Forecasting insect activity by noting plant development and degree days

Questions about this information? Contact us. July 01, 2011

Updated from an original article written by Dave Smitley, Michigan State University Extension, Department of Entomology.

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.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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