The whiteflies likely rode in on another plant, perhaps an annual or other new addition to the garden this summer. As you noted, they can’t overwinter in the northern US, so they shouldn’t be back unless they’re reintroduced. The best preventative technique is to thoroughly inspect any new plants that you have added or will add to the garden. If you find that only a few leaves are infested, remove those leaves. Insecticidal soaps and horticultural oils (neem oil, for example) can help to reduce populations, but they will require multiple treatments and alone won’t be strong enough to completely get rid of an infestation. There are also systemic insecticides, but these are not recommended because they will also kill pollinators and beneficial insects (like ladybugs, which actually prey upon whiteflies).
The whiteflies likely rode in on another plant, perhaps an annual or other new addition to the garden this summer. As you noted, they can’t overwinter in the northern US, so they shouldn’t be back unless they’re reintroduced. The best preventative technique is to thoroughly inspect any new plants that you have added or will add to the garden. If you find that only a few leaves are infested, remove those leaves. Insecticidal soaps and horticultural oils (neem oil, for example) can help to reduce populations, but they will require multiple treatments and alone won’t be strong enough to completely get rid of an infestation. There are also systemic insecticides, but these are not recommended because they will also kill pollinators and beneficial insects (like ladybugs, which actually prey upon whiteflies).