Greenhouse - Fungus Gnat Control on Poinsettias

The following is a good article on fungus gnat control on poinsettias from the University of Maryland.

Fungus Gnat Control on Poinsettias

As we move into mid October, those of you who chose to grow poinsettia this fall are keeping the temperature lower than normal to conserve on your heating costs. Cool, wet soils make ideal conditions for Pythium root rot. Keep in mind that fungus gnats play a role in spreading Pythium. Fungus gnat larvae feed on tender young roots, providing an entryway for pathogens. Plants with succulent stems such as poinsettias, geraniums, sedum, and coleus are especially prone to injury and suffer serious losses.

To monitor for larvae, place potato wedges on the soil surface and check after 48 hours. First look at the growing media under the potato, and then on the potato wedge itself. The larvae will be clear to white colored and have a black head capsule.

Cultural Control

If you keep the plants on the dry side, fungus gnat populations will have a harder time surviving.

Biological Control

Bacillus thuringiensis var. israelensis, sold under the trade name of Gnatrol, is most effective against the young first instar larvae. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insect's gut. The larvae stop feeding and die. Two or three applications at high rates may be needed to provide effective control. Beneficial nematodes are effective in controlling fungus gnat larvae. Steinernema feltiae (ScanMask, NemaShield, Nemasys or Entonem) can also be applied as a drench treatment against fungus gnat larvae. After entering the host insect through various openings, the nematodes multiply and release a bacterium whose toxin kills the target insect. Fungus gnat larvae are killed in one to two days. The nematodes will then exit the dead body and search for new hosts to infect.

Insect Growth Regulators

Insect growth regulators (IGR’s) are most effective against the young developing larvae and will have no direct activity against adults. Commercially available IGR's that can be used on poinsettia include Azatin XL, Aza-Direct, Ornazin, and Distance. Distance it is labeled for fungus gnat and shore fly larvae, but warns that there may be malformation of newly expanded leaves on certain varieties of poinsettia including ‘Freedom Bright’, ‘Freedom Bright Red’, ‘Winter Rose’, and ‘Jingle Bells’. The label also states do not apply after bract formation.

Chemical Control

DuraGuard ME (chlorpyrifos) is a microencapsulated product that may be applied as a soil drench or foliar spray for controlling larvae and adults.

Fungus gnat larvae. Photo from the 10/10/08 Greenhouse TPM/IPM weekly report from the University of Maryland.

Reprinted from the October 10, 2008 edition of the Greenhouse TPM/IPM Weekly Report from the University of Maryland Cooperative Extension, Central Maryland Research and Education Center.