This is from Cornell Univ.:
"How it works:
Spinosad is a fast-acting, somewhat broad-spectrum material that acts on the insect primarily through ingestion, or by direct contact with a spray droplet or a newly treated surface. It activates the nervous system of the insect, causing loss of muscle control. Continuous activation of motor neurons causes insects to die of exhaustion within 1-2 days.
Foliar applications of spinosad are not highly systemic in plants although some movement into leaf tissue has been demonstrated. The addition of a penetrating surfactant increases absorption by tissues and activity on pests that mine leaves (Larson 1997)."
This is from "Naturalyte" (spinosad product)
Coverage notes:
Good coverage of insect feeding
sites is essential as Spinosad is
non-systemic.
Apply min volume of 250L/ha and
increase spray volume as crop
grows. Use nozzles that give
droplets in the size range of 150-
200 microns. High water volumes
definitely help.
Can add wetting agent to improve
coverage.
Ensure spray dries completely before rain.
Repeat applications 7-14 days as new
infestations occur.
Witholding period 3 days in most
cases.
Dow Agroscience says this:
"Spinosad works by contact and by ingestion. Contact occurs either by direct application to the insect or by movement of the insect onto a treated surface.
Ingestion occurs as insects feed on treated substrate (such as foliage). While control via contact is highly effective, control via ingestion is 5 - 10 times more effective."
This last quote tells me that if you wash the product off too early, you are not killing the majority that die by ingestion.