July 21, 2022

Common Houseplant Pests and How to Treat Them: Fungus Gnats Vs Thrips

Houseplants can be a great addition to any living space, but they can also get infected with some pests. Yellowing leaves and infestation of the leaf tissue with little white dots are symptoms that might mean you have infected plants on your hands. Diagnosis is key in getting rid of these critters before it’s too late! Let’s talk about what causes them—and how we treat our plants accordingly.

THRIPS

Thrips are tiny, slender insects with fringed wings. They mostly feed on foliage by puncturing and sucking up the contents of their victim’s cell. Thrips can cause silvery scars on leaves and fruit. They leave behind small patches on your plants, easily recognizable net-like patterns that are fairly visible. You can find them, mainly on the stems and undersides of leaves. Thrips can be harder to spot as they often hide on plant tissue. In their larvae stage, they look like little white sticks, crawling along the plants. And in their adult stage, they can fly. Adult thrips are often brown or black in color. You may notice thrips around spring since they overwinter in soil and emerge when it’s warmer. Their life cycle lasts about one month.

HOW TO TREAT THRIPS

If you suspect your houseplant has these pests then immediately quarantine it until after treatment. Insecticidal soap and neem oil are effective treatments against thrips. Also rinsing plants with water can help reduce thrips populations. My favorite treatment method is Houseplant Systemic Granules by Bonide. Thrips like to lay their eggs inside the tissue of leaves. The Bonide is a sure fire way to eradicate that larvae and the adults. If this systemic isn’t available in your area, look for an insecticidal spray - Captain Jack’s Dead Bug Brew is a great one. And since the spray can’t attack the eggs, you’ll need to spray and treat the plant on a bi-weekly basis until you no longer see anymore thrips.

SPIDER MITES

Spider mites are one of the most common houseplant pests. They’re in the arachnid family and relatives to regular spiders. Truly a nightmare! The tiny little pests are hard to see with your naked eye, but you will notice them if they crawl along plant leaves or make webs on the underside of leaves where petiole meets leaf. Their damage will look like yellow stippling.

HOW TO TREAT SPIDER MITES

While I will recommend systemic granules for many common plant pests, it’s important to note that they will not work for mites. In many cases, it may even make things worse. Mites are best treated with an insecticidal soap specially made for mites. You can find them in big box stores or almost anywhere that sells plants. Mite-X is the one I generally reach for. Also, try to work in cleaning your foliage monthly. Reducing the amount of dust that accumulates on your plants really helps keep the problem at bay.

SCALE

Scale are small, round insects that thrive in warm, dry environments. Often they’ll be either brown or white and usually look like little round dots along the stems of a plant. Scale are vastly different from other houseplant pests because they’re immobile. Once they latch onto a spot, they stick there. So if you notice bumps along your plant, it’s most likely scale.

HOW TO TREAT SCALE

Dip a cotton swab in isopropyl alcohol and press it firmly on the scale and they’ll burn right off. You can follow up with the systemic granule or an insecticidal soap. Beneficial insects can be used natural biological control agents as a preventive way of minimizing pest issues but this requires you to be okay with leaving some bugs on the plant.

MEALY BUGS

Mealy bugs are a soft-bodied, wingless insect that have a cotton-like appearance. They cause damage by sucking the juice from their host plant. They typically like to go after new growth and will cause yellowing leaves. In masses, the waxy excreations they produce will cause sooty mold fungus to develop.

HOW TO TREAT MEALY BUGS

Since mealies are relatives to scale, I recommend the same treatment as scales. Infested plants can be treated with isopropyl alcohol and the systemic and/or insecticidal spray.

FUNGUS GNATS

The most common pest you’ll encounter. Fungus gnats cause signs of stress in plants without directly damaging the leaves. Fungus gnats are usually small, black flies flying around the soil. You may even see their slender, black bodies crawling on the top of the soil. The good thing about gnats is that they’re more of a nuisance to us than to our plants unless their larval populations become large. Fungus gnat larvae feed on young plant roots. Fungus gnat larvae can stunt plant growth. Fungus gnats are attracted to damp, overwatered potting mixes rich in organic matter.

Fungus gnat adults are about 1/8 inch long. A female fungus gnat can lay up to 200 eggs.

HOW TO TREAT GNATS

For the short term, yellow sticky traps work by kiling the adult flying gnats. For the larvae, the most effective way I treat for gnats is to mix equal parts of water and hydrogen peroxide and water any affected plant. This will wash any existing gnat and their larvae. You can also water the plants with a spoon of mosquito bits or do a soil drench with mosquito bit treated water. You can also completely repot the plant, making sure all of the old, infested soil is gone before replacing it with fresh soil. And try to cut back on watering. Gnats are attracted and thrive in moist soil, so letting that soil dry a bit more will drastically reduce population.

Adult Thrips vs fungus gnats and fungus gnat larvae

Fungus gnats and thrips can both harm plants but do so in different ways.

The quickest way to tell the difference between thrips and fungus gnats is to understand their behavior, especially since thrips are one of more than seven thousand species worldwide. Thrips will always be on the foliage and in their adult form can fly around from plant to plant. Adult thrips are 1-2mm long slender winged insects that can cause significant damage to plants. In contrast, thrips adults are active on leaves and stems, while adult fungus gnats aren’t the best at flying and usually stay lower around the pot and soil surface.

You can also use visual inspection along with yellow sticky traps. The pests are attracted to them, and adult thrips are often caught on them so you can confirm whether they are present. If you notice long thin bodied pests, then its thrips, while the immature stages are found on foliage as nymphs or larvae and the winged form is what you see on traps or moving plant to plant. Western flower thrips are a major pest in greenhouse production systems and can spread diseases such as the impatiens necrotic spot virus. Their thrips pupae often drop into the potting media, so targeting that area can help break the life cycle, and a predatory mite such as Hypoaspis is often used for that step.

Small, round bodied bugs hovering near the soil surface are fungus gnats, while thrips are found on leaves and stems, and insecticidal soaps, horticultural oils, or other sprays work best when the active ingredients are chosen to target the exposed adult stage on foliage.

Final Thoughts:

Some other things to consider is that stressed plants are more likely to get attacked, and young plants are especially vulnerable to root and foliage pests. So if a plant is being severely neglected it’ll eventually get some sort of pest. Also, make sure to check every new plant you get, especially seedlings or other young plants. New plants are often the culprit of an outbreak, whether you keep houseplants or indoor crops as commercial growers. And lastly, know that pests are normal. You can do everything right and still get attacked by spider mites. So take a deep breath and know it’ll be okay because now you know how to treat common houseplant pests.