My Bonide Thuricide container says "Do not store diluted spray mix for more than 12 hours". I do not know why but I follow those directions. Yep, it bugs the crud out of me to have to mix it every time I need to use it.

Maybe if you mix the bacillus with water they drown? Geez.

On the other hand, maybe the liquid concentrate they are immersed in is stuff that keeps them alive, and if you dilute it, they can't survive? Anyone have any references about the biology of Bt? This makes little sense.

In the diluted form, the bacteria revive and start trying to grow. Also, competition - other bacteria and fungi - will start growing unless you manage to have perfectly sterilized everything.

That's very interesting. Thanks. One wonders to what extent one can cultivate and propagate Bt at home. It's pretty cheap in the store, but maybe it's easy? Is there a recipe around?

Also, I see it noted that Bt spray degrades in sunlight pretty quickly, so I suppose spraying is best done towards the end of the day.

I don't know how long the BT will stay viable once diluted but one thing I do know is that you don't want to accidently leave a spray bottle with BT out in the sun for very long. I forgot and left a BT spray bottle out in the garden one day and when I found it the next day and opened it to dump it out, I was gagging from the stench...very nasty!

Well, the instructions seem to be pretty consistent that you should not use day-old diluted BT. I assume that's because it is no longer viable. Interesting that dilution makes the BT revive and start to grow. Adding water makes things grow when they're dry. But here, adding more water to things already wet does the same. I guess that rained on Dipel (BT powder) losses efficacy pretty fast.

Making the dilution with distilled or aerated water is really no big deal, so I think chlorination is irrelevant. Also, where I store the diluted solution is right next to where I store my concentrate. So storage conditions are not an issue. My diluted solution is in a 1 qt sprayer. That will fit anywhere.

The issue is that we use this spray weekly, so if the dilution doesn't last more than a week, it costs extra effort every time you use it. I don't really care about odor. I don't think my plants do either.

I have to wonder if the shelf-life of the dilution could be prolonged with refrigeration. These are microbiology questions. Bt biologists, where are you?

I don't think anything catastrophic will happen, unless you count ineffective or less effective spray to be catastrophic (which it could be if your pest problem is bad enough, I guess), i.e., I can't see how it would be toxic or anything. You could always try using old spray and see what happens, if you're ok with the possibility that there might be greater crop loss. Like I said, I think those instructions are mostly butt-covering behavior by companies who don't want to constantly be dealing with complaints and claims from dissatisfied customers when it's not their products' fault. Personally, I don't have a rain barrel or anything, so aerating the water or obtaining distilled would be much more of a pain than working with smaller batches as I go. And refrigerating a tank of old solution is more inconvenient to me due to limited refrigerator space than mixing up small batches as I go. But I have a pretty small garden.

I don't think "catastrophic" is a word that pertains. But it is a bit silly to go to the effort to spray the stuff if it doesn't work, isn't it? Most water supplies these days are disinfected with chloramines, and the chlorine residual from those is pretty tiny. I doubt if tap water is an issue in preserving Bt bacteria. That being said, I won't even try to disentangle butt-covering from real facts here. The real facts are what count. Again, I'd just like to get a straight story from a microbiologist who has some expertise in Bt.

I think the issue isn't just knowing Bt but knowing what is in commercial Bt concentrates. That information is usually proprietary and not available to the public. For example, does it contain ingredients that keep bacteria in a dormant state which, when diluted, no longer inhibits their growth and reproduction? If so, is the issue that bacteria start to rev up their metabolisms, reproduce, etc. once the solution is diluted and there is a limit to how long they can survive without a direct food source/suitable host, therefore any time the solution spends in dilution reduces the amount of time it's effective on the plant? That's just one factor to consider.

It would be great if the forum had someone on it who works in developing commercial Bt concentrates, but that's not likely. You're more likely to get practical answers about whether or not saving the solution decreases effectiveness by trying it yourself.

That's a good point, but I'd like to get some insight from microbiologists about what ingredients might actually keep Bt dormant, and how dilution interferes with that dormancy. Kind of remarkable that, for such a widely used insecticide, there is little real science that I can find about it. It's gotta be out there. Proprietary mixtures are based on established scientific understanding, especially since the stuff is marketed by many different suppliers.

To be clear, I was just coming up with examples of different possibilities. Another possibility is that the food supply just gets too diluted to keep the bacteria alive for more than a limited period of time, so maybe the longer the solution sits in the tank diluted, the less time it has to be viable on your plants. Once again, just brainstorming why the question may not be as simple as you seem to think.

A microbiologist might not know all that much about volume packaging of these bacteria and what is involved in giving them a long shelf life while in liquid in conditions that gardeners typically keep these products.

There's a ton of available science on Bt safety, effectiveness under lab conditions, etc. There's less on what commercial products containing Bt do and how they're formulated, though, because those are probably industry secrets that aren't covered by laws that mandate transparency. BtK is marketed by many different suppliers, but I would be surprised if there was more than one or two manufacturers of the base Bt formula that goes into all of those brands. The difference between brands is probably in a very small fraction of the final formula, such as surfactants, etc.

Good brainstorming. But someone has answers. Maybe not details of industry Bt concentrate recipes, but just an overall understanding of the organism and the processes that pertain, that at least hint at some of the complexity of the answers to the question.

There's a lot of that going around in gardening. I call it the "magic sauce" strategy. Whereby someone comes up with a crazy idea, and it goes viral to the extent that it becomes an accepted fact -- well, EVERYONE knows that! Now, Bt works, no question about that, but subtleties abound.

From some research, it looks like pH is important to the shelf life of Bt liquid concentrates. Diluting with water obviously raises the pH a great deal, so that probably plays a role in degradation of the product.

Exactly right. Would be interesting to know more about this. That's a good reference, and deserves some careful study. 3.3.3b contains some clues, one of which is that refrigeration may be effective in prolonging activity.

That does look like a good reference. Thanks for researching and posting gorbelly!

I am just happy that I don't have to spray BT any more this year as our weather has cooled enough that the cabbage butterflies and loopers are done for the year. I was down to just an ounce or 2 left in the bottle. All the newer leaves on my Brassicas are pristine but the lower leaves look like hades.

I wonder how long it will be before GMO brassicas will be developed with the BTK gene for commercial farms.

Wow, GMO brassicas with Bt. What a great idea! I suspect the economical value of that won't justify its development though. It'll be a while before the economic value of brassicas approach that of corn and cotton.

You are probably correct on the economic value of brassicas. I wonder how the commercial farmers treat their brassicas for worm pests currently.

I believe the range of individual cabbage moths is pretty substantial. Many miles. As a result, it isn't sufficient to just kill them off in a limited area. That's a strategy that works well for dealing with some insect pests in large farms. That is, once you kill them off, you don't have to worry about them for many generations.

I'm wondering how it manages to keep when you buy the "ready to use" type that is already watered down. That clearly sit on the store shelves for more than 24 hours.

Fair question. BTW, Bt is *dead* bacteria, so it isn't about keeping anyone alive. That being the case the stuff is probably bacterially nutrient-rich, and is prone to colonization from other bacteria, which may interfere with its effectiveness. So it may be an issue of how well sterilized you keep everything when you dilute the concentrate. Ready-to-use is probably sterile.

I will say that Southern Ag simply says that chemicals tend to lose potency after being mixed with water, and that's why they suggest the 24-hour expiration when using their concentrate. They don't sell ready-to-use, so won't make any claims about that stuff. I would suggest getting in touch with the manufacturer.