Poinsettia, Euphorbia pulcherrima, is widely grown in greenhouses throughout the U.S.A., and like many greenhouse-grown horticultural crops, poinsettias are susceptible to a number of insect and mite pests. Therefore, it is important to effectively manage insect and mite pests on poinsettias, especially before bract formation, because very few insecticides and miticides are labeled for poinsettias when in bract. The insect and mite pests of poinsettias feed on the roots, leaves or stems, and bracts:
Roots: fungus gnat larvae;
Leaves and stems: whiteflies, western flower thrips, broad mites, Lewis mites, and mealybugs;
Bracts: western flower thrips;
This article provides a brief description of the damage to poinsettia plants associated with each insect and mite pest. In addition, the article discusses the plant protection strategies that should be implemented to manage insect and mite pest populations, which will help to prevent damage to poinsettia crops.
1. Fungus gnats
Fungus gnat, Bradysia spp., larvae (Figure 1) feed on root hairs and small roots, which inhibits the ability of poinsettia plants to obtain water and nutrients. Fungus gnats are primarily a problem under propagation when low larval numbers can kill cuttings or young plants. Cultural control and scouting practices associated with fungus gnats include the following:
Cultural: 1) avoid keeping the growing medium excessively moist, 2) remove weeds and old growing medium from around the production area, and 3) use either pasteurized or bagged growing medium.
Scouting: 1) use yellow sticky cards to capture fungus gnat adults, which will help to detect early infestations, and 2) use potato wedges to detect the presence of fungus gnat larvae.
A number of insecticides can be used to suppress fungus gnat larval populations in the growing medium, including insect growth regulators [e.g. pyriproxyfen], contact insecticides [e.g. chlorfenapyr], and microbials [Bacillus thuringiensis subsp. israelensis]. In addition, there are three biological control agents that are effective against fungus gnat larvae, including the rove beetle, Dalotia coriaria, the predatory mite, Stratiolaelaps scimitus, and the entomopathogenic nematode, Steinernema feltiae. The three biological control agents must be released before fungus gnat larval populations reach damaging levels.
The sweet potato whitefly, Bemisia tabaci, is the primary insect pest of poinsettias, feeding within the vascular system (phloem sieve tubes) and removing plant fluids with their piercing-sucking mouthparts. In addition, the nymphs excrete honeydew, which is a clear, sticky liquid that serves as a substrate for black sooty mold. Whiteflies typically feed on the underside of poinsettia leaves (Figure 2). Furthermore, all the life stages (eggs, nymphs, pupae, and adults) are located on the leaf underside. The cultural control and scouting practices affiliated with whiteflies are the following:
Cultural: 1) remove weeds from within and around the greenhouse, 2) avoid over-fertilizing poinsettia plants with water-soluble nitrogen-based fertilizers, and 3) remove leaf debris from production areas.
Scouting: 1) use yellow sticky cards to capture whitefly adults, which will help to detect early infestations, and 2) visually inspect the underside of leaves for the presence of eggs, nymphs, and pupae.
Contact insecticides [e.g. cyfluthrin], insect growth regulators [e.g. pyriproxyfen], selective feeding blockers [e.g. pymetrozine], and systemic insecticides [e.g. dinotefuran] can be used to suppress whitefly populations. In addition, the parasitoid, Eretmocerus eremicus, can be released early in the production cycle to regulate whitefly populations. Furthermore, entomopathogenic fungi, including Beauveria bassiana and Isaria fumosoroseus, are effective against whiteflies when applications are made early in the crop production cycle. However, be sure to thoroughly cover leaf undersides with spray applications where all the life stages (eggs, nymphs, and adults) are located.
3. Western flower thrips
Western flower thrips, Frankliniella occidentalis, larvae and adults cause direct damage to the leaves and bracts of poinsettia plants that results in leaf scarring and distortion (Figure 3). Cultural control and scouting practices, which are designed to minimize problems with western flower thrips, include the following:
Cultural: 1) remove heavily infested or damaged plants, 2) screen greenhouse openings, such as vents, with insect micro-screening, and 3) remove all weeds from within the greenhouse.
Scouting: 1) use yellow sticky cards to capture adults, which will help detect early infestations, and 2) shake leaves over a white sheet of paper (8.5 x 11 inches) to detect the presence of larvae and adults.
A number of insecticides can be used on poinsettias prior to bract formation to suppress populations of western flower thrips, such as spinosad, chlorfenapyr, pyridalyl, and abamectin. Insecticides that can also be used include Beauveria bassiana, Isaria fumosoroseus, and spirotetramat. Always apply insecticides early in the crop production cycle and repeat applications, if needed, based on scouting information
4. Broad mites
The broad mite, Polyphagotarsonemus latus, is 1/100 inches long and its eggs have bumps on the surface. A dissecting microscope will allow you to actually see the mites on plant leaves. Broad mite feeding causes distortion or deformation of the terminal leaves of poinsettias (Figure 4). If you suspect broad mites are responsible for the damage observed on poinsettia plants, send samples to a diagnostic clinic for verification. Cultural control and scouting practices for broad mites include the following:
Cultural: immediately remove all plants that are suspected to be or are exhibiting damage caused by broad mites.
Scouting: routinely inspect plants for any visible damage that may be associated with broad mites.
Pesticides with miticidal activity may be used to manage broad mite populations, including abamectin, bifenazate + abamectin, chlorfenapyr, fenpyroximate, pyridaben, spiromesifen, and spirotetramat. However, these pesticides must be applied before damage is observed, otherwise it may be too late.
5. Lewis mites
The Lewis mite, Eotetranychus lewisi, is not a common pest problem of poinsettias, but greenhouse producers should be aware of this mite pest. The Lewis mite resembles the more common twospotted spider mite, Tetranychus urticae, in appearance, but is smaller. Lewis mites feed on the lower leaves and cause damage on poinsettias that may resemble a nutritional deficiency (Figure 5). The cultural control and scouting practices for the Lewis mite are the following:
Cultural: 1) remove all weeds from within the greenhouse that may serve as an alternate host plant for Lewis mites and 2) avoid over-fertilizing plants with water-soluble nitrogen-based fertilizers.
Scouting: 1) visually inspect leaf undersides for the presence of Lewis mites and 2) shake leaves over a white sheet of paper (8.5 x 11 inches) to detect the presence of Lewis mites.
Miticides with contact or translaminar activity (material penetrates leaf tissues and forms a reservoir of active ingredient within the leaf that provides residual activity after spray residues have dissipated) can be used against Lewis mites, as well as certain biological control agents, including predatory mites and the predatory midge, Feltiella acarisuga. Check miticide labels to determine those that are registered for use against the Lewis mite. Furthermore, consult with your biological control supplier to determine which predatory mites should be released to regulate Lewis mite populations.
Mealybugs are not a major pest of poinsettias; however, they have become more of a problem. One possible reason is that systemic insecticides applied to the growing medium may not be effective against mealybugs on poinsettias. Therefore, mealybug populations may build up and potentially displace other insect pests, such as whiteflies, that are susceptible to systemic insecticides. Mealybugs feed on the leaf underside, on the plant stem (Figure 6), and near the base of the petioles. Like whiteflies, mealybugs feed within the vascular system, removing plant fluids. Mealybugs can cause leaf distortion, plant stunting and wilting. Also like whiteflies, mealybugs excrete copious amounts of honeydew, which serves as a substrate for black sooty mold. The cultural control and scouting practices associated with mealybugs are the following:
Cultural: 1) immediately dispose of heavily infested plants and 2) avoid over-fertilizing plants with water-soluble nitrogen-based fertilizers.
Scouting: visually inspect plants for mealybug life stages, including nymphs (crawlers) and egg-laying females. Mealybug populations may be suppressed on poinsettia plants before bract formation using insecticides such as acetamiprid, pyrifluquinazon, flupyradifurone, mineral oil, and buprofezin. However, multiple applications will be required, and thorough coverage of leaf undersides and plant stems is important.
A number of insect and mite pests may feed on poinsettia plants during the growing season. However, greenhouse producers can alleviate problems with these insect and mite pests by implementing a multitude of plant protection strategies, including cultural control, scouting, pesticides, and/or biological control. Another strategy used by many greenhouse producers involves installing Yellow Stiky Tape™ among a poinsettia crop, which will capture adult fungus gnats, whiteflies and western flower thrips. The use of the Yellow Stiky Tape may help reduce insect pest populations below plant-damaging levels. Always be proactive when dealing with insect or mite pests that feed on poinsettias, especially before bract formation, as options are limited once poinsettias are forming bracts.