You sprinkle it on the leaves and on the ground around the radishes. I am in a dry area so I usually don't have to worry about rain washing it off.
"Diatomaceous earth is one of the more effective repellents, applied as a dry powder to the plants. Horticultural oils and some neem insecticides also have some repellent effect on this insect."
http://www.ext.colostate.edu/pubs/insect/05592.html
"Diatomaceous earth is one of the more effective repellents, applied as a dry powder to the plants. Horticultural oils and some neem insecticides also have some repellent effect on this insect."
http://www.ext.colostate.edu/pubs/insect/05592.html