Description, life history, and management continued…

Aphids

Aphids are small insects often associated with new growth and the underside of leaves. They range in size from, depending on species, and have piercing-sucking mouthparts for penetrating plant tissues and extracting fluids. Aphid species vary in color from green, black, red, yellow, brown, or grey, and measure 2-4 mm (1/16-1/8 inch) in length. All aphids are parthenogenic and can give birth to nymphs rather than laying eggs. They often reproduce asexually as nymphs are born pregnant, which allows populations to build rapidly under favorable environmental conditions. All aphids secrete honeydew which contains sugars and is attractive to other undesirable insects in the home such as ants and wasps. Excessive accumulation of honeydew can also lead to the growth of sooty mold and can accumulate on furniture below the infested plant. Collectively, aphids attack many different species of houseplants including ornamental peppers, hibiscus, and chrysanthemums.

Heavy aphid feeding causes wilting or curling of leaves, distortion of new growth, and excessive honeydew accumulation. Plants can be inspected regularly for the presence of aphids or honeydew, and yellow sticky traps can be deployed to capture winged aphids. Regarding management, individual aphids can be crushed, and plants can be periodically hosed with a jet of water to dislodge aphids. The larvae of green lacewings are voracious aphid predators and are commercially available for aphid management. Many aphid species are highly resistant to insecticides. However, insecticidal soaps can be effective.

Scale insects and mealybugs

Scale insects and mealybugs are small, fluid feeding insects. Adult scale insects range in size from 3.2-13 mm (1/8-1/2 inch) long and cover themselves in a protective waxy or cotton-like covering that varies in color, depending on the species. Females are wingless, while mature males produce wings and are smaller than females. Two important families of scale insect include the soft scales (family: Coccidae) and armored scales (family: Diaspididae). Both soft and hard scales produce protective coverings that vary in color and serve as the egg laying site for adult females. Soft scales excrete honeydew and the adult insect cannot be separated from the protective covering, while armored scales do not produce honeydew and can be separated from their protective covering. Nymphs are called crawlers and are often the target for chemical treatments since they do not produce a protective covering until completing their development. Scale insects tend to occur on fig, citrus, ferns, and ivies.

Mealybugs are soft, oval, wingless insects that measure up to 3 mm (1/10 inch) long. These insects excrete honeydew and are most active in dry, warm weather. The exact appearance varies depending on species. Some cover themselves with fluffy wax while others have long tail-like projections of wax. Females are wingless, while males resemble gnats and have four eyes, two wings, and long tails of white wax. A single female can lay 200-600 eggs in an ovisac, which is a dense, fluffy, white mass of wax. Low densities of mealybugs are inconspicuous as they tend to reside within plant crevices. As the eggs hatch, the infestation may become more noticeable as different life stages feed on various surfaces of the plant. Mealybugs tend to prefer coleus, cactus, lantana, hoya, jade, and poinsettia.

Feeding of scale insects can lead to stunted plant growth and death when infestations are heavy. Soft scales excrete honeydew which can lead to the growth of sooty mold. Feeding injuries produced by mealybugs include yellowing of leaves, dieback, stunted growth, honeydew accumulation, and death of the plant. Plants can be inspected regularly to detect scale insects and mealybugs.

Scale insects can be crushed or physically removed from plants by handpicking. A cotton swab can be used to apply rubbing alcohol directly to scale insects or mealybugs. One species of lady beetle, the mealybug destroyer (Cryptolaemus montrouzieri), is a predator of mealybugs eggs and nymphs. This predator is commercially available and is most effective at warmer temperatures when mealybug populations are high. Leptomastix dactylopii is a commercially available species of wasp that can parasitize a wide range of mealybug species.

For chemical control, thorough coverage of plants with horticultural oils or insecticide soaps can be effective against scale insects. Crawlers of both soft and armored scales are susceptible to most insecticides registered for use on houseplants, but coverage should be maintained throughout a two-to-four-month period to eliminate further infestations. Systemic insecticides can be effective against crawlers and adults of soft scales. Applications of systemic insecticides, such as soil applications of imidacloprid, can be effective against mealybugs.

Thrips

Thrips are usually less than 1.5 mm (1/16 inch) long and are brown, black, pale, or yellow. These insects have elongated bodies and can fly short distances. Their tiny size allows them to easily pass barriers that exclude larger insects, such as window screens. Given their small size, thrips are difficult to spot and are more easily detected by the feeding injuries they produce (see below). Most species of thrips spend a portion of their development in soil, which can make management challenging. Under favorable environmental conditions, thrips can complete their entire life cycle in two weeks. Some thrips species in Colorado can also transmit tomato spotted wilt virus and impatiens necrotic spot virus, both of which can infect a plant simultaneously. These insects frequently attack lilies, peonies, roses, and vines.

Feeding of thrips causes characteristic irregular silver patches on leaves caused by their feeding, in which thrips scrape plant tissues from the leaf surface, and their excrement which appears as tiny dark spots on the leaf surface. These feeding injuries are larger than the injuries produced by spider mites and can result in the distortion and scarring of petals when thrips infest flower buds. Deploying yellow sticky traps can be an effective approach for detecting thrips since they are small and unlikely to be spotted on the houseplant.

Removing any nearby weeds will eliminate alternative hosts for thrips, who often migrate to other plants when weeds begin to dry in spring or summer. Some natural enemies, such as the insidious flower bug (Orius insidiosus), are commercially available predators of thrips. Many thrips are resistant to insecticides, and targeting individuals during the soil-dwelling life stage is challenging. Some insecticides, such as pyrethroids, can be effective against certain thrips species, such as onion thrips (Thrips tabaci), while insecticide soaps or mineral oils can kill exposed life stages.

Spider mites

Spider mites are small arachnids that have eight legs, two body segments, and produce webbing. They are about 0.5 mm (0.02 inches) long and are easier to spot with a 10X hand lens. The two-spotted spider mite (Tetranychus urticae) is a common pest of household and greenhouse plants. When densities are low, spider mites tend to be located on the underside of leaves but will spread to the entire plant as the population builds, which can occur rapidly under low humidity and the presence of dust. These mites feed on plant fluids by producing small wounds on the surface of plant tissues. The entire life cycle of a spider mite can be completed in one or two weeks, depending on environmental conditions. A mature female can lay as many as five eggs per day, and after a couple of days the nymphs emerge and begin feeding continuously. Spider mites have a wide host range that includes ivies, dracaenas, figs, hibiscus, Norfolk Island pine, and scheffleras.

Spider mite feeding causes small wounds on plant tissues that appear as white flecks. Initially, these injuries are confined to small patches on the underside of leaves and near the base of leaf veins. The leaves gradually turn grey or bronze as the infestation builds and the feeding injury progresses, and eventually the leaves drop prematurely. Plants can be inspected regularly for the presence of webbing, which is abundant at high densities.

Plants can be periodically hosed with a jet of water to dislodge spider mites. Increasing humidity around the plant is also recommended. Several species of predatory mite will feed on spider mites and are commercially available. It is worth noting that predatory mite species require different environmental conditions for optimum performance. Foliar sprays of horticultural oils can be effective against the twospotted spider mite.

Whiteflies

Whiteflies are more closely related to aphids, scales, and mealybugs than true flies. These insects feed on plant fluids with piercing-sucking mouthparts and excrete honeydew. One species, the greenhouse whitefly (Trialeurodes vaporariorum), is a common indoor pest in Colorado. Adults are about 1.6 mm (1/16 inch) long and have yellow bodies with white wings, which allows them to fly when disturbed. The nymphs have oval bodies that are wingless, flat, lack legs, and are nearly transparent. Adult females can lay 25 eggs per day, and the entire life cycle can be completed in as little as three weeks. Most infestations occur through the transport of infested plants to clean areas, as whiteflies cannot overwinter outdoors in Colorado but may become abundant in warm weather. The greenhouse whitefly frequently attacks poinsettia, ivy, Hibiscus, and Lantana.

Heavy infestations of the greenhouse whitefly can cause premature leafdrop, stunted growth of foliage, and reduced plant vigor. These insects also excrete honeydew, which can attract ants and promote sooty mold growth. Visual inspections should be conducted regularly, and yellow sticky traps can be deployed to capture whiteflies.

Hand-held vacuums can be used to remove adult whiteflies from plants, and removing suitable hosts for at least two weeks can starve whiteflies in an area. For this method to be effective, it is important to consider whether whiteflies can survive on any nearby weeds. A small parasitoid wasp, Encarsia formosa, is commercially available for whitefly control. However, this parasitoid requires specific environmental conditions to be effective. Chemical control of whiteflies can be accomplished with pyrethrins, imidacloprid, horticultural oils, neem oil, or insecticidal soaps.

Fungus gnats

Fungus gnats are flies (order: Diptera) that fly when disturbed, making the adults easy to spot. They are about 3mm (1/10 inch) long, black flies with long legs and antennae. These flies also have a distinct “Y-shape” venation on the forewing. Adults tend to remain near infested plants but can fly short distances and often congregate around window frames. Females lay eggs in potting soil or moist organic debris, and adults typically live for seven to ten days. The soil-dwelling larvae are white or transparent, legless, have a shiny black head, and feed on decaying plant matter, fungi, algae, and plant roots. While usually considered a nuisance pest, heavy infestations can reduce plant health. Any houseplant is susceptible to infestation since fungus gnats reproduce in moist potting soil with decaying organic matter.

Heavy fungus gnat infestations are characterized by sudden wilting, loss of vigor, poor growth, foliage loss, and discoloration of plants. Plants and moist potting soil should be inspected regularly, and yellow sticky traps can be deployed to capture fungus gnats.

Fungus gnats are best controlled by avoiding excessive watering. Soils with lower concentrations of organic matter or fish emulsion fertilizers are less attractive to fungus gnats. Soil drench applications of Bacillus thuringiensis or neem products can kill fungus gnat larvae.

Springtails

Springtails are small insects with grey- or cream-colored bodies. These insects are often smaller than 2 mm in length and feed on decaying organic matter in soil with chewing mouthparts. Immature springtails resemble adults but are much smaller. Springtails can jump when disturbed and are more easily seen in potted plants after watering. Springtails can be found in any potted houseplant given their affinity for decaying organic matter in soil. While high densities of springtails can injure roots or tender foliage, they are typically considered nuisance pests and do not cause significant injury to houseplants. Like fungus gnats, springtails can be detected by carefully inspecting moist potting soil and are best controlled by avoiding excessive watering.

References

Cornell University. 2010. Springtails and Snowfleas. Cornell Cooperative Extension. Available https://www.maine.gov/dacf/php/gotpests/bugs/factsheets/springtails-snowfleas-cornell.pdf

Cranshaw, W. 2013. Greenhouse Whitefly. Colorado State University – Extension. Available https://extension.colostate.edu/topic-areas/insects/greenhouse-whitefly-5-587/

Frank, S., and J. Baker. 2022. North Carolina State University – Extension. Available https://content.ces.ncsu.edu/mealybugs#:~:text=Overall%20length%20maybe%201%2F4,appear%20to%20be%20born%20alive.

Hahn, J. 2019. Aphids in home yards and gardens. University of Minnesota – Extension. Available https://extension.umn.edu/yard-and-garden-insects/aphids#:~:text=Aphids%20are%20small%2C%201%2F16,but%20are%20a%20little%20darker

ISU. 2022. Scale Insects. Iowa State University – Extension. Available https://hortnews.extension.iastate.edu/scale-insects#:~:text=The%20size%20of%20scale%20insects,generally%20divided%20into%20two%20categories%3A&text=Soft%20scales%20produce%20a%20soft,separated%20from%20the%20insect%20body

Lewis, D., 2022. Scale Insects on Houseplants. Iowa State University – Extension. Available https://hortnews.extension.iastate.edu/scale-insects-houseplants#:~:text=Scale%20insects%20are%20a%20common,body%20of%20the%20sessile%20insect.

UCIPM. 2015. Whiteflies. University of California Agriculture and Natural Resources. Available https://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html

USU. 2008. Houseplant Pest Blues. Utah State University – Extension. Available https://extension.usu.edu/pests/files/up-newsletter/2008/UtahPests-Newsletter-winter08-09.pdf

White, J. (n.d.). Whiteflies in the Greenhouse. University of Kentucky – Extension. Available https://entomology.ca.uky.edu/ef456#:~:text=General%20Biology,the%20undersides%20of%20plant%20leaves.