Insects and the Cold

 

Insects and the Cold

 

Every fall and winter, particularly following cold fronts, I am inundated with the statement, “Well, that should wipe those (insert target insect or pest) shouldn’t it Blayne?  At least until next year?”  My answer, however, is never as fulfilling or as conclusive as one would expect or even hope.  The truth is, frustratingly for us, insects usually deal with winter and seasonal weather patterns as a natural and expected part of their lives.  The common misconception that a freeze ‘wipes out insects’ is blatantly incorrect.  It should be noted that insects don’t feel cold.  At least not like we understand it.  They experience various inputs that initiate instinct and behavior responses.  This can initiate anything from initiating diapause and other complex life cycle alterations to a simple move to cluster, ground, or shelter during extreme cold snaps.  But the insects are never really gone.  We do see and interact with fewer insects during the winter months, cold weather impacts insects differently even than other cold ‘blooded’ animals.  Their reactions to the weather might be driven more by instinct and short life cycles than freezing pain from the cold but rest assured, insects have a ‘plan’ to survive and ensure the survival of their particular species.  A plan that is as variable as the over 30 million species of insects that use them.

Some insects do not make changes in behavior during the winter months.  There is no need to.  Pests such as lice, bed bugs, and adult fleas live so close to us or their other hosts that they really have no need to adapt to the cold.  Yet there are insects and other pests outdoors that carry on with their life cycles with no major hibernation adaption either.  Spider mites, Russian wheat aphids, and green bugs are examples of all summer pests of corn, and sorghum crops, but also wheat and other small grain crops in the winter.  While aphid species are known to alter their life cycle from asexual to sexual and lay eggs to overwinter plenty are capable of surviving without an adaptation.  These pests move from summer to winter crops via grass green bridges or directly from host to host during the short windows when both are green and active.  These species are very adaptable to whatever acceptable host plant is available.  Likewise many beneficial insects such as ladybugs and green lacewings do not make major adjustments to their life cycle either, but mostly follow their prey as best they can.  Several species of flies, ants, bees, and many others overwinter similarly but without major food source shifts.  In most of these cases, the number of these insects is reduced during the winter and their visible activity during severe cold periods is greatly reduced but that is only because the environmental conditions and availability of food is not ideal.  So, for most species managing this way for the winter are only reduced in number but the survival of the species depends upon plenty of generations surviving and thriving through the winter months.  Many of these insects can be found doing the same things they do during the summer on the warmer winter days.

Many insects such as most grasshoppers and preying mantids have only one generation per year with their life cycles revolving around the seasons.  In these cases, gravid females will lay eggs in preferred, usually protected, sites as the days shorten.  The resulting eggs overwinter to hatch as days begin to lengthen and food becomes more available.  These freshly hatched nymphs mature over the summer to repeat the process throughout repeating seasons, ensuring long-term species survival even without generations ever interacting with each other.  These are some of the few species that do ‘disappear’ during the winter.  A few other pests only have one generation per year also but manage differently and are far from disappearing during cold weather.  Some are even more economically important during winter.  The army cutworm, the more prominent species of ‘miller moths’ we experience here, lay their eggs in well-established wheat during late summer and early fall.  The larva hatch during winter and spend much of their time below ground where it is warmer emerging nightly feasting on wheat.  During the late winter and early spring, the enlarging and engorging larva can do considerable accumulative damage until mid-spring when they emerge as moths.  These moths are highly attracted to lights and are known to the general public to make general nuisances of themselves for weeks on end.  During the summer, these moths feed on nectar and other harmless food sources to hunt for early planted wheat the next fall to invest the next generation into. 

Plenty of insects overwinter with diapause or hybridization specializations.  Many of the moth members of the Noctuid family, which contain most of our Lepidopteran pest species such as bollworm (a.k.a. corn earworm, sorghum headworm), southwestern corn borer, European corn borer, cabbage loopers, and most armyworm species, have multiple generations per growing season.  In warmer climates, where reliable food sources are easily available, overwintering is easy for these species.  They are very mobile during the summer migrating north as host plants become available.  Not all of the summer populations of these pests migrate to temperate climates during the summer, however.  Most of these species successfully overwinter here on the Texas High Plains and even farther north in great numbers.  As the last generation of the summer larva near pupation, they sense a trigger in the shortening of daylight or a drop in temperature which triggers a response.  These last stage larva all pupate in the soil several inches below the soil surface.  These late summer larva do tend to take on a bit more fat, so that they may diapause as a pupa throughout the winter emerging as a moth late the next spring to begin repopulating. 

Other species of insects are a bit more particular about annual survival.  All species of mosquitoes require water to lay their eggs into and for their larva, referred to as wigglers, to develop in.  A few select species can survive the winter as wigglers in water, but the vast majority of mosquito eggs can, will, and do survive the winter and more.  These eggs will not hatch until conditions are right, which can easily include months but even many years until an area is flooded, standing in water again, or the temperature is sufficient.  Yet the same species can complete several generations in a fairly short period of time, as long as conditions remain favorable. 

A list of exact insectile overwintering methods can be endless.  They even include some artic species that can freeze during long winters and reanimate once temperatures rise.  Or there is the example of the monarch butterfly, whose annual migration across North America following developing milkweed and back south again just ahead of the winter is legendary.  Most species we deal with on the High Plains daily do not go into these extremes.  Most species that are actively overwintering locally will go shelter, underground, or a protected food source nearby during the most extreme weather to reemerge unscathed once temperatures rebound. 

We had some interesting trial results from weather recorders a few years ago just outside of Hale Center.  We were trying to determine if the sugarcane aphid overwintered on the High Plains.  The recorded air temperatures were totaled to be below freezing a little over 21 days for the winter.  That is not all at once, but in typical High Plains fashion, accumulated into a large total with warm and cold days.  Meanwhile, at ½-inch, the temperature only dipped below freezing for an added total of about 7 days added for the entire winter.  For that year, the soil surface only froze for less than an accumulated week.  But, at only 2-inches, the temperature only spent less than an hour below freezing that winter.  It never even reached freezing temperatures at a 4-inch depth for the entire winter.  With this understanding, we can see how insects can find some leaf litter, a tiny crack in the ground near some juicy roots, or a sheltered shrub out of the wind and never be hindered by cold snaps.  Especially if there is a nice fluffy blanket of snow to insulate them from colder air temperatures and winds.  

 Perhaps now after a cold snap and a fly buzzes your nose or even a mosquito bites you on a warm January day, you don’t have to wonder how they are still here.  There is some serious biology at work around us. 

 

Blayne