Hello Barbara,

In Michigan, blister beetles are considered somewhat rare, but they are still present and can be found in gardens and lawns. While they are not as common as some other beetle species, they are still a pest in gardens and can be irritating to humans and pets due to their skin-releasing liquid. If you have had blister beetles appear in your yard for four years, they are likely present, but their occurrence may vary by location and season. More information at this link:

Blister beetles: Handle with care - Gardening in Michigan

Blister beetles, including the black oil beetle, are considered rare in Michigan, but they can appear in large localized outbreaks. Their toxic secretion can cause skin blisters, and while spraying works, there are safer and more practical control methods.

Here’s a breakdown of what’s going on and what you can do:

Rarity and Behavior in Michigan

Blister beetles are generally rare in Michigan, but they can become locally abundant under favorable conditions.

The black oil beetle is a type of blister beetle that releases a yellowish toxin called cantharidin from its joints when threatened—a behavior known as reflexive bleeding.

Your observation of large numbers of chives in the garden suggests a localized outbreak, which can happen due to ideal breeding conditions or food availability.

Toxicity and Skin Risk

Cantharidin is a potent irritant that causes painful blisters on human skin upon contact. More information at this link:

Blister beetles: Handle with care - Gardening in Michigan

While uncomfortable, these blisters are typically not medically serious unless the toxin is ingested (a major concern for livestock).

Avoid handling them directly. If you must remove them manually, wear gloves and use tools like tweezers.

Why They’re Showing Up 

Blister beetles are gregarious and often cluster in large numbers on specific plants, especially flowering herbs like chives.

Late summer emergence is typical, as adults appear after pupating underground.

They may wander indoors (like your garage) seeking shelter or warmth.

Control Strategies (Beyond Spraying Individually)

Here are safer and more scalable approaches:

Physical removal: Use a handheld vacuum with a disposable bag to suck up beetles from plants and surfaces. Dispose of the bag immediately.

Barrier methods: Install fine mesh or row covers over vulnerable plants during peak beetle season.

Habitat disruption: Remove weeds and flowering plants near garden beds that may attract beetles.

Trap crops: Plant sacrificial crops like alfalfa nearby to lure beetles away from prized plants, then treat or remove those trap crops.

Insecticidal soap or neem oil: These are gentler alternatives to harsh insecticides and can deter beetles without harming beneficial insects.

Professional pest control: If infestations persist, consult a pest management service for targeted treatment.

You can find more guidance from Michigan State University Extension at this link:

Blister beetles: Handle with care - Gardening in Michigan

I hope this helps!