Southwest MN IPM STUFF 2018 Issue 2
Volume 21 number 2
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 2018-02
It is still winter-like out there but hopefully not for long. Contrary to some of the rumors spread by farmers returning from the south, I have not been fortunate enough to be able to spend the winter twiddling my thumbs and otherwise taking it easy.
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.
So… the last half of the winter and early spring was a major bummer if you were trying to get fields worked and crops planted early. Air and soil temperatures started to warm in late April.
At the SWROC, soils were finally frost free under bare, open ground on May 1. Field work is very slow in getting started, but I am starting to see a few, very few, fields worked and planted this week. The cool weather, late persisting frost, and late-season snow delayed fields being fit to work and I have heard of floaters getting stuck. Rumor has it that progress is a little better in other areas. I suspect that 2018 will be like most years; at some point, it will be too wet, too dry, too cold, too hot, and etc.
Most crops need to experience death (technically a near-death experience) three times before harvest. Patience - creating a bad seedbed can use up more than one of those experiences.
So, it’s been cold. What does that mean for insects?
Effects on the 2017-18 winter on insects that winter here
Some insects migrate south to avoid cold weather. Some of the insects that are known or suspected to migrate south in the fall include monarchs, black cutworm, and potato leafhoppers. The insects that stay in the north country have several strategies for surviving freezing temperatures.
They can choose environments that insulate them from the coldest temperatures. They move deep in the soil, under leaf litter, and some species have found houses as a great place to spend winters.
Some species avoid freezing tissues by minimizing moisture or purging particles that would form sites for ice crystal formation. Some insect species produce antifreeze compounds. The sugar alcohol glycerol is one example of insect antifreeze. Glycerol was once used as an automobile antifreeze too!
Some species allow their tissues to freeze but minimize cell damage of frozen tissues. Like freeze avoidance, a conditioning period is typically needed to increase freeze tolerance.
Many species use a combination of tactics to survive our Minnesota winters. In spite of their winter survival strategies, extreme cold, prolonged cold, and wide springtime temperature fluctuations can do in all but the hardiest insects. Here’s a quick rundown on probable 2017-18 winter effects on several MN insect pests.
In the areas of Minnesota where Japanese beetles are present, freezing temperatures under sod, likely caused mortality of a portion of the overwintering larva. Other native white grub species that can cause crop damage winter deeper in the soil profiles and will be less effected. White grubs caused damage in some west central MN soybean fields during 2017.
Mortality models indicate bean leaf beetles would have experienced very high mortality this winter. However, populations in southern MN were already at very low levels. Minnesota bean leaf beetle populations were high in the early 2000s declined dramatically since. I have not seen a single bean leaf beetle at Lamberton for several years. In addition to a couple of tough winters, 2013-14 in particular, the emergence of F1 adults in soybeans and soybean aphid insecticide applications had occurred at the same time many years.
European corn borer - Mature larvae winter in corn stalks, often moving lower in the plant as they prepare for winter. Once larvae are large enough to overwinter (5th instar and sometimes 4th) and have acclimated to cooler temperatures, they can survive temperatures of -4°F for several months (Hanec and Beck, 1960). Once this winter diapause is broken, the larvae can freeze quite easily. So far, they should be doing fine.
Northern and western corn rootworms spend the winter as eggs in the soil. Winter survival is increased with a long fall and a pre-chill period of more than 2 weeks at 39-40. Northern corn rootworm eggs are cold tolerant and probably minimally effected by the past winter temperatures. Western rootworm may have suffered greater egg mortality. The data on temperatures required to reduce western corn rootworm survival varies, but egg survival has been shown to be reduced below 20°F (Godfrey et. al., 1995) and temperatures of 0.5°F have produced 100% mortality in laboratory conditions (Ellsbury and Lee, 2004).
Abundant 2017 mid-late summer to moisture in most areas of Minnesota increases the odds that eggs were placed at shallower depths where winter temperature extremes are greatest. If you monitor winter soil temperatures at several depths (doesn’t everyone) some estimates of winter egg survival can be made.
The good news, if you grow non-rotated corn, is that some western corn rootworm egg mortality might have occurred in areas where snow cover was delayed until late in the winter. Larval and beetle observations in June and July will ground truth our guesses.
Soybean aphid Laboratory studies indicate soybean aphid eggs can survive air temperatures as low as -29°F (McCornack et al., 2005). They might do even better with optimal fall conditions. Also, any eggs on lower buds could be protected from temperature extremes by snow cover. Canadian research correlated soybean aphid egg hatch with the swell and opening of buckthorn in the spring (Bahlai et al., 2007). This process has just begun in SW, SC, and NW Minnesota. After surviving the 2017-18 winter and predation and disease on buckthorn this spring, the next challenge for soybean aphids will be finding and successfully colonizing soybeans. I hope a large percentage fail.
They are not crop pests but I expect this will be a banner year for blackflies, a.k.a. buffalo gnats, in areas where streams have experienced prolonged high water flows.
2018 Spring Migrants
Mostly because it was too wet to do anything else in the field, I have been occasionally taking sweep net samples of alfalfa and winter cereals. I started to pick up a few aster leafhoppers on May 1. Currently populations are very low and less than 0.01/sweep. I have not found potato leafhoppers yet, but they can arrive on the next system from the gulf.
A few red admiral butterflies have also started to arrive. Unless you are a fan of stinging nettles, the larvae are harmless.
Things that go bump in the night
We picked up a single true armyworm in the SWROC black light trap on May 3rd. Not enough for concern but an indication that some of the insect riffraff has started to move back after spending the winter in the south.
At this point the biggest migrant insect issue is black cutworm moths. Early this week, several pheromone traps in southern Minnesota have had significant captures of immigrant moths. This means that there will be some areas at risk of later crop damage from cutworm larvae. The lack of spring tillage has compounded this risk.
The Minnesota Corn Research and Promotion Council is supporting this trapping effort. 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.
The first report of when, and where, significant black cutworm moth flights occurred and predictions of when 2018 larvae will be large enough to cut corn will be available by May 9th.
Some things to consider right about now
There is not much to do pest scouting-wise at this time. But if you are looking for something to while waiting for field work, most of the following will help make your post-plant pest management decisions easier.
Final stand evaluations of alfalfa and winter rye/wheat stands. Fertilize where needed and the ground will carry.
Don’t skip pre-emerge herbicides, especially on soybeans, during the spring planting rush.
Note winter annual and early emerging spring weeds when working fields or applying a burn-down herbicide.
When field conditions are fit to work fields and plant, get after it. The calendar says it’s time, but it is not too late in the spring so be patient. If you’re getting to feeling pressed for time and anxious, remember, others are under pressure too.
When you start planting – remember to check planting depth, row cleaners, etc. for each field. 2 inches deep is a perfect depth for corn. Remember to plant soybeans shallower.
When placing corn hybrids, consider that first generation European corn borer moths are attracted to the earliest, tallest corn, and second generation moths prefer later pollinating corn.
Be careful out there!