Common Greenhouse Pests

Common Greenhouse Pests Western Flower Thrips Thrips feed by using their mouthparts to pierce plant cells and suck out their contents. Damage appears as small scars or silvered patches. Most plants can outgrow mild thrips infestations but seedlings can be severely damaged in heavy populations. Thrips also vector plant viruses such as impatiens necrotic spot and the tomato spotted wilt virus. Control of thrips in greenhouses can be difficult due to pesticide resistance, but with proper crop rotation and sanitation along with good IPM procedures populations can be kept at a minimum. Breaking up the pest population by cleaning out greenhouse rooms and avoiding growing multiple stages of plants is key to keeping populations at a minimum. Biological control consists of predatory bugs, mites and nematodes that search out and kill thrips as well as biochemical sprays. Aphids Aphids are small, soft-bodied insects with piercing mouthparts that they use to suck out fluids from the plant. There are many different species of aphid that feed on a variety of different plants. Aphids can cause stunting, leaf curling and transmit viruses from plant to plant. They also produce a sticky exudate called honeydew that can attract other pest such as ants and can develop into a growth of sooty mold fungus. There are a number of natural predators of aphids including parasitic wasps, and lady bird beetles. Keeping surrounding areas clean of debri and weeds is also an important part of pest management. Spider Mites Mites can cause leaf damage by sucking the cell contents. Mites are very tiny and difficult to see with the naked eye. Plant damage usually first appears as stippled or yellow leaves. As the pest becomes established you will see webbing on the leaves. Spider mites have many natural enemies, including predator mites, and certain lady beetles which naturally limit their numbers in many yards and gardens, especially when undisturbed by pesticide sprays. Keeping debri and weeds clear of growing sites and proper crop rotation can be critical in breaking up existing pest populations. Chemical controls with oils and soaps are also good at keeping pest populations down. Whitefly Whitefly, like aphids, have piercing mouth parts and can cause some leaf distortion. They also create honeydew and can attract ants as well. Small populations do not usually cause damage and most plants can withstand moderate populations. Whitefly populations can be kept in check with natural predators and a clean environment. Natural predators include, parasitic wasps, mites and some true bugs. Chemical sprays can be effective if caught early enough and all life stages are targeted. Fungus Gnat and Shore Fly Adult fungus gnat (left) and shore fly (right). Photo by Jack Kelly Clark Fungus gnats are small flies that are found in moist soil and potting mix. They are more of a nuisance then a pest, but in high numbers can cause problems. The adults do not feed on plants but their larvae will feed on fungi and organic matter in the soil including plant roots. Eliminating excess moisture and debris in the planting media and around the plants is an effective control of the larval stage. Shore Flies are found in overwatered conditions where algae is present. Neither the adults nor the larvae feed on plants and are only considered to be a nuisance. Controlling excess moisture and algae growth in and around the plants will help keep populations down. Powdery Mildew There are many different species of powdery mildew fungi and each species only attacks specific plants. Powdery mildew first appears as white, powdery spots that may form on leaf and shoot surfaces. Powdery mildew spores are carried by wind. Humidity requirements vary but all powdery mildew species can germinate and infect in the absence of free water. Temperatures between 60° to 80°F and shady conditions are the most favorable for powdery mildew development. Spores and fungal growth are sensitive to extreme heat (above 90°F) and direct sunlight. The best method of control is prevention, planting resistant varieties, and following good cultural practices.