Using degree days to predict pest and crop development in blueberries

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

April 30, 2019 - Author: and Jacquelyn Albert, Michigan State University, Department of Entomology; and Keith Mason, MSU Enviroweather

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

Van Buren County

Ottawa County

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

Tags: blueberries, enviroweather, fruit & nuts, msu extension


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