Biljka se bori protiv lisnih uši.

Boriti is the dictionary form, meaning to fight / to struggle. In the sentence, the verb is conjugated in the present tense:

• ja se borim = I fight
• ti se boriš = you fight
• on/ona/ono se bori = he/she/it fights

So Biljka se bori means The plant fights / is fighting.

Se is a reflexive particle. With boriti se, it is part of the verb itself.

So you should learn this verb as:

• boriti se = to fight / to struggle

In many cases, se does not need to be translated literally as itself in English. Here, se bori simply means fights / struggles.

Biljka is the subject of the sentence, so it is in the nominative singular.

• biljka = plant

It is a feminine noun, and the basic singular nominative form ends in -a, which is very common for feminine nouns in Croatian.

So:

• Biljka = the plant / a plant (depending on context)

Protiv means against.

This preposition requires the genitive case. That is why the phrase after it is not in its basic form.

• protiv
  • genitive

So:

• protiv lisnih uši = against aphids

This is a very common pattern in Croatian.

Because protiv requires the genitive, and here the meaning is against aphids in general, so Croatian uses the genitive plural.

The basic singular phrase is:

• lisna uš = aphid

Its genitive plural is:

• lisnih uši

Both words change:

• lisna → lisnih
• uš → uši

The adjective must agree with the noun in case, number, and gender, so lisnih matches uši here.

It looks the same, but here it is from a different noun.

• uho = ear
• uši = ears

But:

• uš = louse
• uši can also be a plural or case form of uš

In lisnih uši, it comes from uš, not uho.
So this phrase means aphids, not anything related to ears.

In Croatian, se is a clitic, which means it usually appears near the beginning of the sentence, often in the second position.

So:

• Biljka se bori... is the normal order.

This is more natural than putting se later. Croatian clitics often follow this second-position pattern.

Croatian does not have articles like a or the.

So biljka can mean:

• a plant
• the plant

The exact meaning depends on context.

In a standalone example sentence, English might translate it as either The plant fights against aphids or A plant fights against aphids, depending on the situation.

Yes, it is a very common and natural combination.

• boriti se protiv nečega = to fight against something

Examples:

• boriti se protiv bolesti = fight against disease
• boriti se protiv korupcije = fight against corruption
• boriti se protiv lisnih uši = fight against aphids

So this sentence uses a very standard Croatian structure.