Archive for Thursday, February 23, 2006

Weather offers clues about potential insect woes

February 23, 2006


With a record $365 million lottery jackpot going to a lucky winner, it only took a dollar and the correct guess of a few random numbers to win. In the landscape, many gardeners apply this same philosophy to predict the pest problems that we will encounter this year. And most of the time their dollar bet is based on the type of winter we had.

A mild winter means a heavy bug infestation and vice versa. However, predicting bugs is just like predicting lottery numbers. Eventually someone will be right, but there will be a whole lot of guesses that are not. Here is what you need to know about bugs and how soon they will become a problem this year.

There are thousands of different insect species in Kansas. And because insects are cold-blooded organisms, temperatures are especially important in regulating their growth and development. But other environmental factors play an important role as well. Factors such as rain, humidity, evaporation and day length also influence insect development activities. But temperature seems to be the most used and easily measured factor to work with. With that said, insects have a lower threshold for development. There is a baseline temperature above which development occurs and below which there is no development. While lower-development thresholds have been determined for some insect pests with economic importance, precise data is lacking for most pest species commonly found in the landscape.

The lower-threshold temperature is used to calculate degree-days for development. Degree-days is a tool that can be used to predict insect development. A pre-determined number of degree days must be accumulated before an insect can progress to the next growth stage. Degree days are roughly determined by averaging the daily high and low temperature and subtracting the lower threshold for development. For example, several weeks ago gardeners enjoyed the balmy 62-degree afternoon temperature in hopes that it would stay longer. However, that daily high was reached in mid-afternoon and lasted only until the sun dropped low in the sky. And at night, the low temperature dropped to 20 degrees. The total degree days accumulation for that day would be calculated as 62 degrees plus 20 degrees, which equals 82 degrees, divided by two, which equals 41 degrees. Once the lower-development threshold of 50 degrees is subtracted, the result is minus 9 degrees. No degree days were accumulated. Degree-day accumulations generally build during the spring when night temperatures become warmer and complement daily highs. In theory, we should not be finding insects in the garden for several weeks to come.

Another factor to consider in population size is where a particular insect overwinters. For example, if eggs are deposited in the soil, they are protected from harsh temperature extremes, and development is on hold. Soil temperatures build slowly and do not fluctuate like air temperatures. However, eggs laid in areas open to the elements are much more vulnerable to the winter chill and are less likely to survive a sudden cold snap.

The bottom line is this: Despite the arguments of a warm versus cold winter, insect populations will pretty much occur about the same time this year. More importantly, insect populations were influenced by all of the factors in the environment and not just temperature. Rather than trying to predict the future, people should revisit the past. List those insects that caused problems last year and assume that they will, more or less, be a pest again this year. Read up on them and begin scouting before or shortly after their usual occurrence in the landscape.


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