We need to know more about this tree. How long have you had it? How long has it been in the same container? What kind of potting medium do you have? Does your greenhouse remain steadily at 71 or is that a day OR night temperature? Is your plant watered adequately/properly?

Insect infestations can cause stress for a plant. They have to use considerable energy resources to withstand and recover such attacks. My favorite method of mealybug control on citrus (and most other plants) is to keep a misting bottle filled with a solution of one part isopropyl alcohol to three parts water so that you can attack the critters on a regular basis. You can also go after them with a cotton swab dipped in the alcohol out of the bottle PLUS mist them.

Don't use the alcohol on bright, sunny days...or anything else, for that matter.