I was thinking, bug spirits are pretty much dedicated to the destruction of humanity.
Toxic spirits are pretty much opposite to their base spirit.
So, would a toxic bug spirit be benevolent?

Toxics are not the opposite to their base spirit. They are a perversion of it to coincide with themes considered anathema to living beings. Considered, that is, by those living beings. The distinction is not particularly subtle when we consider beings from different gaiaspheres such as the Bugs.

The inverse of a Fire spirit would not be Radioactivity in any sane worldview. That's silly. Even in "elemental" systems the opposite of Fire is still not Radioactivity - typically it's Earth, or, in more modern times (after most people forgot that Elemental Water is flammable), Water. Radioactivity could be considered a perverted version of the idea of Fire that is inimical to human life. That's why the Toxic version of Fire is Radioactivity.

So, the Toxic versions of Bugs would be... I dunno. Bug Shamans may well be summoning Toxic Bugs half the time and not even realise it because Bugs are so alien to metahumanity.

A bug spirit is dedicated to the proliferation of it's species. It doesn't care about mankind, except as a source of viable hosts and possibly food. Toxic bugs would somehow pervert the goal of expanding the bugs's access to this world, to the detriment of the bugs but not necessarily to the benifit of us. A toxic bug spirit might believe in wiping out any non-bug animals on earth because only inhabited creatures deserve to live, but in the process removes the supply of available hosts for more bugs to use.

Or maybe toxic bugs would decide that the best way to make room for more bugs here is to use the astral gateway power and toss the people that are here back into their world.

I guess the next question would be what's the difference between a twisted path bug and a toxic bug?

Toxic Mantis would most likely be killing other Mantis thingies i guess.
Everything else would simply not care about finding hosts for others.
Only for themselves. And destroy everything else getting into their path.

As fot the difference between Toxic and Twisted:
Toxic=Mages/Shamans, Twisted=Adepts, right?

A few ideas tossing around...

All bugs spirits are the Toxic versions - Totems long lost to the Deep metaplane in madness (Horror tainted if you like your Earthdawn timeline crossover). I like the idea of going on a "oh hell" astral quest trying to rescue Mantis, sue me.

The courrupt versions aren't trying to replace humanity - enslave it. They make no effort to hide and never try for "Good merges" - only "bug monsters". I like campy 60-70 horror films. ANTS!!! Sue me.

IIRC Toxics (mages and possibly adepts) are a subgroup of the Twisted. These contain all sorts of corrupted philosophies like faustian mages. See Street Magic p. 137 ff.

I never really understood bug spirits. They have a desire to keep reproducing, yet they need human hosts in order to remain on this plane. So what happens when every metahuman on the planet is turned into a host? What then? Do the bugs just suddenly lose their drive to reproduce? Do they start taking other mammals as hosts?

I could be wrong, but I don't think there's any documented cases of bugs using non-metahuman critters as hosts.

Man, that would be rough. Imagine a barghest-bug flesh form. *shudder*

pretty sure you are wrong. one of the threats books i think details an ares facility with bugs that have inhabited animals.

Well, then it's official. I won't be able to sleep tonight.

I don't understand metahumans. They have a desire to keep their economy growing, yet they need natural resources in order to keep the growth. So what happens when every resource on the planet has been used? What then? Do the humans just suddenly lose their drive to grow?

Humans don't plan ahead, you think bugs do?
Anyway, no metahuman understand bug spirits. They're just too different from us.

If they will run out of hosts, they'll try to find new portal to next plane.
As it's said in "Street Magic": bugs are trying to become dominant species on the plane, so they use humans (actual dominant species)as hosts. And yes, they can use animals.
Forget barghest, think about DRAGON host... if it's possible.

Back to the topic: toxic bug probably will turn against every lifeform, including his own hive (and bugs on their native plan).

I would honestly say that the standard Bug spirits are the toxic ones. Which opens up the question of what non-toxic versions would be like. Probably quite weird but not mindlessly consumptive. They also summon 'normal' spirits that take insect like shapes, rather than inhabiting spirits. Also rare as dragon's toenails.

It's said that some Australian Aborigines shamans have good relationships with some insects totems. These may be the non toxic ones.

You're thinking of Threats 2. I think it was called "Bug Bomb" or something to that effect. Had a great Prescott illustration of a runner being attacked by a bunch of bughounds. Scary, scary shit.