How Cultural Factors Impact Fungus Gnat Populations

We have noticed that greenhouse operations vary greatly in the prevalence of fungus gnats. Why does one operation have few while a neighbor has swarms? Curious to determine what could be the reason behind this, we’ve conducted a series of experiments at Cornell University to look at how cultural factors impact fungus gnat populations. In this article, we’ll look at some of our findings and their implications for fending off fungus gnats in your operation.

Fungus gnats (Bradysia spp.) are among the most common greenhouse pests. Adults are small (⅛- inch) grey-black gnat-like flies with long gangly legs and transparent wings (Figure 1, see slideshow). Larvae live in the substrate and are transparent/white and legless with a shiny black head. They are are ¼-inch long or less. These gnats are often more than just a nuisance. Larvae feed on roots, sometimes to the point of weakening or killing seedlings. Both larvae and adults can transmit root diseases such as Fusarium and Thielaviopsis.

 

 

Substrate Amendments
Ever since the 1960s, peat, perlite and vermiculite have been primary substrate components. Lately, there has been increased interest in alternative substrate components and organic sources of fertilizer, such as compost, vermicompost, blood meal, kelp and composted poultry litter. These components may have several plant growth benefits, including providing more nutrients, increasing the water-holding capacity of the substrate and encouraging the microbial community in the substrate. Fungus gnats are well-named in that the larvae need microbes such as fungi in their diet. And female fungus gnats are attracted to microbial activity as a place to lay their eggs. As microbes can feed on the organic components in the substrate, their increased abundance could result in higher fungus gnat populations.

We conducted experiments at Cornell University with several conventional and organic substrates to determine their attractiveness to adult fungus gnats and to see if fungus gnats reproduced in higher numbers in some substrates than others. The list of substrates tested is in Table 1.

 

In our experiments, we watered pots filled with each of the substrates to container capacity and randomly placed one pot of each mix in eight circles (replicates) on a greenhouse bench (Figure 2, see slideshow). We released at least 80 adult fungus gnats in the center of each circle daily across five days. Small (2×2-inch) clear sticky cards were placed on the surface of each pot to trap fungus gnat adults attracted to that substrate. Sticky cards were replaced daily for five days. The number of captured fungus gnats was summed for each container across all five days.

Interestingly, we found that none of the substrates we tested were more attractive than the others to fungus gnat adults during this five-day period. Regardless of substrate, we caught a total of about seven fungus gnat adults per container.

We next wanted to see if certain substrates produced more fungus gnats (eggs laid that survived to emerge as adults) than others. To answer this question, we used the same pots that were used in the adult attraction experiment, because there were no differences in numbers of fungus gnats attracted to any mix. Following the five-day adult release period, the pots were covered with hairnets to prevent adults from laying additional eggs in the pots and to prevent escape of emerging adults. A small yellow sticky trap was placed inside each pot to capture adults that emerged from the mix. Sticky traps were replaced as they became covered with fungus gnats.

By the end of the six-week period, there were large differences in the number of adults that emerged based on substrate (Figure 3, see slideshow). On average, more than three times as many fungus gnats emerged from the organic substrates than the two conventional substrates. Since all substrates originally attracted the same number of adults, the organic substrates either led the attracted adults to lay more eggs in substrates with microbially active components or more of the eggs that were laid successfully developed into adults.

Substrate Water-Holding Capacity

We noticed that some substrates in our trial held more water than others with consistent watering. The soil moisture of the organic mixes was generally higher than in the conventional mixes, perhaps due to the use of compost and fewer aggregates (such as perlite) in some of them. In general, substrates that held more water produced more fungus gnats (Figure 4); however, this was not a hard and fast correlation with every substrate. Our assumption is that wetter substrates encourage the growth of the microbes on which the larvae feed.

Plant Attraction

Next, we were curious if fungus gnat adults were attracted to or reproduced in higher numbers in containers with certain plant species over others. To answer this question, we used the same methods as before, except we used a single conventional substrate (Lambert LM0111) in all pots but with different plant species in each pot, plus a pot that contained no plant as a control treatment. The plants used were four-week-old seedlings of pansy, dianthus, petunia and snapdragon.

We monitored fungus gnat emergence over six weeks as before. There was no difference in fungus gnat attraction, nor fungus gnat production, due to the plant species or the control treatment evaluated. So it appears that these plant species do not significantly influence fungus gnat attraction or production. Our findings suggest that container substrate and water content are more important concerns than crop species in terms of fungus gnat populations.

What Should A Grower Do?

Our results suggest that fungus gnats are not specifically attracted to organic substrates more than conventional ones, but that larger populations may develop from some organic substrates. This may be due to amendments and a wetter environment that favors microbe growth — a food source for developing larvae.

Does this mean that organic substrates should be avoided? We don’t necessarily think so. The advantages of using organic substrates may outweigh the possible fungus gnat problems. It does mean that growers need to be ready with a fungus gnat management program, regardless of growing medium. Keeping pots as dry as the plants will tolerate can reduce numbers. Our research also shows that growers can expect greater numbers of fungus gnats as greenhouse temperatures increase.

Beyond these cultural effects, fungus gnats can be managed by biological controls such as nematodes, Atheta rove beetles and Hypoaspis (Stratiolaelaps) mites as well as insecticides.

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2 comments on “How Cultural Factors Impact Fungus Gnat Populations

  1. Fungus gnats nearly killed my season last year. I start vegetables for sale as transplants for sale and for growing in my fields. I am an organic grower. I have been paying closer attention to excessive moisture in my peat/perlite/vermiculite/dolomitic lime mix and have started with the use of “Gnatrol”, a form of BT. So far so good, but populations seem to always increase as more and more cells and plants are started in the greenhouse. We’ll see.

  2. I have been pleased with a fair measure of control through the practice of spreading lime beneath greenhouse benches, thus raising the pH and therefore that environment becomes less hospitable for egg laying/larve development

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