Forecasting insect activity by noting plant development and degree days

Predict insect activity by watching plant development and checking the accumulation of degree days. Here’s how.

At the Master Gardener conference in Grand Rapids earlier this week, several Master Gardeners asked me to print a table of insect events correlated with degree days and plant phenology. Unfortunately, we have a nice set of events for spring and early summer, but not much after July 1. Still, you will find the table below as promised. Notice that we are now approaching peak activity for European chafer, which follows catalpa bloom. The adult beetles will be flying at dusk for the next two weeks in the mid-Michigan area. Below is a review of the plant phenology and degree day concept, followed by a table of insect and plant events.

Scouting for vulnerable stages of insect pests of trees, shrubs and perennials is much easier if you know when to look for them. One of the best ways to predict insect activity is by plant development and degree days. The best set of observations we have for Michigan was made by Dan Herms over a five-year period when he was at Dow Gardens in Midland.  Phenology is the study of biological events in relation to weather.  Plant development can be tied to insect development. Dan 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.

Another way to predict insect events 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

So, 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. For example, the high today as I am writing this is 55 and the low 45, so (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.

We can make this easier for you if you visit Enviro-weather where you can get maps showing the current degree day accumulations. At the homepage, click on a station near you and choose “Current degree days” from the degree day tools section.

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

Midland, MI Phenological Sequence: Dow gardens 1985-1989

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

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

European Chafer

First adults

Jun 20 – 30

785 - 1100

25 - Jun

950

Greenspire’ Littleleaf Linden

full bloom

Jun 21-Jul 4

896-1092

26-Jun

985

European Fruit Lecanium

egg hatch

Jun 20- Jul 12

861-1407

29-Jun

1073

Japanese Beetle

First adult

June 27-July 2

7-Jul

Panicled Goldenraintree

First bloom

July 5

Dr. Smitley’s work is funded by MSU‘s AgBioResearch.