Poinsettias (Euphorbia pulcherrima) are seasonal bloomers that grow outdoors in U.S. Department of Agriculture plant hardiness zones 9 through 11 but are more usually kept as a houseplant for the holidays. Finding small black flying insects on poinsettias is a telltale sign your plant has an infestation of fungus gnats. You can get rid of these pests and prevent them from returning.
Despite the name, fungus gnats look more like a tiny mosquito than gnat. They measure about 1/8 inch long and have dark bodies. Fungus gnats can be found flying around the infested plant or crawling on the leaves or soil surface. Fungus gnats generally cause no long-term damage to most plants. University of Connecticut Integrated Pest Management notes that poinsettias are prone to injury from fungus gnat feeding and can suffer serious damage. As the fungus gnats feed on young stems and feeder roots, the leaves of the infested poinsettia will yellow and drop.
Providing the poinsettias with proper care will help to control fungus gnats as well as other pests and problems. Damp areas with an abundance of decaying matter provide the ideal environment for fungus gnats. You can reduce the number of fungus gnats by merely keeping the pot or area around the poinsettia free from fallen plant debris and avoiding over-watering. A good general rule of thumb to avoid over-watering is to water the plant when the top few inches of soil are dry.
Successfully controlling fungus gnats doesn’t always require chemicals. For example, yellow sticky cards placed near poinsettias will trap adult fungus gnats. Cutting a peeled potato into chunks and inserting the chunks into the soil attracts the fungus gnat larvae. Remove the potato pieces every few days and replace them with fresh pieces. Using these methods -- combined with cultural control -- will get rid of the fungus gnats without filling your home with pesticides. Another option to controlling fungus gnats organically is to drench the soil with Bacillus thuringiensis. Bacillus thuringiensis contains a bacterium found naturally in the soil. This bacterium is not harmful to humans, mammals or beneficial insects. It causes the fungus gnat larvae to stop feeding and die. Every brand of Bacillus thuringiensis has specific application instructions that you should follow. For example, one brand recommends mixing 1 to 8 teaspoons of the Bacillus thuringiensis with 1 gallon of water before treating the plant.
Although several insecticides are available to control fungus gnats, they should only be used as a last resort after you have exhausted other control options. For example, you can treat with a ready-to-use pyrethrin insecticide every seven to 10 days, or as directed on the label. If you grow your poinsettia indoors, make sure you choose a product labeled for use indoors.