To get rid of level 7 or virtually any pest, you can use the Summon Pixiespell on the plant in question available from Charley at The Oasis in Krokotopia. He sells it for 175,000 gold pieces.
The pixie additionally lasts for a total of 48 hours and is a "like" by every plant within good proximity of it. You can also get treasures at the Bazaar.
Ant Lion is the next Gardening Spell that kills pest of any rank. It is available from Barley in Silent Market (after completing a few of his quest in Last Wood Khrysalis of course).
