of soybean leaves.  Some producers monitor for this pest by holding colored paper under the leaf and tapping the leaf surface with a pen or pencil.  Under drought conditions, treatment should be considered if plant spotting (stippling) is found and live mites are present.  Entomology researchers from the University of Illinois report that spider mites can reduce yields to near zero under extreme conditions, but often reduce yields 40 - 60 % in heavily infested areas.  Dr. Chris DiFonzo, MSU Field Crops Entomologist, reported in the August 20, 1998 Field Crops CAT Alert that when an infestation is identified early, spot treatments of the affected area plus a border strip may be enough.  For soybeans, estimate the percentage of leaf surface damaged by mites. Treatment thresholds (based on percent defoliation) vary with plant stage.

pre-bloom = 40% defoliation
bloom R1 to pod fill R5 = 15% defoliation
pod fill R5 to early maturity R7 = 25% defoliation
after R7 = do not spray; defoliation at this point has
little effect on yield.

A systemic product like dimethoate is a good choice for control in beans because of cost and systemic activity. Full coverage is not necessary. Later in the season, spraying obviously becomes more difficult due to closing of the canopy, and less effective as plants mature (yield no longer affected).

What about natural controls? Several predators feed on mites, but may not be able to keep up during heavy infestations. A heavy rain may reduce the population, but don’t depend on
rain alone. The real key is high humidity, since fungal pathogens that kill mites require high-sustained humidity to grow and spread through the population. A heavy rain followed by quick drying does not provide a long enough period for the diseases to spread and kill a significant portion of the mite population. But if you recently received or expect rain, and the humidity remains high, hold off on spraying for a few days to see if the mite population is decreasing.

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Last modified: July 27, 1999