SW MN IPM Stuff 2016 Issue 7

Volume 19, Issue 7 - June 2, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-07

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

Crop Weather

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Figure 1. Estimated cumulative precipitation for May. Those of you located in the dark red areas know what it means. Portions of WC, NC and NE MN are below normal for precipitation. Source National Weather Service http://water.weather.gov/precip/.

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 372 degree-days (base 50°F) and 5.56 inches of rainfall May 1 - June 1. Yes, it’s been wet. And it sounds like more on tap for the 3rd.

There were 105 degree-days (base 50°F) accumulated from April 11-30.

The largest corn on the SWROC is V5-V6 (5-6 collar counting the seed leaf) stage.

Some fields are suffering from excess water and many are concerned about nitrogen losses. Yellowing of 5-6 collar and larger corn is not conducive to high yields but is not necessarily fatal. Many fields will recover on their own with warm, dry weather, but some growers may choose panic or another proactive method. Some fields may need additional N and there is still time. For more information, read Brad Carlson's recent MN Crop News article.

The largest soybeans planted April 13 are at V3 (3rd trifoliate open).

Winter Wheat

The few fields in the area are heading to mid-flower depending on maturity. Wheat stripe rust is a serious concern on some varieties. Fungicide applications for Fusarium head blight and rust to winter wheat should be on or scheduled (http://smallgrains.org/2016/06/01/fusarium-head-blight-update-mn-053116/.) To keep things in perspective, rust levels at the SWROC may be higher than other areas because of susceptible varieties and excessive rainfall.

Spring Wheat and Oats

Spring wheat and oats are at or near jointing. The crop looks excellent at this point. It has been too wet to properly scout any spring planted cereals this week. English grain aphids and bird cherry-oat aphids are present but appear to be at levels less than those observed on winter cereals earlier. Wheat stripe rust, Fusarium head blight and bacterial streak are potential diseases of concern on wheat. Crown rust of oats will need to be watched as the season progresses. I am starting to see scattered oat plants with barley yellow dwarf virus symptoms.

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Figure 2. Alfalfa caterpillar larvae, alfalfa weevil adults and larvae in bloom stage alfalfa sweep net sample.

Alfalfa

Alfalfa that has been cut shows good re-growth. Alfalfa weevil populations are variable by planting here and range from overwintered adult to 4th instar larvae. Last week, Matt Bruyette reported similar variability in central MN with some re-growth treatment needed.

Based on the populations of larvae in uncut alfalfa, there will be an abundance of the yellow butterflies of the alfalfa caterpillar within a few weeks. Pea aphid populations have declined and there are numerous predators and parasitic wasps present. Unusually abundant are populations of the predatory minute pirate bugs.

Potato leafhopper adults are present at low levels in SW MN alfalfa and soybean. Scouting for this insect should be a priority for 2nd and 3rd cutting alfalfa.

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Figure 3. Initial shotholing in the whorl, a sign of 1st generation European corn borer larva presence.

Corn

Based on degree-day models and the status of the local peony bloom, European corn borer flight should have started in Southern MN. Corn borers winter as mature larvae in corn stalk residue and pupate in the spring. Degree-day accumulations for corn borer are started on January 1.

When more than 370 degree-days accumulate, moths start to emerge from pupae. For an estimate of corn borer development in your area see European corn borer degree-day map. For those of you in more northern and western parts of Minnesota, the univoltine strain of European corn borer develop at a different rate with respect to degree days.

Hybrids without above ground Bt protection should be scouted for corn borer infestations. When scouting 1st generation corn borer, focus on the tallest corn. Corn less than 18 inch extended leaf height are protected by a compound called DIMBOA.

Later scouting for univoltine and second generation corn will focus on corn near tasseling.

Stay tuned for more information on corn borer scouting.

Common stalk borer will be leaving grassy weeds and herbicide killed (hopefully) ragweed soon.

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Figure 4. Early season soybean aphid.

Soybean

Soybean aphids were observed on SWROC soybeans June 1. The aphids were found in an experiment with simulated volunteer soybeans. The V2-3 soybeans were planted April 13th with corn. These aphids were much more abundant than I  expected in light of  the rainfall we have received. Winged females had evidently been sneaking into soybeans between the rains, and, based on the ages of aphids, some colonies had been there for several days to a week. They are a long ways from being able to cause yield loss.

There is no benefit and there may be a downside to treating small, early season infestations like this. By destroying existing populations of beneficial insects, you leave the field open to attack by aphids arriving later. Because some SW MN soybean aphids have somehow managed to once again find soybeans in the spring, some may choose to ignore economic thresholds based on extensive research and decide to just add a cheap insecticide when spraying an herbicide. This is one possible aphid management decision but one without empirical data to back it up. "Just in case insurance" insecticide applications are likely one of the reasons that  pyrethroid resistant aphid populations have developed in parts of Minnesota.

While insurance insecticide applications have more potential risks than benefits, they are not illegal. However, the comedian Ron White's statement on the inability to repair a lack of intelligence probably applies to soybean aphid management. 

MN Soybean Checkoff dollars have provided research funding for insecticide resistance. We will be evaluating these Lamberton aphids for susceptibility to pyrethroid insecticides. 

Please let Bob Koch or myself know of any early season soybean infestations you come across as we need to screen more early season populations across Minnesota for insecticide resistance. This will help develop better recommendations in the event aphid populations build to economic levels.

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Figure 5. Headless, legless cranefly larvae are harmless to crops. (Photo E. Leman)

Cutworms

The larger corn will soon be safe from black cutworm and other cutting and tunneling into growing points (dead heart). Degree-day accumulations indicate that larvae from all flights should be large enough to cut. Larvae from the earliest flight will be able to cut corn less than 6 leaves for another 7 to 10 days.

It is important for proper ID when making a cutworm, or any insect treatment decision.

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Figure 6. Black cutworn larva. Note the head capsule. Cutworms have 3 pairs of true legs and 5 pairs of prolegs.

Happy Trails,

Bruce Potter