Chrysanthemum Stool Miner (Psila nigricornis) often confused with sciarid fly. 

Biology.
Adults are small black-brown shiny flies, similar and closely related to carrot fly.  They first appear outdoors during May-June and lay eggs in soil near chrysanthemum plants.  Eggs hatch in 2 weeks and the larvae feed for 1-2 months before pupating in the soil or in tunnels in plant material.  A second generation emerges in August- October, the larvae for these cause most damage. These feed for up to 4 months.

The problem is often worse on early flowering stools brought into the greenhouse for propagation.

Treatment.
A spray strength insecticide can be watered into the soil before lifting stools or after boxing up.

A watering with Imidacloprid would give control and protect the stools from further attack while controlling aphids, leaf miner and other pests as well.