Swain, A.F., & Duggan, E. 1928 Significance of mid-season units counts on resistant black scale.. Journal of Economic Entomology 21: 532-542.

Notes: In the resistant black scale area in the citrus belt of southern California, where the "unit system" is used to determine the degree of scale infestation, it has been applied only to mature scale in the spring. No method, other than one of estimation, has been used to determine this at any other season of the year. The writers have carried out observations throughout the past season to secure data on the significance of unit counts on immature scale. Their methods and observations are recorded herewith.