August 3, 2010
Many people have noted impressive moth populations flying at night. The second generation of eastern corn borer moths is flying now, but most of the moths appear to be other moths, such as the green cloverworm moth and the celery leaftier moth.
European Corn Borer Moth – female and male – Wingspan of about 1 inch (Picture from Iowa State University)
Green Cloverworm Moth - Wingspan of about 1.25 inch (Picture from Iowa State University)
Celery Leaftier Moth – Wingspan of about 0.75 inch (Picture from Tom Hillyer, Hillyer AgriService)
Eurpoean Corn Borers
Even though most moths being observed are not European Corn Borers (ECB), it is still the better part of valor to scout corn that does not carry the Bt gene for corn borer for egg masses. Examine the undersides of the middle seven leaves (ear leaf and the three leaves above and the three leaves below the ear leaf) on 20 plants at five locations in the field.
ECB Egg Mass (Picture from Iowa State University)
Multiply the number of egg masses found by 1.1 to correct for eggs that are elsewhere on the plants. Each egg mass, on average, will produce 4.5 surviving corn borer larvae, and each larva on a plant will reduce yield by 4% if the plant is pollinating and 3% once kernels are initiated. So if 25 egg masses were found on 100 plants, that would average 0.25 egg mass found per plant, which would be multiplied by 1.1 for an average of 0.275 egg mass per plant. Multiply that by 4.5 to arrive at an average of 1.2375 live corn borers per plant. If kernels have been initiated, multiply 1.2375 by .03 (per cent loss per borer) and multiply that by the expected yield. So 1.2375 borers per plant on 200 bushel corn would lower the yield 7.425 bushels (1.2375 X .03 X 200). However, an insecticide application is only about 80% effective, so an insecticide can save only 5.94 bushels (7.425 X 0.80). If the corn is priced at $3.80 per bushel, then the insecticide can save $22.57 of corn (5.94 X $3.80) per acre. In this example, if the cost of the insecticide plus application is less than $22.57 per acre, then making the application is a good investment. However, in this example, if the cost of the insecticide plus application is greater than $22.57 per acre, the producer is better off to live with the corn borers. (If the calculated population is initially low, re-scout the field in 5 to 7 days.)
Gray leaf spot is showing up in many fields, especially in the south. There has also been a report of southern corn leaf rust in the southern part of the area. Southern rust is much more virulent than common rust. Eyespot is more common in the north. If a fungicide is to be sprayed, it would be best if it could be done soon. Fields with hybrids that are more susceptible to foliar diseases that are showing fungal disease lesions within three leaves of the ear leaf are the fields that are most likely to respond to a fungicide application.
Green cloverworms and celery leaftiers are laying eggs for the next generation. Some new celery leaftier larvae are already being found. In the past the cloverworms that hatch in August have not been very destructive because they are killed very soon by fungus, and celery leaftiers may encounter a similar fate. However, it is still prudent to be vigilant. Remember that a fungicide application to soybean may also kill fungi that infect insects, thereby promoting insect health.
Soybean aphids are still at low levels in the area. The warm weather we have had recently should help to slow down their reproduction and allow beneficial insects to keep on top of them. Insecticides sprayed when aphid numbers are very low can actually create an aphid problem by killing all of the beneficials, allowing the aphid populations to explode. So far it is looking like this may be an “off year” for aphids, but the next 2-3 weeks will tell.
Soybean Sudden Death Syndrome is showing up in some fields. I haven’t seen any white mold yet, but suspect it will be showing up soon. Brown spot has been common on the lower leaves in many soybean fields. Frogeye leafspot is also showing up in some fields. Fungicide application can help manage both brown spot and frogeye leaf spot if the disease level is significant. Downy mildew and bacterial blight are also showing up in some fields. Fungicides do not prevent these two diseases because downy mildew does not respond to available fungicides and bacterial blight is caused by bacteria. Most soybean fields are now in the R3 stage, which is usually the best time for fungicide applications if they are going to be made. For pictures of frogeye leafspot see X.B. Yang’s website at http://ssnavi.public.iastate.edu/frogeyeleafspotpics.html.
LATE SUMMER (FALL) SEEDING OF FORAGES
Late summer (fall) seeding of forages should be done by August 10 in the northern third of Iowa, August 20 in the central third of Iowa, and by September 1 in the southern third of Iowa. For more information, see http://www.extension.iastate.edu/Pages/eccrops/latesummerforageseed.html and http://www.extension.iastate.edu/CropNews/2008/0710barnharts2.htm.
FALL MANAGEMENT OF FORAGES
This year’s weather has thrown forage harvesting schedule “out the window” for many. As we approach fall and anticipate future harvesting for 2010, remember that, if the field is to be kept for 2011, it needs a rest period from the first week in September through freeze-down (24 degrees) or through late October, whichever come first. See http://www.extension.iastate.edu/Pages/eccrops/latesummerforagemgt.html.
DEALING WITH FLOOD DAMAGED CROPS
Several thousand acres were flooded this past week along the Wapsipinicon, Maquoketa, and Mississippi Rivers. There were reports of only corn tassels showing above the floodwaters. For corn and soybean fields that were totally submerged, it is doubtful that there will be much of a crop to harvest, so the focus should be on preparing for future year’s crops. Partially submerged crops may survive, but debris will need to be removed from the field to avoid damage to harvest equipment in the fall. Pastures that were submerged for a few days will likely survive, but mud and debris will need to be removed before the pastures can be utilized.
By the time corn and soybean fields dry out enough to walk in them it will be more apparent how much of the crop will survive. Roger Elmore and Lori Abendroth have written an article about mid-season flooding on corn and the potential effects at http://www.extension.iastate.edu/CropNews/2010/0801abendrothelmore.htm. Work at the University of Missouri showed great variation in soybean yield after flooding, depending on variety, time of flooding, and length of flooding; see the PowerPoint presentation at http://agebb.missouri.edu/irrigate/moconf/2005/soybeanflood.ppt.
One of the first things that should be done is to check out the insurance options on the crop.
Even if the crop is totally destroyed, there are things that should be done this summer and fall to restore the productivity of the soil. After removing debris, any sand and gravel deposits of more than 4” will need to be removed from the fields. Silt deposits can usually be worked in to the original topsoil. Since there is now new soil in the field, the soil fertility level has likely changed so the soil should be tested for fertility before planting the next crop.
To prevent soil compaction problems, avoid running trucks and heavy equipment over the wet soils. Most soils are not dry enough for traffic until the top 5-6 inches crumble, rather than slick over or pack. A cover crop such as oats or rye can help to keep weeds under control the rest of the season and can help to dry out waterlogged soils. New weeds may have been introduced to the field in the flood, so these will need to be managed in future years. There may also have been herbicides, such as atrazine, introduced with the soil. An oat cover crop this fall would also help to identify any potential future problems with herbicides in the introduced soil. The main concern with herbicide contamination would be for fields where a late summer seeding of forages will be attempted this August. Late summer forage seedings should be done by August 10-20. For more information on reseeding flood damaged forage crops in the late summer see Steve Barnhart’s article in the ISU Extension Integrated Crop Management News at http://www.extension.iastate.edu/CropNews/2008/0710barnharts2.htm.
Forage grasses and legumes can withstand a few days to a week underwater, but will not perform well if the soil remains saturated for several weeks after the water recedes. Any flood debris that will harm livestock or equipment should be removed as soon as possible. If flooded areas are recovering slowly and there are concerns about the viability of the stand, dig random plants in several areas and evaluate the condition of the root systems. Legume plants with a firm taproot, creamy white in color with no evidence of root rot, and that have green and visually healthy crowns and crown buds have the greatest likelihood for survival. These plants need a week or more of sunshine and drying soils. Legume or grass plants with watery, mushy, textured roots, yellowish or tannish in color, and those with no evidence of active crown buds will be the least likely to survive, even with good growing conditions during the next few weeks. For other information on dealing with the aftermath of the flood see ISU Extension’s Flood Cleanup website at http://www.extension.iastate.edu/disasterrecovery/flood.htm.
FOR YOUR CALENDAR
Northeast Iowa Research and Demonstration Farm, Nashua
Demonstration Garden Field Day on August 7, 2010 at 4:00 p.m.
Fall Field Day on August 26, 2010, 1:30 p.m.
Details are and will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetnerf.html.
Muscatine Island Research and Demonstration Farm, Fruitland
Demonstration Garden Field Day on August 9, 2010 at 6:30 p.m.
Details are posted at http://www.extension.iastate.edu/Pages/eccrops/meetmusc.html.
Southeast Iowa Research and Demonstration Farm, Crawfordsville
Fall Field Day on September 15, 2010
Tentatively there will be a manure application field day in the morning followed by a “more traditional” field day in the afternoon. More details will be posted at http://www.extension.iastate.edu/Pages/eccrops/meetserc.html.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
Nondiscrimination Statement and Information Disclosures