Answer: First of all, when you move a tree indoors after they've spent the summer outdoors, they will go through a stressful adjustment period. Light levels are lower indoors and temperatures and humidity are different, too. Leaf drop is normal during adjustment periods. What you can do to help is to provide as much bright light as possible, water often enough to keep the soil moist but not soggy wet, and mist the leaves daily to compensate for the dry indoor air. You will also want to make sure your trees are not in a draft or near a heat register.
Whiteflies are difficult to control and I think Malathion is a bit harsh for your citrus tree. That might also be contributing to leaf loss. I'd use Insecticidal Soap against the pests. The fatty acids will coat them enough to suffocate them and their eggs. Don't move your plant back outside or it will have to go through another adjustment period. Instead, try taking it into the shower and using tepid water, give it a shower. This should help rinse off the pests and their eggs and help dilute the residual Malathion. Give your tree a few weeks to overcome its stress and you should see some new growth. Hope so!
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