Barley in the Silent Market of Khrysalis will only sell this spell to you when you've reached level 15 in gardening experience. And, you must have quested far enough through Khrysalis to have found him and sent him to the market.
Itsy Bitsy Spider costs 30 energy and will remove rank 6 pests. Cost to train: $50,000 gold. Ant Lion costs 35 energy and will remove rank 7 pests. Cost to train: $75,000 gold.
If you're not yet at level 15 in gardening experience, or if you've not yet reached Khrysalis ... your only other defense against level 6 & 7 pests is to train the Summon Pixie spell. You will find that in the Oasis of Krokotopia. Charley will let you train the Summon Pixie spell at gardening level 10, for $175,000 gold. Yes, the price is outrageous.
On the upside, it only costs 25 energy to summon a pixie; the downside is, you have to cast it on each individual plant to remove current pests but once you do, the pixie will keep all pests away for 48 hours.
If that isn't an option either, due to the high cost or you not having enough gardening xp, then the last hope is finding a Summon Pixie treasure card for sale in the bazaar.
Alia Misthaven