Answer: The leaf drop sounds like a combination of problems - adjustment to indoor growing and an infestation of spider mites. Moving plants indoors from outdoors, or from a nursery/greenhouse situation can be a real shock to their system because the conditions are usually so different. Yellowing and dropping leaves is a typical reaction. Don't fertilize for a month or so, but continue watering slowly and deeply. The plant might go dormant for a while and then new growth should appear in the spring. Citrus generally need 8-10 hours of full sun, so try to give more direct light. Citrus are also heavy users of nitrogen, so start fertilizing when your plant recovers. Try to find a fertilizer specifically for citrus. In spring, slowly acclimate your plant to moving back outdoors by putting it outside in a sheltered location for a couple hours at a time, gradually increasing the time period. In fall, do that in reverse, rather than abruptly moving from outside to inside, which will help lessen the shock.
The webbing you describe sounds like spider mites. These pests thrive under hot, dry conditions. You can help avoid the problem in the future by periodically hosing the tree down with plain water during the hottest months of the year. You can also control these pests while your tree is indoors - simply give it a tepid shower with plain water.
Although your tree is losing leaves, continue to treat it as though it were healthy - water regularly, give it a shower every couple of weeks to get rid of the spider mites, and provide a bright, warm spot away from heat ducts so it isn't in a draft.
Best wishes with your tree!
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