Stechen vs. beißen
In German, the choice between stechen and beißen depends on the anatomy of the insect's attack. If it pierces the skin with a stinger or a needle-like mouthpart, you use stechen. If it uses jaws or mandibles to pinch or cut the skin, you use beißen. A common mistake for English speakers is assuming mosquitoes bite in German; instead, they sting.
Stechen
The verb stechen translates to to sting, to prick, or to stab. In German, it is used for insects and arachnids that penetrate the skin using a stinger or a needle-like tube, such as bees, wasps, scorpions, mosquitoes, and ticks.
Eine Mücke hat mich in der Nacht in den Arm gestochen.
(A mosquito bit me on the arm during the night.)
Gestern wurde mein Bruder von einer Biene gestochen.
(Yesterday my brother was stung by a bee.)
Man muss aufpassen, dass einen im Wald keine Zecken stechen.
(You have to be careful that no ticks bite you in the forest.)
Hornissen stechen nur, wenn sie sich bedroht fühlen.
(Hornets only sting when they feel threatened.)
Skorpione stechen mit ihrem Schwanz, um Beute zu lähmen.
(Scorpions sting with their tail to paralyze prey.)
Ich trage langes Spray auf, damit mich die Insekten nicht stechen.
(I apply spray so that the insects do not sting me.)
Beißen
The verb beißen translates to to bite. It describes the action of an animal or insect using jaws, mandibles, or teeth to pinch, grasp, or cut into the skin, which applies to ants, spiders, fleas, bedbugs, and beetles.
Die rote Ameise hat mich beim Picknick in den Fuß gebissen.
(The red ant bit me on the foot at the picnic.)
In Europa gibt es nur wenige Spinnen, die durch die menschliche Haut beißen können.
(There are only a few spiders in Europe that can bite through human skin.)
Mein Hund kratzt sich ständig, weil ihn Flöhe beißen.
(My dog scratches himself constantly because fleas are biting him.)
Bettwanzen beißen oft nachts, während die Menschen schlafen.
(Bedbugs often bite at night while people are sleeping.)
Manche Käfer können erstaunlich hart beißen, wenn man sie fängt.
(Some beetles can bite surprisingly hard if you catch them.)
Waldameisen beißen zur Verteidigung und spritzen dann Säure in die Wunde.
(Wood ants bite in defense and then spray acid into the wound.)
Summary
To choose the correct word, think about the insect's biological weapon. Use stechen for needle-like piercing tools like stingers and proboscises, and use beißen for pinching tools like jaws and mandibles. Keep in mind that while mosquitoes and ticks bite in English, they always stechen in German.