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Greenhouse “Bake-Out” to Control Fungus Gnats

If you are going to do a bake-out anyway for sanitation of the greenhouse, there may be some benefit in putting your new pots of soil in the greenhouse at the time of the bake-out.

At our research greenhouses in the Department of Horticulture & Landscape Architecture, we often observe fungus gnat adults as quickly as 10-14 days following potting into fresh, bagged commercial soilless media. Since the life cycle from egg to adult takes 20-26 days, this observation suggests that the immature forms were present in the bagged media when purchased. Anecdotal reports from other research institutions agreed with our observations, and it was documented by at the University of Illinois in a report published in 2004. Another explanation for the infestation at our greenhouses, in cases where the plantings were rooted cuttings rather than seed, was that the fungus gnats were on the roots of plants prior to transplanting into the fresh soil mix.

A routine part of our sanitation protocol in our glass greenhouses is to heat up the room as high as we can achieve for a period of 2-3 days. We can typically reach 50-55°C (122-131°F) during daylight hours. The goal of this “bake-out” is to kill insects and disease spores either through direct exposure to the heat or by drying up all water sources that support them. It has not appeared to damage any equipment in our glass greenhouses, but it is important to note that this practice should never be done in a polyethylene structure. Another part of our production routine for large studies, such as our poinsettia cultivar trial, is to fill pots prior to planting cuttings in order to ensure that we can plant all the cuttings on the same day under uniform conditions. We wondered if placing the pre-filled pots in the greenhouse during the bake-out would kill fungus gnats present in the bagged mix. No plants were in the exposed pots, of course: it was a pre-plant heat exposure to the containers of media, simulating an oven treatment on a greenhouse-size scale.

Treatments

All treatment groups consisted of twenty-five, 6.5-inch azalea style pots of a commercial soilless media. Since poor storage techniques are a likely cause of fungus gnat infestation, we chose bags of mix that had been stored outdoors of our greenhouses for about 2 months, July and August. We applied the bake-out treatment to pots that had been wetted versus pots that had not been wetted. The bake-out treatment was performed in an empty, clean greenhouse where scouting had indicated no fungus gnats present. For a control group, we sealed a group of pots in Ziploc bags immediately after filling and wetting, to see if we could confirm the presence any fungus gnat in the manufacturer’s bag. We also placed sets of pots, both wet and dry, into a greenhouse with growing plants infested with fungus gnats to verify their movement to the new pots. These pots were exposed to the infested greenhouse for 7 days before being sealed.

Heat Exposure Methods

On Friday afternoon 9/7/2007, we turned on heating equipment and disabled cooling in a glass greenhouse room. Vents were closed and shade curtains retracted. Our goal was to achieve a soil temperature of 51.7°C (125°F) for thirty minutes, as was suggested as a killing temperature for fungus gnat by one university entomologist in personal communication. Due to cool weather, temperatures in the greenhouse on 9/8 reached only 43°C, so no further measurements were taken. However, we were successful on 9/9 in sustaining a soil temperature above 51.7°C for about ninety minutes. Conditions inside and outside the greenhouse on that day were as follows:

Outdoors:

Maximum outdoor air temperature: 29°C (84°F)
Average day outdoor temperature: 25 (80)
Average night outdoor temperature: 18 (65)
Maximum light intensity: 1660 µmol/m2/s

Greenhouse:

Maximum air temperature = 58°C (137°F)
Minimum air temperature = 41 (105)
Day average air temperature = 47 (117)
Night average air temperature = 41 (106)
24 hour-average air temperature = 45 (113)
Maximum soil temperature of wetted media = 52 (126)
Maximum soil temperature of dry media = 46 (115)

Measuring Fungus Gnat Population

To count fungus gnat larvae, we placed a potato wedge, roughly 3-cm square, on the surface of each pot. To measure emerging adults, we placed a yellow insect monitoring card on the soil surface, cut to roughly the same 3-cm dimension with its sticky surface exposed. Pots were then sealed in a Ziploc bag. Larvae were counted 2 weeks after treatment.

Results

The set of “control” pots that were not exposed to heat and were sealed immediately had a single pot with 12 larvae found under the potato wedge, giving the set an average of 0.5 larvae per pot. No fungus gnat larvae were captured in any of the pots of wetted media exposed to the bake out. The media that had not been wetted prior to the exposure resulted in excessive drying of the media. We discontinued these dry pots in the study since the method would have precluded growing plants. The set of wetted pots placed in the greenhouse where fungus gnats were present resulted in an average of 2.1 larvae captured per pot. The dry pots under similar treatment yielded an average of 0.9 larvae per pot. No adult fungus gnats were captured on the yellow monitoring cards in any of the treatments by day 14.

It is difficult to confirm if the fungus gnats observed in the single pot of fresh soil mix were from contamination in the bag, or if a fungus gnat penetrated the methods of isolation and sealing to lay eggs in the media after potting. In personal communication with an entomologist who understood the precautions that were taken, we lean toward the commercial bag containing the fungus gnat immatures. This would then suggest the bake out worked since none were found in the treated pots, but a bigger experiment would be required to confirm these results, or an experiment that could ensure the presence of the fungus gnat in the soil prior to the heat treatment.

To no one’s surprise, fungus gnats easily found and laid eggs in pots placed in an infested greenhouse. It is interesting to note that this happened even in pots not wetted. As some of my colleagues have pointed out, this kind of infestation is a far more likely and realistic scenario than having fungus gnats in the unopened bags of soilless mix. A preventative program of biological control for the gnats [also see Atheta coriaria and Hypoaspis miles] may be better time spent than arranging for a bake out. Bottom line: if you are going to do a bake-out anyway for sanitation of the greenhouse, there may be some benefit in putting your new pots of soil in the greenhouse at the time of the bake-out, as long as the plants are wetted beforehand and the glazing and equipment can handle such extremes.

About the Author

Rob Eddy is the Plant Growth Facilities Manager for the Horticulture & Landscape Architecture Dept, Purdue University, West Lafayette, Indiana.

Article References

• Cloyd, R. A., and Zaborski E. R.. 2004. Fungus gnats, Bradysia spp. (Diptera: Sciaridae), and other arthropods in commercial bagged soilless growing media and rooted plant plugs. Journal of Economic Entomology 97(2): 503-510.
• Price, J. F., et. al. 2005. Management of Fungus Gnat in Ornamentals. University of Florida IFAS Extension Bulletin ENY912.

Filed under Best Practices, IPM Savvy —

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