Give Insects the Third Degree

When gathering at turfgrass conferences, lawn care operators (LCOs) often discuss the recent weather. Why? They know from years of experience that weather patterns often correlate with outbreaks of plant diseases and pests during the growing season.

Of course, there also are many other factors that influence the likelihood of a pest outbreak, including the genetic resistance of the host plant, how heavily a turf or ornamental plant is fertilized, the physical characteristics of the soil, and plant location.

Still, if it were possible to predict the severity of certain insect problems during a given year, we could all throw away our proverbial crystal balls. As it turns out, one particular factor has a significant influence on plant and insect development: temperature. Because temperatures fluctuate daily, an insect that normally takes four weeks to mature from egg to adult may require five weeks or more in a cooler year, and only two-and-a-half weeks during an abnormally warm period.

DEGREE-DAYS DEFINED. The system of degree-day calculation was developed to help LCOs get a better handle on how temperature impacts insects. Once LCOs start working with the concept, it becomes a useful tool for calculating and predicting insect activity. It’s a bit like your first experience typing on a computer or riding a bicycle – at first, you are clumsy, but after some effort and experience, it becomes comfortable. So, what is a degree-day? The best way to think of it is as an accumulation of temperature over time. Because degree-days dictate the rate of growth and development of plants and animals, they are also sometimes thought of as “thermal time.” With most pest organisms in the landscape, there is a certain minimum temperature at which activity, growth and development slows and eventually stops, otherwise known as base temperature or developmental threshold. For most organisms, base temperature is 50 F. Other organisms’ base temperatures may be as low as 39 F or as high as 54 F. Defined, degree-day accumulation is the sum of the number of degrees by which each day’s average temperature exceeds the base temperature. Luckily, this is easy to calculate. A 24-hour period in which the average temperature for the day is only one degree above the base temperature accumulates or yields one degree-day. So, every day, the degree-day accumulation can be calculated for a specific pest and then analyzed over time. Most degree-day models start with the calendar year, beginning with Jan. 1. However, if you live in the Northeast, Pacific Northwest or Midwest, you can choose a starting date much closer to the onset of insect activity that you’ve noticed in previous years because very few days early in the season are warm enough to significantly contribute to the insect’s degree-day accumulation. For example, if you notice that pine needle scale crawlers start hatching in mid-May in your area, start your calculations with May 1. The best way to obtain temperature information is to purchase and install a Max./Min. thermometer, available at many home improvement and hardware stores. You also can use weather data from television stations or newspaper reports, however, the equipment used to record the temperatures is likely to be as much as 20 miles away. There could be important differences between local and weather service temperatures, especially if the property you’re inspecting is coastal or near a lake. If you operate a lawn care business, you may need to obtain separate data sets from each part of the city. If you have a large account (i. e., estate, shopping mall) or you work for a golf course or campus grounds department, it may be practical to purchase and install a weather station containing a Max./Min. thermometer specifically for the landscape you work with every day. Here’s how to figure the calculation (for a degree-day example using pine needle scale and a base temperature of 50 F, see page 64):