Anrobes, you might want to invest in a greenhouse thermostat. These are very accurate, much more accurate than the built-in thermostats that you get with inexpensive, electric, space heaters. That way, the heater only comes on when your greenhouse gets down to a certain temperature and shuts off when the temperature is above that low setting. Many of those thermostat controlled heaters are junk, at least when it comes to accurate on/off functions. I run six heaters during the winter (several greenhouses) and have done so for many years. I have never had a fire. My heaters will shut off if they are tilted and they all are of course on breakers.
I know little about growing citrus, having only the single tree that overwintered in a greenhouse last year. I did not have any white flies. I guess IF I had a greenhouse dedicated to citrus and had a pest problem, I would use dormant oil spray on those trees. I am sure others will have experience in growing citrus and can advise you more than I.
