Magical Corruption Methods

When you say species, do you mean sapient species? What happens if you inject them into, say, a horse? Also how are you defining "female" ? If you're doing it....:

• "biologically" i.e. via chromosomes, what about XXY males? XO people? Genetic chimeras? Women with XY chromosomes but their body doesn't react to testosterone, so they developed female?
• "biologically" i.e. via organs/body parts, what about people naturally born without uteruses or have had them removed because of injury/illness? What if the ovaries are not working as they should because of PCOS? And the many, many intersex conditions out there, how does that work?
• "biologically" i.e. hormonally, is it because of low T or high E? Are there different effects depending on when the subject is in their menstrual cycle? What about pre-puberty? Post-menopause? What about eunuchs?
• spiritually via souls, what determines a female soul? What if a creature lost some or all of their soul? Is there a way one can protect their soul or merge it with someone else? Are there non-gendered souls (like those of animals)? Is it possible for people to be born with a soul that doesn't match their body?
• culturally, how exactly do the insects know what is or isn't female? What about cultures that have radically different ideas of femininity? What about species that have nonbinary gender/sexes (bees/ants being a great example).

Also, in certain species, such as birds, bugs and reptiles, they do not use XX = female and XY = male, they have ZW = female and ZZ = male. Do dragons also have XY sex chromosomes? There's also the X0 system, which bugs and bats have, where X0 = male and XX = female. How do the beetles work on radically different anatomies and genetics?

Also: what exactly are these pheromones? Pheromones are just hormones, but they are outside the body and affect things outside-in. If they're being released inside of the subject, then wouldn't they be hormones? Are they related to estrogen and progesterone? If they were injected into a male (by whatever definition) subject, would it induce feminization? There's an interesting avenue there for a story, especially of someone who wants to induce feminization, but it would take a lot of care to do.

 

To be fair though, you could just handwave it and make it up when you need it. As long as you don't touch on it, very few readers will wonder: "is that beetle attacking the hormones or the genes of that female it's corrupting". Just keep it more or less consistent and you're good to go.

It's fantasy. A valid answer can very well be "the demon bugs know a female when they see it and so they corrupt it." And you can decide that it either does nothing for non-sapient species (since they have no soul or whatever) or they get corrupted and become a demon version of themselves (giving you demon horses and the like) or it can just kill them outright. It can even be that the bugs are too rare and expensive to experiment with and no one has tried it yet and you can get away with never answering the question.

It's a cool idea, so just run with it.