Mszyca jest na łodydze.

Because the preposition na in this sentence means on in a location sense, and that usually requires the locative case in Polish.

• basic form: łodyga = stem, stalk
• after na for location: na łodydze = on the stem

So łodydze is the locative singular form of łodyga.

Na often means on when something is physically on a surface or attached to it.

So:

• na łodydze = on the stem

This is the natural choice for saying that the aphid is on the plant stem.

Jest means is.

In this sentence:

• Mszyca jest na łodydze. = The aphid is on the stem.

In Polish, the verb to be is often omitted in the present tense when talking about identity, like To moja siostra = This is my sister. But in sentences about location, jest is normally used.

So Mszyca na łodydze would usually sound incomplete or unnatural as a full sentence.

Not completely. Polish word order is more flexible than English word order.

The neutral order here is:

• Mszyca jest na łodydze.

But you may also hear:

• Na łodydze jest mszyca. = There is an aphid on the stem.
• Na łodydze mszyca jest. = possible, but more marked or emphatic

The usual sentence for a learner to use is Mszyca jest na łodydze.

Polish does not have articles like a, an, or the.

So mszyca can mean:

• an aphid
• the aphid

and na łodydze can mean:

• on a stem
• on the stem

The exact meaning depends on context.

A rough English-friendly guide is:

• Mszyca ≈ MSHI-tsa
• jest ≈ yest
• na ≈ nah
• łodydze ≈ wo-DI-dzeh

A few useful pronunciation notes:

• sz sounds like sh
• y is not exactly English ee; it is a shorter, more central vowel
• j sounds like English y
• ł sounds like English w
• dz sounds like the ds in beds, but pronounced together
• stress is usually on the second-to-last syllable:
  • mszyca → MSZY-ca
  • łodydze → ło-DY-dze

Mszyca is a noun. It is a feminine noun meaning aphid.

In this sentence it is in the nominative singular, because it is the subject:

• Mszyca = the aphid / an aphid

Yes. Łodyga is also a feminine noun.

Its basic singular case forms include:

• nominative: łodyga
• genitive: łodygi
• dative: łodydze
• accusative: łodygę
• instrumental: łodygą
• locative: łodydze

In this sentence, the locative form łodydze is used after na.

Not if you mean on the stem as a location.

• na łodydze = on the stem, located there
• na łodygę would use the accusative and would suggest movement onto the stem, not simple location

For example, accusative after na is used in sentences like:

• Mszyca weszła na łodygę. = The aphid climbed onto the stem.

But with jest, which describes location, you need na łodydze.

This is a normal spelling and sound change that happens in some Polish noun forms.

The stem of łodyga changes in certain cases:

• łodyga
• łodydze

This kind of consonant alternation is common in Polish inflection. For a learner, the important thing is to memorize that the locative singular is:

• na łodydze

rather than trying to predict it only from English-style rules.

Yes, depending on context, it can communicate almost that idea.

• Mszyca jest na łodydze. focuses on the aphid is on the stem
• Na łodydze jest mszyca. more naturally matches There is an aphid on the stem

Both are grammatical, but they put the emphasis in slightly different places.

It is a normal and correct word, especially in gardening, biology, farming, and discussions about plants. If you are talking about pests on plants, mszyca is exactly the word you want.

The plural is:

• mszyce = aphids

So for example:

• Mszyce są na łodydze. = Aphids are on the stem.

The key pattern is:

• na + locative for location

So:

• na stole = on the table
• na ścianie = on the wall
• na łodydze = on the stem

This sentence is a good example of how Polish changes noun endings depending on the preposition and meaning.