Geraniums (Pelargonium spp.), which grow as perennials in U.S. Department of Agriculture plant hardiness zones 9 through 11 and as annuals in all other USDA zones, are popular with more than gardeners; they are also loved by numerous pests. These include aphids, cottony cushion scale, cyclamen and spider mites, caterpillars, mealybugs, thrips, weevils and whiteflies. When it comes to white worms, the larvae of the fungus gnat are usually harming the geranium.
Fungus Gnat Larvae
Tiny fungus gnat larvae are translucent to white, with black, encapsulated heads. These white "worms" live in soil around geraniums and feed on the roots and stems of the plants, causing a great deal of damage. Seedlings, cuttings and young plants are most at risk. The adults are also small, reaching only 1/8 of an inch. They resemble a mosquito with long legs and thread-thin antennae. As their name implies, fungus gnats eat fungus, particularly Botrytis fungi. Females lay eggs in soil near sources of fungus, including geraniums suffering from botrytis leaf spot or botrytis blossom blight.
Bacillus thuringiensis, Steinernema feltiae and Hypoaspis miles are natural predators of the fungus gnat larvae. Bt is a bacterium that kills a fungus gnat larva by releasing a toxic protein crystal in the larva’s gut that causes the larva to stop feeding, effectively killing it. Two to three applications may be required. Steinernema feltiae are beneficial nematodes that are typically applied to counter fungus gnat larvae. These nematodes enter through openings in the larva’s body, where they release toxins that eventually kill the larva. They also reproduce inside the larva’s body before leaving it for a new host. Hypoaspis miles are soil-dwelling mites that prey on the larvae as well. They come in a 1-liter vermiculite or peat carrier with a shaker lid and can be sprinkled over the soil around the infected plant. The mixture may also contain mold mites, a food source for Hypoaspis miles. They are most effective for smaller infestations and may be used in conjunction with the other biological controls.
Other Natural Solutions
To prevent the reproduction of more white fungus gnat larvae, adult fungus gnats may be trapped by putting out bowls of apple cider vinegar or beer. They drown when they dive into the sweet, fermented liquid. Sticky traps placed on the top of the soil also work on the adult pests. Trap fungus gnat larvae using potato slices. Slice a potato into 1-inch rounds that are ¼ to ½ inch thick and set them on top of the soil around the base of the plant. After at least four hours, collect the larvae and dispose of them. Diatomaceous earth is also effective in killing fungus gnat larvae when applied to the soil. Add 1/2 pound to a 5-gallon bucket of water or sprinkle the powder in a thin layer on the soil's surface around the base of the plant.
Prevent fungus gnat larvae from feeding on your geraniums by preventing Botrytis cinerea and other fungi. Make sure that your geranium is planted in soil with good drainage, and avoid overwatering. Fungus grows in soil that stays wet. Also allow for good air circulation around the plants to help the soil dry faster. Remove dead leaves, stems and flowers to further prevent potential fungal growth. Control pests such as whiteflies and spider mites that can also facilitate fungal growth. Boost your geranium’s immunity by giving it liquid seaweed, which contains beneficial bacteria. Add one capful per gallon of water during regular waterings.
- University of California Statewide Integrated Pest Management Program: Geraniums--Pelargonium Spp.
- University of Connecticut Integrated Pest Management: Fungus Gnats Are Serious Pests
- Golden Harvest Organics: Natural Insect Control: Fungus Gnats
- Colorado State University Cooperative Extension: Fungus Gnats
- Iowa State University Extension and Outreach: Geranium Care
- University of Florida IFAS Extension: Pelargonium x Hortorum Geranium
- U.S. National Arboretum: Arboretum Plant Photo Gallery: This Is Pelargonium "Chicago Rose"
- Photo Credit Jupiterimages/Photos.com/Getty Images