Eradication of whitefly is nearly impossible and should probably not be your goal. Gaining control is a reasonable hope! I would suggest that you might want to change your pesticide of choice, or at least take turns with something else. Also, control of whitefly with contact pesticides requires that you get the product to the underside of the leaves where the critters live, eat, work, sleep, breed, etc. If you are spraying the malathion on top of this hedge, the whitefly are protected by the foliage.

You might consider a super fine horticulural oil spray, as long as you can get it to the underside of the leaves. This can help you interrupt this reproduction cycle. It will smother eggs and nymphs. Read and follow all directions carefully, as there will be some temperature warnings. And don't mix it any stronger than the label says. Oil applications could really make a difference.

Do you really have whiteflies? Around here, the more likely pest on beech is a woolly aphid.