Growing degree days (GDDs) are commonly used to predict important events in plant development, such as flowering or crop maturity. For ornamental crops grown outdoors, GDDs can help predict insect activity since temperature plays a direct role in their growth and development.

With warmer weather comes more pests

When temperatures rise, insect development speeds up, just as plants bloom earlier when spring arrives ahead of its traditional timing. Each insect has a threshold temperature below which no growth or development will occur.¹ A tool for measuring this environmental heat can be helpful for determining when to scout for and begin preventing insect pests.

GDDs can be used to predict important events in the life of an insect, which can provide valuable information to maximize the effectiveness of agronomic and rotation programs. These biological events, such as egg laying, egg hatch, scale crawler movement or appearance of symptoms, can help you plan for activities like scheduling insecticide applications.²

Calculating GDDs

Record the daily minimum and maximum temperatures in a table for each day between March 1 and September 30. Then use the following formula to calculate the daily GDD:
(Max. Temp + Min. Temp/2) - Base Temp (50F) = Daily GDD
Example: 65 + 45 = 110/2 = 55 - 50 = 5

For those days where a negative number results for the daily GDD, use 0. 

Date	Daily Min. Temp F	Daily Max Temp F	Avg. Temp F	Daily GDD	Accumulated GDD
March 1	45	65	55	5	5
March 2	35	50	42.5	0	5
March 3	45	59	52	2	7

The daily GDD numbers are then totaled to reflect the accumulated or total GDDs for this time period. This number can then be checked against various pests listed in the Penn State Chart to see when activity may be expected.

Scout and prevent with GDDs

This GDD chart from Penn State shows the typical ornamental pests and the GDD at which they develop. According to the data, Japanese beetle adults become active at a minimum GDD of 1029 and a maximum of 2154. As these GDDs approach, nursery growers should be scouting and beginning to apply preventive insecticides. Mainspring^® GNL insecticide is a non-neonicotinoid option for controlling Japanese and other leaf-feeding beetles in the nursery. When used preventively, it keeps pest populations from building and provides excellent residual control.

Syngenta offers resources on GDDs, including frequently asked questions, temperature threshold updates and GDD email alerts on GreenCastOnline. The site also includes other helpful tools like soil temperature and soil moisture maps. Having this type of information can be beneficial when battling pests and diseases in the nursery.

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¹www.mortonarb.org/trees-plants/tree-and-plant-advice/horticulture-care/degree-days
²www.canr.msu.edu/uploads/files/Research_Center/NW_Mich_Hort/General/CalculatingGrowingDegreeDays.pdf

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