Question
I have a large number of prickly pear cacti in front of my house. They have a white fuzzy stuff growing on them.  When I touch it it turns purple.  It washes off easily but comes right back.  It seems to be harming the plants.  

What is it and what can I do about it??

Thanks,  Frank

Answer
Dear Frank,

You sound like your Opuntias have mealy bug - there are over 300 different species of mealy bug, so I don't know which you have.

Given the size of your infestation, I'd use either Malathion or Diazinon, in two to three weekly applications, and use it as you would apply the insecticide to roses.

Kind regards,

Jeff