The Khrysalis Gardener, Barley, lets you train spells to deal with Rank 6 and 7 pests. In order to access him, you will need to have finished the main quest "Uneasy Allies" and be Rank 15 in Gardening. If you are both of these, talk to Zaltanna the Mirrormask for the quest "Bring the Green" and you'll be on your way to unlocking this gardening trainer.
Make sure you bring a lot of gold! You'll need 50k for the Itsy Bitsy Spider spell to deal with your Rank 6 pests, but the Ant Lion spell (for Rank 7) is another 75k! And if you want the prevention spell, Strawman, that one is a whopping 300k gold! Plus, Barley sells prevention spells for all 5 needs at 125k gold each.
There's a few ways you can eliminate Rank 6 Pests.
You can:
* Learn and use the Summon Pixie on the affected plant. This spell, available for 175,000 gold pieces from Charley in The Oasis, Krok will summon a pixie. The pixie will rotate around the plant for a 48 hour period eliminating and preventing pest of any rank from affecting said plant.
Also, note that all plants strongly "likes" pixies (usually adds about a 25% increase in growth rate to plants) so have at least two or so circulating various sections of your garden. It helps!
You have to be at least Rank 10 to train the spell however, but they are available as Treasures from the Bazaar.
* Learn and use spell Itsy Bitsy Spider available from Barley at a cost of 50,000 gold pieces. The Ant Lion can also work. He's in Khrysalis after completing a few quests for him in Lost Wood.
Treasures too are available at the Bazaar but requires certain ranks for purchase.
I have been trying to find the rank 6 pest, but have been unable to. If any of you would like to help me on this quest, I would most appreciate it.
Since your topic says "Rank 6 Pest Spell", I am guessing you are looking for the spells and not the pest.
There are 2 rank 6 spells, "Itsy Bitsy Spider" (50k gold) and "Strawman" (300k gold). The only way to get those is to quest your way to Barley in Khrysalis.