Use degree days and plant phenology to anticipate timing of pest problems this spring

With the unusually warm weather this spring, degree day accumulation and the timing of full bloom for certain trees will be the best way to know when pests become active this year, because they could be here as much as a full month earlier than usual.

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).

Species

Phenological Event

Range

5 Yr Average

Date

DD50

Date

DD50

Silver Maple

full bloom

Mar 28-Apr 13

7-45

4-Apr

30

Eastern Tent Caterpillar

egg hatch

Apr 1-19

8-67

9-Apr

47

Red Maple

full bloom

Apr 4-22

30-87

13-Apr

67

Border Forsythia

full bloom

Apr 13-May 3

86-106

22-Apr

97

White Pine Weevil

adult emergence

Apr 18-May 8

58-176

25-Apr

110

Star Magnolia

full bloom

Apr 19-May 7

101-130

25-Apr

114

Gypsy Moth

egg hatch

Apr 21- May 10

118-172

28-Apr

148

Norway Maple

full bloom

Apr 23-May 9

119-193

29-Apr

154

Weeping Higan Cherry

full bloom

Apr 23- May 16

120-176

1-May

155

‘PJM’ Rhododendron

full bloom

Apr 26- May 13

138-224

3-May

172

Amelanchier sp.

full bloom

Apr 27-May 14

141-198

3-May

176

‘Bradford’ Callery Pear

full bloom

Apr 28-May 16

154-202

4-May

182

Hawthorn leafminer

adult emergence

Apr 26-May 9

126-221

4-May

183

European Alder Leafminer

adult emergence

May 2-17

118-252

5-May

189

Birch Leafminer

adult emergence

Apr 29- May 9

126-221

5-May

189

Euonymus Caterpillar

first larva

May 4-23

196-255

9-May

227

Japanese Flowering Cherry

full bloom

May 4-20

217-248

9-May

227

Elm Leafminer

adult emergence

May 4-23

178-284

9-May

228

Eastern Redbud

full bloom

May 6-21

225-284

11-May

254

‘Snowdrift’ Crabapple

full bloom

May 6-22

229-276

11-May

255

Pine Scale

egg hatch

May 7-24

255-292

13-May

277

Cooley Spruce Gall Adelgid

egg hatch

May 7-25

255-312

13-May

283

Wayfaringtree Viburnum

full bloom

May 8-24

258-322

14-May

287

‘Coral Burst’ Crabapple

full bloom

May 8-24

263-322

14-May

296

Common Lilac

full bloom

May 10-28

319-325

17-May

323

Lilac Borer

adult emergence

May 13-29

255-386

16-May

324

Lesser Peachtree Borer

adult emergence

May 10-29

258-465

20-May

362

Oystershell Scale

egg hatch

May 13-28

325-459

19-May

363

Doublefile Viburnum

full bloom

May 15-Jun 1

364-449

21-May

398

Vanhoutte Spirea

full bloom

May 18-Jun 3

411-444

25-May

429

‘Winter King’ Hawthorn

full bloom

May 25-Jun 8

430-542

29-May

485

Pagoda Dogwood

full bloom

May 18-Jun 7

444-533

29-May

488

Bronze Birch Borer

adult emergence

May 28-Jun 8

513-589

2-Jun

550

Black Locust

full bloom

May 29-Jun 10

521-630

3-Jun

564

Peachtree Borer

adult emergence

May 25-Jun 18

481-744

3-Jun

573

Euonymus Scale

egg hatch

May 29- Jun10

517-678

3-Jun

575

Juniper Scale

egg hatch

Jun 7-18

624-776

11-Jun

697

Washington Hawthorn

full bloom

Jun 12-27

 794-908

18-Jun

830

Japanese Tree Lilac

full bloom

Jun 16-28

783-916

20-Jun

860

Fletcher Scale

egg hatch

Jun 16-28

813-1052

20-Jun

884

Cottony Maple Scale

egg hatch

Jun 17-30

833-1062

23-Jun

930

Northern Catalpa

full bloom

Jun 5-30

781-1097

24-Jun

937

Greenspire’ Littleleaf Linden

full bloom

Jun 21-Jul 4

896-1092

26-Jun

985

European Fruit Lecanium

egg hatch

Jun 20- Jul12

861-1407

29-Jun

1073

Spruce Bud Scale

egg hatch

Jun 26-Jul 12

1012-1407

4-Jul

1154

Midland, MI Phenological Sequence:
Dow Gardens 1985 - 1989

Daniel A. Herms, Dept. Entomology, OARDC, 1680 Madison Ave., Wooster, OH 44691

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