Oh no, I just had a funny thought. If Spring is so full of insects that might have a overpopulation problem, could the King use them as a massive weapon somehow? I'm just sitting her laughing imagining him screaming, "I AM THE BUG LORD".

OH MY GOD and my mind instantly went here:

Because, obviously, Avatar.

But seriously: if Angra really wanted to create a massive bug army, he could — but it would require a lot of magic and continual concentration, as it isn’t something Royal Conduits are meant to be used for. Same if he wanted to use all the vines in Spring to wrap around his enemies and crush them — yes, magic can be used on plants (through growing/crop type things), but a sustained use of that type of extreme magic would be incredibly taxing and wasteful. 

And actually…SPOILER WARNING BELOW CUT

…later in the trilogy, Meira learns more about using magic in these sorts of ways — harnessing objects/items around her in quick, short bursts of magic that can be used in fighting. 

But, alas, neither she nor Angra creates a bug army.