I agree with lots of posters above, especially the one about communicating our own fears without meaning to. My son is 3.5, and has recently gone through something similar but not as extreme. Mainly focused around bees and wasps. We tend to react when we see a bee or wasp near our young kids with a rescue kind of behavior... Which communicates fear, and all the explaining in the world isn't going to undo that. So for a little while every flying bug became a potential threat. They can't identify an insect that quickly, so the catch all is they all might sting me. A big part of what helped us was emphasizing that nothing in nature is really out to hurt you. Bees and wasps aren't going to randomly fly over and sting you. They sting as their only defense against this huge thing doing something really scary... Like trying to grab me.... Bees often land on people on hot days because they're tired and dehydrated... They get some water and salt from our sweat, which replenishes their electrolytes. Bees especially won't sting unless very agitated, because they die after singing you. It's a suicide attack, which means they see you as such a threat that they need to do something to protect their sister bees. Wasps on the other hand can sting multiple times. Knowing the differences is a good thing. It's surprising how much they can understand at such a young age... And understanding always banishes fear.
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