A Bit Beyond Official Bt Doctrine

If I had used Bt when I saw caterpillars, particularly on my asters, I wouldn't be having these beautiful little pearl crescent butterflies flocking to my butterfly milkweed recently.


In a recent post, I mentioned that Bt (Bacillus thuringiensis) has a wider range of potential targets than usually expected.  I was gently "taken to task" over that statement and given the official party line.  Here is a link to one of the official lists of Bt targets, written by Whitney Cranshaw of Colorado State University Extension Service.  I think this paper is well written and represents the party line accurately.  (I'm sure Dr. Cranshaw will be overjoyed to hear that I approve!)  As far as it goes.

However.

I've researched and reviewed and thought further about what I want to say beyond this official doctrine.  Some of it is hard to explain without going into the opaque depths of biochemistry and entomological physiology that don't seem appropriate for this blog, but I'll attempt to summarize without coming off as too gaga.

First and foremost, compared to many (most?) broad spectrum insecticides, I want to say that Bt seems relatively benign.  If you HAVE to use something to control caterpillars, Bt seems like the least problematic choice. 

With that disclaimer in place, I'd like to share my concerns.

Basic summary:  There are different strains of Bt.  Different strains of Bt produce different mixes of toxins.  Toxins affect different insect species differently.  Different toxins affect different insect species.

Upshot:  In general, Bt toxins only affect insects that go through complete metamorphosis...and not all of those.  The concentration of each toxin matters, so the amount of Bt that an insect eats matters, as does the life stage it's in when it eats the toxin.  An insect eating the plant surface that has been sprayed with Bt will get much more toxin than the insect that simply burrows through the plant surface and eats the plant material below the surface.

Bacillus thuringiensis , kurstaki strain, is the strain used for caterpillar control.  Any caterpillar that eats enough of it will be damaged or killed by it (setting aside developing resistance in a few populations).  Therefore, sprayed non-judiciously on garden plants, Bt (kurstaki strain) will not only kill cabbage worms (cabbage white butterfly), tomato and tobacco hornworms (hawk moths), and all the other "pest" caterpillars listed in the publication above, it will also kill monarch caterpillars, fritillary caterpillars, checkerspot caterpillars, swallowtail caterpillars, and so forth when sprayed on the plants that they are eating.

If you want butterflies, you have to be very careful when and where you use Bt in your garden.  If you kill off all caterpillars, you are effectively killing off all butterflies and moths.

Next, Bacillus thuringiensis, Israelensis strain, is used to control mosquitoes, fungus gnats, and black flies.  All of these are in the insect suborder Nematocera or long-horned flies.  I have not done enough research to know if this strain (or a different Bt strain) affects other flies, but this suborder also contains midges, an important food source for many fish and other aquatic animals and they have been mentioned as being a casualty when this strain is used.  Care in using this strain would also seem to be indicated to avoid accidental wider, food chain consequences.

The next strains mentioned in the above publication, Bacillus thuringiensis, San Diego and tenebrionis strains, are used to control Colorado potato beetle, elm leaf beetle, and cottonwood leaf beetle.  Again, I have not done enough research to find studies looking at other beetles "controlled" by these Bt strains, but I think it would be safe to say that probably any beetle that ingested enough of the Bt toxin on its food, for example a leaf-eating beetle of another plant species such as aster or skullcap, would be affected.  Again, care needs to be taken to avoid accidental action against non-target animals.

The pretty little beetle below, shining flea beetle (Asphaera lustrans), feeds solely on skullcaps.  Bt used, purposely or otherwise, on skullcaps would kill it.  (I saw this small but colorful beetle only one summer, when I had 2 resinous skullcaps growing in my garden.  The leaf damage pictured in this photo is pretty much the extent of the damage I noted - the blooming didn't seem affected at all, and I had to be very close to notice the leaf damage.  Truthfully, I didn't notice the leaf damage until after I had noticed the beetle and taken its photo.)

The upshot is that CARE NEEDS TO BE TAKEN TO AVOID ACCIDENTAL INJURY TO NON-TARGET ANIMALS, even with Bt.

My last point is that some of the research I found discussed the fact that some of the Bt toxins can affect Hymenoptera (bees, wasps and ants) and nematodes.  It also discussed how some of the toxins that are lethal to one group of insects may be nonlethal, but still detrimental, to other "non-target" groups.  (That dose-dependent issue mentioned above.)  The articles I found with these concerns were published outside of the U.S. and I have no real way to judge their validity, but with Bt toxins affecting 3 of the 4 major insect orders that have complete metamorphosis, I have strong suspicions about their ability to affect other insects beyond just the "target" species.  I'd love to see strong research verifying that Bt doesn't affect these other insect orders, suborders and/or families, rather than just blithely assuming that no harm is being done because we're not testing for it.

Once again, in my opinion "innocent until presumed guilty" has absolutely no place in the world of toxic substances.