be prone to 的to 是作介词还是不定式？

根据牛津高阶字典，to在此处既可以做介词也可以做动词不定式的标志，关键取决于它后面跟什么，跟名词或者doing形式就是介词，跟动词原形就是动词不定式了。
另外，凡是介词后面几乎都可以接doing的，可以把doing就当作是一个名词形式

‘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）
http://www.usingenglish.com/forum/ask-teacher/48867-prone-ing-inf.html
另外上面说的不一定正确，但有95%的把握

看后面加的动词是原型还是ing形式，如果是原型，那么就是作不定式；如果是加ing形式，那么就是作介词。

to是个介词，后面跟名词或名词词组

介词