Hi ho. If anyone hadn't noticed, the spells to prevent pests for 48 hours can cost you more energy in the long run if used on higher-level plants.
If you use the spell, your higher plants are protected from rank 1 and 2 pests, sure. However, that will guarantee that if they get a pest they will be rank 3-5 pests, causing you to need to use more energy than if you had to exterminate only a rank 1 or 2 pest.
It just happened to me the other day. Checked the plants that weren't protected, and one of my Maelstroms had a rank 2 pest on it. Only 10 energy, thankyouverymuch.
On a side note, I have yet to get any snacks from the Maelstroms, even at Elder. Yet I keep getting low-level snacks from the Prickly Bear at every harvest. What gives? :-(