Houseplant pests and diseases

By Kim Willis These articles are copyrighted and may not be copied or used without the permission of the author.

Aphid giving birth
Wikipedia

Houseplants don’t suffer as many diseases as outside plants, but they do suffer from several types of insect pests. The most common insect pests are whitefly, aphids, mealy bugs,  spider mites, and scale.  There are several species of each type of these pests, but control is usually the same.  Another pest often found with houseplants is the fungus gnat. This is more of a soil pest and a nuisance although they can damage plant roots in some cases. I have a good article about fungus gnats on another page so they won’t be covered here.

Before you bring a plant home inspect it carefully for insect pests.  It’s always a good idea to have a spot where new plants can be quarantined for a week or two.  And if you notice insects or suspect them on plants you already have it’s good to quarantine them too.  The quarantine area should be a room separate from other plants.  You may have to use a grow light there.

Houseplants get insects from the greenhouse they were grown in and from summering outside.  However, putting insect infested plants outside will often cure the problem since outdoor conditions and natural predators will help control insects.  Of course, tropical plants can’t go out in winter and some people don’t have an area to put plants outside.

Some species of plants are more likely to insect pests than others.  Plants that aren’t being cared for properly are also more susceptible. Signs of insect infestations like yellowing or curled leaves can also be signs of poor cultural conditions.  Other things like fumes, things dumped in the pot, pet or child damage are sometimes mistaken for insect damage too.

Signs of insect pests are seeing the insects and also seeing honeydew, a sticky substance that insects poop out after sucking sweet plant juices.  Aphids, scale, mealy bugs and whitefly can all cause honeydew.  You’ll see it on plant surfaces and also on windows, pots and things the plants are sitting on.  It sometimes gets a black mold, called sooty mold.  Sooty mold doesn’t harm plants, but it’s a sign you have insect pests.

Disease

The most common diseases of houseplants are botrytis or gray mold and crown and stem rot.  Root rot is also common.  All of these are fungal diseases and while they are contracted from other diseased plants they must have the right conditions to develop and that’s usually overwatering in cool environments with poor air circulation.  Botrytis looks like a gray fuzzy mold on leaves, usually on begonias, African violets and some other soft leaved plants. The rots look just as you would suspect, brown mushy rotted areas on plants.

To control these diseases trim off any infected parts if you can.  Repot in a clean potting medium and scrub pots in hot water and soap if you reuse them. Improve growing conditions and don’t over water. If too much of the plant has rotted, you’ll probably just have to discard it as there is no cure.

Some species of houseplants are susceptible to viruses. They probably had them when you bought them, or they were given the virus by pests like aphids.  Viruses usually cause distorted growth, white streaking on flowers, and oddly blotched yellow areas on leaves.  Plants appear stunted and grow poorly.  There is no cure for viruses and plants should be discarded.

Other diseases of houseplants include some leaf spot diseases, both fungal and bacterial and a few other rare disease problems.  If you suspect them have the plant diagnosed by your county Extension office and follow the recommendations given for treatment.

Insects

Aphids are big enough to see with the naked eye and they can be a variety of colors.  Usually they are pale green.  Aphids have a plump body and tiny wings. They can fly but usually don’t, they crawl to new feeding places. They may cluster on stems and the back of leaves.  They like new growth, the tips of stems and flower buds.  Aphids suck plant juices and leave leaves looking yellow and often curled.  The plants are weakened and stunted. Aphids also carry diseases from plant to plant.  Honeydew is a good sign that aphids may be present.

Aphids give birth to live young that are tiny versions of themselves but without wings. The populations can rise rapidly.  Outside aphids are the favorite food of many other insects and often moving a houseplant outside can drop the aphid population down or eliminate it.

Whiteflies
en.wikipedia

Whiteflies are teeny, tiny white moth -like insects.  They produce tiny green larvae that suck plant juices, leaving behind that lovely honeydew. They cluster under leaves and in leaf joints, with the larvae usually on the backs of leaves. When you lightly shake a plant, the adult whiteflies may fly up in a cloud.   Like aphids they cause yellowed leaves and weak, stunted plants.

Mealybugs
UMN Extesion

Mealybugs look like tiny bits of cotton fluff in the leaf joints and underside and along the veins of leaves. The adults have white waxy hairs that cause them to look fluffy.  They too suck plant juices and cause leaves to yellow and fall. Eggs are laid in a bit of fluff on the backs of leaves. This fluff covering protects eggs from some forms of pesticides.

Spidermites and webbing
UMN Extension

Spider mites are very tiny red, yellow or olive colored insects barely visible to the naked eye. With a hand lens they do look like tiny spiders. What is visible is fine webbing, like spider webs, that may appear at branch tips, on the back of leaves and along plant trunks.  They cause yellow stippling of leaves and leaves that dry up and fall.  Heavy infestations can kill plants.

They are most often found when the environment is hot and dry, with low humidity.  To see if they are your problem place a white sheet of paper under some leaves and tap them.  If you see tiny dots that fall on the paper and move around you probably have spider mites, especially if you also see webbing.

scale insects

Scale insects look like tiny brown oval scabs along plant stems and on the back of leaves. Some people may mistake them for bark or natural parts of the plant. They attach to the plant and their hardened shell protects them from insecticides and other insects.  They have a larval stage that is mobile and moves from plant to plant.  They cause lots of honeydew and once again yellowed, falling leaves and weak stunted plants.

When you scrape off a scale insect with your fingernail the backside is usually pale colored. They are pretty easy to scrape off and it’s a good control, but they are often missed.  Because adult scale insects are protected from sprays systemic pesticides should be used on them.  These are pesticides that are poured on the soil and taken up internally by the plant.  The crawling stage can be killed by insecticidal soap or spray pesticides.

For Fungus gnats go to this page; 

https://gardeninggrannysgardenpages.blogspot.com/p/fungus-gnats-inhouseplants-in-winter.html

Pest control

People often get very upset when a houseplant insect pest pops up.  It isn’t the end of the world and most can be saved.  Some people simply throw out the plant and that’s an option, but if the problem is noticed early and the proper treatment started most plants can be saved.  Houseplant pests don’t harm the house, other than the honeydew, (which can be cleaned off windows and other things with soap and water), and they don’t infect people or pets. 

With houseplants one should take a IPM approach to control.  That means identifying the pest and starting with the safest solutions for control and working “up” from there if more radical steps are needed and desired.

The very first thing one should do when insect pests are identified is give the plant a shower.  Yes, a shower.  Move the plant outside or to the shower or basement laundry tub and spray them with a strong stream of warm water.  You don’t need soap, just water. This will dislodge most pests except scale insects, and it will even help with scale. Hit the underside of leaves and along stems and branch joints with the stream of water.  If you are worried about soil washing out of the pot or it getting too wet, cover the pot with a plastic bag.  Repeat the shower every few days until the problem seems to have disappeared.

The second thing to try if showers aren’t correcting the problem is an insecticidal soap spray.  Use a commercial product, not homemade concoctions which may harm plants. This is most effective on aphids, mealy bugs, spider mites and white fly.

For whitefly and aphids, you can try yellow sticky traps, which are hung close to plants and which attract insects, which stick to them. You can buy these or make your own by coating bright yellow plastic or cardboard with petroleum jelly.  Replace them whenever they have a lot of insects on them or get dusty.

For scale insects you can try removing them by scraping them off.  Give them a warm shower first if you can to wash off the larval stage and soften the scales. You can use your fingernail or a pocket knife blade to scrape them off.  Or you can use cotton swabs dipped in a little alcohol to remove them.  Try not to get alcohol on the plant tissue.  This is a tedious chore and you will miss some of the scales, I guarantee it.  Removing most of the population will increase the plants health.  You will need to repeat the treatment at least a few times.

If the more natural remedies don’t work and you are not averse to chemical treatments I will offer some suggestions here.  Remember to follow label directions exactly.  It’s a good idea to take the plants being treated to an unused room or outside until sprays have dried.  Systemic treatments can sometimes be used.  If you have pets that eat plants or for some reason, kids that eat plants, you may not want to use pesticides, although some are pretty safe even with consumption of a small amount on the plant.

Don’t worry about pesticides harming good insects if the plants are inside, or even if they go outside but are not blooming plants.

Remember that pesticides regulations change so this pesticide list could be outdated when you read it.  Look for products that mention they can be used inside and that control the pest you have identified.  Always follow label directions and keep pesticides out of the reach of kids in pets. These are not brand names, look for the ingredient listed below on labels.

Pyrethrins, resmethrin, and permethrin will work on most insect pests except spider mites. Bifenthrin will work on most pests including spider mites. Imidacloprid will work on mealybugs, scale, and aphids but is used in special potting mixes that can be hard to find. Neem oil will work on whiteflies and aphids.

And while we are talking treatments you know I have to caution about some of the oddball home remedies that float around.  Dish soap solutions are one of the most popular home concoctions.  Dish soap should not be used on plants. They have chemicals that strip natural oils and waxes off plant leaves.  An insecticidal soap, one you purchase, has a different formula and doesn’t remove the protective waxes and oils.  Dish soap may cause leaf yellowing, drying up and curling also.  Many of the dish soap “recipes” I see are so diluted they won’t harm the plants, but they won’t harm the insects either.

Other remedies that don’t work on houseplants are Epsom salts, diatomaceous earth, alcohol sprays (very bad for plants), coffee or coffee grounds, cinnamon, baking soda, onion or garlic oil, and other grocery items.  Also, not recommended for inside are predators, like lady bugs or frogs. (Don’t laugh, people suggest this, but house conditions are not good for them and its rarely effective control in a home.) But if you leave regular spiders alone near your plants they may control some pests.

Houseplant pests don’t need to destroy your plants or keep you from having houseplants.  Keeping a watchful eye on your plants and moving quickly to control pests will keep your plants healthy and growing.