Fired my newbie tech after he botched a simple Talstar P ant treatment

So I'm running a small pest operation and just had to let go of this kid I hired last month. He was supposed to do a basic perimeter spray with Talstar P on a residential account, but he mixed the dilution wrong and sprayed way too much, left the yard smelling like a chemical plant. Clients called pissed, said their dog was acting weird, had to comp the whole quarterly service and eat the material cost. Now I'm back to solo until I find someone reliable, anyone got tips on screening techs for basic safety protocols? Saw a similar story on r/pestcontrol about hiring headaches. TBH, it's making me question expanding at all.