Degree days and indicator plants: the pest-timing tables extension services publish
Extension services tie the timing of tree pests to heat accumulation and to what plants are in bloom. This page reports how Penn State, UConn, Kentucky and Rutgers describe the method and what their tables list for a few tree pests.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page describes what four extension publications say. It lists no products, rates or spray dates, and it is not advice on treating a tree. Each table is local to the state that published it. The site's degree-day calculator works out accumulated heat from a visitor's own temperatures, and the guide on plant health care explains where timing fits in a programme.
What a degree day is
Penn State Extension describes a growing degree day as the difference between the average daily temperature and a base temperature below which development stops. Its formula adds the day's maximum and minimum, subtracts the base, and divides by two. A day that comes out negative adds nothing. Penn State says 50 degrees Fahrenheit is commonly used as the base because development of most insects and mites is limited below it, and that other bases are occasionally used, so the base should be checked when numbers from different sources are compared. It also says the totals are cumulative and that data collection has to begin at the same date each season, with March 1 often used.
UConn Extension states that its chart is based on a base 50 model unless it says otherwise.
What an indicator plant is
Penn State credits Donald A. Orton of the Illinois Department of Agriculture with a system that pairs phenology, the study of how climate affects the development of plants and animals, with degree days. It says plants used for observation are called indicator plants, and that good ones are common over a wide area, hardy, easy to recognise, and have short, well-defined bloom periods.
The University of Kentucky's publication ENT-66 says it tracked 33 insect pests over three years in Lexington and matched their emergence to the flowering of 34 landscape plants. It says flowering tended to be a more reliable indicator than calendar date because of year-to-year temperature swings. Its three worked examples: eastern tent caterpillar egg hatch coincides with border forsythia in bloom and the start of star magnolia bloom; oystershell scale egg hatch with Sargent crabapple at 95% bloom and the first bloom of doublefile viburnum; bagworm egg hatch and the first flight of dogwood borer adults with American holly at 95% bloom and the first bloom of Washington hawthorn.
Degree-day ranges in the UConn chart
The UConn chart, by Wallace, Siegel-Miles, Sowizral and Tomis, lists degree-day ranges for Connecticut landscape pests with plants in bloom. These rows are for tree pests.
| Pest | Degree days listed | Plants in bloom listed |
|---|---|---|
| Bronze birch borer | Optimal timing 440 to 800; adult emergence 400 to 600 | Kousa dogwood, cranberry bush, beautybush, mountain laurel, mock-orange, Japanese tree lilac, Washington hawthorn |
| Eastern tent caterpillar | Optimal timing 90 to 190; egg hatch 45 to 100 | Japanese quince, saucer magnolia, bridalwreath, Japanese flowering cherry |
| Dogwood borer | Optimal timing 148 to 700; adult 350 to 850 | Redbud, Sargent crabapple, flowering almond and others, to Washington hawthorn |
| Emerald ash borer | Adult emergence 400 to 500; peak adult activity 1000 to 1200 | None listed |
Dates in the Kentucky table
ENT-66 gives calendar dates for Lexington, Kentucky, as three-year averages from 1992 to 1994, next to the bloom events. These are for tree pests.
| Pest | Event | Three-year average |
|---|---|---|
| Eastern tent caterpillar | Egg hatch | March 16 |
| Oystershell scale | Egg hatch | April 23 |
| Bagworm | Egg hatch | May 18 |
| Dogwood borer | First flight | May 20 |
| Bronze birch borer | Emergence | May 22 |
| Twolined chestnut borer | Emergence | May 29 |
Rutgers: where the method has limits
Rutgers Cooperative Extension, in a March 15, 2021 post by Steven K. Rettke, describes indicator plants as a way to time scouting when a landscape manager does not calculate degree days. It says calendar recommendations can be two weeks or more early or late in a cooler or warmer year, and that cooler spots such as shade, north slopes and higher elevations can run later.
Rutgers also warns that some uncertainty remains, that a single plant will not always coincide with temperature and pest emergence, and that the familiar forsythia and crabgrass rule is not always reliable. Its list gives its own ranges, for example birch leaf miner at 123 to 290 degree days and bagworm at 600 to 900, with catalpa in full bloom. The UConn chart gives 190 to 290 for birch leaf miner. The two publications differ for the same pest, and Penn State's advice, given above, is to check the base when numbers from different sources are compared.
Sources
- Penn State Extension, using nature's signals to manage landscape pests, updated December 2, 2011, read October 9, 2026
- UConn Extension, growing degree days: insect pests of Connecticut chart, read October 9, 2026
- University of Kentucky Cooperative Extension, ENT-66 timing control actions using flowering plants as indicators, read October 9, 2026
- Rutgers Cooperative Extension, accurately timing scouting by using plant phenological indicators, March 15, 2021, read October 9, 2026