It's not that they "become immune," AfaIK. That kind of terminology paints a picture... of a child getting chicken pox and recovering - and, now, safely immune to it, that child can be around other children who have chicken pox without worry.
In
this case: You have a large population and you're trying to poison it all. Mutations happen. One could say - if one said such things
- that Mother Nature doesn't like genocidal events unless she caused them in the first place. So in a population, one or two - or 37, the exact number isn't really significant - will naturally be immune to a given poison.
Maybe that number is zero, and everything is fine. But assume one or two are immune, due to mutations somewhere along the way in its parental lines. You kill 42,000 spider mites, leaving only some small few... that
breed. At that point, there's lots of food and NO competition for it, lol. So they breed, their progeny breeds... and a couple weeks later, you have another 42,000 critters - all from the genetic line that is resistant to "poison #1."
Many people with use multiple weapons against a colony of nasties. Hit them with #1, #2, and #3. IDK, but I'd
guess that it'd be more effective in practical terms to do this simultaneously instead of #1, wait... #2, wait... #3. Because you're not giving the survivor(s) from one poison the opportunity to increase its population.
As
SweetSue mentioned, you need to use multiple treatments, spaced 3 or so days apart, to account for any eggs that'll hatch. This would be true whether you take the... shotgun or the single-shot (lol) approach.
If your population and environment is sufficiently large - say, a 1,200-acre forest - then you could conceivably end up with a surviving population that's resistant to everything in your arsenal.
You can experience the same phenomena writ large by entering a random hospital and staying long enough until you pick up a drug-resistant strain of MRSA or the like :icon_roll .
I'm just rambling. I don't suppose I really said anything :icon_roll .
BtW, this infestation was carried in on clones? I almost never introduce others' clones to a garden for this reason. But when I do, the FIRST stop is either the sprayer at the kitchen sink or the shower. Water-blasting works pretty good, lol. IDK whether this kills the little <BLEEPS> or not - once they reach the sewer system, I don't really care. If the clones are in some sort of container, with much in the way of soil or other media, I will at the least stand there hosing the soil over and over and... And will sometimes try to remove the vast majority of it, if I think the clone might survive the root stress. Water-blasting doesn't depend on poison, so immunity does not play a part.
I read within the past year or so that russet mites - those things that are so tiny you can barely see them even when they clump together in numbers - can even live/travel on
seeds. That has me a little bit worried; approximately 98% of my seeds were gifted to me. I think I'm going to start doing what others do and give my seeds a hydrogen peroxide "bath" before soaking them prior to germination.
These days, I can't see well enough to identify the sex of preflowers, lol / <SIGH> . So the thought of little critters scares the willies out of me.