Below, you will learn about three types of poisonous spiders found in Michigan that are DANGEROUS to people.

Fortunately, the species below only bite when disturbed or provoked. It’s important to remember that spiders would rather run away from you than bite. If you encounter one of the spiders listed below, please LEAVE THEM ALONE!

Lastly, I think it’s interesting to know that, technically, almost ALL spiders are venomous. This is because they have poison glands that release venom to help subdue their prey, typically small insects. Luckily, most species’ venom does not affect humans.

For today’s post, I found the types of spiders that are known to be very dangerous to people when they do bite, or at least “medically significant.”

Venomous Spiders in Michigan:

#1. Black Widow

Identifying Characteristics:

• Females are 6 to 19 mm long—shiny black with a distinctive red hourglass-shaped mark. A row of red spots is sometimes visible above.
• Males are half the size of females.
• Bristles on their hind legs, which they use to cover their prey with silk once it has been trapped.

Black Widows are the most venomous spiders in Michigan!

In addition, they are probably the most popular and recognizable spiders in the world. Almost everyone can recognize the red-shaped hourglass mark that appears on the females.

But even though they have highly toxic venom, 15 times stronger than a rattlesnake, they are not aggressive. Black Widows only bite when in danger or if their web is disturbed. They RARELY bite humans.

But, if you are one of the few unlucky people who are bitten, you should go to the hospital immediately. The venom is dangerous, as it affects your nervous system.

Deaths to healthy adults are VERY rare, but the neurotoxic venom can be fatal to small children if untreated.

Only adult females have chelicerae (mouthparts) long enough to deliver venom, as males are too small to pierce our skin.

The best places to look for these venomous spiders in Michigan include overhanging ledges, woodpiles, under benches or stones, near entrances to abandoned rodent burrows, or around outbuildings. Inside your home, they can be found in dimly lit locations, such as dark corners, closets, or other cluttered areas. They don’t like moisture and prefer dry areas.

Black Widows have a bizarre mating ritual. It begins with the male coming to the female’s web and destroying it. This reduces the female’s pheromones and makes the web less attractive to other males.

The females do not seem to mind the destruction of their property, and it actually seems to make them more receptive to the male. Even so, sometimes, the female kills and eats the male after mating. This gruesome behavior is what gave these poisonous spiders their name.

#2. Black-Footed Yellow Sac Spider

• Cheiracanthium inclusum

Identifying Characteristics:

• Pale yellow-beige color with dark brown markings on its palps, jaws, and the ends of its feet.
• An orange-brown stripe often runs down the top center of its abdomen.
• Also known as the American Yellow Sac Spider. As you can see, it gets its name from its appearance.

Yellow Sac Spiders are venomous and capable of biting humans.

Their bite begins with a bit of pain followed by itching and swelling. Symptoms from the bite typically are gone within 7-10 days. On the bright side, Yellow Sac Spiders rarely bite!

The severity of one of their bites is not nearly as bad as a Black Widow. A study of twenty confirmed Yellow Sac Spider bites found that none resulted in necrosis (decaying of tissue due to cells dying). Instead, it seems like most bites just caused swelling and localized pain.

• Check out this book if you need additional help identifying a spider you found!

These venomous spiders are most often found in Michigan on the foliage of trees and shrubs. But it’s not out of the question to find one in your house or other manufactured structures. Honestly, it’s pretty uncommon to actually see a Yellow Sac Spider unless you’re actively searching for one. The reason is that they are nocturnal and only come out at night to feed and mate.

Lastly, in 2011, Black-footed Yellow Sac Spiders caused quite a big problem for Mazda. The automobile company had to recall 65,000 Mazda 6 vehicles after discovering that webs from this species were clogging fuel system ventilation tubes. It seems that the spiders were attracted to the smell of gasoline, but it’s unclear why ONLY this particular model was affected!

#3. Mediterranean Recluse

• Loxosceles rufescens

Also called the Fiddleback Spider.

• Both sexes are around 7mm long.
• Yellowish to grayish brown. Long legs.
• The head is often darker, usually with a distinct violin-shaped mark.

Luckily, this venomous spider does not naturally live in Michigan!

Mediterranean Recluses are native to parts of Europe and Northern Africa. They prefer warm temperatures and generally dry weather.

But in 2021, during a routine building check in the basement of the University of Michigan’s Shapiro Undergraduate Library, these dangerous spiders were found! The incident made national news, and the building was closed for two days while the spiders were removed.

Mediterranean Recluses are considered one of the most invasive spiders in the world, so it’s not that surprising that they ended up in Michigan.

*WARNING – If you are bitten in Michigan and think the spider is venomous, please go to the hospital!

If you have an allergic reaction, do not use this article to try to identify the spider. If possible, it is beneficial to try to capture the spider that has bitten you so they can determine the correct treatment.

Did you know that spiders are considered venomous, NOT poisonous?

There’s a common misconception between poisonous and venomous; one is ingested, and the other is injected. So, for example, If you eat something that makes you sick, it is considered “poisonous.” But if an animal, like a spider, delivers its toxins when it bites, then it’s considered “venomous.”

Check out these other guides to animals found in Michigan!

• The 24 COMMON SPIDERS Found in Michigan! (ID Guide)

• The 14 Types of SNAKES That Live in Michigan! (ID Guide)

• The 35 MOST Common Birds in Michigan!

Which of these venomous spiders have you seen in Michigan?

Leave a comment below!