Southwest Michigan field crop update – May 3, 2017
An update on armyworm, black cutworm, herbicides and wheat, insecticide and herbicide interactions.
The 2017 growing season has gotten off to a bit of a slow start. Prolonged wet conditions delayed the beginning of field work in many locations in southern Michigan. Drier weather prior to the last weekend in April allowed field work to get underway in many locations. There was even a couple of days of planting activity on limited acreage in some locations. It would appear as though folks were trying to take advantage of the warm conditions to plant some early-season crops, soybeans primarily.
The warmer weather we have enjoyed will soon be replaced by an extended period of cooler than normal conditions, in both the 6-10 day and 8-14 day period. Precipitation trends look drier than normal in both periods, which should get us underway with planting as the soils dry out by the weekend.
The wheat crop has been developing rapidly in the warm, moist conditions. Advanced fields are in Feeke’s Growth Stage 9, where the ligule of the flag leaf is visible. This stage means we are pretty much at end of the herbicide application window for all of the wheat herbicide options in these fields. For more information, see “Timing is everything when controlling weeds in winter wheat” by Michigan State University Extension weed control specialist Christy Sprague. Be sure to check the growth stage of later-planted fields before applying herbicides to avoid costly injury to the crop. Stands and color look good so far. Minor incidence of powdery mildew can be found in the lowest leaves in some areas.
True armyworm moths have been caught in traps in high numbers by Purdue University in Indiana and MSU Extension in west central Michigan during the last two weeks. These pests do not overwinter here, but are deposited by thunderstorm events. Central and west Michigan did receive more thunderstorm activity in mid-April than the southwest, which may account for the high trap counts in these areas.
We have not seen the numbers of moths caught in either Indiana or central Michigan, but it would be prudent for growers to scout wheat fields as we move forward over the next several weeks to avoid being caught by surprise by these pests. The distribution of infestation is notorious for being spotty. The strategy is to protect the upper two leaves from significant feeding damage by armyworms—the flag leaf being the most critical, as it is thought that 70-80 percent of the photosynthate that goes to the developing kernels is provided by the flag leaf.
Alfalfa is growing quite well following the warmer weather last week. We begin scouting for alfalfa weevil damage when 250-300 growing degree-days base 48 have accumulated since Jan. 1. This essentially occurred during the last week of April in most of southern Michigan. In a few fields we have walked, you can find some minor pinhole alfalfa weevil feeding from very small larvae.
Minor acreages of corn and soybeans have been planted in several areas in southern Michigan. Much of the activity was focused on lighter, irrigated soils. Winter annual weed growth was pretty excessive where fields were not burned down. Most burndown herbicides were quite effective, mostly likely due to the warmer than normal conditions at the time of spraying. In addition to chickweed and purple deadnettle, henbit and annual bluegrass are becoming much more prevalent in the winter annual mix.
Purdue University has reported black cutworm moth catches in several of their more northern traps. With armyworm and black cutworm concerns rising with moth trap catches near the region, growers may want to review the level of ground cover that was in the field during the last week of April. Fields that had significant amounts of winter annual weed growth are more likely to have issues with black cutworms; fields that had cereal cover crops may have provided a more attractive habitat for armyworm moths to lay their eggs in. Clean fields will not have damage. The key is to understand the level of plant growth in the field during this period to be able to focus your scouting efforts.
In corn, some of the Bt events have potential to control black cutworm and armyworm larvae, but not all traits will. MSU Extension field crops entomologist Christina DiFonzo has updated her excellent Trait Table for Bt Corn for 2017.
Potential for interaction between some corn herbicides and OP insecticides
With the threat of Bt-resistant rootworms and sporadic white grub damage, some folks have returned to using soil insecticides on commercial corn. Several of the newer corn herbicides and pre-mixes have restrictions on their use if an organophosphate (OP) soil insecticide was used. The main classes of herbicides with the restrictions are ALS and HPPD inhibitor herbicides.
In the simplest of terms, the injury is caused when the plant tries to metabolize the OP insecticide and these classes of herbicides at the same time, which can overwhelm the metabolic pathway used and cause injury. An April 2013 article by University of Illinois weed control specialist Aaron Hager provides a good explanation of why the injury occurs and a list of herbicides and insecticides that have restrictions. Remember that this resource is relatively old, and newer herbicide products and premixes may also present a challenge.
Remember that early-season foliar OP insecticide sprays used to control early-season insects like cutworms and armyworms can also be restricted if these herbicides were used. If you are unsure of the classes of herbicides you applied on your fields, MSU’s corn and soybean herbicide classification chart is an excellent resource in helping you determine which herbicide families you used.
As we move into early May, look for opportunities to plant soybeans where the soils are dry enough to support planting equipment and prevent sidewall compaction and open seed furrows from occurring. It is also important to avoid planting when rainfall is imminent within 24 hours. This, along with some additional practices, can reduce the potential for chilling injury to occur from the first water imbibed into the freshly planted soybean seed.
There is an advantage in getting soybeans planted as early as possible in May, as there is more potential for node production and better use of day length, which can lead to higher yield potential. However, planting into poor conditions can impact yield potential to the point where the advantage is easily lost.
2,4-D planting restrictions from burndown applications
Remember, if you used 2,4-D as part of your burndown program, the 1 quart per acre rate requires 30 days or more prior to soybean planting, and the 1 pint per acre rate requires seven days or more before planting.