How many Phytoseiulus Persimilis, predatory mites, should I throw on my plants?

[keith1975]

I've had a terrible time with mites and this is the next thing in line for me to try and get the populations down or maybe even eradicate them. It's quite expensive, about 35 USD for 2000 mites, but apparently they will destroy the spider mites. Unfortunately, I have to use them as I get them, which is what is going to make it expensive. Would be amazing if you could buy 2000 and release only a few every day for the whole season. Which brings me to my question.

If I buy 2000 mites and I have decent amount of spider mites running around, but no serious webbing or anything, how many mites should I release per plant?

Note: One company told me 500 - 1000 per plant, another told me 10 per plant. I dont even know if you can sprinkle as little as 10 from the container they come in, but that's what they said. So I'm pretty lost as to what the best amount is. If it won't give me problems, I dont mind throwing all 2000 into the room. I just don't know what an overpopulation of predatory mites might do. I dont want to have to do extra budwashing to get the eggs of the predatory mites off, for example, if that is even a concern.

 

[TorturedSoul]

First, those two-spotted spider mites that you are trying to eradicate would have come from somewhere. If the source is local (IOW, your own home and/or grounds), it would be helpful in terms of long-term strategy if you could release "a few" there, too.

The Wikipedia article has the relevant expected statistics, if you want to estimate the number of borg you are dealing with and then do the math. Here's a paste from it:

(typical values @ 68–77 °F) sex ratio: four females/male
average eggs laid per day: 2.4
lifespan: 30–36 days - days from egg to adult: 7.5
average eggs per female: 54
population increase = 44 x in generation time of 17 days
prey consumed per day: 7 adults, 20 immatures or 25 eggs

As you can see (and probably already knew), this is not a "static" pest-control device; it is a self-replicating one. However many you end up releasing in your garden, they will produce a second generation, which will mature and produce a third, larger generation, which will mature and... Et cetera. They won't breed like fleas (those SOBs, I'll tell you what... ) - but they'll certainly put the proverbial rabbits to shame, lol. The actual rate will only approach the thereotical maximum if conditions are very good to perfect (including a sufficiently available food supply, which you apparently have). And that food supply will also be "madly" reproducing.

In nature, this is sort of a self-regulating process, I expect. By that, I mean that there are always going to be "bad" mites around; species which immediately set about consuming all of their food-animals tend to not exist very long. So I would look at this sort of thing as more a "control" than a "final solution." Hopefully, though, it'll be able to knock the numbers down enough that you can hit them two or three times (each) with a couple of different pesticides, applied simultaneously (IOW, spray both products at the same time). Because hot temperatures appear to favor the nasty - and knowing that these predators will happily consume immature prey as well as adults - I suggest you take steps to maintain your garden's lights-on temperature at the 70°F to 77°F range (and definitely not 85°F to 90°F. If I remember correctly, two-spotted spider mites overwinter in the ground. However, I do not know whether this will prove helpful; attempting to create conditions mirroring the time immediately preceding Winter may harm production, and will certainly slow it down. Regardless, if your medium allows it, you might consider adding a thin layer of diatomaceous earth on top of it (but realize that it'll harm any soft-bodied insect/arachnid that happens to crawl across the stuff - so this kind of thing might be best done before releasing predators (IDK if those predators go to ground, lay their eggs in the ground, their immature population lives/moves/etc. along the ground, etc.).

One last thing: Before you take the step of releasing predators, haul all of your plants into the bathroom and spray/clean (sterilize) your garden area. Then place each plant into the shower and hose it off with a strong, thorough blast of water (shaking the plant might help, especially if you can cover the container so that they don't just end up in it) from every direction before you return it to your grow room. You'll most likely knock most of them off your plants, lol. Remember to be careful not to allow the treated plants to come into contact (or even near-contact) with the ones still weighting to get "deloused." And then clean/sterilize that bathroom afterwards, along with the path between it and your grow space.

Oops, lol, another "last thing" - those spider mites thrive in hot temperatures and relatively low humidity. I already mentioned that the predators prefer a somewhat lower temperature range, but they also do better when the humidity is higher. Get your grow room RH up above 60% and keep it there for a period of time beginning when you release the predators if it is at all possible to do so. If you have really dense plants (and/or ones in flower), you may have to take steps to prevent mold while your humidity is high. Hmm... There was something else I just thought of, but it went away as quickly as it-- oh, yeah, consider investing in a UV light source. About the only things that seem to like strong doses of that are reptiles.