Diagnosing Houseplant Problems Caused by Insect Pests

Growing indoor plants is a rewarding hobby that yields beautiful results.  But when problems arise with your indoor plants, it can be difficult to determine how to fix them.  

leaf with scaleWhen encountering a houseplant problem, ask yourself, Are the environmental conditions appropriate for this species?

If problems arise, start by understanding the needs of that particular species for light, soil, water, humidity, temperature, and fertilizer.  If these environmental conditions are not ideal, they must be changed, or the houseplant must be moved to a location where they are ideal.

A single factor does not cause many problems – they result from several factors coming together to cause an unhealthy or unattractive plant.  Always look for more than one factor contributing to the issue and correct all of them to solve the problem.  It is also important to identify the primary issue.  Some problems are caused by other factors, and while you can address the problem, if you don't solve the primary issue, the problem will always return.


Insects & Other Pests of Houseplants

While a wide range of species are grown as houseplants, only a few ubiquitous pests impact indoor plants.  Insects and other pests are often difficult problems for houseplants. When left unmanaged, they can quickly cause extensive damage to plants. 

Monitor for pests frequently and deal with them as soon as they are noticed while populations are small.  When managing any houseplant pest, often the best results come when you pair multiple techniques simultaneously.  For example, rather than just spraying an insecticidal soap to control aphids, pair it with a good rinse and selective pruning to get good results.  If possible, isolate any infested plants from your other plants because most houseplant insect pests can infest many types of houseplants.  


Mealybug  |  Scale  |  Fungus Gnats  |  Spider Mites  |  Aphids  |  Whiteflies  |  More Information  |  Back to Main Page


Mealybug

Mealybug on basil By Julia AdobeStock_569363275
Mealybug infestations are seen as cottony masses under leaves and in leaf axils.  Photo by Julia/AdobeStock

Mealybugs are white, cottony insects often found clustered in leaf axils and the undersides of leaves.  They emerge from egg masses and crawl around the plant to find a place to pierce the plant and suck sap. Mealybugs produce protective wax filaments, which gives them their white cottony look.  Mealybugs also produce honeydew (sugar-filled excrement), which makes lower leaves, tables, and floors sticky, and over time, the sticky deposits turn black with sooty mold.  Plants are weakened from the feeding, leading to stunted growth, yellow and deformed leaves, and death when infestations are large enough.

Management:

  • Mealybug is very difficult to control as they often feed in protected locations and their waxy coating makes them challenging to control with insecticides.
  • Scout often and catch it early.  Deal with it right away to prevent populations from becoming unmanageable.
  • Mealybugs produce honeydew, which causes leaves and areas around the houseplant to have shiny, clear spots.
  • If only a few are found, spot treat with a spray of rubbing alcohol or a cotton swab dipped in rubbing alcohol. 
  • Insecticidal soap, neem, and pyrethrin are all relatively safe insecticides that can be used indoors to treat this pest.  Apply frequently as insecticides are most effective on the immature mealybugs, which hatch every few weeks.
  • Granular insecticides applied to the soil can also be used but may not be a good choice for homes with pets or children who may dig in the soil because of their toxicity.  Be sure the plant species being treated is on the label, and always follow the label directions.
  • While using biological control is not practical at home, it can be considered for greenhouses and botanical gardens.
  • In many cases, destroying heavily infested plants before they infest nearby plants is better.  

Click here for pictures, videos, and more information on mealybugs.


Scale

scale on houseplant By Robert Lessmann AdobeStock_433777504
Scale insects often align along leaf veins or stems.  Photo by Robert Lessmann/AdobeStock

Scale insects look like small bumps or scales clustered or lined up along stems and leaf veins, typically on the underside.  These sucking insects hatch from eggs held under the hard protective covering of their mother, crawl around the plant, and then pierce the plant to suck sap.  Once they set up a permanent feeding site, they remain stationary for the rest of their life.  Scale insects produce honeydew, making lower leaves, tables, and floors sticky; over time, the sticky deposits turn black with sooty mold.  Plants are weakened from the feeding, causing stunted growth, yellow and deformed leaves, and plant death when infestations are big enough.

Management:

  • Scale insects are tough to control as the protective covering makes them challenging to control with insecticides.
  • Scout often and catch them early.  Deal with infestations right away to prevent populations from becoming unmanageable.
  • For light infestations, remove individual insects with your fingernail.
  • If only a few are found, spot treat with a spray of rubbing alcohol or a cotton swab dipped in rubbing alcohol. 
  • Prune out branches or leaves that are heavily infested.
  • Insecticidal soaps, pyrethrin, rotenone, resmethrin, and acephate are insecticides used indoors to treat for scale.  Apply weekly for at least 4 to 6 weeks, as insecticides are most effective on the immature “nymphs”  that hatch on a rotating basis.
  • Granular insecticides to soil can also be used but may not be a good choice for homes with pets or children who may dig in the soil because of their toxicity.  Be sure the plant species being treated is on the label, and always follow the label directions.
  • In many cases, destroying heavily infested plants before the scale can infest nearby plants is better.

Fungus Gnats

fungus gnats on yellow sticky card By DimaBerlin AdobeStock_664482431
Fungus gnats are black flies that become a nuisance. Photo by Dima Berlin/AdobeStock

These small, black, flying insects are very mobile and do not cause notable damage to houseplants but are a big nuisance.  The larvae thrive in wet soils and feed on fungi and decaying organic matter, including root systems that have died from overwatering. Because fungus gnats prefer moist soils, they are good indicators of over-watering.

Management:

  • Allow the soil, especially the soil surface, to dry out between waterings.
  • Never allow plants to sit in saucers or outer pots filled with water.
  • Insectidies are frequently ineffective as the larvae are found in the soil, making it hard to get insecticides in contact with the larvae, and the adults are too mobile to effectively spray.
    • Insecticides are not needed when soils are allowed to dry out.
  • Other recommendations of covering the soil surface with sand or rocks are unnecessary – just let the soil dry between waterings.  This is a good practice for your houseplants anyway!
  • Yellow sticky card traps can catch flying adults but do not prevent new insects from hatching.
  • Fungus gnats do not always originate from plants.  Check unused drains, condensation trays, or other wet areas.

Spider Mites

spider mites on lemon leaf By larisikstefania AdobeStock_482010639 (1)
Spider mites are difficult to see because of their small size, but their feeding causes a mottled appearance on the leaves. Photo by larisikstefania/AdobeStock

These arachnids (not insects) are closely related to spiders and ticks.  They are tiny and difficult to see and live primarily on the undersides of leaves.  Often, the best way to find them is to tap leaves over a piece of paper and look for tiny specks that move around on the paper.  Spider mites prefer dry conditions and produce fine webbing on leaves and stems when populations are big enough. While they feed on nearly any houseplant, some species, like Norfolk Island pine, ivy, and croton, are highly susceptible.  Mites feed by piercing and sucking leaves, causing a mottled appearance.  As infestations worsen, the yellow-green mottling turns tan and leaves eventually fall off.

Management:

  • Scout often and catch spider mites early.  Deal with this pest right away to prevent populations from becoming unmanageable.
  • Since they prefer dry conditions, increasing humidity can slow population growth.
  • Rinse foliage thoroughly in a shower or outdoors.
  • Remove branches or leaves that are heavily infested.
  • A miticide or other pesticides like insecticidal soap or horticultural oil can be applied according to label directions.  Be sure to treat the underside of the leaves thoroughly.  Multiple applications every week for 4 to 6 weeks will be needed for complete control.
  • In many cases, destroying heavily infested plants before they infest nearby plants is better. 

Aphids

aphids By ViniSouza128 AdobeStock_451097568 (1)
Aphids often cluster at growing points  Photo by ViniSouza128/AdobeStock

These small, soft-bodied insects are often green, brown, or reddish in color.  They typically cluster on growing points, underneath leaves, along stems, and on flower buds.  They have sucking mouthparts and produce honeydew which makes lower leaves, tables, and floors sticky, and over time, the sticky deposits turn black with sooty mold.  Plants are weakened from the feeding, causing curled, yellow, and deformed leaves that eventually drop off.  When infestations are big enough, plants can die.

Management:

  • Scout often and catch it early.  Aphid populations can grow quickly.  Deal with them right away to prevent populations from becoming unmanageable.
  • For light infestations, a forceful spray of water can dislodge most aphids. 
  • Remove branches or leaves that are heavily infested.
  • Insecticidal soap and other insecticides labeled for use on indoor plants are options for treatment.  Always follow label directions.
  • In many cases it is better to destroy heavily infested plants before they infest nearby plants. 

Whiteflies

white flies By Jurgen Kottmann AdobeStock_334506982 (1)
Whiteflies are very mobile but, when resting, are often found on the underside of the leaves. Photo by Jurgen Kottmann/AdobeStock

Whiteflies are small white, flying insects that are highly mobile making them difficult to control.  Immature whiteflies look like scale insects and feed on the undersides of leaves.  Adult whiteflies readily fly when disturbed.  They feed by sucking sap and, when infestations are large enough, produce honeydew.  Infested plants have leaves that are stunted or yellow and may drop prematurely.

Management:

  • Scout often and catch them early.  Deal with the pest right away to prevent populations from becoming unmanageable.
  • Inspect new plants carefully and never introduce plants that have infestations to the home.
  • Yellow sticky traps can trap adults as they are highly attracted to the color.  Sticky traps will not effectively eliminate the infestation but are very helpful in scouting the problem.
  • Insecticides, like insecticidal soap and neem, can be used but are typically less effective than you hope.  Focus applications on the undersides of leaves.  Multiple weekly applications are required to gain control.
  • In many cases it is better to destroy heavily infested plants before they infest nearby plants.  This is particularly true for this very mobile pest.

More Information About Diagnosing Houseplant Problems

Improper Environmental Conditions

Improper Environmental Conditions

Poor Culture

Poor Culture

Diseases

Diseases 

wilted peace lily

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