SW MN IPM STUFF 2018 Issue 6

Volume 21 number 6

06/14/2018

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

A printer-friendly pdf version of this newsletter is available to download here: SW MN IPM Stuff 2018-06

Crop weather

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

There has been more rain, with very heavy amounts creating ponded water and flooding in some portions of the SW MN IPM viewing area (Figure 1). In some areas, including the SWROC, wet fields have been a chronic condition. When the water recedes, some low spots will be planted for the 3rd time…No one needs any more planting practice this late in the season. 

We are well ahead of average precipitation this spring (4.92” vs 6.60”) as of June 11. On the other hand, warm temperatures in May and early June have us ahead on degree-days (678 vs. 524).

Excess soil moisture, and anaerobic soil conditions have reduced root function, and we have a lot of yellow crop, of all types, out there right now. Don’t jump to the wrong conclusion. Supplemental N may not be needed once cornfields dry out. Yellow soybeans may be suffering from temporary nitrogen and/or sulfur deficiency or SCN root injury rather than iron deficiency chlorosis (IDC).  

Wet field conditions create herbicide application challenges that can be overcome with horsepower and ground clearance. Challenges created by windy conditions, not so much. 
14-day rainfall totals 

Expect some root disease where wet conditions have persisted. There have been some stand issues where soils remained wet such as low areas and side-hill seeps.
Remember, even if you used a seed applied fungicide, they do not have a long period of effectiveness, nor are they effective against all pathogens.

Diseases such as SDS on soybeans, several species/biotypes of Fusarium and Rhizoctonia on several crops might be worse this year, but their severity will depend on weather during the remainder of the growing season. Finally, injured or dying plants with bad roots do not have to be due to disease. 2018 reinforces the premise that drowning is a perfectly legitimate way for crops to die. 

Wet weather and invertebrate pests of crops

Unlike plant disease, many insect problems tend to be lessened by wet conditions. However, some insects and other invertebrate pests may be a bigger issue with wet conditions, particularly if weather is cool. 

Corn rootworm damage where corn planting was delayed may be more significant if larvae attack smaller root systems of late-planted corn. On the other hand, larvae can be drowned if soils are saturated or flooded before they can infest roots.

Slugs may cause some issues in no-till situations, and based on reports from last year, rye cover crops may make issues worse.  Slugs resemble snails without shells and as gastropod mollusks, they are indeed related to snails. They are not controlled by insecticides and require special pesticides such as metaldehyde. For those with “glass half full tendencies, slugs do not make good fish bait. Predators of slugs include birds, some insects such as ground beetles, and some small mammals. If you had your DNA tested or have other reasons to suspect a French background, you may be tempted to try these with some garlic and butter. Don’t, slugs are not likely to be palatable and some species can transmit a parasite to humans.

I received a picture of millipede damage to corn in Lyon County (thanks for the heads up Jerry). Millipedes, meaning thousand legs, are not insects, and are in the class Diplopoda, named for the two legs/body segment. They are most often found where conditions are cool, damp and in areas with large amounts of decaying organic matter, their typical food. Occasionally, they will attack corn and soybean seedlings. Do not expect good control from insecticides.

“D’oh!” (quote attributed to Homer Simpson)

Typically, we expect problems from seedcorn maggot when crops are planted near the time the adults lay eggs in the soils of fields with particular environmental conditions. The several annual generations of seedcorn maggot flies tend to lay eggs in areas with decaying organic materials. Fields with incorporated solid livestock manure or green manured plant materials incorporated into the soil are most often affected. In these high-risk situations, delaying planting to miss the predictable egg-laying windows or use of a seed-applied or at-plant insecticide is recommended. Damage occurs to below ground tissues after seeds have imbibed moisture. Cool weather delays emergence and increases the length of time seeds and seedlings are vulnerable.

We may need to take previous years’ cropping history into account when planning seedcorn maggot management strategies. I received a report of a central Minnesota soybean field with maggot damage (thanks for the observations Steve C). Portions of the neighboring fields also had some damage. It turns out, that the field had been planted to sweetcorn during 2017. The field was passed and not harvested. The green manured sweet corn stover must have attracted egg-laying adults last fall and subsequently supported a very large population of overwintering pupae. This spring, soybean planting into the still decaying sweet corn was likely well-timed with respect to emergence, mating and egg-laying of the adult flies emerging from these pupae. If you want to learn more about seedcorn maggot biology and management (and who would not?) see the MN Crop News blog at heavy-seedcorn-maggot-infestations.

Some things to consider right about now
Alfalfa

Scout for potato leafhoppers and plant bugs in second cutting. Leafhopper populations have increased in many southern MN alfalfa (and soybean) fields. I have received one report of high plant bug populations (thanks Matt). Thresholds and scouting advice for both insects can be found at: http://www.extension.umn.edu/agriculture/forages/pest/sampling-alfalfa-insects/

Scouting for these alfalfa insects is fast and easy. While spraying insecticides on a cutting-based schedule may seem more simple, it increases costs, encourages the flare of secondary pests, and can speed the selection of insecticide resistant insect populations. 

Dry beans

Potato leafhoppers!

Corn
Weed control

It has not been easy getting this done this spring. Evaluate control. Hopefully you are finding minimal crop injury symptoms. Finding any potential resistance early will make management more effective. I appreciate that you guys in the country keep me in the loop on these issues. We are looking into what might be happening with some hard to control Kochia in SW MN (thanks Regan).

Hopefully you are finding minimal crop injury symptoms. Thanks to all of you who have sent the depressing photos this spring. Please send a few of phenomenally healthy crops to keep my therapy sessions shorter. 

European corn borer

Based on black light trap captures, first generation, multi-voltine, European corn borer flights have been very low this spring: https://www.vegedge.umn.edu/moth-data/ecb-info.

UMN entomologists Anthony Hanson & Bill Hutchison have been working on degree-day maps for insect development using European corn borer as the pilot species (Figures 2,3). These corn borer development maps display predictions for several corn borer life stages important to management. These maps will be updated weekly at the UMN Veg Edge site: https://www.vegedge.umn.edu/moth-data/mndd. These maps are show corn borer development in an approximate way only - don’t start or end scouting too late.

Estimated European corn borer development

Projected European corn borer developement

Start scouting corn whorls for 1st generation borer shot-holing soon. The largest corn without above-ground Bt trait(s) are most likely to be infested and these fields should be scouted first. DIMBOA, a defensive compound produced by the corn plant is at the highest concentrations in seedling corn and it prevents larvae from surviving on corn less than 18-inches tall.

Soybean

Weed control

Will be the priority for the next week. The cutoff for dicamba applications to soybean in Minnesota remains Wednesday, June 20th

You don’t need to see emerged weeds for an herbicide application to be effective but you do need to pay attention to label restrictions on growth stage, wind speed, temperature, tank mix partners and spray adjuvants, and calendar date.

Soybean aphid watch

In the process of working in research trials soybean aphids have been found at UMN Research and Outreach Centers at Lamberton, Rosemount, Morris and Waseca. Rosemount had the largest populations but even there, aphids were not abundant.

Soybean aphid populations at Lamberton have been hindered by driving rains, high soil moistures and late soybean planting and perhaps late tillage and corn planting removing volunteers after they were colonized by aphids moving from buckthorn.

Surviving volunteer soybean

You have some time before you need to worry about controlling this insect. If you want to find a few just for practice, look in smaller fields in areas near buckthorn, fields with V2 or larger soybeans and coarser textured soils. Let ants and lady beetles help you scout!

Look at the rainfall map in Figure 1. I suspect aphids will be most easily found in fields matching the above criteria and located in areas of MN with lower to moderate rainfall and where planting occurred relatively early. I am not providing any guarantees with these predictions.

Happy trails,

Bruce