A popular weapon in the anti-pest arsenal is biological control — i.e., the introduction of a natural enemy. It’s considered a nice environmental alternative to pesticides.
But it can still disrupt the local ecosystem and have serious consequences, like this example from a NY Times article: The knapweed is widespread in the West. The gall fly was introduced to control it. The deer mouse likes gall fly larvae, and now the mouse population is exploding. The droppings of deer mice can cause hantavirus, an infection that can be fatal to humans. Whoops!
What’s most significant is that the chain leads very clearly and obviously to human impact, and it stands as a strong testament to the way actions that harm the environment can in turn harm us.
There are numerous instances throughout history of unintended consequences like this, such as the mongoose in Hawaii, introduced to control rat populations. Instead, it killed off several rare ground-nesting bird species.
Perhaps the most famous example of poor judgment in bio-control is the old lady who swallowed a fly. Her attempts at controlling the fly population inside her have been well-documented, and tragically led to her stomach rupturing and an entire menagerie springing from her torso.