The circular scale, Chrysomphalus aonidum is a tropical species, native to the Oriental region but it has been dispersed widely in tropical and subtropical parts of the world, mostly in association with the citrus industry. In more northern countries of its range, it occurs in greenhouses. In addition to citrus, it occurs on many other plants.
Life cycle and appearance of Circular scale
Female nymphs and adults have flat to moderately convex, circular scales up to 2 mm in diameter with a slightly raised, sub-central point which is sometimes pale. When the scale is lifted, the insect beneath is yellow and up to 1.7 mm long. The adult male is 0.7 mm long and has a single pair of wings. First-instar nymphs are 0.3 mm long and have legs, but soon settle to form circular, initially white scales.
Circular scales reproduce sexually. Adult females lay 50-150 eggs under the scale. Eggs hatch under the scale and the crawlers walk about to find a suitable feeding site before settling to a sessile lifestyle. The second-instar nymphs are the main feeding stage in both sexes. Development to adult takes 7-16 weeks depending on temperature.
Chrysomphalus aonidum has a preference for humid environments. In citrus, it tends to prefer the lower and central parts of mature trees. Circular scale prefers the leaves, but in high infestations it may spread to fruits, stems and trunks, and may cause yellowing of leaves, premature leaf and fruit drop, and stem dieback.
First signs of an infestation are dark-purple to reddish-brown or black spots with paler margins, on both surfaces of shaded leaves of the host plant.