Diatomaceous Earth for the Garden
What is it?
Diatomaceous earth is made from the fossilized remains of tiny, aquatic organisms called diatoms. Their skeletons are made of a natural substance called silica. Over a long period of time, diatoms accumulated in the sediment of rivers, streams, lakes, and oceans. (source: http://npic.orst.edu/factsheets/degen.html#whatis)
How does it work?
Diatomaceous earth causes insects to dry out and die by absorbing the oils and fats from the cuticle of the insect's exoskeleton. Its sharp edges are abrasive, speeding up the process. It remains effective as long as it is kept dry and undisturbed. (source: http://npic.orst.edu/factsheets/degen.html#whatis)
it can be used to kill bed bugs, fleas and ticks but I use it in my garden to repel squash bugs, potato bugs and Japanese beetles. I found that a regular dusting of the powder along with hand picking bugs keeps the pests under control.
How to apply.
Diatomaceous earth can be applied by sprinkling it on the ground encircling the plant you wish to protect. I found this to not be very effective in my garden. Another method is to dust the foliage with a fine mist. I first tried to is a home made shaker but the powder came out in clumps. A youtube video suggested a plastic water bottle with a piece of nylon stocking over the opening. I found that a dish washing liquid bottle works well.
Apply the dust on a calm day or one with a slight breeze. You should wear a mask because it can cause irritation to your lungs if you breathe it in. How much to use is a matter of judgement, I like just a light visible dust. I squeeze the bottle below the foliage as well as on top. Rain will wash it away and make it ineffective so it should be applied often.
Diatomaceous earth also contains almost 20% calcium so it can be used to combat blossom end rot.