Mealy Bugs

My wife keeps her citrus trees in the greenhouse over the winter. This year, they had a really bad infestation of mealy bugs, which leave a powdery white residue on the leaves, as pictured in the link below.
https://hosting.photobucket.com/images/p691/pavel314/0/DSC04734.JPG?width 20&height80&fit=bounds
Does anyone have experience with this? If so, what is a good way to prevent it or destroy it? When we move the trees outside in the spring, the problem quickly goes away.
Thanks for any help,
Paul
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Pavel314 wrote:

Mealybug Destroyer beetles, but there may also be other biological controls.
songbird
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