The white moldy material is excretion from the mealybugs. It’s a trademark sign of a particularly bad infestation.
Horticultural oil is an excellent natural pesticide. It will not harm your plants but it will suffocate any pests that are already on your plant. One application is typically enough to prevent non-visible infestations from growing. You can apply an additional layer after 7 days if you’d like, though.
Mealybug eggs can take months to hatch. It’s not realistic to quarantine your plant for that long, and 1 week is typically long enough for you to recognize signs of adult mealybugs.
If you find the mealybugs on an outdoor plant already in your garden, quarantine it. Then, treat the plants that were surrounding the infected plant, even if you don’t see any signs of mealybugs. If mealybugs are infesting a large number of plants in your garden, it’s not worth digging them out and quarantining them. Simply deal with the problem where your plants happen to be.
Use pine oil instead of bleach or disinfectants if you want a less abrasive option. Do this every time that you use your tools to prune, plant, or dig.
Do not make the water pressure on the nozzle so high that you destroy the plant. Use a nozzle setting that is firm enough to put some pressure on the plant, but not so hard that it rips leaves or flower buds off.
Alternatively, you can fill a spray bottle with 10-25% isopropyl alcohol and mist the affected plants. Do this once a week until the mealybugs are gone.
Neem oil will also suffocate some pests that are already firmly established in your garden. Unfortunately, it may also kill ladybugs and other insects that are good for your garden. Neem oil is completely natural and it is extracted from neem trees. It won’t damage your plants on its own, but some plants may not thrive when covered in too much oil. If any of your plants struggle to flourish after you apply the neem oil, stop using it.
Never apply horticultural oil if it’s hotter than 80 °F (27 °C) or colder than 40 °F (4 °C).
This is not a particularly good preventative measure, since the mealybug ladybirds will simply leave your garden if there isn’t any food there for them. While they have the word “mealybug” in their name, mealybug ladybirds are not actually a species of mealybug. They won’t harm or eat your plants.
