SW MN IPM STUFF 2018 Issue 4

Volume 21 number 4


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 2018-04

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 SWROC Weather.  

A lot of field work got done the last half of last week and through the weekend. Rain in some areas stopped field work after Sunday.

While corn planting, and in some cases soybean planting, has wrapped up for some, areas near I-90 in SC and SW Minnesota still have standing water in some fields. Planting is being patched in fields where it can and there will be prevented planting for some. At the SWROC, there are still corn studies to be planted. This spring’s delay and range of planting dates will influence weed (waterhemp) and insect (corn rootworm, corn borer, soybean aphids) pressure later this season.

Another bout of rain (over 2 inches at the SWROC) has turned SW MN IPM corn rootworm research into a race. Will the studies get planted, germinate and develop root before rootworm larvae hatch and starve to death…or drown?

The early-planted corn fields are up and, from the road, most look good. By the way, road hunting for crop stands and pest problems is not a SW MN IPM endorsed scouting method.

Things that go bump in the night

Other than black cutworm, other migratory insect pests have arrived at low levels or not at all. Armyworm flights have been low and aster leafhoppers, while present are at low numbers here. I have had a very hard time finding any cereal aphids in winter grains. Potato leafhoppers have not yet shown up and even red admiral and painted lady butterflies have been scarce. Why? Either these species had poor survival in areas where they winter or the weather systems needed to help transport them have not been in the right place and time.

I have been curious as to how many species of May beetles (white grub adults) we have at Lamberton. Intern Adam Hass has been pinning May beetles captured in the light trap; it sounds like he was attacked by hordes of May beetles at late night campfire. In eight days I suppose we should call them June beetles or June bugs.

Some things to consider right about now


Alfalfa weevils egg laying continues. Larvae are also present in SWROC alfalfa stands. Mid to late 1st cutting and early 2nd cutting alfalfa should be scouted.

Alfalfa weevil in 1st crop alfalfa

Start scouting for alfalfa weevil (AW) at 250 degree-days (base 48°F) using a sweep net and by looking for feeding damage on the tips of first crop stems. Taking 30 or more sweeps will determine whether weevil adults or larvae are present in an area. Sweep net samples can detect but do not accurately assess populations when larvae are small or it is windy.

Twenty (20) larvae/180-degree sweep is an economic threshold used in some states, but this has not been validated for Minnesota conditions. Note: that this is a different sweep net sampling method than used for potato leafhoppers and most other alfalfa insects.

An easy to use 1st cutting threshold is 30% or more of stem tips with feeding damage (and weevils still numerous). 

Stem sampling AW is easily accomplished and provides better estimates of potential yield loss. Avoid field margins and select 5 areas of the field representing a range of topography and alfalfa growth. In each area, record the average alfalfa height and collect ten (10) stems by cutting or snapping them near the ground. Place the stems tip down in a bucket (a white five-gallon pail is an excellent choice). Beat the stems against the pail to dislodge the AW larvae so they fall to the bottom of the bucket. As you remove the stems, check the tips for feeding damage and any small larvae that might remain within rolled upper leaves (For those of you that missed it last week, here’s another shot at it: alfalfa weevil stem sampling video).

More comprehensive thresholds are based on alfalfa height, number of weevil larvae/stem and the economic factors of expected value of hay and control costs. They are based on research from NE in the early 1990s. With current hay prices, this calculation typically works out to 1 or more larvae/stem with control costs under 8 dollars and late vegetative to early bud alfalfa. More detail can be found at: Alfalfa Weevil (pdf).

If an insecticide is used, select a longer residual insecticide if treating early in the first crop. Adults moving into the field after application and un-hatched eggs may not be controlled with short residual compounds. Residual becomes less important to applications late in the first crop or second crop. Residual, in fact, becomes undesirable from a standpoint of protecting pollinators when plants are near bloom. Also, Pre-Harvest Interval (PHI) needs to be carefully considered as applications are made closer to harvest.   

Do not use insecticides where they are not needed to protect yield. You can flare other insect problems such as aphids. Development of insecticide-resistant populations of target and non-target insects is always a possibility. During 2017, poor control of pea aphids with pyrethroid insecticides was reported from fields in western MN and eastern SD. Insecticide resistance has not yet been documented in MN pea aphids, but the 2017 reports are a reminder of the potential for unintended consequences.

Depending on the alfalfa growth stage, it may be better to cut rather than to use an insecticide treatment for an AW infestation. If you are within a week of a scheduled cutting it is usually more economical to take the first cutting of hay early rather than to spray.

Cutting and crimping and windrowing can kill the larvae by direct mechanical injury or by desiccation. Avoid, where possible, leaving windrows on fields for an extended period as the alfalfa weevil will congregate there. 

Selected references:

  • Pellissier, M.E., Z. Nelson, and R. Jabbour. 2017. Ecology and Management of the Alfalfa Weevil (Coleoptera: Curculionidae) in Western United States Alfalfa. Journal of Integrated Pest Management. 8(1): 5; 1–7
  • Peterson, K.D., S.D. Danielson, L.G. Higley. 1993. Yield Responses of Alfalfa to Simulated Alfalfa Weevil Injury and Development of Economic Injury Levels. Agronomy Journal. 85:595-601.

Black cutworm

Information on black cutworm trap captures and predictions of when and where cutting is most likely to occur can be found in weekly reports at MN Cooperative Black Cutworm Reporting Network.

Cutworm leaf feeding on common lambsquartersFigure 1. Cutworm leaf feeding on one of the common lambsquarters leaves on upper right. Early feeding signs can be subtle.

The most recent projections are available at 2018 Black Cutworm Network Report #5. Initial leaf feeding in corn and weeds should be visible now. Pay particular attention to cutworms when scouting corn and other crops this year. Scouting tips, cutworm ID (important with cutworms in corn) and management advice can be found in the magnum opus Black cutworm.

I have had reports of feeding by cutworms from McLeod and Renville counties so it is definitely time to start scouting. Thanks Curt! Some larvae from the earliest 2018 flights should be large enough to cut corn next week and they will be active well into next month. Cutting of emerging sugarbeet and small weeds can occur sooner.

Happy trails,