There have been lots of questions on corn aphids late last week and this week. Some fields have very few aphids and some fields have lots of aphids, says Paul Kassel, an ISU Extension field agronomist at Spencer in northwest Iowa.
There are several species of aphids that are causing this damage, he says. Some of them are called corn leaf aphids, bird cherry oat aphid, English oat aphids, green bugs and others. The exact species may not be important because they all do similar damage. They damage corn by using plant moisture, creating honeydew and decreasing the absorption of sunlight.
Kassel says you should consider using an insecticide treatment for corn aphids if the following conditions exist:
* Corn aphids are present at the ear leaf and above on 80% of the plants.
* Numbers of aphids and aphid colonies appear to be on the increase.
* Dry weather/low soil moisture will increase the potential damage to the corn
* Chlorpyrifos may be the product of choice because of the vapor/fume activity.
* Corn development is at or before the early dent stage.
* Corn that is in the hard dent stage will not benefit much from treatment.
Other issues in crop production emerged over the past two weeks
One big issue is the rapid development of the 2011 corn crop. The week of August 8 to 12, many farmers saw ears of corn starting to dent early, way ahead of schedule. "This rapid development seemed to occur during the week that ended August 12, as it was easy to find corn in the dough stage (cob turns pink in non-white cob hybrids) early in the week. And by week's end, it was fairly easy to see corn that was dented," says Kassel.
Why is this 'not good'? The dent stage of corn (called the R5 stage) is supposed to take about 31 to 33 days, he explains. This year it took about 21 days, if you consider that the average pollination date in the Spencer area was about July 20.
Why did this rapid corn development happen? The Growing Degree Day (GDD) accumulation is about 100 above normal this summer. "Usually we expect that corn would require about 600 GDDs to reach early dent stage (R5)," he answers. "It is interesting to note that we have had nearly that amount in my area of northwest Iowa from July 20 to August 12. You can go to the ISU Mesonet site to look up specific information from different locations in Iowa."
Warm nights in late July and early August were also a factor
The warm nights that occurred in late July and early August was clearly another factor that hurt this year's corn yield potential. This means that the corn plant uses energy during the night that it would normally keep if night time temperatures were cool, says Kassel. Also contributing to corn crop stress was the high daytime temperatures, which contributed to stress degree days (also available on the Mesonet). The high temperatures – in spite of high humidity – caused a lot of stress on the corn crop.
Farmers seeing shallow kernels and some kernel "tip back" on ears
It appears that the advanced development of the corn crop this summer is showing up as shallow kernels and ear tip-back. "This will mean reduced corn yields," he notes. Pictures show this point. True, you could find this symptom most any year somewhere, but it seems pretty common this year.
Kassel adds, "I'm guessing we will see a lot of 175 bushel per acre corn yields this fall, based on ear counts and kernel counts I'm now seeing and hearing about. A yield of 175 bushels per acre is not bad, but will be very disappointing to many local farmers. I hope I am wrong on all of this--as time will tell."
Recent hot weather may have benefitted the 2011 soybean crop
The recent hot weather may benefit the 2011 soybean crop. Soybeans appeared to stay in the full bloom (R2) stage for a longer time period this year, says Kassel. This may benefit soybean yields by lengthening the time soybeans are in the reproductive stages and adding nodes. Additional nodes usually means more yield potential. Currently many soybean fields are in the full pod (R4 stage) to the early beginning seed stage (R5 stage).
Many fields were treated for soybean aphids the first two weeks of August 2011. "We are encouraging farmers to continue to check for soybean aphids," says Kassel. "Soybeans will be susceptible to damage from aphids until for about another three weeks (or late R5 growth stage of soybeans, which is when the seed is is 1/8th inch long in the pod at one of the top four nodes). Also, it is important to check for aphid re-infestation in soybean fields where insecticides were applied in late July."
Goss's wilt is a lingering corn disease concern this summer in Iowa
Goss's wilt is still a concern. This disease has become more apparent in fields in northwest Iowa and in other areas of Iowa too. The recent dry weather should limit the development of this disease. Heavy rainfall followed by hot humid conditions would favor Goss's wilt development. Further info on Goss's is available from Extension plant pathologist Alison Robertson at ISU and also here is some information from the University of Nebraska.
Kassel adds, "Goss's wilt will be more prevalent in corn-on-corn fields, in fields that had Goss's wilt in 2010 and in fields with susceptible corn hybrids. Check local test plots for hybrid susceptibility to Goss's wilt. Also check seed company literature to evaluate corn hybrids for Goss's wilt resistance for next year.