Extended Drought Affects our Corn and Soybean Insect Expectations
By Sam Custer
Extension Educator, Darke County Ohio
August 15, 2012 

Aspergillus Ear Rot and Aflatoxins 

Most of our corn this year has experienced extremely hot, dry conditions and as a result grain development has been severely hampered. In addition, such drought-like conditions have had several producers concerned about Aspergillus ear rot and aflatoxin contamination of grain. Although we have not yet received reports of Aspergillus ear rot and aflatoxin in Ohio, these concerns are understandable and justified. Normally, aflatoxin contamination of corn is not a major issue in Ohio, but the dry conditions experienced across the state could lead to such a problem this year. Drought-stressed corn is more susceptible to infection by Aspergillus flavus, an ear rot fungus that produces a very potent group of carcinogenic (cancer-causing) toxins called aflatoxins. However, the fungus can infect grain and produce aflatoxin under a wide range of conditions: temperature between 54 and 108F; kernel moisture between 15-25% and relative humidity above 80%. But it should be noted that although Aspergillus ear rot is a good indicator of potential aflatoxin contamination, there are no guarantees that: 1) moldy ears will be contaminated with aflatoxins; 2) ears without visual signs of fungal infection will be free of aflatoxins, and 3) the amount of Aspergillus ear rot will provide an accurate measure of the levels of aflatoxin contamination. In addition to weather conditions, the levels of ear rot development and toxin contamination depend of the strain of the fungus, with some members of the Aspergillus flavus group being capable of producing more or less toxins than others. 

Producers should start checking for Aspergillus ear rots by stripping back the husks and examining the ears of 80-100 plants sampled from across the entire field for a yellow-green or gray-green mold. Since not all ear rots are associated with mycotoxin contamination, it is important to properly identify ear rots before harvest in order to determine if mycotoxin will be a concern and to make adequate marketing and storage decisions. Aspergillus ear rot tends to be more severe in insect- or bird-damaged fields or sections of a field and is much easier to identify in the field than in harvested grain. Samples from suspect fields should be sent to an approved laboratory for testing to determine whether aflatoxins are present and whether they exceed thresholds established by the U.S. Food and Drug Administration. More information on aflatoxin testing and FDA thresholds are available at: http://www.rma.usda.gov/fields/il_rso/2012/aflatoxin.pdf  (Originating Author - Pierce Paul). 

Late Season Insect Expectations 

With the growing season coming to an end, there are a few insect pests that we should discuss, including which ones are still a concern and those of interest. The main insect that growers should continue to monitor are bean leaf beetles on soybean, especially in fields that remain green into September. The final adult generation will feed on pods and seeds, resulting in both fewer seeds as well as quality issues. Beetles will tend to go to late maturing fields to feed prior to overwintering, and often cause more damage to seeds than expected. See fact sheet http://ohioline.osu.edu/ent-fact/pdf/0023.pdf for information on pod feeding by bean leaf beetles. This is probably a more widespread problem than realized. 

Another pest to watch out for, albeit we do NOT expect it to be a concern, is the brown marmorated stink bug. This is a potentially new crop pest in Ohio that so far has not caused many problems except for homeowners and renters in the fall. However, we expect it to sooner or later begin feeding on soybeans in late summer. Growers might want to monitor their fields, especially along field edges nearest wooded areas, for the presence of this new pest. 

See fact sheet http://entomology.osu.edu/ag/images/Marmorated_Stink_Bug.pdf for more information. Anyone finding the brown marmorated stink bug in their soybeans is asked to let us know about it by sending an email to hammond.5@osu.edu. 

As we predicted, this past summer was a “low” soybean aphid year (C.O.R.N. Newsletter 2012-08, April 10-16, 2012), with very few aphids being found. If things hold true like in past years, we will begin to see some aphids on soybeans over the coming weeks. However, these will NOT reach levels to cause concern, especially at the late growth stages that soybeans will be at. But it will be these aphids that will produce the eggs that will later be found on buckthorn that will overwinter. We would point out that trying to control any late occurring soybean aphids to prevent an overwintering population will NOT work; so do NOT apply insecticide sprays for these aphids. We will monitor the buckthorn this coming fall, and more importantly, remind growers to keep a watch for aphids next summer. 

Finally, Ohio is now experiencing cooler and wetter conditions, both of which appear to be causing two spotted spider mite populations on soybeans to diminish. Before making any further treatments for mite control, definitely check to see if most mites are still alive. We think you will find that most mite numbers have finally dropped.  (Originating Authors -Ron Hammond,Andy Michel) 

The information in this article is part of the C.O.R.N. newsletter published weekly and the link can be found at the OSU Extension, Darke County web site http://darke.osu.edu/.