Coleus plants are cherished for their vibrant foliage, making them a popular choice for greenhouse cultivation. While greenhouses offer optimal conditions for growth, they also create an environment where certain pests can thrive. Understanding these common greenhouse pests and their impact is important for maintaining healthy plants and managing potential infestations.
Why Greenhouses Attract Coleus Pests
Greenhouses inadvertently create ideal conditions for pest proliferation. Consistent temperatures and high humidity levels inside a greenhouse offer pests a stable habitat year-round, unlike outdoor settings where natural fluctuations might limit their populations. This enclosed space also often lacks the natural predators that typically keep pest numbers in check outdoors, allowing pest populations to increase rapidly without interruption.
Continuous plant growth cycles and the close proximity of plants within a greenhouse offer an uninterrupted food supply and easy movement for pests between host plants. Stagnant air and poor ventilation can contribute to humid pockets, which are favorable for many pest species. When plants are stressed by unsuitable conditions, they can become more vulnerable to pest attacks, creating a cycle where pests flourish and plant health declines.
Key Pests Affecting Coleus in Greenhouses
Aphids
Aphids are small, soft-bodied insects that can appear in various colors, including green, black, white, or brown. These sap-sucking pests typically gather on new terminal growth, shoot tips, flower buds, and the undersides of younger leaves. Infested coleus plants may show stunted growth, distorted or curled leaves, and a general weakening of the plant. Aphids excrete a sticky substance called honeydew, which can lead to the growth of black sooty mold on leaf surfaces. They thrive in greenhouse conditions due to abundant lush growth and the absence of their natural enemies.
Spider Mites
Spider mites are tiny, oval-shaped arachnids, usually yellow-brown with two dark spots on their bodies, and are often no larger than a pinhead. They feed by sucking plant sap, causing small yellow spots or stippling on leaves, which can eventually lead to entire leaves turning yellow. A clear sign of a heavy infestation is the presence of fine webbing, particularly on the undersides of leaves and between leaf axils. Spider mites are especially problematic in hot, dry greenhouse environments with low humidity, as these conditions accelerate their reproductive cycles.
Whiteflies
Whiteflies are small, moth-like insects, about 1/16 inch long, with four white, waxy wings that give them a mealy appearance. They tend to cluster on the undersides of coleus leaves and will fly up in a cloud when disturbed. Whiteflies damage plants by sucking sap, which results in a lack of vigor, wilting, and yellowing of the leaves. Like aphids, they excrete sticky honeydew, leading to sooty mold development. These pests can complete their life cycle quickly in warm, humid greenhouse conditions, allowing populations to build rapidly.
Mealybugs
Mealybugs are soft-bodied, oval insects ranging from 1/20 to 1/5 inch long, covered in a white, powdery, waxy material. They often appear as cottony masses on leaf surfaces, in leaf axils, and along stems. Mealybugs feed by withdrawing plant fluids, causing stunting, leaf yellowing, and distortion of plant parts. Their feeding also results in honeydew excretion and subsequent sooty mold growth, which can hinder photosynthesis. Mealybugs are resilient and can overwinter indoors, making them a persistent problem in greenhouses.
Fungus Gnats
Fungus gnats are small, dark, mosquito-like flies, approximately 1/8 inch long, that are often seen flying erratically around the soil surface or resting on plant media. Their larvae are slender, whitish maggots with distinct black heads, growing up to 1/4 inch long. While adult gnats are primarily a nuisance, the larvae feed on plant roots, organic matter, and fungi in the soil. This root feeding can lead to stunted growth, wilting, and yellowing of lower leaves in coleus, particularly in young plants. Fungus gnats thrive in damp, moist soil conditions, especially those rich in organic matter, making overwatered plants particularly susceptible.
Managing Pests on Coleus in Greenhouses
Controlling pests on coleus in a greenhouse involves a combination of proactive measures and responsive treatments. Regular inspection of plants is important for early detection, allowing for intervention before infestations become severe. Checking the undersides of leaves and new growth frequently can help identify pests in their early stages. Prompt action can prevent widespread issues.
Promoting good cultural practices significantly reduces pest appeal. Ensuring proper plant spacing improves air circulation, which can deter pests that prefer stagnant, humid conditions. Quarantining new plants before introducing them to the main greenhouse population helps prevent the introduction of new pests. Maintaining cleanliness by removing plant debris and controlling weeds both inside and outside the greenhouse can eliminate potential pest breeding grounds.
When pests are detected, various management approaches can be employed. Manual removal, such as wiping mealybugs with a cotton swab dipped in rubbing alcohol or washing aphids off with a strong stream of water, can be effective for small infestations. Insecticidal soaps and neem oil are general treatments that can control many common pests by smothering them or disrupting their life cycles. The concept of Integrated Pest Management (IPM) combines these strategies, using monitoring, preventative measures, and targeted treatments to maintain pest populations below damaging levels. Utilizing yellow sticky traps can also aid in monitoring and trapping flying pests like whiteflies and fungus gnats.