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I saw a ladybug on my balcony one day and I just counted the dots.

There was 18 black dots on a red/orange elytra.

The location is in Europe (Geneva, Switzerland) and usually we have quite different endemic ladybugs and also the invasive asian ladybug Harmonia axyridis.

I did not found any local ladybugs with such dot counting except articles saying the Harmonia axyridis can be found with a lot of variations (elytra color, points or stripes, etc...).

The question is: Can I be sure that a ladybugs with 18-points (found in Europe area) is of the genus Harmonia axyridis?

Note that I have a picture of the animal but I want to focus my question on the number of dots exclusively.

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No, most of the time you cannot. There is a large diversity of ladybug species (several dozens in western Europe), and many of them can have the same number of spots. Moreover, as you already said, there can be intraspecific variation in the number of spots.

I do not know if there are specific numbers of spots that would be associated to only one species - but for most numbers, it is not the case. For instance, yes, it is common for Harmonia axyrids to have 18 spots (source). But Anisosticta novemdecimpunctata, which can also be found in Europe - and can also be orange ! - may have between 15 and 21 spots (though it more frequently has 19) (source).

Thus, the number of spots cannot be used as a sole criterion to identify ladybugs: other criteria usually include the general shape, colour of elytra, size, shape and colour of the pronotum, length of antennae, etc. (See here for examples.)

First picture: Harmonia axyridis

Harmonia axyridis

Second picture: Anisosticta novemdecimpunctata

Anisosticta novemdecimpunctata

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