Using degree days to predict pest and crop development in blueberries

Rufus Isaacs<isaacsr@msu.edu> and Jacquelyn Albert, Michigan State University, Department of Entomology; and Keith Mason, MSU Enviroweather - April 30, 2019

MSU’s Enviroweather system can help you predict crop and pest development.

Aphids on leaves — Aphid colonies on blueberry leaves. Photo by Rufus Isaacs, MSU Entomology.

What are growing degree days?

Growing degree days (GDD) are a useful tool for blueberry scouts, consultants and growers because they can provide early warning of key crop and pest events through the growing season. Degree days are a measure of accumulated heat, and by knowing the threshold temperature that a specific crop or insect develops at, we can calculate the amount of development (growing degree days) that will accumulate each hour or each day. A detailed explanation of degree days is available in the Michigan State University Extension article, “Understanding growing degree-days.” It is written for a different crop but the general ideas apply to blueberry bush development and insect pests, whose development is driven by temperature.

Finding local degree day values

Enviroweather map — Locations of Enviroweather stations (yellow dots) where growing degree day information can be accessed.

MSU maintains a network of weather monitoring stations across Michigan. These stations connect to the Enviroweather system that provides free and automatic calculation of degree days for each station. By clicking on the weather station nearest to your farm, you can scroll down to click on a line titled “Temperature, rainfall and degree-day summary”. Once the new page loads, scroll down and you will see a table that lists degree days for five different base temperatures (the lower threshold temperature for calculating degree days). This shows degree days up to today and for the week ahead. The different columns are used because different plants and insects have different temperature thresholds above which they will develop.

We have found that blueberry bushes develop above ~50 degrees Fahrenheit and most insects develop above thresholds of 42 or 50 F, so we provide target values for these two base temperatures in the table at the end of this article.

Crop phenology monitoring

We monitored pests and crop development for four years at multiple Michigan blueberry farms to develop the table below. Based on our earlier sampling, first bloom of Jersey is predicted to start at 310 GDD base 50 F. For today’s situation in Grand Junction on April 30, 2019, we are at 138 GDD with 171 GDD predicted by Monday, May 6. For earlier cultivars, these target numbers would be smaller, and bloom will be sooner. Further north or west in the state, the accumulation is slower, so we are currently at 93 GDD at the South Haven Enviroweather station and 92 GDD at the West Olive Enviroweather station.

Pest phenology monitoring

Degree days can also be used to predict pest emergence, when to put out monitoring traps and when to optimally time insecticide applications. As shown in the table below, the first cherry fruitworm moths are predicted to start their flight on average at 262 GDD base 50. This is before predicted Jersey bloom, so a good rule of thumb is to get your fruitworm pheromone traps set out in your fields in late April or early May before any of the cultivars bloom. This provides a chance to have a week or two with zero catch to be sure you detect the start of moth flight.

The table provides some GDD predictions for other key blueberry insect pests. We suggest you print this table and keep it handy during this season. With this and the Enviroweather station information, growers and consultants should be able to keep ahead of the pest development, even if it is early or late depending on the type of growing season we experience. We plan to expand the list of pests in this table based on a new project running this season and next. More blueberry pest information is available online at MSU Extension’s Blueberries website.

We stress that these are guidelines and are not validated models (which is what we are studying this season). If you are trapping for pests and also tracking GDDs in your area, your feedback on the accuracy of these values would be greatly appreciated. Please send comments to Jackie Albert at albertj@msu.edu.

Average dates and degree days of Jersey cultivar blueberry development and key insect pests.
Event (growth stages)	Date first seen		GDD 42	GDD 50
Event (growth stages)	Van Buren County	Ottawa County	GDD 42	GDD 50
Bud break	April 17	April 18	224	108
Bloom	May 14	May 15	591	310
Petal fall	May 27	May 28	768	407
First harvest	July 15	July 15	2,060	1,313
Cherry fruitworm moths
First	May 10	May 10	511	262
Peak	May 28	May 30	804	431
End	June 12	June 16	1,180	683
Cherry fruitworm eggs
First	June 1	June 2	872	472
Peak	June 9	June 9	1,074	612
End	June 21	June 18	1,337	797
Cranberry fruitworm moths
First	May 24	June 1	758	412
Peak	June 16	June 17	1,267	747
End	July 17	July 11	2,018	1,285
Cranberry fruitworm eggs
First	June 6	June 11	1,235	732
Peak	June 9	June 13	1,264	776
End	June 19	June 15	1,401	856
Blueberry aphids
First	June 5	June 4	949	525
Peak	July 4	July 4	1,715	1,062
End	Aug. 8	Aug. 13	2,804	1,853
Parasitized aphids
First	June 29	June 23	1,503	904
Peak	Aug. 4	July 30	2,571	1,692
End	Aug. 28	Aug. 13	3,314	2,246
Obliquebanded leafroller moths generation 1
First	June 15	June 14	1,208	695
Peak	July 1	June 28	1,607	994
End	July 29	July 28	2,434	1,600
Obliquebanded leafroller moths generation 2
First	Aug. 18	Aug. 12	2,968	1,986
Peak	Sept. 6	Sept. 7	3,574	2,396
End	Sept. 27	Sept. 30	3,980	2,666
Japanese beetle adults
First	July 4	July 15	1,908	1,211
Peak	Aug. 14	Aug. 11	2,738	1,832
End	Sept. 4	Sept. 1	3,452	2,319

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).