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Why are Fungus Gnats Attracted to Yellow? What you Need To Know

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Fungus gnats have long been the topic of photopraxis studies, investigating the movement of simple organisms in response to light and color.

Managing fungus gnat populations in a greenhouse using light and colored sticky cards is tricky because the light intensity and color spectrums can impact plant stage development and growth. One study showed that fungus gnats have a distinct preference for greenish-yellow cards with UV light support.

Studies Showing Fungus Gnat Response to Color

The study concludes:

The highest numbers of adult Bradysia difformis were attracted to UV radiation (382 nm), followed by green‐yellow light (532–592 nm). The responses to UV and the green‐yellow range were relatively unspecific and primarily intensity-dependent.

Combining UV and yellow LEDs improved trapping efficacy compared to a single UV or yellow LED trap and a standard yellow sticky trap.

When both wavelengths were compared to a black surface to increase contrasts, the black surface was preferred over yellow but was less attractive than UV.

Thus, B. difformis displays two, probably wavelength‐specific, behaviors to UV radiation and green‐yellow light, with UV being the most attractive stimulus.

Cite: Stukenberg, Niklas et al. “Visual orientation of the black fungus gnat, Bradysia difformis, explored using LEDs.” Entomologia Experimentalis et Applicata 166 (2018): n. pag.

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Studies Showing Fungus Gnat Response to Light Intensity

A further study shows the response of the fungus gnat to color and light intensity.

Under conditions of darkness, fungus gnat adults migrated randomly with no significant differences among the six sample compartments.

Fungus gnat adults responded positively to light intensities less than 0.08374 mmol -2 s -1. In addition, adults responded to light intensities below the detection threshold of a photosynthetically active radiation light sensor.

A higher percentage of fungus gnat adults (22% to 39%) were captured on yellow sticky cards in the sample compartments closest to a directional light source compared with sample compartments located further away from the light source (2% to 9%).

Fungus gnat adults exhibited a significant response when exposed to two distinct ranges of light intensities (0.12 to 0.26 versus 0.87 to 1.02 mmol -2 s -1 ), with adults significantly more attracted to the highest light intensities (0.87 to 1.02 mmol -2 s -1 ).

The results obtained in this study indicate that fungus gnat adults are positively phototactic, and as light intensity increases, they display a preference for those higher light intensities.

Modifications in light intensity may be a feasible management strategy for alleviating problems with fungus gnats in greenhouses.

Cite Dickinson, A. A. et al. “Phototaxis of Fungus Gnat, Bradysia sp. nr coprophilia (Lintner) (Diptera: Sciaridae), Adults to Different Light.” (2007).

Using the Fungus Gnat’s Attraction to Yellow to Your Advantage

In a greenhouse setting, yellow sticky cards (3 x 5 inches; 76 x 127mm) provide information on insect pest population changes or trends, and presence or absence throughout the growing season,  helping approximate insecticide application timing.

Yellow sticky cards should primarily be used to determine the presence and absence and assess the population of adult insect pests both in space and time because yellow sticky cards do not consider the egg, larval and pupal populations.

Nonetheless, yellow sticky cards may be the most cost-effective and least time-consuming method to determine the number of insect pests compared to destructive sampling, which is impractical.

Sticky card placement

When using yellow sticky cards, place them horizontally near the growing medium surface because adults are most active in this region of the plant canopy.

In addition, yellow sticky cards can be placed on the rims of flats or containers. One side of the yellow sticky card can be used during the first week by leaving the protective wax paper on the unused side of the yellow sticky card for use during the second week, allowing the use of one yellow sticky card for two weeks.

Square yellow cards are so yesterday. To make them less noticeable, marketers now offer yellow sticky cards in the shape of butterflies and flowers, helping you keep things pretty.

Combining UV and Yellow for Max Fungus Gnat Management.

With the availability and low-cost LED light strips, you can combine UV lighting with yellow sticky cards for maximum impact.

Our visible colors are:

  • Violet: 380–450 nm (688–789 THz frequency)
  • Blue: 450–495 nm
  • Green: 495–570 nm
  • Yellow: 570–590 nm
  • Orange: 590–620 nm
  • Red: 620–750 nm (400–484 THz frequency)

Violet light has the shortest wavelength, which means it has the highest frequency and energy. Red has the longest wavelength, the shortest frequency, and the lowest energy.

While humans can only see violet, bees, fungus gnats, and other insects can see ultraviolet light, which is commonly reflected by flowers.

Summarizing Why Fungus Gnats Are Attracted to Yellow

While much research is happening, we still have a limited understanding of some of the finer nuances of insect behavior. We know that fungus gnats love greenish-yellow and UV light, allowing us to use this knowledge to protect our indoor plants better.

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