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Fungus Gnats (Sciaridae)

 

Substrates that contain a high level of microbial activity and abundant organic matter such as coir and peat attract fungus gnats. For this reason, fungus gnats are a common problem when growing in coir.

 

Adult Fungus Gnats are small, delicate bodied, long legged mosquito like insects that commonly develop in organic growing mediums.  The adults don’t damage plants per se. Their larvae, however, while feeding mainly on decaying plant material and fungi also feed on healthy plant roots and tunnel into stems of young cuttings and seedlings. Therefore, the larvae cause plant damage by feeding on the roots, thus interfering with the ability of plants to uptake water and nutrients, which results in wilting and stunted growth. Significant root damage and even plant death has been observed where high populations of Fungus Gnat larvae are present.

 

Additionally, larvae feed on fungal root zone pathogens and are capable of directly transmitting these pathogens, including Pythium spp., Fusarium spp., and Verticillium spp., from diseased to non-infected plant’s.

 

Fungus Gnat adults selectively deposit their eggs on the fungal mycelium. Phytopathogenic fungi such as Botrytis cinerea, Fusarium species and Phoma betae are preferably used as egg laying sources. Conversely, beneficial fungi such as Trichoderma are non-preferred nutrient sources, albeit fungus gnat larvae will also feed on Trichoderma and other beneficial fungi species to some extent.[1] Fungus Gnat adults are capable of carrying the aerial conidia of certain foliar and soil-borne plant pathogenic fungi such as Botrytis cinerea Pers.:Fr., F. avenaceum, F. acuminatum, T. basicola, V. dahliae Kleb., and V. albo-atrum, which can then be transmitted to healthy plants. The potential for both adult and larvae of Fungus Gnats to transmit disease means that the tolerance level for the presence of this pest may be very low, which is similar to other pests that vector diseases.[2]

 

The Fungus Gnat life cycle consists of eggs, four larval (instar) stages, pupa, and adult. In total, 21- 40 (typically 21-27) days, dependent on substrate and air temperature, are necessary to complete the entire life cycle. The adult lays its eggs in the substrate and the larvae inhabit the upper 3cm of the substrate before pupating and emerging from the substrate as adults.[3]  The egg laying Fungus Gnat adults have a very short life cycle (7-10 days); however, in this time they can lay hundreds of eggs which hatch as larvae in five to six days. The larvae will feed on any organic matter present, including roots, for 10 to 14 days. See following image that shows the lifecycle of the Fungus Gnat.

 

 

Fungus-gnat-lifecyle

 

 

Because the Fungus Gnat life cycle is very short and because female adults can lay so many eggs over a short space of time a Fungus Gnat population can quickly explode under the right conditions.

 

Fungus Gnats thrive in warm, wet, nutrient rich, organic environments. This environment is readily supplied in indoor hydroponic grow rooms where organic media is used.  Because Fungus Gnats are attracted to organic media, infestations are a common problem for coir growers. For this reason, it is imperative that coir growers understand how to identify and combat this all too common coir loving pest.

 

 

Fungus Gnat Identification

 

Adult Fungus Gnats are about 2–5 millimeters long, gray to black, slender, small mosquito like flies with long legs and antennae, and one pair of wings. The adults are considered to be weak fliers and they often appear to dance about – flying in an irregular pattern – while taking short flights. Because adult Fungus Gnats do not fly well they tend to reside near the surface of the growing media.

 

Larvae are tiny (hardly visible to the naked eye) and have a shiny black head and an elongated, whitish-to-clear, legless body.

 

 

fungus-gnat-adult

Fungus Gnat Adult

 

 

Yellow Sticky Traps are Imperative in ANY Grow Room

 

Yellow sticky traps act as the best early warning system for trapping and identifying adult Fungus Gnats. These traps are supplied through hydroponic stores and are cheap to purchase. Fungus Gnat adults are attracted to the colour yellow and when they land on the traps – which are covered in a thick glue like substance – the gnats become stuck. When large amounts of Fungus Gnat adults are present in the grow room the traps will literally be speckled with hundreds of gnats that have become stuck to the surfaces of the traps.

 

Studies have shown that sticky traps laid flat on the media surface capture about 50–60% more adult Fungus Gnats than cards placed vertically. Because sticky traps act both as an early warning system and as an efficient means of mass-trapping adult females, thus reducing the number of larvae in the next generation, I tend to place the traps both vertically and horizontally, at media level (horizontal on top of the substrate and vertically at the top of pots, underneath the plant canopy) in my grow room. Traps should be monitored daily to ensure the presence of Fungus Gnats in the grow room is identified early.

 

As a rule, the more adults that are trapped daily the higher the population of Fungus Gnat larvae in the substrate. Therefore, if the numbers of adults trapped daily increases on a daily basis (e.g. 5 one day and 10 the next day) this probably indicates the population is increasing quickly and measures should be taken to control the population.

 

An effective way to detect the presence of Fungus Gnat larvae is to insert slices or wedges of potato onto the surface of the coir. Slice raw potatoes into about approximately 1-inch by 1-inch by 1/4-inch pieces. Place the slices next to each other on the surface of your potting media to attract Fungus Gnat larvae. Larvae will migrate to the potato and start feeding within a few days. The potato slices should be turned over to look for larvae present on the underside.

 

Prevention and Control

 

A point I stress again and again in IH is that prevention is a far better practice than cure. That is, by the time you identify a problem, plant health and growth may already have been impacted. This, among other things, applies to pest control. Therefore, practices that prevent Fungus Gnats from taking hold in a crop should take priority over eliminating them once high numbers are present.

 

The first line of defense against Fungus Gnats comes down to screening inlet air (to keep Fungus Gnats found outdoors out of the grow room environment), grow room moisture management and sanitation practices. Where clones or seedlings are obtained from outside sources these should be quarantined from the grow environment and treated with an insecticidal drench two or so days before being introduced into the grow room.

 

Fungus Gnats are primarily a problem under conditions of excessive moisture. Maintaining nutrient temperatures at ideal levels, the use of well-drained growing media and not over-watering plants may help avoid issues with water-borne plant pathogenic fungi which act as the food source for Fungus Gnats, reducing the possibility of disease transmission by the larvae and adult. Additionally, the accumulation of water and presence of algae may lead to abundant populations of Fungus Gnats. This means keeping the nutrient run off tank/reservoir tightly sealed, cleaning up any spills, and keeping surfaces dry at all times.

 

It is also important to remove plant material and growing medium debris immediately from the grow room as Fungus Gnat adults may emerge from disposed growing medium or plant debris and migrate onto the crop.

 

Keep Insects Outdoors by Filtering the Air that is drawn into the Grow Room 

 

Screening inlet air with HEPA filters is an ideal means of filtering out all insects and high degrees of airborne microbial contaminants (bacteria, fungi, viruses). This said, placing any filter over an intake port or inlet fan works well as long as the filter is fine enough to act as a barrier against pests.

 

 

Contaminated Substrates

 

It is important to note that one potential (and not necessarily rare) source of Fungus Gnat infestation is contaminated substrates that are purchased through garden supplies or hydroponic stores. This is important to understand because even where best practice (such as screening and sanitation) is employed, Fungus Gnats may find their way into the grow room through contaminated bagged products of coir.  For this reason, always purchase a reputable brand of coir which is produced to quality assurance standards (e.g. RHP standard). These products in some cases undergo sterilization/pasteurization during manufacture which kills any living biological matter (e.g. insects, larvae, bacteria and fungi) in the substrate prior to bagging. In other cases high quality production standards are applied to ensure a quality, albeit biologically active, non-sterilized product. For example, Canna claims to have taken the latter non-sterile route so as their product can, among other things, maintain viable Trichoderma which is naturally occurring in coir.

 

However, there is the potential, no matter what the treatment or quality assurance standard that Fungus Gnats could have laid eggs in a bagged coir substrate during shipping or storage where entry, through even the smallest holes in a bag, can be gained by adult Fungus Gnats. Thus, contamination can potentially occur along the supply chain (i.e. while the product is stored with a producer/supplier, wholesaler or retailer). Not to be alarmist but it is something coir growers need to be aware of.

 

If you are concerned about the possibility of a bagged coir substrate product being a potential source for a Fungus Gnat infestation, one option is to treat the coir before it enters the grow room. This really comes down to drenching the coir with the pesticide/insecticide spinosad prior to taking it into the grow room. I cover how to do this on page…..

 

Control – Dealing with a Fungus Gnat Infestation

 

Okay, so you have filtered inlet air and been fastidious about grow room sanitation and you still have a Fungus Gnat infestation. Don’t panic… dealing with them is relatively easy!

 

Because Fungus Gnat larvae are of the greatest concern, due to causing direct root damage, and because most of a Fungus Gnat’s life cycle is spent as a larva, the most effective control method targets larvae rather than attempting to directly control the mobile, short-lived adults.  Therefore, substrate drenches that target the larvae prove to be the most effective means of controlling a Fungus Gnat infestation.

 

Time of Application Considerations when Choosing a Control Option/Product/Approach


If you identify the presence of Fungus Gnats, how you combat them (i.e. insecticide choice etc) should be done with careful consideration. This applies to which point of the crop cycle you are at (i.e. time before harvest), and how severe the infestation is. Key factors that need to be considered are the withholding periods of the drench/insecticide that you use and its toxicity potential to the end consumer if trace amounts remain residual in the harvested product.

What I will do now is outline two different approaches I take – one using spinosad, an OMRI listed (organically certified) agrochemical product, and a second option using a combination of predatory nematodes (Steinernema feltiae) and neem oil – the latter being for the die hard “I would never use an agrochemical insecticide no matter how safe they claimed it was” ‘hydro’ growing crew.

Further to this, it is important to note that regardless of its OMRI organic listing, Spinosad (option #1 for controlling Fungus Gnats) is not permitted for use on medical marijuana products as outlined by Washington State regulators (here and here)

This, I have been informed, really comes down to labeling compliance regulations. Basically anything not FIFRA (The Federal Insecticide, Fungicide, and Rodenticide Act) exempt, or that does not contain language on the label that allows for use on “other, not listed” crops, is not permitted. In other words the labels currently used on Spinosad products are too specific regarding what crops it is labeled for use on, without actually stating cannabis.  I expect at some point, Spinosad may be labeled according to meet codes that will allow for its use with medical marijuana. However, until then be aware that if Spinosad residues are found in lab ‘Medicinal’ cannabis tests this will result in a fail re pesticide compliance. 

For this reason, Spinosad should not technically be used in the production of “medical” marijuana and option #2 should be used by “Med” growers.

 

Option #1

 

Spinosad Drench

 

Spinosad, produced by Dow AgroSciences, is technically an organic (considered nonsynthetic) product and certain formulations are listed for use by the Organic Materials Research Institute (OMRI) for organic use in the US and various other countries.  Due to its low effective use rate, safety to the environment, safety to mammals, and safety to beneficial insects, spinosad was registered under the US EPA’s reduced risk program. Spinosad was also awarded the Presidential Green Chemistry Challenge Award in 1999.

 

Spinosad is produced by aerobic fermentation of the actinomycete (bacteria) species Saccharopolysora spinosa. Spinosad contains two chemicals, spinosyn A and spinosyn D. These are crystalline solids with low odor, no volatility, and with low water solubility. The half-life of these compounds on a plant leaf is about 2-16 days. Spinosad is slowly and poorly absorbed through the skin. Dermal exposures in the rat for 24 and 120 hours resulted in only 1 and 2% absorption, respectively. Spinosad, which is absorbed via the oral or dermal routes of exposure, has been found to be rapidly metabolized and eliminated from the body. For example, 95% of the spinosad residues in rats are eliminated within 24 hours (U.S. EPA). Trace residues of spinosad that may be absorbed from food or water by terrestrial and aquatic organisms have been found to be readily metabolized and excreted and, as a result, spinosad and its metabolites do not accumulate in living tissues. Spinosad elimination half-lives of around 4 days are observed in fish. Basically, spinosad has very low toxicity to non-target organisms (e.g. humans) and by agrochemical standards is the safest choice of insecticide that, arguably, can be used.

 

Spinosad is shown to have high efficiency when dealing with Fungus Gnat larvae. For example, one study in an insecticide non-resistant cohort showed spinosad, trichlorphon, deltamethrin, spintoram, permethrin, and malathion had the greatest effect on Fungus Gnat larvae populations respectively (i.e. spinosad provided the most effective control) . [4] Spinosad is a fast-acting material that acts on the insect primarily through ingestion, but also by direct contact. It activates the nervous system of the insect, causing loss of muscle control. Continuous activation of motor neurons causes insects to die of exhaustion within 1-2 days.

 

When used as a drench in low CEC substrates’ such as rockwool or expanded clay, spinosad has systemic properties and is uptaken and distributed throughout the plant, offering protection against pests such as whitefly[5] and spider mite[6]  for up to 30 days after treatment. One author concluding that apparently, spinosad has systemic properties and quantities as low as 1 mg/plant could protect tomato plants from mite infestation.” This author also concluding that the persistence (withholding period) of systemically applied spinosad to tomato was up to 45 days.[7] However, what’s interesting about this is that when spinosad is applied to substrates with varying percentage of clay and organic matter these systemic properties aren’t apparent, or are greatly reduced. An immediate explanation for this is that organic matter, due to its CEC properties, binds spinosad making it unavailable for plant uptake. While more research is needed, given coir is an organic substrate and has moderately high CEC this possibly means that very little spinosad is uptaken by coir grown plants and this reduces the possibility of spinosad residues being present in the harvested product.  Further, given the low levels used, low absorption rate, low toxicity and rapid metabolization by mammalian species this makes it a very efficient and safe option for killing Fungus Gnat larvae in organic substrates.

 

Use

 

There are several spinosad products/brands on the market (e.g. Conserve, Entrust, Monterey, Greenlight) with varying percentages of spinosad as the active ingredient.

 

For hobby growers who are producing on a small scale, the easiest way to go about things is to purchase small volumes, rather than having to store large amounts.  For this reason, I typically purchase Monterey Garden Insect Spray which contains 0.5% spinosad as the active ingredient. This is used at 4 tablespoons per US gallon when applied as a media drench. This converts in metric to 15ml/L or 75ppm of spinosad in the diluted drench solution.

Depending on your locale spinosad products are available under different brand names. For example, in Australia, Yates sells Nature’s Way Fruit Fly Control, Active: 0.24g/L spinosad (200ml) or Yates Success Natralyte Insect Control, Active: 1.0% spinosad (200ml). These are easily accessible options for Australian growers and would be used at 8 tablespoons per US gallon or 30ml/L (Nature’s Way Fruit Fly Control) or 2 tablespoons per US gallon (7.5ml/L) with Success Natralyte Insect Control. 

When mixing and applying the drench be sure to adhere to any safety warnings on the product label and wear gloves and eyewear. Mix with mains water and pH adjust to 5.8 before applying the drench to the substrate. It is important that you thoroughly drench the entire surface area of the substrate evenly to ensure all larvae come in contact with the spinosad (spinosad is most effective as a contact spray/drench). Drench the media evenly and well. Apply when the lights first turn off and leave in substrate overnight before flushing with a 1/2 strength pH adjusted nutrient solution an hour before the lights come on. Go back to your normal nutrient irrigation regime thereafter. What I also do is spray around the tops of the pots with a permethrin or deltamethrin based fly spray to kill adult Fungus Gnats that reside on the substrate surface.

Pest Resistance to Insecticides

It is important to note that because of the rapid reproductive rate of many pests a generation of many insects can take place in a few weeks and many generations can be produced in a single season or year. Repeated use of the same class of pesticide to control a pest can cause undesirable changes in the gene pool of a pest leading to another form of artificial selection, pesticide resistance. When a pesticide is first used, a small proportion of the pest population may survive exposure to the material due to their distinct genetic makeup. These individuals pass along the genes for resistance to the next generation. Subsequent uses of the pesticide increase the proportion of less-susceptible individuals in the population. Through this process of selection, the population gradually develops resistance to the pesticide. The faster the development rate of the pest species, the faster the pesticide resistance occurs.

For example, resistance to two organophosphorus insecticides chlorfenvinphos and primiphos-ethyl among Fungus Gnat populations has been reported in the UK where these chemicals were commonly used.[8] Similarly, studies have shown that some populations have become resistant to malathion and permethrin treatments due to the heavy use of malathion and permethrin which created resistance problems. Therefore, chemical drenches that are effective in one location (e.g. Australia) may not be as effective in another location (e.g. the U.S.). For this reason, three days after drenching I replace all the yellow sticky traps in the grow room with new ones and monitor these closely for the next few days to see if the treatment has been effective; i.e. spinosad will kill the larvae very quickly and adults will stop emerging from the substrate almost immediately.   As a result, the number of adult flies caught on the traps should be absolutely minimal to nil several days after drenching. If reasonably high numbers, 4-5 days after treatment, are still being trapped this indicates the drench has been less effective than desirable and another type of drench should be employed. To date, I haven’t found any resistance problems regarding spinosad use; however, it is something that you need to be aware of. For example, some years ago I used permethrin as a drench and found over time that the Fungus Gnat population was becoming resistant. The permethrin still worked reasonably well; however, more and more Fungus Gnats were surviving treatment. At that point I switched to another drench (spinosad) and this worked extremely well (100% control).

 

That’s pretty much it. You’re now up to speed on controlling a Fungus Gnat population with spinosad. Job done… let’s move on to option two.

 

Option #2 – Neem Drench and Foliar Spray + Predatory Nematodes

Okay so you’re a diehard anti-agrochemical pesticide proponent and you don’t like the idea of using spinosad no matter how safe it appears on paper.  It is an agrochemical after all…  What to do?

 

Well you have a couple of good botanical and biological pest control options. Firstly, you can reduce and control the population by applying a neem drench and foliar spraying with neem every three days, or you can combine this very safe organic treatment (neem) with predatory nematodes.

 

Neem Oil

 

The chemicals isolated from neem can be categorized into two groups: isoprenoids and non-isoprenoids . Non-isoprenoids are amino acids, carbohydrates, flavonoids and others, while isoprenoids contain compounds such as azadirachtin. The most important compound in neem for pest control is azadirachtin, which approximates the shape and structure of hormones vital to the lives of insects. The bodies of insects absorb the neem compounds as if they were their real hormones, and this blocks their endocrine systems. The resulting deep-seated behavioral and physiological aberrations leave the insects so confused in brain and body that they cannot reproduce and their populations plummet. Additionally, their feeding cycle is interrupted. Therefore, azadirachtin acts to break the feeding and breeding cycle in many species of insects, including fungus gnats.

 

One quite amazing feature of neem is that it presents low toxicity to many beneficial insects which prey on plant pests. What this means is that neem can be used in conjunction with fungus gnat predators such as Steinernema feltiae[9]. In fact, studies have shown that a higher degree of control is exhibited when neem and predatory insects are applied together.

 

It is important to understand that neem doesn’t provide the same level of control that spinosad does when used as a substrate drench for control of larvae.[10] However, when used correctly it will offer good control and reduce the Fungus Gnat population. Neem needs to be applied every 3 days as a drench. Additionally, spray the plants at the same time to target the adults, ensuring good coverage until run off. Pay particular attention to spraying the underside of leaves and spray thoroughly around the surface of the growing media where adult Fungus Gnats reside in large numbers.  Always spray plants when the lights are off to avoid plant burning.

 

Most hydroponic stores stock neem oil under various brand names. Neem products can vary significantly in quality and purity. Speak to your hydroponic supplier and ask him/her about product options and recommended usage rates etc.

 

As a tip, neem can leave the final produce with a bitter taste if applied too close to harvest. For this reason, cease drenching and foliar feeding neem at least a week before harvest.

 

Predatory Insects

 

Predatory controls, such as soil-dwelling predatory mite, Stratiolaelaps scimitus, a rove beetle, Dalotiacoriaria Kraatz, and the entomopathogenic nematode, Steinernema feltiae, are shown to provide a high degree of control in suppressing or regulating Fungus Gnat populations. For example, one study (2008) found that Steinernema feltiae (S. feltiae) gave 90% control of the third instar L. ingenua larvae when S. feltiae larvae were incorporated in the growing medium at the rate of 74 nematodes/cm2.[11]  An earlier study by Nickle and Cantelo (1991) reported 72–81% mortality to the second instar to fourth instar L. ingenua larvae where S. feltiae at the rate of 620 nematodes/cm2 was applied as a drench treatment.

 

It is important to note that the use of predatory insects is most successful where alternative plant protection strategies (moisture management, sanitation, repellent materials), and/or where compatible insecticides are implemented simultaneously in the case of an infestation.[12]

 

One highly successful approach here is to use neem as a media drench in conjunction with S. feltiae.  One study by Krishnayyaand et al (2002), showed neem at 5- 10 mL L -1 is compatible with the use of S. feltiae. The authors concluded that neem can be safely mixed at the field recommended concentrations with juveniles of S. feltiae for application. [13]

 

On this note, a Fungus Gnat predator I have found very effective is Steinernema feltiae; however, there are a few things growers need to be aware of when using this predatory nematode.

 

Steinernema feltiae (S. feltiae)

 

Predatory nematodes such as S. feltiae can be used to effectively control Fungus Gnats. However, there is a caution here re hobbyist/novice indoor growers. That is, often fungus gnat populations aren’t discovered by many growers until the population/infestation is very high and ordering biological controls can mean waiting several days for them to arrive, allowing the Fungus Gnat infestation to explode in the interim. This presents as a problem because the application of Steinernema feltiae soon after Fungus Gnats are first detected, while the population is relatively low, provides the best control. However, with the application of a neem drench and foliar spray to begin attacking the Fungus Gnat population, while you wait for your Steinernema feltiae to arrive, this presents as less of a problem. I.e. a neem drench and foliar spray will begin controlling the Fungus Gnat population and when you receive the Steinernema feltiae the population of Fungus Gnats is already under attack by neem. Because the use of neem and Steinernema feltiae are compatible you simply add the Steinernema feltiae to the substrate when you receive them.    

 

Steinernema feltiae control the Fungus Gnat larvae by infecting, feeding, reproducing inside the fly larva and ultimately killing the larva. Nematodes such as Steinernema felitae that infect Fungus Gnats can be ordered by telephone or ordered online. Additionally, some hydroponic stores will order them for you. They arrive in a plastic container, cooled by an ice brick during transport and should be kept in the fridge (not the freezer) until use (if ordering through a hydroponic store be sure to tell the supplier to store the nematodes in a fridge until collection). Nematodes must be used within two weeks of receiving them.

 

It is important to note that Steinernema feltiae are most active/effective when used in air temperatures of below 28°C. For example, Koller (2011) found that control of Fungus Gnats with Steinernema feltiae was most successful, with an efficacy of 69–90% at 24°C.[14] For this reason, ensure that your grow room environment is not overheated and that ambient air temperatures are conducive for Steinernema feltiae (<28°C). Additionally, what I would advise if dealing with a high population of Fungus Gnats is to apply the Steinernema felitae at 2-3 times the supplier recommended rate to achieve a large and stable population quickly.

 

A word of caution about a few commonly recommended Fungus Gnat control options 

 

Various Fungus Gnat control methods are recommended by growers on forums. However, some of these methods are shown to be largely ineffective in studies. For example, some recommend mixing diatomaceous earth (DE) into the substrate to control Fungus Gnat larvae.

 

DE is composed of ancient siliceous skeletonized diatoms, which remove the insect cuticle waxes, absorb oils and waxes on the outer cuticle, or disrupt the integrity of the insect cuticle resulting in extensive loss of water from the insect body. However, the use of DE relies on the insect to make direct contact with it, and while some larvae may come into contact with DE others may not. Further, when diatomaceous earth becomes moist, it loses any abrasive properties.[15] Thus, using diatomaceous earth in a moist substrate is an ineffective means to combat Fungus Gnats.

 

 

Others recommend the use of neem cake amended coir substrates. However, while this can prove to be an effective method for controlling Fungus Gnats, while the research is somewhat variable, several studies show that neem cake’s active constituent azadirachtin potentially disrupts beneficial microflora (bennies) and enzyme activity in soils and substrates.[16], [17], [18], [19] Therefore, while offering effective control over Fungus Gnats, neem cake amended coir may not provide a conducive environment for bennies such as Trichoderma spp. Other than this, the potential for azadirachtin to interrupt enzyme activity is not a good thing.

 

Another issue with neem cake is that it provides relatively high amounts of nutrients; i.e. neem cake contains more nitrogen (2-5%), phosphorus (0.5-1.0%), calcium (0.5 -3%), magnesium (0.3 – 1 %) and potassium (1 – 2 %) than farm yard manure or sewage sludge.[20] Neem cake also provides varying levels of micronutrients. What this means is that while neem cake is a good source of fertilizer for organic growing, its use in hydroponics, where the nutrients supplied to the plants can be highly controlled through ppm of each nutrient species in solution, is less than ideal where hydroponic nutrients are used.

 

 

Another method that is commonly recommended is to place sand or diatomaceous earth on top of the media to create a barrier, which is thought to interfere with the ability of adults to lay eggs and to stop adults from emerging from the substrate. However, studies have shown that placing diatomaceous earth or sand on the substrate surface has little effect on fungus gnat adult emergence or inhibiting females from laying eggs because these physical barriers contain small openings that allow larvae to pupate, and adult females to lay eggs.[21]

 

Yet others recommend the use of the bacterium Bacillus thuringiensis (BT). However, my own experiences with using BT and that of several studies are that, at best, BT offer only limited control and, at worst, are largely ineffective. One study that compared BT and predatory nematodes against the efficiency of pesticides concluded BT effectiveness would be reliant on the BT being applied before the Fungus Gnat populations build up and before overlapping generations develop.[22] In another study (2011) that compared the efficiency of Steinernema feltiae, neem oil and BT to control fungus gnats the author concluded that control with the nematode Steinernema feltiae was most successful, with an efficacy of 69–90% at 24°C air temperature. Azadirachtin (Neem-seed oil) could be an alternative under hot conditions (>28°C). Bacillus thuringiensis israelensis (BT), however, showed only a minor effect (1-51% efficacy).” [23]

 

Yet others state that hydrogen peroxide (H2O2) can be used for Fungus Gnat control. However, some issues present. These being; 1) H2O2 is a potent oxidising agent that attacks and breaks down all organic matter (e.g. Fungus Gnat eggs and larvae) including the roots of plants. This means that when used at too high levels it can lead to root damage/burning; 2) H2O2 is also uptaken by plants. Studies show that when H2O2 is applied to soils endogenous levels of H2O2 increase in the plant tissue. This can lead to phytotoxicity when application rates are excessive[24]; 3) H2O2 products can vary widely in composition, which effects the required dilution rates; 4) H2O2 reacts strongly with organic molecules rendering its oxidising potential ineffective in a short space of the time. For example, one study showed that 10 g·L-1 of peat reduced the amount of H2O2 and peroxyacetic acid from activated peroxygens by 33% and 50% respectively after 4 hours contact time.[25] There would therefore be some concerns about whether, 1) Fungus Gnat larvae would be exposed to enough oxidising agent (ppm in solution/substrate) for enough time to ensure high mortality rates (i.e. effectiveness is reliant on time of exposure and the levels of hydrogen peroxide the Fungus Gnat larvae are exposed to) and; 2) whether the exposure time and the levels of H2O2 required to achieve a high mortality rate would not also prove damaging to the crop through root burning and/or phytotoxicity.

 

Bottom line on H2O2 … my advice is that more reliable Fungus Gnat control options exist.

 

References

[1] Kühne, F. and Heller, K (2010) Sciarid fly larvae in growing media – biology, occurrence, substrate and environmental effects and biological control measures

[2] Cloyd, R.A. (2015) Ecology of Fungus Gnats (Bradysia spp.) in Greenhouse Production Systems Associated with Disease-Interactions and Alternative Management Strategies, Insects 2015, 6, 325-332; doi:10.3390/insects6020325

[3] Evans, M.R. et al (1998) Fungus Gnat Population Development in Coconut Coir and Sphagnum Peat – based substrates

[4] Muhammad Hussnain, B. et al (2014) Efficacy of Different Insecticides Against Mushroom Sciarid Fly (Lycoriella auripila) in Punjab, Pakistan

[5] Van Leeuwen, T. Van de Veire, M, Dermauw, W. Tirry, L. (2006) Systemic toxicity of spinosad to the greenhouse whiteflyTrialeurodes vaporariorum and to the cotton leaf worm Spodoptera littoralis

[6] Van Leeuwen T Dermauw W, van de Veire M, Tirry L (2005) Systemic use of spinosad to control the two-spotted spider mite (Acari: Tetranychidae) on tomatoes grown in rockwool.

[7] Van Leeuwen, T. Van de Veire, M, Dermauw, W. Tirry, L. (2006) Systemic toxicity of spinosad to the greenhouse whiteflyTrialeurodes vaporariorum and to the cotton leaf worm Spodoptera littoralis

[8] Shamshad, A. Clift, A and Mansfield, S. (2008) Toxicity of six commercially formulated insecticides and biopesticides to third instar larvae of mushroom sciarid, Lycoriella ingenua Dufour (Diptera: Sciaridae), in New South Wales, Australia

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[13] Krishnayyaand, P.V.; Grewal, P.S (2002) Effect of Neem and Selected Fungicides on Viability and Virulence of the Entomopathogenic Nematode Steinernema feltiae. Biocontrol Science and Technology, Volume 12, Number 2, 1 March 2002, pp.259-266(8)

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