Experts and BHG readers answer.
Insects on butterfly garden
Thanks for writing. With a problem such as scale and whitefly, the key is to keep reapplying the product such as Organicide or Captain Jack's Deadbug Brew every week or two for a month or so. Note: You'll need to be really careful about this as there are few products that will kill whitefly/mealybug/etc. but not affect caterpillars.
---Justin, Senior Garden Editor, BHG.com