Why Are My Tree Leaves Disappearing?
What are late-summer tree defoliators? Late-summer tree defoliators are a group of insects—most notably Japanese Beetles, Fall Webworms, and Sawfly larvae—that feed on tree foliage in Minnesota and Wisconsin between July and September. While a single year of defoliation rarely kills a mature hardwood, repeated attacks in the St. Croix Valley weaken a tree’s energy reserves, making it highly susceptible to secondary pests and the region’s harsh “winter kill” temperatures.
The 5 Most Common Tree Defoliators of the St. Croix Valley
In the St. Croix Valley, these five pests are responsible for the vast majority of canopy loss. Understanding which one is on your property is the key to effective Plant Health Care.
- Identification: Blue stripes along the sides with a row of white, “keyhole” or “footprint” shaped spots down the center of the back. (Note: They do not build tents).
- Damage: These tree defoliators are aggressive eaters that consume the entire leaf, often leaving nothing but the mid-rib.
- Local Impact: These follow a cyclic outbreak pattern in Washington County. During “boom” years, they can move through wooded neighborhoods like an army, defoliating entire blocks.
- Identification: Formerly called “gypsy moths,” these are dark caterpillars with five pairs of blue dots followed by six pairs of red dots on their backs.
Damage: Highly destructive and indiscriminate tree defoliators. They can completely defoliate a tree in a single season, forcing the tree to use its emergency energy reserves to survive.
Local Impact: St. Croix County, WI is a high-monitoring zone. Because they are invasive, they have few natural predators in our region, making professional PHC intervention essential.
- Identification: Distinctive metallic green bodies with copper-colored wing covers. They are often found clustering together in the upper canopy.
Damage: They are “skeletonizers,” meaning they eat the soft tissue between the leaf veins, leaving behind a brown, lacy appearance.
Local Impact: Highly active in Stillwater and Lake Elmo from July through August. They are a primary stressor for Linden and ornamental fruit trees.
- Identification: Small, pale yellow or green caterpillars covered in long white hairs, living inside large, silken “tents” or webs.
Damage: They build their nests at the tips of branches and consume all foliage within the web. As they grow, the web expands to enclose more leaves.
Local Impact: Extremely common along the St. Croix River bluffs in late summer. While they look alarming, they rarely kill mature trees like Walnuts or Maples.
- Identification: Often mistaken for caterpillars, these “slug-like” larvae often feed in large, synchronized groups. Common varieties include the Pear Sawfly and Pine Sawfly.
Damage: They can rapidly strip needles from evergreens or “windowpane” the leaves of hardwoods, often leaving only a thin membrane behind.
Local Impact: A major concern for Hudson and North Hudson homeowners with established Pines or Oaks. They can strip a young tree in less than a week.
| Pest Name | Diagnostic Feature | Primary Hosts | Peak Activity | Risk Level |
|---|---|---|---|---|
| Forest Tent Caterpillar | White “keyhole” spots | Aspen, Oak, Maple | May – July | High |
| Spongy Moth | Red & blue dots | Oak, Aspen, Willow | May – July | Extreme |
| Japanese Beetle | Metallic green/copper | Linden, Birch, Fruit | July – August | Moderate |
| Fall Webworm | Silken nests on tips | Walnut, Cherry, Apple | August – Sept | Low |
| Sawfly Larvae | “Slug-like” appearance | Pine, Oak, Rose | June – Sept | High |
How Late-Summer Damage Affects Winter Survival
In the St. Croix Valley, trees use the late summer months to store energy (starches) in their root systems to survive the sub-zero winters. When a tree is defoliated in August:
Reduced Photosynthesis: The tree cannot produce the energy needed for the dormant season.
Forced Re-leafing: If the tree tries to grow new leaves in September, it wastes precious energy that should have been saved for winter, leading to “Winter Dieback.”
PHC Treatment Options
When it comes to treating defoliators, an approach known as Professional Horticultural Care (PHC) is often the best solution. A PHC plan focuses on the overall health of the tree to help it resist and recover from pest attacks. Treatment options may include:
- Insecticides: For severe infestations, a professional may recommend targeted insecticide treatments. These can be applied as foliar sprays or, for larger trees, as systemic trunk injections or soil drenches. It is important to use these treatments mindfully to protect beneficial insects.
- Biological Control: Some biological treatments, like Bacillus thuringiensis (Bt), can be effective against specific caterpillars without harming other insects.
- Mechanical Removal: For smaller trees, physically removing egg masses or caterpillars can help reduce the population. The webs of Fall Webworms can be pruned and disposed of to prevent further damage.
- Tree Health Management: Since healthy trees are more resilient, a PHC program may also include deep-root fertilization and other measures to boost the tree’s vigor and improve its ability to withstand stress.
DIY Homeowner Tips
Watering is Critical: A defoliated tree is a thirsty tree. Ensure your trees receive 1-2 inches of water per week through the first hard frost.
Manual Web Removal: For Fall Webworms, you can often use a long pole to break open the webs, allowing birds and predatory insects to handle the rest.
Is Your Canopy at Risk?
Don’t let a late-summer pest infestation turn into a dead tree next spring. If you live in the St. Croix Valley or Western Wisconsin, our team of Plant Health Care (PHC) experts can diagnose and treat your valuable trees to protect them from these worrisome pests!
