HOW TO: Forecast pest problems

Ever notice how some pests occur consistently and cause problems every year, while others are more sporadic? Sometimes the more sporadic pests are the more difficult to control, and they may occur unexpectedly. Obviously, it would be beneficial to be able to predict outbreaks, especially those that don't seem to occur with much regularity. Or, at least, detect the pest populations at an earlier stage so as to provide more control options.

For example, newer insecticides targeted at grubs such as imidacloprid and halofenozide generally are more effective when directed against earlier stages. Some “natural” products, such as those containing the naturally-occurring growth regulator azadirachtin, will work best when applied against early instar caterpillars. A general rule for mole cricket management is that almost all products work most effectively when applied against the smaller, early instar crickets.

Obviously, there is a need to be able to predict and characterize insect life stages so that you can target specific products against certain stages. In addition, some pest controls operators are searching for ways to use many of these newer products with lower rates. Either way, finding a method to accurately time the applications is a challenge. This is particularly true for soil insects such as white grubs and mole crickets because these insects spend much of their life in the soil and remain somewhat hidden. It's often hard to know exactly what is going on.

Scouting and monitoring pests is a good way to stay on top of pest abundance and development. Techniques used to keep track of pests range from light traps for moths to pheromone traps for pests like Japanese and Oriental beetles, to soapy water flushes for mole crickets and caterpillars, to digging in the soil for white grubs. These techniques can be time-consuming, all the more so if you don't have at least a rough idea of when to search.

Fortunately, there's a better way than continuous sampling.

Insects: facts of life

All insects are cold-blooded. That is, their growth and development depends on the temperature of their environment. The warmer it is, the faster the insect develops and the cooler the temperature, the slower the development. There is usually a lower threshold for development, and often this is roughly 50°F (10°C), although some insects may actually develop at temperatures slightly below that level. Some insects have an upper threshold as well. In some cases, once it gets warmer than the upper threshold, the insect's developmental rate may not increase any further.

Other factors may play a role in an insect's overall development, such as day length, relative humidity, soil moisture and others. However, temperature is usually the main element that influences insect development. In cold regions, winter temperatures bring all pest development to a standstill. In areas farther to the south, some development may continue during warm spells in the winter.

Research has shown that many insect populations can be forecast or predicted by using either soil or air temperatures. However, because plant growth is also regulated by temperatures (as well as day length) it is sometimes possible to relate some stage of plant development (such as flowering) to the development of a certain stage of a specific insect. This is called phenology, and can be much easier than constant monitoring and logging of data (although this information is made available from universities and cooperative extension offices).

A specific example of relating plant development to insect activity can be found in the northeastern United States. The adult annual bluegrass weevil migrates to turf when forsythia is in full bloom and begins laying eggs just before dogwoods flower. In some areas, the bloom of the Van Houtte spirea, the horse chestnut, and the rose of Sharon synchronize with adult egg laying of the black turfgrass ataenius. If you use indicator plants, be sure to confirm their reliability in your area. Not all such relationships are valid universally.

Degree-days: A simple way to measure

The use of degree-days is a relatively simple, but often highly accurate means of monitoring how “warm” it has been. As a general rule, 50°F is the base threshold for insect development. Some insects may vary from this, but any time the temperature is over 50°F, some insect development usually takes place.

Degrees are typically calculated by adding the maximum temperature for a day to the minimum temperature for that day. The sum is divided by two and then 50 is subtracted. The resulting number is the degree-day accumulation for that day. For example:

The maximum temperature for the day was 82°F

The minimum temperature was 56°F
56 + 82 = 138
138 ÷ 2 = 69

69 - 50 = 19, which is the degree-day accumulation for that day.

This method is prone to potential inaccuracies. For example, it might have been a cloudy day and it was cool except for one hour when the sun was out. In such a situation, the maximum temperature is accurate but the actual “warmth” of that day may be overstated with such an average.

A more accurate means to calculate degree-days is to use what is called the sine-curve method which measures daily temperatures at regular intervals (see illustration, page 44). Then the actual degree-days can be calculated. This is not something we can do by hand, but many computerized weather stations determine degree-days in this manner. Some crop reporting services also figure degree-days in this manner.

One word of caution: some plants have relatively low upper thresholds for development (upper 80s) as compared to insects (above 100). Therefore, degree-days reported for crops may be less than that actually acquired by insects because the warmer temperatures are not included in their calculation. Be sure you know whether the degree-days figures you're relying on are appropriate for your situation.

It is also important to consider that 50°F is commonly used as a base threshold temperature for insects, but it is only an average. Soil insects typically develop at temperatures slightly below 50°F and some foliar insects may require temperatures slightly above that level.

Another area of variability is the exact start date for accumulating degree-days. Many published forecasts start at January 1; others at February 1. Some may start with the detected presence of a specific insect stage (such as the presence of adults). In more northern areas, there may be some flexibility in that most days are consistently cold enough to prevent any insect development until spring. Areas to the south may see some development during the winter months.

Despite these sources of “error,” degree-days still provide useful insight into pest development. Examples of degree-day accumulation dates are found on page 45. These indicate the approximate degree-day accumulations from January 1 for beetle emergence and for black cutworm development.

Note that masked chafers begin emerging a little ahead of the Japanese beetles — this is important. For those turfgrass managers who battle Japanese beetles on various ornamentals each year, the need to forecast these pests might be questionable. After all, it is quite obvious when they are present. You don't need a forecast to tell you that Japanese beetles are eating your prized ornamentals. However, the adults of the masked chafer grubs are much less conspicuous. The same is true for the Oriental beetles. These beetles don't present themselves in as obvious a fashion as do the Japanese beetles. Monitoring degree-days can help you keep track of what these and other pests are doing.

Today's newer insecticides often require more accurate timing to get maximum benefits from the application. Some of the newer pest products work best for white grubs when applied at egg laying or egg hatch. In the absence of adults flying around, the degree-day indication of beetle emergence could help. The recent detection of Oriental beetle grubs in western North Carolina helps emphasize the importance of timing. Our studies indicate the beetles (which are much less obvious than Japanese beetles) emerge several weeks earlier (see bar graph, above). Insecticides directed toward the grubs of these species would need to be applied early. We are currently working to develop degree-day accumulation for this beetle.

When soil is very dry, beetle emergence may be delayed. Despite the accumulation of a sufficient number of degree-days, the newly formed beetles may stay in the soil. Emergence will follow immediately after a good soaking rain. Thus, environmental extremes such as dry weather can affect the accuracy of such predictions. Understanding some of the factors that influence the appearance of the insect can help you correct for these events.

Insects that have multiple generations in a year get tricky. Cutworms require approximately 730dd (with a base of 50°F) to go from an egg to fully developed larva. But when do you start counting the degree-days? Over-wintering is important, as is the number of generations per year. We have tried the use of pheromone traps for moths of cutworms and armyworms, but have not been too successful in correlating these trap catches with the time to begin accumulating degree-days for a particular generation.

How does one use degree-days to forecast pest outbreaks? It all starts with record keeping. If you have a computerized weather system, your work may be a little easier. The goal is to simply build a more precise database with each passing year through experiences and observations. Keep track of degree-days from January 1 and use a base temperature of 50°F. Do the same for the various insects. Take note of when you first see Japanese beetles, or white grubs, or anything else that gives you trouble. Do the same thing when you use soap flushes for mole crickets. Good record-keeping of pest occurrence and weather data is the key to this process. By doing this for a number of years, you can adjust and refine your forecasts.

After several years you can use a fairly reliable forecast that has been personalized to your location. When you approach the target degree-days each year for a particular pest, then you know it's time to begin your scouting or a monitoring program. This allows you to use your time more efficiently by eliminating monitoring when degree-days can tell you there's no point in doing it.

The obvious value in spending the time to develop such forecasts is that it helps you better understand the pests you must deal with each year. It also helps prevent being caught off guard. Such forecasts perhaps play the most significant role in predicting insects that are small and not readily seen in advance of the damage, insects that can't effectively be trapped or insects that require management strategies to be implemented at a very specific stage of the life cycle. The latter is becoming more common and is particularly true of biological control agents.

If insect pests sometimes surprise you, give forecasting a try. Contact your extension office or consultant to see what prediction models are available for your area. Using good records of weather and insect abundance will soon allow you to forecast pests and improve your timing for scouting and management. And make life a little easier.

Dr. Rick L. Brandenburg is a turfgrass entomologist at North Carolina State University (Raleigh, N.C.)

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