‘To’ is not an infinitive marker but a preposition in your sentences.What follows a preposition should normally be a noun or noun phrase.‘Prone’ is one of those few adjectives like ‘inclined’ or ‘consent’ used with ‘to’ and followed by a bare verb like an infinitive
I would say ;This plant is prone to attack (as a noun not a verb) by insects
翻译就是
to是个介词,后面跟名词或名词词组.Prone是极少数能和to连用,后面跟不带to的动词不定式（动词原形）.
be prone to eat(eat是不带to的动词不定式,做名词） junk food
另外意思也有区别
infinitive:This plant is prone to attack by insects.
meaning,insects might attack the plant.（昆虫也许会侵害树木）（以前没侵害过）
gerund:This plant is prone to being attacked by insects.
meaning:insects have attacked the plant before.（昆虫也许会侵害树木）（昆虫以前就侵害过树木）
BE + past participle:This plant is prone to be attacked by insects.
meaning,the plant will definitely be attacked by insects.（（昆虫一定会侵害树木）（以前侵害过）
另外上面说的不一定正确,但有95%的把握