Have a read in here or try googling "whitefly" http://en.wikipedia.org/wiki/Whitefly
. I am not sure but I think they drink the sap from your plants. Here's what it said for control: Whitefly control is difficult and complex, as they rapidly gain resistance to chemical pesticides. The USDA recommends "an integrated program that focuses on prevention and relies on cultural and biological control methods when possible."  Use of yellow sticky traps to monitor infestations and only selective use of insecticides is advised.
'there was a young man named jobby,
who haboured an illicit hobby,
with renknowned growing fame,
he put our plants to shame,
that bright young man named jobby'