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Cold Weather Impact on Insect Pests

April 25, 2007

The last couple of weeks of cold weather that followed unseasonable mild weather brings up some questions as to how insect pests will react to this situation. Spring-occurring insects have adapted to the various situations that can occur with early, late, wet, and dry springs.


Insects are cold-blooded animals, meaning that their body temperature is similar to that of their surroundings. They do not maintain a body temperature as do mammals and birds. Most insects do not develop or function well at temperatures below 50?F. This base temperature varies a few degrees up or down, depending on the insect species, but is near 50? for most insects. When the temperature hangs between freezing (32? F) and 50?F, insects typically go into a dormant state in which body functions are greatly slowed down.


An interesting feature is that plants appear to develop at temperatures somewhere in the 30s and above. When temperatures hang in the 40s for an extended period in the spring, plants continue to expand leaves and grow stems, and they can get ahead of their insect pests that are present but not developing. As a result, insect pest populations may be typical, but plant damage caused by those pests may be much less than normal.


At temperatures below freezing, many insects that are present during the summer die. However, many spring occurring insects survive temperatures in the 20s, becoming active again once temperatures rise above 50?F. Fewer species survive temperatures that drop into the teens. With the temperatures that we have experienced over the past few weeks, many of the insect pests and their natural enemies that are insects will have died. However, many of them will have survived.


Insect populations typically do not emerge at once. Some hatch from overwintering eggs or emerge from overwintering pupae early, and some of the same species emerge later. This is obvious each spring with the different sizes of eastern tent caterpillar tents and corresponding different-sized caterpillars within them. The smaller tents and caterpillars represent egg masses that hatched later than others. A similar feature can be easily seen in the summer with fall webworm.


Thus, when the cold weather occurred, not all spring- occurring insect pests and natural enemies had hatched or emerged from overwintering stages. Even if the early-hatching and -emerging insects were killed by the cold weather, some will hatch or emerge after the cold temperatures. This may result in less damage to plants due to the loss of the early insects, reducing the overall number of insects attacking the plants. This year, much of the plantsí foliage was killed by the cold weather, and the later-hatching and -emerging insects will be present to feed on the replacement foliage as it emerges.


In summary, the insect pests will survive but will probably be less numerous. They will be present to feed on the replacement leaves, so that insect damage will be found. However, the damage due to spring-occurring insects will probably be less than normal. This weather will have little, if any, effect on insects that typically emerge later in the growing season.