SW MN IPM STUFF 2019 Issue 1

Volume 22 number 1


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

A print-friendly version of this newsletter is available to download here: SW MN IPM STUFF 2019-01

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.   

So…. It’s cold and it’s getting colder. What does that mean for insects?

I looked through some previous newsletters for some information on this as we wait out the current Polar Vortex. 

Insect species have adapted to MN weather with the strategy of spending their winters in the south migrating back north each spring. Not a bad approach and mimicked by many Minnesotans. 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. A layer of snow provides tremendous insulating value for wintering insects. They move deep in the soil, under leaf litter. Some species, like Asian lady beetle and brown marmorated stink bug, 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! The antifreeze approach may even have human parallels. The beer and cheese saturation method most commonly employed by Green Bay Packer fans is just one example.  

Supercooling is another mechanism insects use to avoid freezing. Water remains liquid at much lower temperatures when particles that are sites for the formation of ice crystals are removed. 

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.

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, the low populations of corn borer in MN are likely still OK.

In the areas of Minnesota where Japanese beetles are present, freezing temperatures under sod cause 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.

Northern and western corn rootworms spend the winter as eggs in the soil. The open winters of 1976-77 and 2012-13 are suspected to have greatly reduced western corn rootworm (WCR) populations through most of Minnesota. Winter mortality of rootworm eggs depends on several other factors including fall temperatures (Chiang 1973). A long fall and a pre-chill period of more than 2 weeks at 39-40ºF increases winter survival of eggs.

The data on temperatures required to reduce WCR 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). In lab experiments, WCR egg hatch dropped dropped to 43-50% after one week at 14°F and no hatch occurred after one week at 5°F (Guston 1981). Duration of the cold temperatures is important to the eggs of both species but northern corn rootworm (NCR) eggs can tolerate lower temperatures than WCR (Woodson and Gustin, 1993; Woodson and Ellsbury, 1994). NCR also seem to seem to tolerate supercooling to lower temperatures and desiccation better than WCR (Ellsbury and Lee, 2004).  

Abundant 2018 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.

Soybean aphid. Laboratory studies indicate soybean aphid eggs can survive air temperatures as low as -29° F (McCornack et. al, 2005). They might even do a bit  better with optimal fall conditions. Much of Minnesota should see these conditions. Snow cover can protect any eggs on buds close to the ground from temperature extremes. 

Fall 2018 populations of soybean aphids on buckthorn were low to the point of non-detection. The current cold snap may further reduce spring populations.  Unfortunately, the ability of soybean aphid to rapidly reproduce and move long distances means that a long-term widespread mass extinction event is unlikely.

The eternal optimist in me is hoping that a fitness cost for pyrethroid resistance is reduced cold hardiness. Perhaps the Koch lab would be able check that out with some SW MN aphids.

Keep warm and…

Happy trails,  

Bruce Potter

References cited

Chiang, H.C. 1973. Bionomics of the northern and western corn rootworms.  Ann. Rev. Entomol.18:47-72

Ellsbury, M.M. and  R.E. Lee, Jr. 2004. Supercooling and cold-hardiness in eggs of western and northern corn rootworms. Entomologia Experimentalis et Applicata. 111: 159-163.

Gustin, R.D.  1981. Soil temperature environment of overwintering western corn rootworm eggs. Environ. Entomol. 10:483-487.

Hanec, W and S.D. Beck. 1960. Diapause in the European corn borer, Pyrausta nubilalis (Hubn.). Journal of Insect Physiology 5 (3-4):169-80.

McCornack, B.P., M.A. Carrillo, R.C. Venette and D.W. Ragsdale. 2005. Physiological Constraints on the Overwintering Potential of the Soybean Aphid (Homoptera: Aphididae). Environ. Entomol. 34(2): 235-240.

Woodson, W.D. and  R.R. Gustin.  1993.  Low temperature effects on the hatch of western corn rootworm eggs (Coleoptera:Chrysomelidae).  Journal of the Kansas Entomological Society. 66(1):104-107.

Woodson, W.D. and  M.M. Ellsbury.  1994. Low temperature effects on the hatch of northern corn rootworm eggs (Coleoptera:Chrysomelidae).  Journal of the Kansas Entomological  Society. 67(1):102-106.