Doug Johnson, Extension Entomologist, University of Kentucky
The onset of colder than normal weather always brings up the question of how the cold will affect insect populations in the upcoming growing season. Though the answer to this question seems simple it is actually very complex.
Certainly, temperature has a large impact on insects. If the temperatures are too high (generally greater than 85-90o F) insects may die because their cellular activity is moving so fast. It’s like humans having an elevated temperature, except much worse. When the temperature is too low, (generally less than 60o F) insects are less able to move, eat and reproduce but they often do not die even when it is very cold; they just remain sluggish or immobile until temperatures warm back up. The mechanisms that allow this survival are too complicated to describe here, but basically insects have two main protective mechanisms. They can seek shelter from the cold and they can produce compounds in their bodies that are very similar to the antifreeze that we humans put in our cars. These compounds keep them from freezing solid. It is not the cold but the freezing/thawing processes that usually causes insect death.
When thinking about how winter affects insects we need to remember that we are talking about population size. An individual insect is either alive or dead, there is no in between. But an insect population, a group of insects, can have different rates of survival. So, in a “mild” winter, perhaps 10% of the overwintering insects die leaving 90% of them to live into the spring. On the other hand, in a “hard” winter, perhaps 80% of the population will die leaving only 20% of the population to survive into the spring. So, it is not so much a matter of if insects can survive as it is how many insects will survive.
Remember that snow and ice are insulators. Though what they coat will be at freezing (32o F) and temperature will drop slowly, this is often much better than the air temperatures from an arctic blast! So, for example, when there is a -10o F day, things that are covered with snow and ice will remain much nearer to 32o F, providing a measure of protection from the ultra-cold temperatures. This and protection from water loss is why wheat producers like to have a snow cover when severe cold is present.
The status of water in the area of overwintering is also very important. For many insects the presence of liquid water, combined with very cold conditions, is quite dangerous. With most species the over wintering stage can survive much lower temperatures if they are dry and not in direct contact with ice crystals. This may sound confusing but greater insect survival rates occur when the overwintering insect is dry, and not touching an ice crystal. They may be insulated by surrounding ice so long as they are not touching the ice.
So, what can we say about insects in Kentucky? We can generally classify our insect pests as those that are: 1) cold adapted, 2.) warm adapted or 3.) migratory. Here are some examples.
Fully cold-hardy European corn borer (ECB) larvae can survive -4o F for three continuous months, while the egg stage of the soybean aphid can survive to about -29o F in a single severe cold event. Clearly, European corn borer, imported in the 1700’s from northern Europe, and soybean aphid which arrived in KY from northern Japan in 2000, are not likely to face many winters in Kentucky that they cannot survive; especially when there is snow cover.
Southwestern corn borer (SWCB), which migrated to the central states from southwestern US states and Mexico, is a more warm adapted pest, (though there is some evidence the species is adapting to cooler areas). In laboratory studies larvae can survive temperatures of 14o to 19o F for several days if they are dry. The importance of seeking a hiding place is illustrated by a study done near Evansville, IN, which represents the likely northern range edge for southwestern corn borer. Temperatures inside intact corn root crowns, the overwintering location of SWCB, were measured over three winters. Although air temperatures fell below −2.2° F for up to five consecutive days, root crown temperatures only approached 17.6o F for periods of a few hours, due primarily to a moderating effect of freezing soil water. So, though the air temperature was cold enough to kill the SWCB, they were not experiencing that cold in their hiding place.
Migratory pests like fall armyworm cannot survive a Kentucky winter any time. They can only live in an area where their food grows continuously all year round. Thus, we only see them after the weather has become warm enough for: 1.) The insect to survive the cold and 2.) Their food source becomes available. As those conditions expand northward each year we see the population expanding into our area and beyond.
What conditions are likely to cause the greatest decrease in an insect population in Kentucky?
• It appears that cold, wet weather, particularly with wet but not frozen soils for insect overwintering in the ground, and lack of snow or ice cover will cause the greatest or at least the most common decrease in survival of overwintering insects in Kentucky,
• Extreme cold that is not moderated by snow or ice cover increases mortality but is a rather rare event.
• Repeated wide swings in temperature, alternating warm and cold during the traditionally cold months (e.g. Dec – Mar).
• A long, warm period late in the traditionally cold season followed by a quick return to very cold conditions.
• Consistently cold but “average” temperatures are not as damaging as most people expect.
What will possibly happen to grain crops insect pests in the 2015 production year because of the 2014-2015 winter conditions? This is skating on thin ice; but let’s take a go!
• “Cold adapted” insects like European corn borer will not be much affected,
• Migratory insects like armyworm and fall army worm: Given how far south the cold weather has penetrated, these insects may show up later in the year and in smaller numbers, but this may be more dependent on how long the cold and the snow cover lasts, rather than how cold it gets.
• Warmer adapted insects like southwestern corn borer may have less overwintering survival. The severe cold weather in the late 1970s decimated this pest in Kentucky which did not reappear in an economic way until the early 1990s.
• The more warm adapted southern green and redbanded stinkbugs that are on our boarders but not yet recorded in Kentucky, are unlikely to make it to us in 2015.
• Kudzu bug populations in the south were much smaller and appeared later in the 2014 growing season, partially because of the 2013-14 winter. So, I would expect this to happen again in 2015.
• Brown Marmorated stink bug is a more difficult call. This insect did not seem to be much affected by the 2013-14 winter and my guess is the same for this winter. There appeared to be increases in BMSB populations in central KY during 2014. This is one I would look out for in central KY this year.
• Our normal Green & Brown stinkbug complex was much larger in 2014 than in the previous year. So, I don’t see that the 2013-14 winter had much impact on them, thus I don’t think the 2014-15 winter will either.
Baskauf, S. and D. McCauley. 2001. Evaluation of low temperature mortality as a range-limiting factor for the southwestern corn borer (Lepidoptera:Crambidae) Environ. Entomol. 30(2): 181-188.
Hanec, W. and S. Beck. 1960. Cold hardiness in the European Cornborer, Pyrasta nubilalis (Hubn.) J. Insect Physiology.5(3-4)169-180.
MCCornack, B., M. Carrillo, R. Venette and D. Ragsdale. 2005. Physiological constraints on the overwintering potential of the soybean aphid ( Homoptera: Aphididae). Environ. Entomol. 34(2):235-240.