Fungus gnats, Bradysia spp., are important insect pests in greenhouses. The fungus gnat life cycle consists of an egg, four larval stages, pupa and adult. Fungus gnat adults typically live 7-10 days, flying near the growing-medium surface and causing minimal direct plant damage. Damage to plants mainly occurs from larvae feeding on root tissue in the growing medium. Fungus gnat larvae depend on various bacteria and fungi, including yeasts, as a food source, and the food source available to fungus gnats determines abundance and fitness of mature adults and the reproductive capacity of females.
Fungus gnats feed on a wide range of ornamental plants, including poinsettia, gerbera daisy, gloxinia, cyclamen, impatiens, salvia, geranium, etc. Seedlings and young plants are particularly susceptible to injury from larval feeding. Fungus gnats are most commonly found in moist environments, such as those during propagation and plug production in greenhouse operations. In addition to direct plant injury through feeding, fungus gnat larvae also vector soil-borne pathogens.
As growing-medium components become less uniform, porosity of the growing medium increases, resulting in a higher number of open spaces on the growing-medium surface. These spaces provide more sites for female fungus gnats to lay eggs. Fungus gnat females tend to lay eggs in crevices, which are a more humid environment, resulting in higher egg survival rates compared to eggs that are laid directly on the surface.
Growing media that have a high degree of microbial activity are ideal for fungus gnat breeding. Additionally, growing media with high levels of organic matter tend to have larger pore spaces, and fungus gnats tend to lay eggs in growing media with large pore spaces. Any differences in fungus gnat egg-laying preferences among growing media may be associated with the species and activity of microbial colonies in the growing medium.
Sphagnum peat moss is used as a component in commercial growing media due to the high water-holding capacity, good aeration properties and resistance to decomposition. Coconut coir is a by-product of the outer shell of coconut fruit and has been marketed as a substitute for sphagnum peat moss, due to their similar characteristics. These same desirable characteristics that work well to produce plants may also enhance the development of fungus gnat populations. Hardwood bark (nursery-mix) decomposes during the growing season, which increases the water-holding capacity, and enhances the possibility of promoting high fungus gnat populations.
There is some evidence that growing medium choice can influence fungus gnat populations with fungus gnats tending to prefer growing media containing peat moss that is abundantly moist. The purpose of this study was to determine whether female fungus gnats, Bradysia sp. nr. coprophila, when given a choice under laboratory conditions prefer certain growing media in which to lay eggs.
Fungus gnats used in this study were obtained from a laboratory colony started with flies from the growing medium of a potted geranium in a commercial greenhouse. Á The colony is maintained in moist growing medium supplemented with shredded potato and oatmeal.
Fungus gnat pupae were collected from a 10- to 12-day-old fungus gnat colony, in which adults had just started to emerge, by sieving 150-200 grams of growing medium. Growing medium was rinsed through a sieve with pupae and growing medium collected on a sieve and rinsed into a 6-inch petri dish. Pupae were then harvested and placed into a beaker with filter paper moistened with deionized water. Three to four pieces of hardwood bark chips were placed in the beaker to provide dry landing sites for fungus gnat adults once they emerged, because excessive moisture in the beaker produced condensation that could trap and kill the fungus gnat adults.
The beakers were covered with plastic wrap, secured with an elastic band, and placed into an environmental growth chamber set at 72-79° F, 50- to 60-percent relative humidity and 24:0 (light:darkness) hour photoperiod. Beakers were checked every 12 hours for adult fungus gnat emergence.
As adults emerged, they were removed using an aspirator and placed into plastic vials where adults were sexed and allowed to mate. Mated pairs were then placed into another vial with a piece of filter paper moistened with deionized water. The use of moistened filter paper was essential in making sure the fungus gnats survived.
After the male was removed, the female fungus gnat was placed into an experimental chamber. The experimental chamber consisted of four 21?2-inch glass petri dishes inside a 6 x 6 x 2-inch plastic container. Each experimental chamber had nine ventilation holes. Fifty experimental chambers were used; each contained three petri dishes with different growing media and a petri dish with filter paper moistened with deionized water (control), for a total of 200 petri dishes. There were 50 replications for each growing medium and filter paper control. The three growing media used in the study were Metro-Mix 560 with Scotts Coir (The Scotts Company LLC), Sunshine LC1 Mix (Sungro Horticulture, Inc.) and Universal SB 300 Mix (Strong-Lite Horticulture Products). The glass petri dishes were filled with each of the three growing media or filter paper. The growing media and filter paper were saturated with deionized water, and any standing water was decanted. Wet growing-medium weight was determined before the start of the experiment.
Petri dishes were arranged in a completely randomized design. A newly enclosed female fungus gnat (less than 24 hours old) was released by gently tapping the vial to ensure she was not near the lid. The vial lid was removed, and the vial was quickly inverted into the center of the experimental chamber. The vial was allowed to remain in place until the female was on the floor of the experimental chamber (approximately 3 minutes), after which the vial was removed and the experimental chamber lid was tightly sealed to prevent the adult from escaping. No female fungus gnats escaped during this procedure. The experimental chamber containing the released female was placed into a growth chamber. The chamber was set at 72-79° F, 50- to 60-percent relative humidity and 24:0 (light:darkness) hour photoperiod.
After 48 hours, the growing media were processed using a flotation/extraction technique. The filter paper was rinsed, using deionized water, through a sieve, and eggs were collected on a sieve. Samples were rinsed into another set of petri dishes and stored in a refrigerator set at 29-38° F for 5-7 days, until they were counted using a dissecting microscope.
The experiment was set up as a completely randomized design, with each experimental chamber representing a replication. Data were analyzed, and means were separated using a randomization test.
There were no significant differences among the three different growing media in the number of eggs laid by female fungus gnats; however, all three growing media were significantly different than the control (see Figure 2, above). The overall mean number of eggs laid per female across the experimental chambers was 104.9 ± 5.7 (mean ± standard error).
The three growing-media moisture contents were significantly different from each other and were also significantly different from the filter paper control (see Figure 3, page 44). Universal SB 300 Mix had a higher moisture content than Metro-Mix 360 and Sunshine LC1 Mix. The moisture content of the growing media ranged from 18-20 grams, the filter paper had a moisture content of 0.9 grams (see Figure 3, page 44).
Growing-medium type, based on the components, may be a factor that determines where female fungus gnats will lay eggs. There was no significant difference among the growing media tested (Metro-Mix 560, Sunshine LC1 and Universal SB 300) in the number of eggs laid by the female fungus gnat. However, there is conflicting data on the effect of coir on fungus gnat behavior. Several studies have found growing media containing coir repels fungus gnats, whereas other studies have demonstrated that coir has no repellent activity against fungus gnats.
In our study, coir did not appear to have any repellent activity nor was it more attractive to female fungus gnats than either Sunshine LC1 or Universal SB 300; however, it should be noted that fungus gnat females tended to lay eggs more often in Metro-Mix 560 (86 percent) than Sunshine LC1 (66 percent), Universal SB 300 (52 percent) or filter paper (18 percent), which may suggest a certain level of preference by female fungus gnats.
Growing media with high moisture levels are generally microbially active and oftentimes produce an abundance of fungal growth that is attractive to female fungus gnats for egg laying. Moisture measurements were taken prior to releasing fungus gnat females. This was done to determine if fungus Á gnat females preferred a certain growing medium but avoided confounding effects of moisture. The moisture contents of the three soilless growing media needed to be similar initially in order to prevent biasing the female to a particular growing medium. The moisture content of the three growing media, although statistically significant based on the analysis, may not have been realistically significant with only a difference of 1.7-2.1 grams (see Figure 3, above). Similarities in the moisture contents among the growing media likely removed the possibility that moisture content was responsible for the number of eggs being laid in the different growing media. It should be noted that fewer eggs were laid by female fungus gnats in Universal SB 300 (see Figure 2, page 41), the growing medium with the highest moisture content (see Figure 3, above), which suggests there may be other factors involved that influence egg laying.
Research designed to measure the effects of different moisture levels on the attractiveness of growing media and determine which moisture contents fail to support fungal growth may prove useful in managing fungus gnat populations in greenhouse production systems. In addition, the practice of pasteurizing growing media may reduce fungus gnat populations by killing the larvae that may already be present. It is possible that pasteurized growing media may contain lower levels of fungal growth than unpasteurized growing media, which means these growing media may be less able to sustain fungus gnat populations.
This study has demonstrated, based on the choices of growing media provided under laboratory conditions, that female fungus gnats may not prefer a specific growing medium for egg laying. However, fungus gnats may not depend on the growing medium components alone in the decision to lay eggs but a combination of factors. Studies are needed to assess other aspects of growing media that may influence egg laying, such as the impact of moisture on attractiveness and the effect of volatiles emitted from growing media. A better understanding of fungus gnat behavior as it relates to survival and reproduction in different types of growing media with variable moisture contents and microbial activity may increase the likelihood of effectively dealing with this pest. This may lead to effective management strategies that reduce problems with fungus gnats in greenhouses.