Poisonous things to look out for?

Hey I'm wondering if there are any poisonous insects, animals or plants to look out for in Japan? I'd like to go explore Japanese nature and I don't want to be unprepared

You can catch Lyme Disease in Japan. It is a good idea to check you entire body for ticks after you have been in any area where deer are around. The chances of getting it are pretty small, but the impacts can be devastating if a case is allowed to go untreated.

Japan has not many poisonous animals and plants. I lived here for many years and explored the seas and mountains extensively, but was never in a dangerous encounter with a poisonous animal or plant.

I believe statistically the Japanese hornet is the most dangerous animal in Japan, but you rarely encounter it, and even if you do, they do not usually attack unless attacked.

There is also a centipede with a very painful - not fatal - bite, but I have never encountered it.

In the seas there are quite a few poisonous animals, such as sea snakes (but they never bite) and the habu jellyfish (but you have to be very unlucky to encounter it - in Okinawa).

Okinawa is also home of the habu snake, while the main islands have mamushi snakes, but you have to step into bushes and be very unlucky in order to be bitten. Even if they bite, you have good chances to survive.

There are also bears, but they tend to be more afraid of you than the other way around.

There are no dangerous spiders.

By far the most dangerous animal is the human behind the steering wheel. Traffic accidents outnumber accidents by animal encounters by a huge factor.

As the person above me mentioned, Japan is pretty safe when it comes to dangerous animals.

Honestly your biggest worry is the Japanese hornet. If you spend most of your time in cities and well hiked mountains you will never encounter them.

However they are pretty common in the countryside and you will definitely encounter a few if you are hiking mountains in the middle of nowhere. Your best defense is just to listen. They are huge and super loud when they fly. They also generally go about their business if you leave them alone. Just stay far away from a nest, they are known to get really aggressive if you pass into the "danger radius" around their hive.

I got stung by a jelly fish in Okinawa on the face, it was quite painful for a few hours but luckily I had some antihistamine tablets (just normal allergy pills can buy without prescription). I believe the jelly fish was probably a dead one and just some stray tentacles brushed my cheek because I was snorkelling and I looking about me the whole time. Anyway it cleared up within a few hours.
If you go to Okinawa, Yakushima, Tokyo 7 Islands etc. I recommend carrying antihistamine pills in your first aid kit to reduce the effect of mild stings and bites. If you get a serious bite/sting of course seek medical help.(However as people said the chances are very slim and possibly no more likely than in your home country)

Of course, many of the most dangerous animals are in the ocean. In addition to the box or habu jellies, be aware of the Blue Ring Octopus (deadly!!), Moray eels, Lionfish, Stonefish, and of course, sharks. It's easy to avoid these if you stay out of the ocean.

It's not poisonous animal though it's advisable to buy a bear bell if going to mountains. It jingles loudly to scare bears away when hiking. I haven't met any bear in Japan but think I saw some footprints near Mt. Kumotori.

The Australian redback spider has migrated in containers to Japan and is surviving and spreading. It's a relative of the American "Black Widow" and inhabits the same kind of places - dry, dark sheltered areas and under stuff that keeps them dry. Presently, they are spreading the most from Fukuoka and Osaka. There is a Japanese "Redback" spider that is non-poisonous but is not the same.
http://en.wikipedia.org/wiki/Redback_spider

If you're heading off-pavement into areas of tall grass or loose rocks, you should watch out for the "mamushi (Gloydius blomhoffii)," a pit viper common in East Asia. Every year about 2000 people in Japan are bitten by this snake but with rapid treatment, there are few deaths. The coloring that I've seen is a brown to gray with patterns similar to a western diamondback in the US. However, it's not nearly as aggressive as the diamondback and usually only strikes as a last resort or when stepped on. They are reported to be most common around farming areas where there are lots of rodents for food.
http://en.wikipedia.org/wiki/Mamushi

The habu of Okinawa is better known so there is something to say about the mongoose vs habu shows (as well as the habu in a bottle of shochu) in educating the public.

Although it sounds like a good idea, bear bells may not be as safe as it is thought. In the mid-80s, a bear research from Colorado State University told us that bears (in the western US, anyway) tend to think of hikers wearing bear bells as a dinner bell. They, the bears, found that whenever they heard a hiker with the tinkling bell, the hiker usually had food also.

I would recommend just being noisy. Talk loudly, and just let the bear(s) know you are headed their way.

asukachiaki,

It depends on what environment you are used to and where you are going.

For example, growing up in L.A. and then coming to Tokyo and then hiking/camping in mountains mainly in Nagano Pref., I've felt no danger related to poison.

The closest I got to was probably more than 20 years later when I was bit by a gnat in a farm in Nasu, Tochigi. Within a day a "mountain" arose on my thigh and I felt the most terrible itch for days. Since I missed the initial treatment that I should've done, no medicine can heal it later until it gradually went away.

Just pay attention to health issues and you'll be fine. But keep in mind that locals are educated to cover their skin when hiking.

Several years ago, I was bitten over the eyebrow while I was sleeping in a futon in Mashiko, Tochigi-ken. When I awoke, I thought I had a mosquito bite, but eventually, the skin around the bite turned black and finally died and fell off. I still have a small "crater" where I lost the skin. About 3 cm to the right of the bite was another small spot, either a second bite or "drain" from the first one. There's a mini-crater left there. It seems like a spider bit me, but I've not heard of poisonous spiders there. It was surely a poisonous insect of some sort!

Bees and hornets - there are signs in some places.

Nobody mentioned the poisonous snakes. Miyajima has signs for these too.

And Pine Processionary Caterpillars 　(毛虫／ケムシ）

I saw a lime green snake with white stripes at Kamikouchi. Other than that, some of the cats look pretty unpleasant, the hawks that flock around Enoshima might give you a nasty scratch especially if you have food, the monkeys in the parks can be nasty in some circumstances, and the mosquitos give a bite like you wouldnt believe.

Out of everything, I think the mozzies would be what I would be most on the lookout for.