Because Romanian usually puts the definite article at the end of the noun, not in front of it like English does.
So -ul here is the masculine singular definite article.
Here, pe means on.
So:
English speakers often learn that pe can also mark certain direct objects, especially people, but that is not what is happening here. In this sentence, it is simply a preposition of location.
This is a very good question, because English often expects the here.
Romanian often leaves a noun without the definite article in prepositional phrases, especially in descriptive, technical, or general statements about location.
So:
If you want to make the stem clearly specific, Romanian often adds more detail, for example:
So the version without the article is normal and natural in many contexts.
The most neutral word order here is:
That is basically:
Romanian word order is fairly flexible, though. You can move things around for emphasis:
This sounds more marked, as if emphasizing where the pest is.
So the original sentence is the most straightforward and natural version for a learner.
A rough pronunciation guide is:
duh-oo-nuh-TOH-rul ES-te pe tool-PEE-nuh
A few useful notes:
If you want a more exact IPA-style pronunciation, it is approximately:
/də.u.nəˈto.rul ˈes.te pe tulˈpi.nə/
It can refer to any harmful organism in context, not only an insect.
In agriculture or gardening, dăunător can mean:
So the exact type of pest depends on the situation, not on the word alone.