Learn French: L'insecte est plus nuisible qu'il ne paraît. - The insect is more harmful than it seems.

This is the ne explétif. It doesn't make the sentence negative. Instead, it is used after affirmative comparisons of inequality (using plus, moins, or mieux) to emphasize the gap between reality and expectation. The insect is actively more harmful than its physical appearance suggests.

Is it grammatically wrong to say qu'il paraît without the ne?

Not at all! The ne explétif is completely optional. It adds a formal or literary elegance to your speech and writing, but in everyday spoken French, you will almost always hear qu'il paraît without the ne.

Could I use semble instead of paraît in this sentence?

Yes, paraître and sembler are practically interchangeable here. Both mean 'to seem' or 'to appear.' However, paraître often relates slightly more strictly to objective physical appearance (how the insect looks), while sembler can imply a more subjective judgment from the speaker.

I've sometimes seen a neutral le in sentences like this. Could we say qu'il ne le paraît?

Yes, you can! Adding the neutral pronoun le to stand in for the adjective nuisible (making it qu'il ne le paraît) is also perfectly correct and very common in formal French. It acts similarly to saying 'than it appears to be' in English.