Eliminating Gnats in Arrowwood Viburnum Shrubs

Seeing tiny flying insects hovering over your arrowwood viburnum’s soil? Chances are you’ve got an infestation of fungus gnats or fruit flies. These diminutive pests are drawn to the moist, organic environment of container plants and overwatered soils. Left unchecked, they can stunt plant growth and spread disease. Getting rid of gnats in arrowwood viburnums requires drying out the soil and using traps and organic treatments to destroy all life stages.

What Are Fungus Gnats and Fruit Flies?

Fungus gnats and fruit flies are often mistaken for each other because of their small size and habit of swarming wet potting mix. But they are different insects.

Fungus Gnats

Fungus gnat adults are about 1/8 inch long, with long legs and antenna and one pair of clear wings The leggy larvae are translucent, up to 1/4 inch long, with black heads. They feed on plant roots and organic matter in damp soils.

Fruit Flies

Fruit fly adults are 1/16 to 1/8 inch long and yellow-brown Larvae are cream colored, about 1/4 inch long They infest overripe and decaying produce but also lay eggs in wet potting mixes.

While fungus gnat larvae directly damage roots fruit fly larvae are more of a nuisance. But both insects can spread plant diseases like pythium root rot in overwatered soils. Getting rid of their breeding sites is key.

Signs of Fungus Gnats and Fruit Flies

Watch for these signs of gnat and fruit fly infestations:

• Swarms of small flying insects when you water or disturb the soil
• Larvae visibly crawling in the top few inches of soil
• Adults emerging from drain holes of containers
• Stunted, yellowed growth or wilting of plants
• Webbing on soil surface from feeding larvae
• Root damage from larvae feeding

Inspect soil closely around arrowwood viburnum rootballs for the larvae. Adults often spread to nearby plants, so check all containers and borders.

How to Get Rid of Gnats in Arrowwood Viburnums

An integrated pest management approach combining cultural controls, traps, and organic treatments will eliminate gnats in potted or planted arrowwood viburnums.

Allow Soil to Dry Out

Fungus gnats and fruit flies need moist soil to breed. Allow the top few inches of soil to dry out slightly between waterings to discourage larvae. Don’t overwater. Improve drainage by amending soil with compost or perlite.

Use Sticky Traps

Traps placed at soil level or just above catch adults, reducing breeding. Use yellow sticky cards or make DIY traps from apple cider vinegar in jars covered with plastic wrap and a few drops of dish soap.

Remove Top Inch of Soil

Removing the top layer of infested soil can eliminate 50% or more of larvae. Discard the soil in a sealed bag. Replace with fresh, sterile potting mix.

Apply Sand Layer

Sprinkling a 1-inch layer of clean, dry play sand atop container soil prevents adults from accessing soil to lay eggs. It also deters larvae from emerging.

Treat Soil with BTi

Bacillus thuringiensis israelensis (BTi) is a natural bacterium that kills fungus gnat larvae when ingested, but is safe for people and plants. Apply as a soil drench every 5-7 days until adults and larvae are gone.

Use Horticultural Oil Spray

A lightweight horticultural oil sprayed onto adults and soil suffocates eggs and larvae on contact. Spray soil until just saturated, and coat plant stems to 15 inches. Repeat weekly for 3-4 weeks. Avoid plant leaves.

Apply Diatomaceous Earth

The sharp edges of diatomaceous earth dust scratches the exoskeleton of larvae and dries them out. Dust a thin layer on the soil surface. Wear a mask to avoid inhaling. Reapply after watering.

Use Beneficial Nematodes

Beneficial nematodes are microscopic parasites that attack fungus gnat larvae. Apply to moist soil and avoid treating with insecticides for 6 weeks so nematodes establish. May take 2-6 weeks to see results.

Set Out Pitcher Plants

The carnivorous pitcher plant (Sarracenia sp.) traps and digests fungus gnat adults in its fluid-filled leaves. Place several in a badly infested area for natural control.

Preventing Future Infestations

Here are some tips to avoid fungus gnats and fruit flies in your arrowwood viburnums:

• Inspect new plants and avoid purchasing those with larvae in the soil.
• Quarantine new plants for 2-3 weeks to prevent spread.
• Water arrowwood viburnums thoroughly until water drains from holes, then allow soil to partially dry before watering again.
• Provide good drainage with soil amendments if needed.
• Allow the top 1-2 inches of soil to dry slightly before watering.
• Avoid excess fertilization which promotes succulent, gnat-favored growth.
• Clean up fallen leaves, rotten fruit, and debris that breed gnats.
• Use sticky traps continuously to monitor for adults.
• Treat plants preventively with BTi drenches.
• Cover container soil with 1 inch layer of sand.

With vigilance and persistence, you can break the reproductive cycle of fungus gnats and fruit flies and safely protect the health of your arrowwood viburnums. Consistently allowing the soil to dry out between waterings and eliminating places where adults breed are essential to long term control.