Spring Tree Pests to Watch for in Northwest Indiana
- Austin Cowen
- Apr 30
- 3 min read
What to look for — and when — using Growing Degree Days
As trees leaf out across Northwest Indiana, so do the pests and pathogens that threaten their health. Some are easy to spot — others work quietly, weakening trees before symptoms appear. Knowing what to watch for (and when) can make all the difference.
At NWI ArborCare, we rely on more than just the calendar.
We track Growing Degree Days (GDD) — a weather-based model that helps us anticipate exactly when pests become active. Understanding this timing is essential for early detection and proper planning.
What Are Growing Degree Days (GDD)?
GDD is a way to track seasonal heat accumulation. It’s calculated by taking the daily average temperature and subtracting a base temperature (typically 50°F for tree pests). This gives us a much more accurate predictor of insect and disease activity than using the calendar alone.
Most pests emerge or become active at specific GDD thresholds. By tracking these in real time, we can identify what to watch for and when to inspect.
Insects Emerging in Spring
Eastern Tent Caterpillar
GDD Range: 90–192
Tree Hosts: Crabapple, cherry, plum
First Signs: Webbed tents in branch crotches; small caterpillars feeding on early foliage
Peak Activity: Late April to early May
Oystershell Scale
GDD Range (crawler emergence): 375–530
Tree Hosts: Ash, lilac, dogwood, maple
First Signs: Tiny gray-brown scales crusted on bark; twig dieback often seen later
Activity Notes: Crawlers emerge just once per year, making timing critical for monitoring
Spongy Moth
GDD Range (larval hatch): 192–400
Tree Hosts: Oak, aspen, willow, crabapple, birch
First Signs: Fuzzy caterpillars feeding on leaves; egg masses on trunks or outdoor structures
Activity Notes: Feeding begins when leaves are still tender
Emerald Ash Borer
GDD Range (adult emergence): 450–550
Tree Hosts: Ash trees (all species)
First Signs: Canopy thinning, bark splitting, D-shaped exit holes appear later in the season
Activity Notes: Adults emerge and begin mating around GDD 500
Springtime Fungal Diseases
Apple Scab
GDD Range (spore release): 150–300
Tree Hosts: Apple, crabapple
First Signs: Olive-green or brown spots on young leaves; possible early leaf drop
Weather Trigger: Cool, wet weather greatly increases infection risk
Rhizosphaera Needlecast
GDD Range (spore dispersal begins): 220–500
Tree Hosts: Blue spruce, white spruce
First Signs: Yellowing or browning of older needles, usually on lower branches
Weather Trigger: High humidity or prolonged rain supports fungal spread
Anthracnose (Maple, Oak, Sycamore)
GDD Range (infection window): 100–300
Tree Hosts: Various hardwoods (maple, oak, ash, sycamore)
First Signs: Leaf blotches, curling, and premature drop — especially on young or tender leaves
Weather Trigger: Wet, cool springs encourage spread
Why This Matters
Most pest and fungal problems start before symptoms are obvious. And since each species follows its own schedule tied to temperature—not the calendar—it’s easy to miss early warning signs without expert timing.
By tracking GDD thresholds for our region, NWI ArborCare:
Identifies what pests or diseases are likely to emerge next
Plans inspections based on precise development stages
Provides proactive tree health monitoring at the right time
What You Can Do Right Now
Watch for tents, leaf damage, discoloration, or unusual bark growths
Take note of which tree species are showing symptoms
Stay informed about your local GDD — or let us do it for you
When in doubt, contact an ISA Certified Arborist to evaluate your trees
Let's Keep Your Trees Ahead of Spring Problems
You don’t need to guess when pests are coming or wait until there’s visible damage. With the right timing, trees can stay healthier and more resilient throughout the year.
Click the button to schedule a spring inspection:
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