SW MN IPM Stuff 2016 Issue 6

Volume 19, Issue 6 - May 26, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-06

This newsletter and the advice herin are free. You usually get what you pay for.

Crop Weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN can be found at the SWROC weather webpage.

  • We have received 260 degree-days (base 50°F) and 2.09 inches of rainfall May 1-25.
  • There were 105 degree-days (base 50°F) accumulated from April 11-30.
  • The largest corn on the SWROC is V3-V4 (3-4 collar counting the seed leaf) stage.
  • The largest soybeans planted April 13 are at V1 (1st trifoliate open).

Looks like another stormy, rainy stretch. Some areas to the north and east have already received hail from round 1. You might want to save a copy of this recent University of Minnesota Crop News article and the contained links in for current or future reference: Hail damage and replant options for corn and soybean.

Southwest MN was spared from frost fatalities to corn and soybean crops. There has been some replanting of soybeans and cornfields in the southeast and will likely be some soybean replants in areas receiving hail on the 24th.

Meanwhile, in SW MN, the problem has usually been too much water. Corn replants have been directed to low spots and crusted portions of fields. Another shot of rain earlier this week has once again, left water standing in some areas.

Corn may be replanted late-season to maintain a crop rotation and to provide competition for weeds. Some growers may choose to plant soybeans instead, herbicide permitting. Replanting corn to soybeans may have some nematode and disease consequences. Read the label for the corn herbicide applied for any rotational restrictions to soybean (or other crop). 

Raising the dead

When experiencing a catastrophic crop injury, such as frost or hail, dismay, frustration and a feeling of helplessness is an understandable first reaction. For growers, this is quickly followed by: is there anything I can do to help my crop recover?

There is a simple biological fact: some wounds are fatal and some are temporary setbacks from which your crop can recover. The severity and timing of the injury to your crop, crop genetics and physiology, and the subsequent weather will determine whether the plants survive. With the exception of variety selection, these factors are beyond human control. 

Visiting a local watering hole or the internet may provide a source of diverse agronomic opinions...and some might even be correct. While applying a fungicide to a damaged crop might initially seem like a good idea, there are some points to consider. Missing or dead plant tissue cannot absorb and translocate fungicide. Fungicides do not control bacteria that enter wounded plant tissues. There is little conclusive evidence for fungicides speeding recovery or increasing yield of hail injured corn and soybeans. Less evidence exists for their ability to return mortally injured plants to life - corn and soybean zombies would be an interesting development.

Whether or not to apply a fungicide is a crop input management decision that must be made by the grower. Some might want to let nature take its course while doing nothing may not set well with some more nervous types. Applying fungicides to hail injured crops will certainly stimulate the economy, but it is not always the grower's bottom line that receives the stimulation. 

cut corn leaf

Figure 1. Cutting corn above the soil line and growing point is not fatal. Soybeans and other dicot plants will be killed if cut below the cotyledon. Cutting above the ground level is more representative of climbing cutworms like dingy cutworm. Unless soils are very wet, black cutworms tend to cut at or below the soil surface. It is important to identify which species is causing the damage. Black cutworms can cause significant stand loss while dingy cutworms will kill soybeans but seldom kill corn


Reports of populations of black cutworm threatening corn have been very scattered. Steve Commerford reported a field with economic threshold levels of damage in Nicollet County. This complements the previous report from Benton County and could have originated from a March 28 or more likely an April 12 flight. Most agriculturalists I have talked with have seen few signs of cutworms and it would be good if it stays that way. In some cases, corn may outgrow cutworms’ ability to cause yield-reducing damage.

Degree-day based projected cutting and pupation (end of feeding) dates are listed below. If you are unfortunate, a field may have had more than one cutworm flight and multiple cutworm species present.

Date* of significant trap capture Trap location Degree-days/estimated development stage Projected date to corn corn cutting Projected date to end cutting
28-Mar Swift Co. 437 DD/5th instar cutting 19-May 9-Jun
12-Apr Rock Co. 366 DD/5th instar cutting 23-May 12-Jun
19-Apr Murray Co. 270 DD/3rd instar leaf feeding 29-May 18-Jun
20-Apr Sibley Co. 278 DD/3rd instar leaf feeding 28-May 18-Jun
24-Apr Rock Co. 262 DD/leaf feeding 29-May 18-Jun
24-Apr Meeker Co. 232 DD/leaf feeding 31-May 21-Jun
*first date if two-night significant flight Degree-day accumulations to 5/19/16

Table 1. Dates and locations of significant 2016 captures of black cutworm moths in 2015. The estimate of projected cutting date is based on accumulating 300 Degree Days (base 50ºF) after a significant capture. It uses 30-year historical temperature data for projecting future degree-days. Projections run with temperature data current to May 19, 2016. Source: https://mygeohub.org/groups/u2u/gdd

corn with feeding damage weed with feeding damage

Figure 2. Small cutworm larvae are hard to find in the soil. Leaf feeding is similar to many other insects.In this case, a cutworm was not found and several insects may have caused damage. Look under soil clods and residue starting at the damaged plant and working down the row both ways. Cutworms often rest at the boundary between moist and dry soil. Larger cutworms moving from weeds to a Bt trait effective against cutworms will feed and might be able to cut a plant before they die.

Figure 3. Look for cutting and leaf feeding damage on weeds as well as the crop.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    


Figure 4. Slug collected from SWROC alfalfa.

Over on the eastern front, Mark Bernard found low levels of slug damage to soybeans in high residue situation. We have been picking up a few of the slimy critters in sweeps of SWROC alfalfa this spring, which is unusual. Slugs are not related to insect or arthropods being kin to snails, clams, octopods and such.

Slug damage to crops in MN has occurred in the past but rarely so. This may change as more residue is left on the surface or cover crop plantings increase.

They require cool moist conditions and dense residue or vegetation ground cover help provide that environment.

slug damage on plant

Figure 5. Slug damage to soybean seedling. (Photo by Mark Bernard)

Slugs damage plants by rasping and ingesting plant tissues. Slugs are hermaphrodites. While mating usually takes place between two adults, it only takes one to tango. The fall of 2015 was long with plenty of rainfall in much of southern Minnesota; favorable for slugs, slug social interactions and subsequently, large numbers of slug eggs.

Slug damage might easily be confused with cutworm or other insect feeding. Soybeans can be killed when the hypocotyl is damaged during or after emergence. While the stem is not always severed, similar to hail damage, the feeding scar leaves a weak spot that can kill the plant later by breaking or by cutting off water transport.

Slugs are not insects and special molluscides are required as pesticides.

Ron Hammond, entomologist at the Ohio State University conducted much of the research on slugs in Midwest corn and soybean crops. This 2014 fact sheet, co-authored by Dr. Hammond before he retired, provides more information and photos:  Slugs on Field Crops.

Speaking of wet weather and dead plants

Many crop diseases are favored by wet weather. Root diseases, in particular, can be very hard to identify by symptoms. The U of MN Plant Disease Clinic can help. They provide a range of diagnostic services for plant disease. Sample submission instructions can be found on their website.

decaying plumule

Figure 6. Note mites near decaying plumule on seed. Photo by Trey Hopwood

Soil biodiversity anyone?

From fields in low-lying areas in the wetter parts of southern Brown County, agriculturalists were looking at dead corn seed/seedlings with numerous tiny, round, white hyaline critters. Some of these seeds had signs of insect tunneling but radicle (root) or plumule (shoot) were decayed or missing in most. The animal's small size made it hard to ID from a photo, but when Brandon provided some live specimens a 10x hand lens revealed mites. The eight-legged mites are arthropods,  but not insects, more closely related to ticks and spiders. I am by no means a mite taxonomist, but I suspect that these globular soil mites are likely one of many Orabatid mite species that feed on decaying organic material and fungi. They are a common part of the complex and diverse soil fauna that assist in decomposing dead organic material.

mites on corn seed

Figure 7. Close-up of soil mites infesting a dead corn seed. Note the fungal hyphae above the missing scutellum or cotyledon. (Specimen provided by Brandon Wendlandt)

Why were the mites there?

There are some soil dwelling mites that can attack live plant tissue, but most species feed on decaying organic matter or are predators. In this case, the corn likely expired from lack of oxygen and in a few cases perhaps insects  or disease. The mites were there just to clean up. There are other soil dwelling mites that are predators, and some mites can attack living tissue, bulb mites for example. The insecticide on the corn seed did not kill the mites because they are not neonicitinoid susceptible insects.

Production agriculture events at the SWROC

There will be a crop and soil school for agricultural producers at the U of MN SWROC on Monday, July 11th. Details to follow.

Happy trails,

Bruce Potter