Dealing with Buggy Indoor Plants in Winter (Or any time, really) By Paula Szilard It’s easy to deal with buggy plants when you can just take them outside to dispatch the pests. In the winter months, you usually have to find some way of dealing with them indoors. You don’t have bugs? We’d love to know your secret. Most of us have waged decades long wars against them. If you are skittish about spraying pesticides in your home, you are not alone. Many of us would prefer not to use them in areas where we relax, cook and sleep. Especially during winter, when we don’t want to open our windows, the lowest impact methods of pest control suddenly become much more attractive. The most frequently seen indoor plant pests are mealy bugs, scale, whitefly, aphids, spider mites and fungus gnats. Most of them injure plants by sucking fluids out of leaves stems and branches. Below are some effective low impact ways of dealing with them. The Arsenal Water Whitney Cranshaw, CSU Extension entomologist and stand up comedian (If you‘ve heard him speak, you know what I mean.) recommends chiefly low impact methods. He emphasizes using strong jets of water to dislodge pests, particularly spider mites and aphids. Applications must be done repeatedly, especially in the case of aphids, which have a very short life cycle. You can also knock scale off leaves and branches if your jet of water is strong enough. I use our upstairs shower fitted with a shower massage and a double laundry sink in the basement plant room fitted with an outdoor sprayer. The really large ones like the banana get a special preventive treatment in the fall before they are brought indoors and if they need further attention in the winter I wipe affected areas with alcohol and wait for a 60 F day to take them outside and treat them. Of late, there have been a number of such warm winter days. Trapping Trapping can also be an effective technique for reducing the populations of flying insects like the greenhouse whitefly and fungus gnats. The yellow sticky traps available at your garden center are perfect for this, but for best results you must use them in conjunction with another method of control, like horticultural oils, or in the case of fungus gnats, a BT soil drench in the form of Gnatrol* or Knock Out Gnats.*(See below.) Pruning Consider pruning out some of the badly infested branches. This enables you to focus on saving the rest of the plant and just like surgery in humans, it might help to save its life. If a plant forms new shoots easily from the stems, consider cutting it almost to the ground, leaving a short stub. Fine horticultural oils** These oils are also quite effective against spider mites, scale and even mealy bugs, but not fungus gnats. Because most people find mealies so disgusting, you will want to rinse them off first with a strong spray of water and let the plant dry. My sister who is an interiorscaper recommends knocking off not only mealies, but also scale and other insects, allowing the plant to dry and then treating it with the oil spray. After all, the oil just suffocates the insects. It does not remove the dead bodies. Be forewarned about fine horticultural oils**. They should not be used indoors unless you have an enclosed area where they will not cause oil stains on walls, floors and even shower grouting. Nor should they be used in the sun. Also, do not put a treated plant back in a sunny spot, even indoors. The sun could scorch the plant. In the winter months I usually wait for the 60 F days to do this outdoors, unless the plant is small enough to fit into my shower or double sink. Many plants are damaged by oil spray. If the plant is hairy like an African violet, for instance, horticultural oil spray is a good way to hasten its demise. Study the label carefully. It will tell you what plants are damaged by oil spray. Alcohol Although labor intensive, Alcohol has been an old standby for removing mealies and scale. The first time you use it on a plant, apply just a little to a test spot. I apply it, not with Q-tips, but with the 2-inch cotton rounds used to remove make-up. You can wipe larger surfaces, which saves you a lot of work. Alcohol has the reputation of causing damage, especially to leaves. Sometimes a 25% or 50% dilution of alcohol with water can be sprayed directly on the plant to make the process a little easier. Be forewarned, though, it is possible to kill a plant with very tender thin leaves using this method. Also, never use alcohol on a cactus or succulent. It will remove the waxy coating on the outside surface and not only will the plant look unattractive, it will lose some of its protection against water loss. I use alcohol to remove scale from my oleander plants. Oleander is very attractive to scale insects, but it is also very sensitive to oil spray. I have actually killed an oleander plant by spraying it with fine horticultural oil**. I have read that people who use oil spray on oleanders outdoors go back an hour later and rinse it off. With one oleander death on my conscience already, I have not had the courage to try this. Insecticidal Soaps Insecticidal soaps, which contain potassium salts of fatty acids, are an often recommended method of dealing with houseplant pests. I personally think they are overrated, especially when it comes to dealing with spider mites and scale. Just a reminder, they are only effective if they make direct contact with the insect. I seem to rely more on just sprays of water and treatment with horticultural oils. If you do try it, then be sure to test first because some of these soap sprays have the potential to damage plants. Bacillus thuringiensis, var. israelensis (H-14 strain) BT israelensis is the treatment of choice against those pesky fungus gnats that breed in the top layer of potting mix when it’s kept too moist. It’s used as a soil drench and is sold as Gnatrol and Knock Out Gnats.* The latter is available mail order from Gardens Alive. There are also cultural controls. To begin with, water less! Water from the bottom and replace the top 2 inches of potting mix with clean, washed sand, which lacks the organic material the larvae need for food. With sand on top, it is difficult to determine when the plant needs water, so use a moisture meter to determine if the soil is dry. Neem Oil This oil can be applied much like the fine horticultural oils and like these oils, it does a good job of suffocating the insects. However, it also interferes with the insect’s growth, something other oils don’t do. So, if you have the opportunity, give it a try, however do a test application first. Remember oils can be harmful to certain plants! Pyrethrins These are natural insecticides manufactured from the pyrethrum daisy. They are considered effective against the greenhouse whitefly. Pyrethrins are ingredients in many natural insecticides used in the garden and on indoor plants and are permitted as inputs in organic agriculture. They act rapidly, upon contact, but are not considered toxic to humans, nor do they persist in the environment. However, they are considered highly toxic both bees and fish and moderately toxic to birds, so don’t use them outdoors in when bees and birds are around. Nor should you flush them down the drain or pour them out where they could be carried through storm sewers to streams and rivers. There are also a number of synthetic pyrethroids, such as resmethrin, tetramethrin, and sumithrin which are effective against the greenhouse whitefly and other insects. Another one, bifenthrin can be effective against spider mites. Like the natural pyrethrins, the synthetic pyrethroids are also highly toxic to bees and fish and somewhat toxic to birds, so the cautions above also apply. Imidacloprid This systemic insecticide is used in many different products formulated for outdoor use. It has also been formulated as a systemic for indoor use sold under the trade name Marathon. It is a neonicotinoid insecticide which kills bees and other pollinators. It is taken up by the plant and translocated to the nectar and pollen. Germany and France are about to ban it, but we are still fiddling while Rome burns. Use it only when you are totally desperate to save a valuable plant and have tried all other options. When you dispose of the soil, make sure to put it directly into the trash. Do not put it anywhere in the garden and heaven forbid, not the compost pile. Also, don’t bring any treated plants outside for the summer. Act Quickly! The old adage, “He who hesitates is lost,” is never more true than when applied to houseplant pest control. Inspect your plants regularly. Use a magnifier, if necessary. Do not wait until your plant is covered with the fine webbing indicative of large populations of spider mites, or your sunroom floor gets sticky with honeydew from an aphid, mealybug, or scale infestation. I waited once until some of the fungus gnats wound up in our morning coffee. My husband was not pleased! Or worse yet, before I knew much about indoor plant pests, I once waited until the top layer of soil actually moved when the small larvae of the fungus gnats began to stir. By the way, honeydew is a thick, sticky clear liquid excreted by aphids, mealy bugs, scale and the greenhouse whitefly. It is easily washed off wood floors and vinyl, but could be more difficult to remove safely from fine furniture and upholstery. It is quite sweet and can attract ants. When it builds up on branches of woody plants, it encourages the build up of a black fungus called sooty mold. Timing is Key! In planning your treatments, do pay attention to the life cycle of the insect in question because this suggests appropriate treatment intervals. If you really want to disrupt their life cycle you have to do more than kill of the adults. It is helpful if you get the other stages like the larvae, immature insects, even eggs (assuming your method of treatment is actually effective in controlling these). This means that in the long run you’ll probably make fewer applications because they will be more effective. It also means you need to know a little about the life cycle of each insect, so you can to hit it when it really counts. For instance, if your targeted insect hatches a new generation within 3 days, the best time to strike is before that new generation has a chance to spread, or if you are using a control that also kills off eggs and larvae, you can apply it during these stages. The Pests Aphids Aphids, Whitney Cranshaw is fond of saying, are born pregnant! Such an incredible reproductive feat could have given this insect an evolutionary edge. Maybe that’s why they’re so widespread! It only takes 14 days for a new generation to mature in temperatures above 60 F. If you have ever wondered why aphids suddenly appear after you bring plants indoors in the fall, the warm temperatures in your home could be the reason. Greenhouse whitefly This insect is the scourge of greenhouses and sunrooms and is notoriously difficult to eradicate. Adults lay their eggs, which hatch in 3-7 days, on the bottom of the leaf. These young are the immature nymphs. They then go into a pupal stage from which they emerge as adults. Whitney Cranshaw considers the pyrethrin-containing insecticides (tetramethrin, resmethrin, sumithrin) the most effective controls for whitefly. He also considers the systemic insecticide imidacloprid highly effective. According to Cranshaw, however, horticultural and neem oils are only effective on the nymphs on the leaves. Mealybugs We think of mealies as stationary, but they’re not. The eggs, hidden in all that white cottony stuff, hatch in just 3 days. So timing the treatment to destroy the next generation is important. The young crawlers, which look like little white specks, do move around on the plant and can colonize other parts of the plant and then move on to nearby plants. After you have treated a plant, make sure you clean the pot thoroughly inside and out, paying particular attention to the lip. If insects are left on the pot, it is easy for them to reinfest the plant. It is surprising how little these critters need to survive. I had left a book near an infested plant for a couple of weeks and they started to spread on the spine of the book. If I didn’t have photographic evidence, no one would believe me! Certain mealybugs can also attack a plant’s root system. When this happens, you only have two alternatives—either get rid of the plant or use a systemic insecticide formulated specifically for indoor plants, such as imidacloprid effective at controlling them. Please look below in the paragraph dealing with imidacloprid about proper use and proper disposal. Spider Mites Spider mites are not really insects. They are arachnids related to the spiders. Unfortunately, most insecticides meant for other pests are not affective against them. Their whole life cycle is completed in 1-2 weeks. If you are into chemicals, you could use a miticide, a pesticide which targets them specifically. However, if you want to use a low impact method of control, then fine horticultural oil* is effective. Often, affected plants have been watered insufficiently or located in an overly dry and hot place, so it’s wise to correct these growing conditions. Whitney Cranshaw also considers the binfenthrin effective against the two spotted spider mite, the most common spider mite attacking houseplants. Scale Many different types of scale insects attack indoor plants. Many are soft, but some have a hard shell and are referred to as armored scales. Sometimes the soft types are very difficult to see unless you use a magnifyer. They are shiny light brown or pale yellow insects with darker brown markings. On a small branch of citrus, for instance, these insects are virtually invisible. They can attack woody branches and the bottom surfaces of leaves along the leaf vein. You can tell they’re there because you see the tell tale signs of the honeydew. The armored scales produce less honeydew than the soft ones. Like the soft scales they are also found on branches and the bottom surfaces of leaves along the veins. Contrary to popular opinion, scale insects are not stationary throughout their life cycle. As with mealybugs, there is a mobile ‘crawler’ stage. If you can apply your horticultural oil during this phase of their life cycle, it is more effective because they have little protection. Females can give birth to live young, which is not in your favor, but relative to other common houseplant pests, they have a long life cycle, so you have some time to work on them. It takes them about 1 to 6 months to grow to maturity. Indoors, there may by anywhere from 3 to 6 generations a year. Fungus Gnats Although fungus gnats are small flies, they look a little like tiny mosquitoes. They don’t cause much damage to plants, but they are extremely annoying. Fungus gnat eggs hatch in 6 days. The entire life cycle lasts about 1 month. These insects like to feed on and breed in decaying organic material in consistently moist soils. You can use the soil drench called Gnatrol* or Knock Out Gnats*. It is relatively slow acting. If you have the stomach for it, a handout produced by CSU Extension (Plant Talk N o. 1445) describes a novel approach to reducing the number of larvae, i.e. maggots. They suggest placing a ½ inch slice of potato on the surface of the soil. The larvae will come out and feed on it. You can catch them when they are on the soil surface or feasting on the potato. Presumably this would have to be repeated! You may also need to pay attention to some of the cultural practices that created the situation. Chances are you have been overwatering, thereby creating the perfect environment for fungus gnats. Knowing When You’re Licked If the effort to save a plant is going to be gargantuan, you might want to reconsider. How unusual is the plant? Will it be difficult to replace? If it’s in some way valuable to you, a family heirloom, for instance, you might want to give it your all. If on the other hand, the plant can be found at garden centers or readily ordered through the mail, it might not be worth the effort. Consider, too, if you could get a cutting from a friend or a fellow garden club member and start a new plant. If a plant is badly infested and you keep treating it without good results, consider throwing it out. I recently treated a plant with a bad infestation of mealies for the better part of a year, only to give up on it eventually anyway. I could have saved myself a lot of work by just getting rid of it earlier. Sometimes you’re just not ready to admit defeat! Preventing Trouble Since most infestations begin with the purchase of buggy plants, examine plants very carefully before you purchase them, even using a magnifyer if necessary. Then, when you are satisfied that you are probably not bringing an infested plant into your home, isolate it for a month or so to be certain. Many of us don’t have a good place to isolate plants. If you have a spare bedroom, that will work. Just like humans, indoor plants like a summer vacation. Taking your plants out for the summer gives many of them optimal conditions for growth and flowering. This also keeps the insect infestations down. Unless you are a heavy pesticide user, you probably have an abundance of beneficial insects flying about just waiting for a quick snack. Before you know, most of your pests have disappeared. While it’s true you might occasionally pick up an aphid or other insect, the benefits far outweigh the costs. You can always rinse the plants off before bringing them in. When you move your plants out, don’t just put them directly into the bright sun. Start them out on the north side of your house, then move them to the east side and finally, if they need a lot of sun to the west or south side. As mentioned above, careful and regular inspections of the plants in your collection will make it possible for you to nip a problem in the bud. Also, making sure that plants are grown according to their individual needs of soil, water, light and fertilization will assure that plants have a better chance to resist infestation. Author’s Note: I would like to thank my sister Irma Kristel, who runs a small interiorscape business, for reviewing the draft of this article and for making useful suggestions for changes and additions. Footnotes: *Knock Out Gnats is available from Gardens Alive, www.gardensalive.com. Gnatrol is only available locally at wholesalers in very large quantities. **The label on my 2 ½ gallon container says as follows: Ultra-Fine Oil—All Season Horticultural Insecticide, Miticide and Fungicide. There are other brands out there. Just check with your garden center. Sources: Caldwell, Brian, et al. Resource Guide for Organic Insect and Disease Management. Ithaca, NY: Cornell University Extension Service, 2005. Cranshaw, W.S. Managing Houseplant Pests. Fort Collins, CO: Colorado State University Extension Service. Last reviewed: 11/06. (Fact Sheet No. 5.596). Available at the CSU Extenstion website, www.ext.colostate.edu. Fungus Gnats. CSU Extension. (Plant Talk Colorado no. 1445). Available at the CSU Extension website: www.ext.colostate.edu. Krischik, V. “Imidacloprid, found in most homeowners insecticides is translocated to nectar and pollen and kills good bugs.” Yard and Garden News, University of Minnesota Extension, August 2010. McCollum, Susan and Teena Risley. Plant Basics: A Manual for the Care of Indoor Plants. Fairfax, VA: Garden Tapestry, Inc., 1994.

Dealing with Buggy Indoor Plants in Winter

(Or any time, really)

Paula Szilard

It’s easy to deal with buggy plants when you can just take them outside to dispatch the pests. In the winter months, you usually have to find some way of dealing with them indoors. You don’t have bugs? We’d love to know your secret. Most of us have waged decades long wars against them.

If you are skittish about spraying pesticides in your home, you are not alone. Many of us would prefer not to use them in areas where we relax, cook and sleep. Especially during winter, when we don’t want to open our windows, the lowest impact methods of pest control suddenly become much more attractive. The most frequently seen indoor plant pests are mealy bugs, scale, whitefly, aphids, spider mites and fungus gnats. Most of them injure plants by sucking fluids out of leaves stems and branches. Below are some effective low impact ways of dealing with them.

The Arsenal

Water

Whitney Cranshaw, CSU Extension entomologist and stand up comedian (If you‘ve heard him speak, you know what I mean.) recommends chiefly low impact methods. He emphasizes using strong jets of water to dislodge pests, particularly spider mites and aphids. Applications must be done repeatedly, especially in the case of aphids, which have a very short life cycle. You can also knock scale off leaves and branches if your jet of water is strong enough. I use our upstairs shower fitted with a shower massage and a double laundry sink in the basement plant room fitted with an outdoor sprayer. The really large ones like the banana get a special preventive treatment in the fall before they are brought indoors and if they need further attention in the winter I wipe affected areas with alcohol and wait for a 60 F day to take them outside and treat them. Of late, there have been a number of such warm winter days.

Trapping

Trapping can also be an effective technique for reducing the populations of flying insects like the greenhouse whitefly and fungus gnats. The yellow sticky traps available at your garden center are perfect for this, but for best results you must use them in conjunction with another method of control, like horticultural oils, or in the case of fungus gnats, a BT soil drench in the form of Gnatrol* or Knock Out Gnats.*(See below.)

Pruning

Consider pruning out some of the badly infested branches. This enables you to focus on saving the rest of the plant and just like surgery in humans, it might help to save its life. If a plant forms new shoots easily from the stems, consider cutting it almost to the ground, leaving a short stub.

Fine horticultural oils**

These oils are also quite effective against spider mites, scale and even mealy bugs, but not fungus gnats. Because most people find mealies so disgusting, you will want to rinse them off first with a strong spray of water and let the plant dry. My sister who is an interiorscaper recommends knocking off not only mealies, but also scale and other insects, allowing the plant to dry and then treating it with the oil spray. After all, the oil just suffocates the insects. It does not remove the dead bodies. Be forewarned about fine horticultural oils**. They should not be used indoors unless you have an enclosed area where they will not cause oil stains on walls, floors and even shower grouting. Nor should they be used in the sun. Also, do not put a treated plant back in a sunny spot, even indoors. The sun could scorch the plant. In the winter months I usually wait for the 60 F days to do this outdoors, unless the plant is small enough to fit into my shower or double sink. Many plants are damaged by oil spray. If the plant is hairy like an African violet, for instance, horticultural oil spray is a good way to hasten its demise. Study the label carefully. It will tell you what plants are damaged by oil spray.

Alcohol

Although labor intensive, Alcohol has been an old standby for removing mealies and scale. The first time you use it on a plant, apply just a little to a test spot. I apply it, not with Q-tips, but with the 2-inch cotton rounds used to remove make-up. You can wipe larger surfaces, which saves you a lot of work. Alcohol has the reputation of causing damage, especially to leaves. Sometimes a 25% or 50% dilution of alcohol with water can be sprayed directly on the plant to make the process a little easier. Be forewarned, though, it is possible to kill a plant with very tender thin leaves using this method. Also, never use alcohol on a cactus or succulent. It will remove the waxy coating on the outside surface and not only will the plant look unattractive, it will lose some of its protection against water loss.

I use alcohol to remove scale from my oleander plants. Oleander is very attractive to scale insects, but it is also very sensitive to oil spray. I have actually killed an oleander plant by spraying it with fine horticultural oil**. I have read that people who use oil spray on oleanders outdoors go back an hour later and rinse it off. With one oleander death on my conscience already, I have not had the courage to try this.

Insecticidal Soaps

Insecticidal soaps, which contain potassium salts of fatty acids, are an often recommended method of dealing with houseplant pests. I personally think they are overrated, especially when it comes to dealing with spider mites and scale. Just a reminder, they are only effective if they make direct contact with the insect. I seem to rely more on just sprays of water and treatment with horticultural oils. If you do try it, then be sure to test first because some of these soap sprays have the potential to damage plants.

Bacillus thuringiensis, var. israelensis (H-14 strain)

BT israelensis is the treatment of choice against those pesky fungus gnats that breed in the top layer of potting mix when it’s kept too moist. It’s used as a soil drench and is sold as Gnatrol and Knock Out Gnats.* The latter is available mail order from Gardens Alive. There are also cultural controls. To begin with, water less! Water from the bottom and replace the top 2 inches of potting mix with clean, washed sand, which lacks the organic material the larvae need for food. With sand on top, it is difficult to determine when the plant needs water, so use a moisture meter to determine if the soil is dry.

Neem Oil

This oil can be applied much like the fine horticultural oils and like these oils, it does a good job of suffocating the insects. However, it also interferes with the insect’s growth, something other oils don’t do. So, if you have the opportunity, give it a try, however do a test application first. Remember oils can be harmful to certain plants!

Pyrethrins

These are natural insecticides manufactured from the pyrethrum daisy. They are considered effective against the greenhouse whitefly. Pyrethrins are ingredients in many natural insecticides used in the garden and on indoor plants and are permitted as inputs in organic agriculture. They act rapidly, upon contact, but are not considered toxic to humans, nor do they persist in the environment. However, they are considered highly toxic both bees and fish and moderately toxic to birds, so don’t use them outdoors in when bees and birds are around. Nor should you flush them down the drain or pour them out where they could be carried through storm sewers to streams and rivers. There are also a number of synthetic pyrethroids, such as resmethrin, tetramethrin, and sumithrin which are effective against the greenhouse whitefly and other insects. Another one, bifenthrin can be effective against spider mites. Like the natural pyrethrins, the synthetic pyrethroids are also highly toxic to bees and fish and somewhat toxic to birds, so the cautions above also apply.

Imidacloprid

This systemic insecticide is used in many different products formulated for outdoor use. It has also been formulated as a systemic for indoor use sold under the trade name Marathon. It is a neonicotinoid insecticide which kills bees and other pollinators. It is taken up by the plant and translocated to the nectar and pollen. Germany and France are about to ban it, but we are still fiddling while Rome burns. Use it only when you are totally desperate to save a valuable plant and have tried all other options. When you dispose of the soil, make sure to put it directly into the trash. Do not put it anywhere in the garden and heaven forbid, not the compost pile. Also, don’t bring any treated plants outside for the summer.

Act Quickly!

The old adage, “He who hesitates is lost,” is never more true than when applied to houseplant pest control. Inspect your plants regularly. Use a magnifier, if necessary. Do not wait until your plant is covered with the fine webbing indicative of large populations of spider mites, or your sunroom floor gets sticky with honeydew from an aphid, mealybug, or scale infestation. I waited once until some of the fungus gnats wound up in our morning coffee. My husband was not pleased! Or worse yet, before I knew much about indoor plant pests, I once waited until the top layer of soil actually moved when the small larvae of the fungus gnats began to stir. By the way, honeydew is a thick, sticky clear liquid excreted by aphids, mealy bugs, scale and the greenhouse whitefly. It is easily washed off wood floors and vinyl, but could be more difficult to remove safely from fine furniture and upholstery. It is quite sweet and can attract ants. When it builds up on branches of woody plants, it encourages the build up of a black fungus called sooty mold.

Timing is Key!

In planning your treatments, do pay attention to the life cycle of the insect in question because this suggests appropriate treatment intervals. If you really want to disrupt their life cycle you have to do more than kill of the adults. It is helpful if you get the other stages like the larvae, immature insects, even eggs (assuming your method of treatment is actually effective in controlling these). This means that in the long run you’ll probably make fewer applications because they will be more effective. It also means you need to know a little about the life cycle of each insect, so you can to hit it when it really counts. For instance, if your targeted insect hatches a new generation within 3 days, the best time to strike is before that new generation has a chance to spread, or if you are using a control that also kills off eggs and larvae, you can apply it during these stages.

The Pests

Aphids

Aphids, Whitney Cranshaw is fond of saying, are born pregnant! Such an incredible reproductive feat could have given this insect an evolutionary edge. Maybe that’s why they’re so widespread! It only takes 14 days for a new generation to mature in temperatures above 60 F. If you have ever wondered why aphids suddenly appear after you bring plants indoors in the fall, the warm temperatures in your home could be the reason.

Greenhouse whitefly

This insect is the scourge of greenhouses and sunrooms and is notoriously difficult to eradicate. Adults lay their eggs, which hatch in 3-7 days, on the bottom of the leaf. These young are the immature nymphs. They then go into a pupal stage from which they emerge as adults. Whitney Cranshaw considers the pyrethrin-containing insecticides (tetramethrin, resmethrin, sumithrin) the most effective controls for whitefly. He also considers the systemic insecticide imidacloprid highly effective. According to Cranshaw, however, horticultural and neem oils are only effective on the nymphs on the leaves.

Mealybugs

We think of mealies as stationary, but they’re not. The eggs, hidden in all that white cottony stuff, hatch in just 3 days. So timing the treatment to destroy the next generation is important. The young crawlers, which look like little white specks, do move around on the plant and can colonize other parts of the plant and then move on to nearby plants. After you have treated a plant, make sure you clean the pot thoroughly inside and out, paying particular attention to the lip. If insects are left on the pot, it is easy for them to reinfest the plant. It is surprising how little these critters need to survive. I had left a book near an infested plant for a couple of weeks and they started to spread on the spine of the book. If I didn’t have photographic evidence, no one would believe me! Certain mealybugs can also attack a plant’s root system. When this happens, you only have two alternatives—either get rid of the plant or use a systemic insecticide formulated specifically for indoor plants, such as imidacloprid effective at controlling them. Please look below in the paragraph dealing with imidacloprid about proper use and proper disposal.

Spider Mites

Spider mites are not really insects. They are arachnids related to the spiders. Unfortunately, most insecticides meant for other pests are not affective against them. Their whole life cycle is completed in 1-2 weeks. If you are into chemicals, you could use a miticide, a pesticide which targets them specifically. However, if you want to use a low impact method of control, then fine horticultural oil* is effective. Often, affected plants have been watered insufficiently or located in an overly dry and hot place, so it’s wise to correct these growing conditions. Whitney Cranshaw also considers the binfenthrin effective against the two spotted spider mite, the most common spider mite attacking houseplants.

Scale

Many different types of scale insects attack indoor plants. Many are soft, but some have a hard shell and are referred to as armored scales. Sometimes the soft types are very difficult to see unless you use a magnifyer. They are shiny light brown or pale yellow insects with darker brown markings. On a small branch of citrus, for instance, these insects are virtually invisible. They can attack woody branches and the bottom surfaces of leaves along the leaf vein. You can tell they’re there because you see the tell tale signs of the honeydew. The armored scales produce less honeydew than the soft ones. Like the soft scales they are also found on branches and the bottom surfaces of leaves along the veins. Contrary to popular opinion, scale insects are not stationary throughout their life cycle. As with mealybugs, there is a mobile ‘crawler’ stage. If you can apply your horticultural oil during this phase of their life cycle, it is more effective because they have little protection. Females can give birth to live young, which is not in your favor, but relative to other common houseplant pests, they have a long life cycle, so you have some time to work on them. It takes them about 1 to 6 months to grow to maturity. Indoors, there may by anywhere from 3 to 6 generations a year.

Fungus Gnats

Although fungus gnats are small flies, they look a little like tiny mosquitoes. They don’t cause much damage to plants, but they are extremely annoying. Fungus gnat eggs hatch in 6 days. The entire life cycle lasts about 1 month. These insects like to feed on and breed in decaying organic material in consistently moist soils. You can use the soil drench called Gnatrol* or Knock Out Gnats*. It is relatively slow acting. If you have the stomach for it, a handout produced by CSU Extension (Plant Talk N o. 1445) describes a novel approach to reducing the number of larvae, i.e. maggots. They suggest placing a ½ inch slice of potato on the surface of the soil. The larvae will come out and feed on it. You can catch them when they are on the soil surface or feasting on the potato. Presumably this would have to be repeated! You may also need to pay attention to some of the cultural practices that created the situation. Chances are you have been overwatering, thereby creating the perfect environment for fungus gnats.

Knowing When You’re Licked

If the effort to save a plant is going to be gargantuan, you might want to reconsider. How unusual is the plant? Will it be difficult to replace? If it’s in some way valuable to you, a family heirloom, for instance, you might want to give it your all. If on the other hand, the plant can be found at garden centers or readily ordered through the mail, it might not be worth the effort. Consider, too, if you could get a cutting from a friend or a fellow garden club member and start a new plant.

If a plant is badly infested and you keep treating it without good results, consider throwing it out. I recently treated a plant with a bad infestation of mealies for the better part of a year, only to give up on it eventually anyway. I could have saved myself a lot of work by just getting rid of it earlier. Sometimes you’re just not ready to admit defeat!

Preventing Trouble

Since most infestations begin with the purchase of buggy plants, examine plants very carefully before you purchase them, even using a magnifyer if necessary. Then, when you are satisfied that you are probably not bringing an infested plant into your home, isolate it for a month or so to be certain. Many of us don’t have a good place to isolate plants. If you have a spare bedroom, that will work.

Just like humans, indoor plants like a summer vacation. Taking your plants out for the summer gives many of them optimal conditions for growth and flowering. This also keeps the insect infestations down. Unless you are a heavy pesticide user, you probably have an abundance of beneficial insects flying about just waiting for a quick snack. Before you know, most of your pests have disappeared. While it’s true you might occasionally pick up an aphid or other insect, the benefits far outweigh the costs. You can always rinse the plants off before bringing them in.

When you move your plants out, don’t just put them directly into the bright sun. Start them out on the north side of your house, then move them to the east side and finally, if they need a lot of sun to the west or south side.

As mentioned above, careful and regular inspections of the plants in your collection will make it possible for you to nip a problem in the bud. Also, making sure that plants are grown according to their individual needs of soil, water, light and fertilization will assure that plants have a better chance to resist infestation.

Author’s Note:

I would like to thank my sister Irma Kristel, who runs a small interiorscape business, for reviewing the draft of this article and for making useful suggestions for changes and additions.

Footnotes:

*Knock Out Gnats is available from Gardens Alive, www.gardensalive.com. Gnatrol is only available locally at wholesalers in very large quantities.

**The label on my 2 ½ gallon container says as follows: Ultra-Fine Oil—All Season Horticultural Insecticide, Miticide and Fungicide. There are other brands out there. Just check with your garden center.

Sources:

Caldwell, Brian, et al. Resource Guide for Organic Insect and Disease Management. Ithaca, NY: Cornell University Extension Service, 2005.

Cranshaw, W.S. Managing Houseplant Pests. Fort Collins, CO: Colorado State University Extension Service. Last reviewed: 11/06. (Fact Sheet No. 5.596). Available at the CSU Extenstion website, www.ext.colostate.edu.

Fungus Gnats. CSU Extension. (Plant Talk Colorado no. 1445). Available at the CSU Extension website: www.ext.colostate.edu.

Krischik, V. “Imidacloprid, found in most homeowners insecticides is translocated to nectar and pollen and kills good bugs.” Yard and Garden News, University of Minnesota Extension, August 2010.

McCollum, Susan and Teena Risley. Plant Basics: A Manual for the Care of Indoor Plants. Fairfax, VA: Garden Tapestry, Inc., 1994.