Assassin bugs to start with. Make sure they carry the Chagas parasite. They are small enough to get in through any medieval armor. Have them target the eyes.
The most deadly animal in the World, the Mosquito loaded with Malaria and Dengue Fever. Again, get in to the helmet, near the eyes. That is your second wave.
Your Anti Cavalry Send the scorpions to take on the horses. Direct them to places that will make the horse react violently to the sting. With luck the horse will throw the rider. If not, have the scorpion sting in a way to get the horse to kick the humans on the ground. Make sure the critter is seen.
The point of going for the eyes and face and being seen is to reinforce that this is not random, that a bug is doing the deed, and that you are not to be triffled with.
Mosquitoes especially can keep the infection rate up with multiple bites, and each infected guy serves as a source for the other mosquitos to keep transmitting the disease.
Keep it going with wasps, hornets, ants, and centipedes. Go in waves and go for the head. You get a high mortality rate for the diseases, pretty good horse related damage, and occasional anaphylaxis.
Hide as deep in the forest as you can and have your little helpers just attack in small wave after small wave. Maybe let the knights see hints that you are there, but never let them see you directly. Hey, get some cockroaches together to pile up in a humanoid shape then scatter so another wave of Assassin bugs attacks
100% mortality is probably not going to happen, but any Survivors are going to get really scared,especially when you chase them out with spiders