Hello Mark,

Japanese beetles are relentless feeders, and ash trees—while not their top choice—can still suffer significant defoliation in Michigan’s peak beetle season. Based on university extension sources, here’s a science-backed strategy tailored for both immediate relief and long-term suppression:

Short-Term Control (Immediate Action)

Hand Removal: In the early morning, when beetles are sluggish, shake branches and drop beetles into a bucket of soapy water. It’s surprisingly effective for small populations.

Targeted Insecticides: Use contact insecticides like carbaryl (Sevin), cyfluthrin, or permethrin on infested foliage. These offer fast knockdown but require reapplication after rain or heavy irrigation. Avoid spraying during bloom to protect pollinators.

Avoid Pheromone Traps: These attract more beetles than they catch and can worsen infestations near ash trees. READ AND FOLLOW THE DIRECTIONS ON THE LABEL WHEN APPLYING THE INSECTICIDES!

Long-Term Management (Sustainable Suppression)

Milky Spore & Beneficial Nematodes: Apply Paenibacillus popilliae (milky spore) or Heterorhabditis bacteriophora nematodes to turfgrass areas in late summer to target beetle larvae (also known as grubs). These biological controls build up over time and reduce future adult populations.

Deep Watering Practices: Infrequent, deep watering discourages beetle breeding and supports tree resilience.

Diversify Plantings: Japanese beetles are highly attracted to particular species (roses, grapes, linden). Reducing these in your landscape can lower overall pressure from beetles.

Monitor & Rotate Treatments: Use MSU’s degree-day models to time interventions. Rotate insecticide classes to prevent the buildup of resistance. More information at this link:

GDD of Landscape Insects - Integrated Pest Management

Integrated Pest Management (IPM) Tips

Regular scouting during peak emergence (late June–August) is key.

Prioritize high-value or vulnerable trees for treatment.

Combine cultural, biological, and chemical controls for best results.

For more detailed guidance, MSU Extension’s Japanese beetle management page - More information at these links:

Japanese Beetles - MSU Extension

Japanese beetle - Integrated Pest Management

Managing Japanese beetles in fruit crops - MSU Extension

How to choose and when to apply grub control products for your lawn - Turf

The University of Michigan’s Matthaei Botanical Gardens guide is an excellent resource. More information at this link:

How to Control Japanese Beetles | Matthaei Botanical Gardens & Nichols Arboretum

I hope this helps!