I usually find locusts by one of the trees near the waterfall in the National Park. The spot closest to the waterfall spawns dragonflies, but there is another spawner just a bit farther to the right of the falls. If you camp out there, locusts will generally spawn first thing in the morning. Sometimes, the mantis or monarch butterfly spawns in that spot; collect them and wait. Dragon Dragonfly spawns just to the left of the waterfall, also early morning.