Despite record cold temperatures in January, many insects will survive the wintery blast and will be poised and ready to impact crops in 2007.
University of California Integrated Pest Management Farm Advisor David Haviland says many factors influence the ability of insects to survive the winter and reach pest status the following year.
“Predicting the influence of cold weather on insects is a tricky business and in most cases, nearly impossible. 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.”
Insects have several coping skills that help them to avoid temperature extremes. Insects know when to come in out of the cold. 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 air temperature. For example, temperatures in Shafter, Calif., dropped to 20 degrees or colder for four straight nights from Jan. 13 to 16, while soil temperature probes at the same sites just six inches in the soil never dropped below 43 degrees,” explains Haviland.
“Many other insects such as bark beetle larvae also buffer themselves by spending the winter inside living plants, 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.”
Insects that are unable to avoid cold weather have two 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 but is also a huge advantage since their bodies can tolerate a wide range of body temperatures. Insects stop moving when they are cold, and as long as they don't freeze solid, most can 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,” says Haviland.
“While this means that the insect does not eat, drink, grow, or move for a period that can extend to 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, meaning that insects can tell that winter is coming by the ratio between hours of sunlight and darkness, and use this information 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.
Tropical insects such as whiteflies and fruit flies are the exception. 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. “These tropical insects are susceptible to extreme cold temperature,” says Haviland.
“It's likely that initial spring populations of olive fruit flies, silverleaf, giant, and other species of whiteflies and some scale insects will be significantly lower than in most years. However, many tropical insects have exponential growth rates so that even low initial populations in the spring can quickly increase to a point where they can cause crop losses and must be controlled.”
When adding in the complexity that the prevalence of many insects depends on other insects, such as the relationship among pests and beneficial insects, there is no telling what may happen.
Haviland says, “It's 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 Statewide Integrated Pest Management Program Web Site at www.ipm.ucdavis.edu.