Bt

[applestar]

www.entomology.wisc.edu/mbcn/fea207.html

[rainbowgardener]

Good information, thanks.

But I really wish in there they had put in warnings about over use and indiscriminate spraying leading to resistant populations. Since the advent of GMO plants with built in Bt, more insects are becoming resistent to the Bt pathogen. If we are not careful, we will lose another good tool. The problem is more with the acres of Bt-GMO's, but home gardeners can avoid adding to it. Don't spray your yard, spray where the problem is after you know what the problem is (what kind of pest you have). But in that area, be thorough, which may take repeat applications.

It's like taking all your prescription of penicillin -- If you spray with Bt and kill 90% of your targeted pests, the 10% you didn't get are the most resistant ones. Now they are the only ones remaining and you removed a lot of their competition, so they will multiply and you will have a resistant population. So keep working not to leave the 10%.

[GardeningCook]

Oh for heaven's sake - now we're not supposed to use Bt??

Talk about the start of sending organic gardening down the tubes.

[rainbowgardener]

Nope, I didn't say that. I said we should use it with some care. Don't spray your whole garden with it. Use it only in targeted areas where there is a real problem. Don't use it "preventatively." Don't build it in to the genetics of your plants to that whole square miles of plants have it built in to every plant.

Leave some areas of the garden unsprayed where non-resistant individuals of the target pest can continue to thrive and be part of the population, so that the whole population doesn't become resistant.

If we use it with some care, it will continue to be a useful garden tool. If we use it indiscriminately, it will become useless as we breed resistant organisms, just as we now have lots of antibiotic resistant disease pathogens, due to indiscriminate use of antibiotics, as in cattle feed.

[Peter1142]

I am using it preventatively against SVB, which I had in abundance last year, and despite probably only being marginally effective I am 100% OK with that decision.

The use of anything will breed resistance, whether used discriminately or not. I agree you shouldn't mass spray your garden with no rhyme or reason, but I don't imagine many BT users would do that.

[GardeningCook]

If one is really against using Bt, there's a very easy remedy that works even better. Protect all of your brassicas from Day 1 first seeding/seedling-transplanting with insect-weight floating row cover. Period. If the caterpillar moths can't reach your crops to lay their caterpillar-hatching eggs, then Bt - which only works against already-hatched caterpillars - won't be necessary. Since brassica crops don't require pollination, there's no need to ever remove the row cover except for harvest &/orbrief checkups as to how things are going. End of story.

[rainbowgardener]

Re: "The use of anything will breed resistance, whether used discriminately or not" I don't agree. Short term, targeted use may or may not breed resistant individuals. Resistant populations only come from wide-spread and chronic use.

Resistant populations are mainly what we are worried about.

[Peter1142]

Short term targetted use does the same thing. Any pests that survive the pesticide now have the evolutionary advantage and so might future generations they breed. In fact targetted spraying is just as likely or more so to create resistant populations IMO, because there is a large presence of the pest you are exposing to the pesticide. If you indiscriminately spray and the pest isn't actually there, it is not possible to breed resistance, in a non existant population.

That's like saying if you take antibiotics when you actually have a bacterial infection for the prescribed short term period you won't breed resistant germs... and that is simply not true.

[rainbowgardener]

" Any pests that survive the pesticide now have the evolutionary advantage and so might future generations they breed."

"Any pests that survive the pesticide now have the evolutionary advantage" this part is true.

"and so might future generations they breed." This part is not necessarily true. If there are non-resistant individuals around for them to breed with and the resistance is not a dominant trait, then their offspring are not necessarily resistant. And the fewer resistant individuals there are as a percent of the population, then the less likely the resistance is to spread through the population.

Actually, since the resistance is not something we are putting in them, it is just that by treating the plants we are selecting for the strongest, most resistant individuals, it is more accurate to say that the less chronic and widespread the treatment, the less likely we are to be left with only/ mainly the resistant individuals who will then be able to pass along the resistance and who will have less competition for resources.

Interesting discussion though. I like it when people are willing to delve deeply into an issue like this.

wiki has a nice article on resistance: https://en.wikipedia.org/wiki/Pesticide_resistance

It shows this:



Left column is before pesticide application; right column is after. Top row is first generation; bottom row is later generations.

"Pesticide application can artificially select for resistant pests. In this diagram, the first generation happens to have an insect with a heightened resistance to a pesticide (red). After pesticide application, its descendants represent a larger proportion of the population because sensitive pests (white) have been selectively killed. After repeated applications, resistant pests may comprise the majority of the population."

If there are not repeated applications and if there remain non resistant individuals to breed with, then the population does not become resistant.

Here's another article: http://jenny.tfrec.wsu.edu/opm/displays ... php?pn=-70

It notes that the longer time between repeat applications, the less likely for increasing resistance in the population.

[Peter1142]

Yes I definitely agree that frequent repeated applications are more likely to breed resistant populations.

That is one reason they suggest rotating pesticides and fungicides, and I agree that is a good idea - but I also agree it is best to avoid their use and not spray unnecessarily. Even mass spraying of profilactic fungicides as is commonly recommended on garden forums I think is a bad idea for numerous reasons. It is bad for your soil, bad for the environment (this includes copper), breeds resistant germs, is only marginally effective, and costs $$.