‘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
翻譯就是
to是個(gè)介詞,后面跟名詞或名詞詞組.Prone是極少數(shù)能和to連用,后面跟不帶to的動(dòng)詞不定式（動(dòng)詞原形）.
be prone to eat(eat是不帶to的動(dòng)詞不定式,做名詞） junk food
另外意思也有區(qū)別
infinitive:This plant is prone to attack by insects.
meaning,insects might attack the plant.（昆蟲(chóng)也許會(huì)侵害樹(shù)木）（以前沒(méi)侵害過(guò)）
gerund:This plant is prone to being attacked by insects.
meaning:insects have attacked the plant before.（昆蟲(chóng)也許會(huì)侵害樹(shù)木）（昆蟲(chóng)以前就侵害過(guò)樹(shù)木）
BE + past participle:This plant is prone to be attacked by insects.
meaning,the plant will definitely be attacked by insects.（（昆蟲(chóng)一定會(huì)侵害樹(shù)木）（以前侵害過(guò)）
另外上面說(shuō)的不一定正確,但有95%的把握