MSU Extension Landscaping

Record heat accumulation in March 2012 has red maple trees blooming already. This means eastern tent caterpillar eggs could be hatching by the time you read this (in Detroit area), white pine weevils will be active next week, and gypsy moth eggs could begin hatching in two weeks. This is three to four weeks ahead of an “average” year, so it is going to catch a lot of people by surprise. One way to keep up with the record-early pest activity it too scan the list of pests in the degree day table at the end of this article, and match them with plant phenology events that you are familiar with, like red maple bloom.

How to use degree days and plant phenology tables

Scouting for vulnerable stages of insect pests of trees, shrubs and perennials is much easier if you know when to look for them. For example, euonymus scale is a very destructive pest of evergreen euonymus (Euonymus fortunei), a popular landscape shrub. Euonymus shrubs can be protected from damage caused by euonymus scale by spraying infested shrubs with a 2 percent concentration of horticultural oil (SunSpray, Volck Oil, or others) when the crawlers emerge. Although the oil spray kills about 50 percent of the overwintering scale insects when it is applied in April or May, it is much more effective when applied after the crawlers emerge in June. If sprayed after most of the crawlers have emerged, a horticultural oil will kill more than 90 percent of the crawlers, providing adequate plant protection from a single spray.

The ideal timing of this oil spray is zero to two weeks after 90 percent of the crawlers have emerged. This can be determined by clipping infested twigs once per week starting the third week of May, putting them in a Zip-Lock bag labeled with the date they were sampled, and observing them on your desk. Because your office is much warmer than the average daily temperature outdoors in late May, the crawlers will emerge much sooner in your office, giving you some warning as to when to carefully examine the shrubs outdoors. When you see so many crawlers that it looks like a yellow dust on the twigs and leaves, plan on spraying with a horticultural oil about one week later.

The exact week that euonymus scale crawlers emerge each year is different because plant growth and insect activity depend on temperature. In a very warm spring, the euonymus scale crawlers could emerge as early as the middle of May; or in a very cold spring they may not emerge until late June. Also, if you live in Gaylord, Mich., euonymus scale crawlers may emerge two weeks later than they do in Detroit.

To make all of this easier, we can use observations about indicator plants and degree days made by Dan Herms over a five-year period when he was at Dow Gardens in Midland, Mich. (Table 1). The time that euonymus scale crawlers start to emerge each year, and other key pest events, can be predicted accurately by the degree day accumulation in your area or by when certain plants are in full bloom. Phenology is the study of biological events in relation to weather. “Plant phenology” follows easily observed events such as bloom time to track the development of insect pests over the course of the growing season.

Herms uses full bloom (as shown in Table 1) of different kinds of trees and shrubs as a biological calendar to predict when key insects pests are active. Bloom time of plants are good indicators of insect development because plants bloom earlier in a warm spring, and bloom later in a cold spring. Likewise, insects emerge earlier in a warm spring and later in a cold spring. So, one way you can predict when euonymus scale crawlers emerge is to look for full bloom of ‘Winter King’ hawthorn, Pagoda dogwood or black locust.

Another way to predict when crawlers emerge is to keep track of or look up the degree day accumulation in your area, and compare it with the table below. During a five-year period, euonymus scale crawlers emerged each year when the degree day accumulation (base 50) ranged from 517 to 678 (average 575). Degree day accumulation is a way to keep track of accumulated heat units each day of the year, starting March 1. This has proven to be a reliable indicator of when plants bloom and when insects are active. Notice that in Table 1 the degree days are listed as “DD50.” The 50 refers to the base temperature used to calculate the degree days. Fifty degrees Fahrenheit is often used as a general base temperature for insects because most insects do not develop or grow when the temperature is below 50°F. Anybody can calculate degree days if they set out a maximum-minimum thermometer to take daily readings or take the time to write down the maximum and minimum temperature from the newspaper each day. Here is how you do it:

Maximum Temperature + Minimum Temperature
DD50 = 2 - 50

For example, if the high today was 80 and the low 60, the degree day accumulation for today would be (80 + 60)/2 - 50 = 20. Notice that for most days in April and early May, there is no degree day accumulation. Another example: The high is 55 and the low 45. (55 + 45)/2 – 50 =0. Anything less than 0 also counts as 0. Then for each day, starting on March 1, you must add up the degree-days as a running total.

Now you are ready for the bug battles this year. Just save the table below and you will know when to look for insect pests. If you also check the MSU Landscape CAT Alert each week for weekly degree day accumulations for weather stations located throughout the state of Michigan, you will be prepared. Good luck!

Table 1. Prediction of insect pest activity in Michigan from full bloom of trees and shrubs, or by degree day accumulation (DD50).