Winter Survival of Insect Pests

Bryan Jensen
Integrated Pest Management Program
University of Wisconsin - Extension

          Wisconsin has had its warmest winter on record (so far at least).  What effects, if any, might this weather have on overwintering survival of Wisconsin’s field crop insect pests?  That question has been asked repeatedly but unfortunately the answer isn’t clear. True, warm weather may favor survival but many factors contribute to insect population dynamics.  Spring and summer weather patterns, abundance of natural enemies and crop growth and development, are as big of influence on insect populations as winter weather.  For example, high overwintering pest populations may cause little if any economic problems if spring weather isn’t favorable, natural enemy populations are high or crop growth isn’t favorable.  Likewise, low overwintering populations may cause significant damage if conditions are good.  Therefore, careful crop scouting for the insect, their damage and natural enemies is still necessary and will remain an important component of an IPM program.

          Regardless, crop advisors do want to anticipate problems and the following guidelines might be helpful.  Obviously, the first issue you have to address when attempting to forecast insect populations is whether or not that insect overwinters or migrates into Wisconsin each year.  Some of our most important pests do not overwinter but rely on spring weather patterns to assist with their migration.  Those insects that migrate to Wisconsin each year include potato leafhopper, armyworm, black cutworm and corn earworm.

          Understanding the overwintering location and life stage may add additional insight to the potential for damage.  Insects that overwinter in the soil have a tendency to be less affected by winter weather conditions especially if there is adequate snowfall.  Furthermore, insects that overwinter as eggs tend to withstand adverse conditions better than those which overwinter as adults, pupae or in the immature stages. Insects falling into one or both of these categories include corn rootworms (eggs laid in soil), soybean aphids (egg), wireworms (larvae in soil), white grubs (larvae or adults in soil), stalk borer (eggs) and hop vine borer (eggs).  These insects probably are affected little by weather extremes. 

          Insects that may benefit from mild winters include European corn borer and to a smaller degree, alfalfa weevil.  European corn borers overwinter as larvae in their host plant and have the added advantage of entering the winter is higher than average numbers in some areas of the state.  However, before you can assume there will be significant damage from first generation, many other factors have to be considered.  Weather must be favorable during adult flight; abundance of dew and lack of heavy rains would enhance oviposition and influence larval survival rates, respectively.  Corn growth must be advanced (mid-to-late whorl) to attract egg-laying females.  Also, corn at this growth stage would positively influence larval survival.  Larvae hatching on early whorl stage corn have a lower survival rate as a result of higher concentrations of DIMBOA, which is a natural feeding deterrent. To get an early indication of damage potential, monitor early planted corn fields. 

          Alfalfa weevils, which overwinter as adults, have had a slight increase in population during recent years and could benefit from the warm winter weather.  However, spring weather is as likely to play a key role.  Monitor field areas that are likely to warm up earlier, such as south-facing slopes and sandy knolls, to estimate damage potential.

          Corn flea beetles and bean leaf beetles are two insects typically thought of as being “southern pests” and could significantly benefit from this unusually warm winter weather.  Although Corn flea beetles populations may be higher than normal as a result of recent mild winters, they are not anticipated to be a significant problem.  Primarily because little inoculum of Stewart’s Wilt, a bacterial disease vectored by corn flea beetles, is thought to be present in the state (based on Wisconsin Department of Agriculture’s surveys).  However, if high levels of Stewart’s Wilt were noticed in your area it would be important to scout sensitive crops like sweet corn and corn inbreds early. 

          Bean leaf beetles are also known for higher populations following mild winters and should be one insect to be looking for in the southern counties.  Other factors contributing to an increase in bean leaf beetles populations include early planting dates and an increase in soybean acreage.  Although seedling soybean defoliation and late season pod feeding can be a concern, of equal or greater importance is their ability to transmit bean pod mottle virus. 

          Although it is important to anticipate insect damage as a result of winter weather, do not use this information alone to make control recommendations.  Remember, weather is one of many variables that influence insect populations.  A good approach would include spot-checking those field areas most likely to show damage first or those that are most attractive to the pest.  Use this scouting information to gauge potential for other fields.  This method may not be foolproof, but when time is limited you will want to look in those fields most likely to have damage.  Don’t forget to monitor degree days when appropriate and to use other informational newsletters like the Wisconsin Crop Manager and WDATCP’s Cooperative Pest Survey Bulletin to increase you awareness.  Electronic subscriptions to The Cooperative Pest Survey can be obtained by emailing bulletin@datcp.state.wi.us.  Hardcopy subscriptions are $20/year, checks should be payable to DATCP.   Write Cooperative Pest Survey Bulletin, DATCP, Box 93193, Milwaukee, WI  53293-0193.