Growing Things: Slugs do significant damage

Q I purchased 12 super petunias from various greenhouses. I stored them in the garage for two to three weeks and they were flourishing. I put them outside on May 23 and within four days all of the new foliage and blossoms had been eaten off of them or just disappeared somehow. I do not see aphids or anything else on them. Three of the plants still have some leaves that have turned silvery. Are these thrips?  Or what else could they be?  I have had aphids in the past but have never seen