Plants affected
Aster novi-belgii types of Michaelmas daisy.
Symptoms
The most obvious symptom is poor flowering. Many blooms are
converted into rosettes of small leaves due to the mites'
feeding activities within the developing buds. Infested plants
are also stunted and have brownish scarring on the stems.
Cause
The whitish mites that cause this damage are less than 1mm long, and thus not visible without the use of a microscope. They feed by sucking sap inside the developing flower buds and shoot tips.
Control
Infested plants should be destroyed as none of the insecticides available to home gardeners are effective.
Other types of late summer-flowering perennial asters, such as Aster novae-angliae and A. amellus are more resistant, and can be planted instead. Alternatively, if Aster novi-belgii cultivars are required, plants should be obtained from a reputable nursery.