What to Scout for in Late August: A GDD-Based Pest Guide for Trees and Shrubs

Late August can be one of the most important scouting periods of the year for plant health care professionals, arborists, landscapers, and property managers.

By this point in the growing season, many spring pests have finished their most damaging activity, but that does not mean pest pressure is over. Several insects and mites remain active through late summer, while others are entering life stages that can help determine how severe problems will be next year.

The challenge is that late-August pest activity does not occur on exactly the same date every year.

A hot spring and summer can push insect development weeks ahead of schedule. A cooler season can delay it. That is why professional plant health care programs increasingly rely on Growing Degree Days, or GDDs, rather than calendar dates alone.

Using GDDs alongside regular property scouting can help determine which pests are likely active, which plants should be inspected, and when treatment may still be worthwhile.

Why Late-August Scouting Matters

Late summer is a transition period in the landscape.

Some insects are reaching maturity. Others are producing another generation. Mites may increase rapidly during hot, dry weather, and certain scale insects may still have vulnerable crawler stages present.

At the same time, plants that have been under stress all summer may begin showing symptoms that were not obvious earlier in the season.

Late-August scouting can help identify:

• Active insect infestations

• Mite populations

• Scale crawlers

• Late-season defoliators

• Borer activity

• Spotted lanternfly populations

• Drought and heat stress

• Disease symptoms

• Problems that should be added to next year's treatment program

The goal is not simply to find something to spray.

Good scouting helps determine what is actually happening on the plant and whether treatment is appropriate.

1. Spider Mites

Spider mites are one of the most important pests to watch during hot, dry periods in late summer.

They are extremely small and can build populations rapidly when conditions are favorable.

Common symptoms include:

• Fine stippling on foliage

• Yellowing or bronzing leaves

• Premature leaf drop

• Fine webbing during heavier infestations

• A dull or dusty appearance to foliage

Plants commonly affected include arborvitae, spruce, hemlock, boxwood, burning bush, roses, and many other ornamentals.

One simple scouting technique is to hold a sheet of white paper beneath a branch and sharply tap the foliage. Small moving dots on the paper may indicate mites.

It is important to remember that not every mite species behaves the same way. Some mites prefer cooler conditions, while others become most problematic during summer heat.

This is another reason that temperature accumulation and local weather conditions are often more useful than simply looking at the calendar.

2. Scale Insects

Scale insects can be especially difficult to manage because their susceptibility to insecticides changes dramatically depending on their life stage.

Once many scale insects develop their protective covering, contact insecticides become much less effective.

The most vulnerable stage is often the crawler stage, when newly hatched scale insects move across the plant before settling down.

Late summer can be an important scouting period for second-generation crawlers of certain scale species.

Inspect:

• The undersides of leaves

• Small twigs

• Branch crotches

• Bark

• Areas with honeydew or sooty mold

Look closely for tiny moving crawlers or newly settled scale insects.

Because crawler emergence is highly influenced by temperature, Growing Degree Days can be especially valuable when trying to predict scale activity.

Rather than automatically spraying scale insects on the same date every year, GDD tracking can help narrow the window when susceptible life stages are most likely present.

3. Spotted Lanternfly

By late August, spotted lanternflies in many areas of the Northeast are typically reaching or already in the adult stage.

Adults are relatively easy to recognize with their gray forewings, black spots, and bright red hindwings.

While tree-of-heaven is strongly associated with spotted lanternfly, adults may feed on a wide variety of woody plants.

Scout:

• Tree-of-heaven

• Grapevines

• Maples

• Black walnut

• Willow

• Birch

• Fruit trees

• Landscape trees near wooded edges

Large populations may produce significant amounts of honeydew.

Honeydew can coat leaves, decks, vehicles, outdoor furniture, and other surfaces. Sooty mold often develops on the sugary residue.

Late summer is also a good time to identify properties with high spotted lanternfly pressure so they can be monitored for egg masses later in the season.

4. Bagworms

Bagworms become increasingly noticeable as the season progresses because their protective bags become larger.

They frequently attack:

• Arborvitae

• Juniper

• Cedar

• Spruce

• Leyland cypress

• Other conifers and ornamentals

Small bagworms early in the season are generally much easier to control.

By late August, mature bagworms may be significantly more difficult to manage with insecticides.

That does not make scouting useless.

Finding them now helps identify which properties need closer monitoring the following season.

Heavy populations can cause severe defoliation, especially on evergreens that may not recover quickly after losing foliage.

Late-season scouting can therefore be just as important for planning next year's treatment program as it is for making immediate treatment decisions.

5. Japanese Beetles

Japanese beetle activity typically begins earlier in summer, but adults may still be present on certain properties during late August.

Look for skeletonized foliage on plants such as:

• Roses

• Linden

• Birch

• Grapes

• Fruit trees

• Hibiscus

• Other preferred ornamentals

By late summer, however, the more important part of the Japanese beetle life cycle may be occurring below ground.

Females lay eggs in turf, and the resulting white grubs feed on grass roots.

This means a property with heavy Japanese beetle activity during summer may also be worth monitoring for grub problems in turf.

Understanding the pest's entire life cycle is one of the biggest advantages of using phenology and GDD data rather than treating each insect sighting as an isolated event.

6. Late-Season Caterpillars and Defoliators

Several caterpillars and other leaf-feeding insects remain active into late summer.

Depending on the location and host plants, arborists and landscapers may encounter pests such as:

• Fall webworm

• Yellow necked caterpillars

• Walnut caterpillars

• Various sawflies

• Other late-season defoliators

Fall webworm is especially noticeable because colonies build large silk webs around the ends of branches.

Despite their appearance, late-season defoliation is often less damaging than similar foliage loss earlier in the growing season because trees have already completed much of their annual photosynthesis.

Treatment decisions should therefore consider:

• Pest population

• Plant health

• Amount of defoliation

• Host species

• Time of year

• Aesthetic expectations

Not every late-season insect requires treatment.

7. Borers and Bark-Associated Insects

Late summer is also an important time to look for signs of borer activity.

Symptoms may include:

• Frass

• Sawdust-like material

• Sap or resin

• Bark cracking

• Exit holes

• Branch dieback

• Thinning crowns

Borers often attack plants that are already stressed by drought, transplanting, root disturbance, poor soil conditions, or other environmental problems.

Finding borer activity should therefore trigger a broader inspection of the tree rather than simply an insecticide recommendation.

Ask:

Why was this tree susceptible in the first place?

Improving irrigation, soil conditions, root health, or overall tree vigor may be just as important as managing the insect.

8. Lace Bugs

Lace bugs can continue feeding on susceptible ornamentals into late summer.

Common hosts include:

• Azalea

• Rhododendron

• Pieris

• Andromeda

• Hawthorn

• Sycamore

• Other ornamental trees and shrubs

Damage appears as pale stippling or bleaching on the upper leaf surface.

Turn the leaf over and you may find lace bugs, dark fecal spotting, cast skins, or eggs.

Heavy infestations can make foliage appear almost white.

Plants growing in hot, sunny locations frequently experience heavier lace bug pressure than plants growing under more favorable conditions.

Don't Forget About Plant Stress

Not every problem discovered in late August is caused by an insect.

Late summer is often when environmental stress becomes most obvious.

Inspect plants for:

• Drought stress

• Excessive irrigation

• Root problems

• Soil compaction

• Heat stress

• Salt injury

• Nutrient deficiencies

• Poor drainage

• Construction damage

A stressed plant may also become more susceptible to mites, borers, scale insects, and disease.

That is why professional scouting should always evaluate the whole plant and its growing environment, not just search for insects.

Why Growing Degree Days Are Better Than Calendar Dates

Traditionally, many landscape pest-control programs have been organized around calendar dates.

For example:

“Treat this pest in early June.”

The problem is that insect development is primarily driven by temperature.

If one spring is unusually warm, pest development may occur significantly earlier.

During a cool spring, the same pest may emerge later.

Growing Degree Days measure accumulated heat over the growing season.

As temperatures accumulate, plants and insects progress through predictable developmental stages.

Instead of asking:

“What date is it?”

GDD-based scouting asks:

“How much heat has accumulated at this location, and which pests should be active now?”

That can produce a much more precise scouting and treatment program.

GDDs Don't Replace Scouting

Growing Degree Days are not a substitute for inspecting plants.

They tell you when to look.

A strong plant health care program combines:

1. Growing Degree Day tracking

2. Local weather conditions

3. Host plant identification

4. Visual scouting

5. Knowledge of pest biology

For example, if GDD data suggests that a scale crawler stage may be approaching, that does not automatically mean the property should be sprayed.

It means the technician should begin closely inspecting susceptible plants.

Once crawlers are confirmed, a treatment decision can be made based on the actual pest population.

What Should PHC Companies Be Doing in Late August?

For plant health care companies, late August is a good time to shift part of the focus from simply performing scheduled treatments to gathering information.

Technicians should be documenting:

• Plants with recurring pest pressure

• Heavy mite populations

• Scale infestations

• Spotted lanternfly activity

• Borer symptoms

• Severe defoliation

• Plants under environmental stress

• Properties needing different treatment timing next season

Good scouting data collected now can dramatically improve next year's PHC program.

Instead of treating properties according to a generic calendar, programs can be adjusted based on actual pest history and local Growing Degree Day accumulation.

Track Pest Activity by ZIP Code With GDDIntel

The difficult part of using Growing Degree Days has traditionally been keeping track of the numbers and knowing which pests correspond with them.

GDDIntel simplifies that process.

Enter a ZIP code and GDDIntel tracks local Growing Degree Day accumulation while helping identify pests and diseases approaching their monitoring and treatment windows.

For arborists, plant health care companies, landscapers, and homeowners, that means less guessing about when pests may become active.

Late August may feel like the end of the plant health care season, but there is still plenty happening in the landscape.

The right question is not simply:

“What should I spray this week?”

It is:

“What should I be scouting for right now?”

And Growing Degree Days can help provide the answer.