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Despite Record Cold Temperatures, Expect Most Pests to be Business as Usual

February 2007


There are many factors that influence the ability of insects to survive the winter and reach pest status the following year.  Important factors include availability of food, moisture in the form of rainfall, and temperature.  There is no doubt that extreme cold temperatures will negatively impact some species of both beneficial and harmful insects; however, most insects are proficient at knowing how to cope.

Avoidance of temperature extremes is the most common method for insect survival in the winter.  Many insects, including ants and the immature stages of most beetles, spend their winter living below the soil surface.  Temperatures just inches below the surface can be considerably warmer in the winter and cooler in the summer than ambient temperature.  For example, ambient temperatures in Shafter dropped to 20°F or colder for four straight nights (13 to 16 Jan., 2007), whereas data from soil temperature probes at the same sites just six inches in the soil never dropped below 43°F.  Many other insects also buffer themselves by spending the winter inside living plants, such as bark beetle larvae, or in leaf litter where freezing temperatures rarely occur.  These insects should be relatively unaffected by the cold January weather in the San Joaquin Valley.

Many insects that are unable to avoid cold weather have two excellent mechanisms for surviving its effects.  The first is that insects are cold-blooded, and do not regulate their own temperature.  This is a disadvantage to insects that want to warm themselves up.  However, it is a huge advantage in that their bodies are made to tolerate a wide range of body temperatures.  Humans, for example, being warm-blooded can never 'freeze' to death.  Instead, if our body temperatures were to drop twenty or so degrees below optimal we would lose motor skills, and then cease to function.  However, actual 'freezing' would require dropping the body temperature another 50 to 60 degrees.  Insects, on the other hand, just stop moving when they are cold, and as long as they don't freeze solid, most can just wait until temperatures warm back up and then continue doing whatever they were doing.

The other mechanism insects have for tolerating cold weather is to go into a state of hibernation, called diapause.  During diapause an insect becomes almost completely metabolically inactive.  While this means that the insect does not eat, drink, grow, or move for a period that can extend months, it also means that the insect does not expend energy.  During this state of inactivity most insects can tolerate long periods of cold weather.  In many cases diapause in insects is triggered by day-length, such that insects can tell that winter is coming by the ratio between hours of sunlight and dark, and use this to enter a state of diapause before winter weather arrives.  In other insects diapause is entered when temperatures begin to lower in the winter, or when food becomes scarce.  Despite the trigger that causes an insect to go into diapause, this genetically engrained survival mechanism protects many insects from temperatures reaching far below the norm, and well below freezing.

The most notable exceptions to insects that fall into the aforementioned categories are many of the tropical insects such as whiteflies and fruit flies.  In the native habitats of these insects there is no need to go into diapause or hide from cold winter weather, since temperatures never drop below freezing.  As such they are susceptible to extreme cold temperature.  One such example is the olive fruit fly, where an entire generation of this pest has likely been killed.  Other examples include silverleaf, giant, and other species of whiteflies as well as some scale insects.  It is likely that initial spring populations of these pests will be significantly lower than in most years.  However, many tropical insects are recognized for their ability to have exponential growth rates, such that even low initial populations in the spring can quickly reach treatable levels.

Predicting the influence of cold weather on insects is a tricky business and in most cases nearly impossible.  Even when only one insect species is considered at a time, complicated factors related to pest biology, temperature, food availability, and moisture make predictions about as difficult as asking a weatherman for a prediction of the high and low temperatures for a weekend two months in the future.  When adding in the complexity that the prevalence on many insects depends on other insects, such as the relationship among pest and beneficial insects, there is no telling what may happen.  The best way to respond is to treat business as usual.  Learn about the biology of pests you frequently encounter, learn how to monitor for them, evaluate pest densities throughout the year, and make treatment decisions based on established treatment thresholds where available.  Information on how to do this is available for many crops, as well as many household and landscape pests, through the University of California Pest Management Web Site at http://www.ipm.ucdavis.edu.