Species of Cardinium are intracellular endosymbionts of arthropods. They are known to interfere with the mode of reproduction in some parasitoid wasp and mite hosts, and they are associated with parthenogenesis in the oleander scale, Aspidiotus nerii Bouché (Hemiptera: Diaspididae). Here, we survey A. nerii more broadly for presence of Cardinium species. In total, we tested 593 individuals in 301 populations, representing 132 identified species of armored scale insects, by using polymerase chain reaction (PCR) and sequencing of 16S rDNA. We obtained positive PCR results in 67 individuals of 34 populations, representing 20 species and one unidentified individual. Sequencing of these PCR products yields 20 new Cardinium haplotypes, raising the number of known insect-associated Cardinium haplotypes from 15 to 35. The new host-list elucidates the ecology of Cardinium, which in insects seems to be specialized on plant-feeding Hemiptera and associated parasitoid Hymenoptera. A phylogenetic analysis of all known insect-associated Cardinium 16S sequences reveals deep diversity in armored scale insects and implies horizontal transfer of Cardinium among armored scale insects and their parasitoids.