SW MN IPM STUFF 2018 Issue 7
Volume 21 number 7
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 2018-07
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.
It’s still too wet in many areas, but we are above normal on degree-days. Early planted corn is starting to tassel and in some cases silks are out.
Most soybeans are now at R2.
Crop conditions range from very good to darn awful in SW MN, sometimes in the same field.
As mentioned in a previous e-mail, there are fields with economic infestations of true armyworm. Most of the reports have been from Central and WC MN.
Although armyworms feed on grasses, they can occasionally cause a problem on broadleaf crops when a grass cover crop or weedy grasses are removed by cutting, herbicide, or armyworm feeding. When grasses in a field are removed, it typically triggers a mass movement of armyworms out of the field to find new food sources. However, they may be forced to feed on the remaining alfalfa, soybeans, etc. If the armyworms persist on the broadleaf crop, manage the crop defoliation as if they were any other defoliator. For example, the defoliation threshold for reproductive stage soybeans is 20% leaf loss.
The defoliation estimates should be based on upper, middle, and lower leaves, not just the most severely impacted.
Some things to consider right about now
Continue to Scout for potato leafhoppers. Untreated alfalfa at the SWROC is beginning to show hopperburn injury (Figure 1).
- Scout univoltine corn borer
- Rootworm adult emergence has started evaluate root systems. Yellow sticky traps should go out soon.
- Scout corn for fungal diseases. Focus first on non-rotated fields planted with susceptible hybrids.
- Watch for potential weed escapes/resistance while you are out there.
Symptoms of bacterial leaf streak have shown up in some SC MN fields. Some hybrids are showing symptoms of anthracnose on lower leaves.
European corn borer and other lepidopteran pests of corn
UMN entomologists Anthony Hanson & Bill Hutchison have included more degree-day maps for insect development.
Multivoltine and univoltine corn borer degree-day models are available to help your scouting efforts at: https://www.vegedge.umn.edu/mndd.
Seedcorn maggot, a fly, not a moth, and western bean cutworm (tolerant to many of the lepidopteran Bt proteins), projections have also been added.
Light trap and pheromone trap data for several lepidopteran pests are also available at the VegEdge site: https://www.vegedge.umn.edu/moth-data/ecb-info.
Things not to worry about
Corn blotch leaf miner
These leaf mining fly larvae are unusually numerous this spring. Their abundance may be due, in part, to the environment created by this spring’s wet weather.
In spite of the highly visible leaf mines, they are unlikely to affect yield.
A foliar fungicide will not provide any benefit for these insects.
False Japanese beetle
It has been a very good year for these as well. Several have reported swarms of false Japanese beetles (Figure 4).
Unlike the similar but more brightly colored Japanese beetle (Figure 5), false Japanese beetles are not significant crop pests. They will feed on flowers. False Japanese beetles appear to be attracted to white – white flowers, shirts, pickup trucks etc.
- Evaluate fields for fungal diseases
- Scout for aphids and defoliating insects
- Record fields that will require special emphasis for weed control for the next few years.
Populations have started to build in some but not all areas of the state. In areas that have had early soybean planting and more moderate rainfalls, populations are likely to begin to increase rapidly.
Low populations of beneficial insects or protection of aphids from predators by ants, helps produce large winged aphid producing colonies on plants (Figure 6).
Once most of the plants in a field have been colonized, populations can begin to increase rapidly. This is one of the reasons that small fields often reach economic threshold levels sooner than larger fields.
Fields that are uniformly infested without discernable hot spots can be a sign they have been colonized by aphids migrating from other soybean fields.
I have had several reports of aphid population beginning to increase in portions of C (Jason, Phil), WC (Brad), and to a lesser extent SC MN (Steve, Adam). Thanks also to those who have been reporting areas with low aphid populations.
Soybean aphids often do worse in areas where there has been excessive rainfall for several reasons: 1) Soybeans under mild moisture stress are better hosts. The nutritive quality of soybean sap is reduced with saturated soils and also under extreme drought stress, 2) Aphid killing fungal diseases do better when prolonged rain or fog increase humidity, particularly if temperatures are cool, and 3) Heavy rains and wind on seedling soybeans can be hard on early-season aphids. Aphid populations on larger soybeans are protected by the canopy. Unfortunately, for many aphids in southern Minnesota this season, those cornicles (tailpipes) on the rear of the aphid cannot do double duty as snorkels.
Where to start?
Yield damaging soybean aphid populations can occur in any field. However, some fields tend to have more consistent problems with aphid infestations from year
to year. Several factors increase the likelihood of aphid problems. In spring, aphids are often found first in geographic areas with abundant buckthorn. Smaller
fields with wooded borders are often the first to develop high populations. In addition, early-planted fields and fields with slightly droughty coarser textured or lower
potassium testing soils often see aphid populations develop sooner. Early planted soybeans and perhaps uncontrolled volunteer soybeans serve as a source of aphids for other fields. Later in the season, full-maturity soybean or late-planted soybean, such as beans following peas, are often reported to have higher populations.
Rigorous scouting of high-risk ("indicator") fields can provide valuable information on when aphid populations are beginning to increase in an area, which should trigger scouting of other fields.