The following is information on identifying mealybugs in greenhouses, conservatories, and interiorscapes.
Citrus mealybug is recognized by a thin purple stripe tha t runs down the middle of the back. These mealybugs are densely covered with white wax and have very short tails.
Citrus mealybug. Photo from the United States National Collection of Scale Insects Photographs Archive, USDA Agricultural Research Service, Bugwood.org
Longtailed mealybug have seventeen pairs of white waxy filaments, if you care to count them, around the periphery of the body. In mature females there are two long tail-like projections on the rear of the insect that are longer than the length of the body of the insect.
Long tailed mealybug. Photo by David Cappaert, Michigan State University, Bugwood.org.
Madeira mealybug looks a lot like citrus mealybug. They are generally a dull gray color under the white wax and lack the single purple stripe on their back that citrus mealybug posses. Their egg sacs are longer and denser than citrus mealybug and male pupal cases may be found in equal numbers to the females. This is a very hard to control mealybug.
Madeira mealybug. Photo by Sally Tucker, Bugwood.org.
Descriptions from the February 6, 2009 edition of the Greenhouse TPM/IPM Bi-Weekly Report University of Maryland Cooperative Extension Central Maryland Research and Education Center.