1. Good Organic Gardening - January-February 2016

SAFER SOLUTIONS | Dipel
Rose budworm damage
Clever soil bacteria
Bacillus thuringiensis
While organic gardeners prefer to garden as naturally as
possible, sometimes a little help may be needed
Words & photos Angie Thomas
Like our bodies, soil contains billions
of bacteria. In fact, they are crucially
important to the health of soil
as they’re involved in important
processes such as breaking down organic
matter and converting nitrogen from the
atmosphere into plant-available nitrogen.
One particularly special soil bacterium
is Bacillus thuringiensis, a naturally
occurring, aerobic, spore-forming
bacterium that has insecticidal properties.
The history of Bacillus thuringiensis
(Bt) is fascinating. A Japanese biologist
first identified the bacterium in 1901 while
researching the cause of an unknown
silkworm disease. Ten years later in 1911,
a German scientist rediscovered Bacillus
thuringiensis during investigations into a
flour moth disease and, by 1920, farmers
were using Bt as an insecticide.
How does Bt work?
When the Bt bacteria produce spores
they naturally form crystalline proteins
(endotoxins) that affect insects — in
particular, Lepidoptera caterpillars, which
are caterpillars of moths and butterflies.
Bt must be ingested by caterpillars to
be effective. It does not work via contact
action and therefore needs to be applied
when the insects are in their larval stages
and feeding on host plants.
Once ingested the endotoxins paralyse
the cells in the caterpillar’s gut, they stop
feeding and will succumb to starvation or
infection. This usually occurs within a few
hours, though some caterpillars may remain,
inactive, on plant foliage for 3–4 days.
Bt targets particular receptor sites
in the caterpillar and has no effect on
humans as we lack those receptors. Bt
also does not affect beneficial insects,
pets and wildlife. Because it specifically
targets caterpillar pests and is safe
58 | Good Organic Gardening