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Are Rice Hulls Attractive to Fungus Gnats?

Just about anybody who works with plants is familiar with fungus gnats.  They are common in moist, shady areas where decaying organic matter is present and tend to be associated with moist organic soils or potting media that contain peat moss or composted organic materials.  They are familiar pests in greenhouses and nurseries and are also fairly common in homes where houseplants are grown.  Their presence is often associated with overwatering.  Fungus gnats can also be found outdoors where they are commonly associated with mulch, decaying leaves and other organic materials, and compost piles.  Adults are about 1/8 inch long and have a dark-colored body, a single pair of delicate, clear wings, long legs, and distinctive, long, slender antennae.  The larvae (maggots) are about 1/4 inch long when mature and are grayish-white and translucent with shiny, black heads.

Historically, fungus gnats were primarily considered a nuisance, but more recent evidence indicates they can be serious plant pests.  Adult fungus gnats are still primarily considered a nuisance as they are commonly seen on the surface of growing media or flying about, but they may also be involved in the spread of fungal diseases that can attack nursery and greenhouse crops.  Although people tend to be wary of bugs in general and their presence can discourage customers, the adults do not bite animals or humans nor do they feed on plant tissue.  The larvae are general feeders and feed on decaying organic matter, fungal hyphae, algae, and the young roots and root hairs of a wide variety of plants.  They can be difficult to see, but are most often found within one to three inches of the surface depending on moisture levels.  Young plants (seedlings) and plants with succulent stems are most susceptible.  Symptoms include reduced vigor, wilting, chlorosis (yellowing foliage), leaf abscission (dropping leaves), stunting, and even death.  The maggots will also feed on bulbs and other fleshy tissues and storage organs and will sometimes tunnel into larger roots and plant crowns.  Research also indicates they will feed on the developing callus at the base of cuttings and the resulting damage can delay rooting.  More so than the adults, the larvae can also serve as vectors for soil-borne pathogens through their feeding activities and the wounds caused by feeding which can serve as points of entry and infection.  Viable spores of pathogenic fungi have also been found in their feces.

Since fungus gnats have been recognized as a serious pest, research has increasingly focused on their life cycle, environmental and plant preferences, and potential methods of control.  One area of study involves the attractiveness of various growing media and growing medium components to fungus gnats.  Many of you are probably also familiar with rice hulls and some of you may have used them as a growing medium component.  Rice hulls, a byproduct of rice processing, have been studied as a growing medium component since the 1970’s and have subsequently been used in a variety of forms in horticultural growing media.  In general, fresh rice hulls appear to be less suitable than aged, composted, or parboiled rice hulls which have been successfully used alone and in combination with other materials.  Rice hulls are most often used as a substitute for peat, bark, and perlite.

In response to a growing interest in rice hulls as a growing medium component and increased concerns about fungus gnats in horticultural production systems, research conducted at Kansas State University in Manhattan, Kansas, has focused on these issues.  Specifically, several commercial growing media (including media containing rice hulls) and several growing medium components, were evaluated based on their attractiveness to fungus gnats including:

  • Sunshine LC1 Mix (75% sphagnum peat moss, 25% perlite)
  • SunGro RH1 (80% peat moss, 20% rice hulls)
  • Sunshine SB200 (60% peat moss, 20% bark, 20% perlite)
  • Sunshine SB300 (50% composted bark, 20% peat moss, 20% vermiculite, 10% perlite)
  • SunGro RH20 (100% rice hulls)
  • Individual growing medium components – composted pine bark (a component of the SB300 medium), pea gravel, sand, and perlite

A multiple choice experimental procedure was used to quantify fungus gnat preferences for the various substrates in a moist or dry state and was used to measure the attractiveness of the various media to fungus gnats.  Additional research has also investigated whether other growing media are specifically attractive to fungus gnats and how different growing media and environmental conditions influence fungus gnat activities.

Primary findings:

  • Moisture content appears to be a primary factor related to the attractiveness of growing media to fungus gnats; fungus gnats tend to be attracted to moist growing media and are less interested in dry media (moisture contents less than 10%); larval survival can be reduced in media that is too wet.
  • Peat-based growing media tend to be attractive to fungus gnats; the attractiveness of peat-based media may be related to the high water holding capacity of peat moss.
  • Significantly higher numbers of fungus gnats were attracted to the SB300 growing medium (pine bark-based) compared to the LC1 mix (peat-based) and both of these commercial growing media were more attractive to fungus gnats than a variety of individual growing medium components tested; regarding the attractiveness of composted bark, the authors cite other research where lower numbers of fungus gnats were associated with media containing composted bark.
  • The SB300 growing medium (50% composted pine bark) was more attractive to fungus gnats than composted pine bark alone.
  • When used as growing medium component, parboiled rice hulls are no more or less attractive to fungus gnats than other growing medium components and growers should not be concerned that rice hulls will attract fungus gnats.
  • Odor has been suggested as a factor involved in the attractiveness of growing media to fungus gnats; moist peat moss and peat-based growing media tend to have a distinct musty odor and a number of volatile, odoriferous compounds have been identified as being present in peat-based substrates and not in others based on gas chromatography of steam distillates; rice hulls alone have little, if any discernible odor and differences in the volatile constituents of rice hulls have been quantified.
  • Differences in the volatile constituents of a number of other growing medium components have also been identified using steam distillation and gas chromatography; further research is needed to determine if any of the volatile compounds identified are particularly attractive to fungus gnats and whether any differences might be related to fungus gnat preferences for or against specific growing media; if volatile compounds that are attractive to fungus gnats can be identified, it is possible that they might also be useful in managing fungus gnats.
  • Although differences have been reported, further research is needed to confirm whether female fungus gnats prefer to lay eggs in specific substrates based on their composition and whether such information can be used to manage fungus gnats; it is possible that a combination of factors may be involved in the attractiveness of growing media to fungus gnats.

The damage associated with fungus gnats is just one of a multitude of factors that must be considered relative to the myriad decisions involved in the development and management of plant-specific nursery and greenhouse production systems.  Research continues to improve our understanding of fungus gnat behavior and their effects on plant performance in greenhouse and nursery environments, but additional research is needed, including more research on the role of growing substrates on fungus gnat activities, to provide the knowledge required to do a better job of managing these important pests in horticultural production systems.

For additional detail about this body of research see:


Other research related to the attractiveness of various growing media to fungus gnats is cited including the following studies which also may be of interest to people interested in this topic:


To comment on this research update, suggest research topics of interest, or pass along a piece of research-based information that might be of interest to your industry colleagues, please email us at Research@MNLA.biz.

 

 
 
 

 

Photo Credit: Jim Calkins
Figure 1. Rice hulls, a byproduct of rice production and shown here in raw and processed form, have been researched as a growing medium component since the 1970’s.