Not actual danger, but the mosquitos can be real annoyance in Lapland in summer. As a whole, there are more nuisance than dangerous. They just suck a little amount of blood from you, but do not spread illnesses like in Asia.
There are basically two generations of mosquitos between June and August, depending how wet the early summer is.
The first generation comes in the very early of June or late May, when the swamps defroze. These are quite unharmful and dum and they seldom hit the target.
The second generation comes after Midnight sun in the end of June and the beginning of July depending the amount of rain in the beginning of the summer. This invasion is often quite mean, hungry and plentiful. You might consider mosquito head net for longer trekking. Reindeers go to the upper parts of mountains for avoiding them.
In thee mosquito point of view, the best summer time to enjoy Lapland is in the early August. Days are still quite warm, but mosquito swarms have disappeared.
On the other hand, the late August and early September is the most beautiful time of the summer season in Lapland. 'Ruska' starts, which means that all th nature is start to get green, yellow, red and orange.
If you are traveling in Lappi with your own or rented, your most likely traffic accident could be a collision with a reindeer. This might sound like a joke, but it is not.
Reindeers in Lappi do not fear human beings or cars at all. Instead, they just walk from the forests to the highways, sometimes alone, sometimes in 50 unit flocks. Sometimes they just pass the highway, but quite often they will stand still on the highway also.
Above Rovaniemi (look map) you should start to look both sides of the road you are driving. Reindeers do not literally jump or drop from the trees to the highways, but they are quite quick to appear from dense forest.
When you see one - or more - reindeer, start to slow down your car firmly. If a reindeer seems to came just for experiencing the noisy life in the highway from the edge, you can keep on driving.
But, if a reindeer starts to pass the highway, you should try to pass a reindeer behind its back. Reindreers backpedal very rarely. ;-)
If a reindeer decide to stand on the highway, lower your speed and try to bypass the animal from the backside. If this seems to be hazardous or dangerous, bypass it from frontside. Althought male reindeers can have quite challenging horns, they very seldom use horns to sideattacking to the doors of a car.
I found in Saariselka when it's snowing everywhere kind of looks the same, just snow, trees and a few cabins! And in temperatures like there are in the winter, its wise not to get lost! So keep your eyes open and try to take in little landmarks (a signpost, a tractor, a cabin being built) i got so used to walking around in the dark, i sometimes got confused when it was light! Snow and trees can look so different somehow!!
As i unfortunatley found out, it takes appox half an hour for an ambulance to reach Saariselka from Ivalo! And then obviously half hour on top of that to get to the hospital. So if you must injury yourself, be careful you don't do anything too serious!!