SW MN IPM Stuff 2016 Issue 14

August 5, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-14

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

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.

We have received 1656 degree-days (base 50°F) and 15.14 inches of rainfall May 1-August 4. An additional 105 degree-days (base 50°F) accumulated from April 11-30.

We are about 3-4 days above average in degree-day accumulations (more if you had he crop planted in April). In spite of a rain-free stretch, we are nearly 4 inches above average in precipitation.

Mosquitoes - philosophical, biological, medical and historical tidbits

In some areas of Minnesota, numerous and heavy rains created prolonged periods of standing water. These temporary, shallow waters resulted in tremendous populations of mosquitoes. There are 51 species of the tiny winged scourges that have been documented to occur in Minnesota. Some prefer to feed on animals and rarely bite humans and some like the all too common Aedes vexans, feed on both man and beast. The annoying hum of thousands of tiny wings and the blood loss and welts resulting from the feeding of blood-sucking female of the species, are reasons enough to dislike these aggravating members of the order Diptera (flies). Less obvious, but perhaps more troubling is mosquito transmission of diseases. West Nile virus, several forms of encephalitis, and dog heartworm are transmitted by species of mosquitoes that are found in Minnesota.

Mosquito species are specialized in which species can transmit a particular disease - if any. The yellow fever mosquito, Aedes aegypti, and Asian tiger mosquito, Aedes albopictus, were introduced to North America and can transmit the yellow fever and zika viruses. A 1793 yellow fever epidemic in Philadelphia may be partially responsible for United States capitol being moved. Fortunately, A. aegypti is not found in Minnesota and while A. albopictus has been found here but is not known to be established. Yellow fever vaccines are standard fare in the set of shots for travelers to the tropics.

Malaria, is a disease caused by several plasmodium protozoan parasites. It is transmitted by some of the mosquitoes in the genus Anopheles (if the mosquito biting you is doing a headstand, it is probably an Anopheles sp.). Avoiding malaria is not as simple as getting a vaccination and the disease is a significant cause of death in some parts of the world. At one time, Malaria was an endemic disease along the eastern seaboard, the south and much of the Midwestern US. Historically, some early advertisements designed attract immigrants to Minnesota, touted the lack of malaria here.

Quinine from the bark of the Peruvian cinchona tree was found to have curative and preventative effects on malaria. Cinchona plantations developed once quinine became widely used as a drug. The bark extract is very bitter and was usually mixed with sugar to make it palatable. During World War II, Japan and Germany had captured most of the world's quinine production facilities and part of the American war effort involved trying to find South American sources and creating synthetic quinine.

The gin and tonic (G and T if you're a Brit) was developed by members of British East India company in the early 1800s when soldiers’ gin rations were mixed with their quinine and sugar malaria medicine. Even though malaria does not occur in MN, as a precaution, some agriculturalists take to consuming G&Ts during the summer mosquito season. The vitamin C in the lime slice minimizes any chances for scurvy as well.

For those that would like to try to determine which skeeter is biting you this key may be useful http://www.cdc.gov/nceh/ehs/docs/pictorial_keys/mosquitoes.pdf. I would probably stop the ID at the genus level to avoid frustration.

References: From memory and all over the internet... so you know they're correct.

Corn

Any fungicide decisions should have been made a while ago.

Some western and northern corn rootworm beetles were still emerging last week. We may be past peak emergence in SW MN.

Second generation corn borer scouting in non-Bt fields should be happening now. While late-emerging moths are still present, the earliest laid eggs from this generation should be hatching. Second generation corn borer moths are attracted to later planted corn. 

Soybean

Most soybeans are in the R4 and R5 stage.

Bacterial blight is the most prevalent disease in the fields I have looked at. There are some SDS symptoms out there now. In soybeans at the SWROC, a new generation of mated SCN females are beginning to emerge on roots. I talked with a WC Minnesota farmer who had observed a few soybean plants killed by white mold this week. The field was still a long way from a white mold disaster.

Soybean Aphid Watch 2016

Aphids at economic threshold, but not everywhere - Soybean aphids have responded to last week's weather with rapid population increases. 

In some areas of the state, an increasing number of fields reached the 250 aphid/plant threshold over the past week. The weather forecast indicates that this aphid increase is likely to continue. However, many fields and some areas of the state have low aphid populations as aphid numbers vary greatly by geography, planting date and other factors. This year, perhaps more than most, it is important to treat each field individually when making insecticide decisions.

Winged aphids have been present in soybeans season long, primarily produced on plants where the aphids were crowded or had higher predator/parasite populations. Now, as vegetative growth in some fields ends, we are seeing more winged aphids being produced. These winged aphids may be bad news for fields that were treated with insecticide early. Removing beneficial insect population greatly increases the rate of increase for aphid population and populations can double in less than two days.

It ain't over yet - On more mature soybeans, small light colored aphids are left to feed and reproduce on lower canopy leaves. Eventually, as soybeans continue to develop, the nutritional qualities in these lower leaves will become more favorable and these "white dwarves" will produce larger, green offspring. Include the dwarves in your aphid counts.

Unlike last year and other higher aphid population years, most fields, particularly in later planted geographies, were/are colonized late. While field edges tend to have higher populations, aphid populations within fields tend to be fairly uniform.

While the potential for yield losses are starting to decline, there are still several weeks for aphids to cause economic loss in most fields. Don't give up on soybean aphids until R6 stage soybeans (seed completely fills the pod cavities near the top of the plant) or you have evidence of a widespread population crash.

Isaac Newton and soybean aphids - A bit of good news is that the kinetic energy and force of the 2016 soybean aphid population in MN is currently much lower than in 2015. While the rate of population increase (velocity) is currently high, poor "aphid weather" and high rates of predation exerted considerable drag on population increases early this season. Poor early season colonization success has led to the current low overall aphid populations (mass) and spotty pattern of economic threshold populations. However, the wide range in planting dates in some areas will continue to make the remainder of the 2016 aphid season hard to predict. A change in area aphid populations or rate of population change the current equation significantly.

You may be interested in how quickly population can increase or how quickly aphid- days will accumulate with respect to soybean maturity. I cobbled together an EXCEL spreadsheet where you can input population doubling rates and initial aphid populations. It will calculate future aphid populations and aphid-days. Far from perfect and much less exciting than Pokémon Go, the aphid population calculator at http://swroc.cfans.umn.edu/ag/pest-management/soybean-aphid-resources is offered for your entertainment. You will need more than 5000 aphid days and often much more to detect soybean yield loss in most situations. 

Insecticide resistant aphids - There have been a few more reports of soybean aphid populations that were poorly controlled with pyrethroid insecticide and outside the 2015 problem area. Initial signs of resistance continue to be small areas in the field with no control that rapidly increase and expand because the predators and parasites have been removed. For more information on the pyrethroid resistance in MN soybean aphid populations see the previously released Minnesota Crop News blog article: http://blog-crop-news.extension.umn.edu/2016/08/assessing-and-reporting-potential-cases.html

I have been receiving quite a few pictures of yellow and black beetles in soybean. They are goldenrod soldier beetles otherwise known as Pennsylvania leatherwings. They are not soybean pests. Soldier beetles are loosely related to fireflies. They feed on pollen and small insects and the larvae are predacious. They are abundant on flowers in late summer, particularly goldenrod.

Happy Trails,

Bruce Potter