As you know, prone can take either an infinitive or a gerund as its object:
Originally Posted by micaelo
infinitive: prone to get headaches
verb + verb's object
meaning, hasn't had headaches before
gerund: prone to getting headaches
noun + noun's object
meaning, might have had headaches before
gerund's object: prone to headaches With that said, your example sentence, This plant is prone to be attacked by insects, has an infinitive verb, so it should work, but its verb is BE + a past participle, which makes it look like a passive verb, and which is why it reads awkward--or rather, it doesn't seem to fit the pattern prone to + infinitive + noun.
noun's object / ellipsis
default meaning, has had headaches before
=> Is it ungrammatical? No.
infinitive: This plant is prone to attack by insects.
meaning, insects might attack the plant.
gerund: This plant is prone to being attacked by insects.
meaning: insects have attacked the plant or like plants before.
BE + past participle: This plant is prone to be attacked by insects.
meaning, the plant will definitely be attacked by insects.