Degree Day TrackerScience-Based

Predict SLF
Emergence
in Your Area

Degree-day models tell you when SLF nymphs will hatch and when adults will emerge — based on real temperature data from your ZIP code. More accurate than any calendar.

Your Local GDD

Enter your ZIP code to see accumulated growing degree days and your current SLF stage.

Your Area's SLF Status

How to Read Your Results

GDD benchmarks used by Penn State Extension for SLF life-cycle prediction.

Base Temp — Nymph Emergence

37.4°F

3.0°C — lower threshold for egg development

Base Temp — Adult Emergence

50°F

10°C — standard growing degree day base

~450 GDD

First eggs begin hatching

base 50°F

~900 GDD

Peak nymph activity

base 50°F

~1,350 GDD

Adults begin to appear

base 50°F

~2,400 GDD

Peak adult activity / oviposition begins

base 50°F

What the Numbers Mean for You

Match your current GDD to the action window below.

< 450 GDD— Egg Season

Focus on egg mass scraping — still viable. Every mass scraped = 30–50 fewer nymphs.

450–900 GDD— Nymph Window OPEN

Best time for circle traps and systemic treatment. Nymphs are small and vulnerable.

900–1,350 GDD— Prepare for Adults

Contact sprays for nymph knockdown. Scout for early adult emergence.

1,350+ GDD— Adult Management

Adult management + egg mass watch begins (late season). Circle traps still effective.

Why Degree Days Matter

Calendar dates are a poor predictor of insect emergence because temperature varies significantly by location and by year. A warm spring in Philadelphia can push SLF hatching two to three weeks earlier than a cool spring in the same county.

Degree-day models solve this by tracking accumulated heat instead of elapsed time. Each day above the base temperature contributes GDD proportional to how warm it was. When enough heat has accumulated, the biology triggers — regardless of what month it is.

Penn State Extension researchers validated these thresholds for SLF using field observations across multiple states. Using GDD rather than fixed calendar dates can improve your treatment timing by weeks.

Official Reference

Penn State GDD Tool

The Center for Environmental Informatics at Penn State maintains the definitive online degree-day tool for SLF, with county-level historical data and forecasts.

Open PSU Tool →