Use the Right Organic Pesticide

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Squash bug. Photo by Adobe Stock/Darek.

When faced with a pest problem, gardeners often wish for a solution that comes in a spray bottle. It’s true that many poisons sold in garden centers will kill any, and often all, insects in your garden. But pesticides are hazardous to humans and wildlife, and most will kill beneficial insects along with the problem pests.

Even organic pesticides can kill beneficial insects, so they should be used carefully and only as a last resort. Learn about insect pests often seen in your area and their natural enemies, as well as cultural methods that keep populations low, before you decide to grow a food crop.

The organic pesticides described in these pages pose minimal harm to organisms you don’t want to hurt, including humans. These products are also accepted under the National Organic Program standards, which serve as a framework for certified-organic food production. In turn, the nonprofit Organic Materials Review Institute (OMRI) evaluates whether or not specific products comply with those standards. OMRI uses a panel of experts that includes farmers, scientists, environmentalists, and businesspeople to decide which products should be approved for use by organic growers. Approved products usually display the OMRI seal on their label, or you can check the OMRI website to see whether a product is allowed, allowed with restrictions, or prohibited.

Allowed products may be used freely at the grower’s discretion; most of them are soil amendments and organic fertilizers. Many organic pesticides carry a restricted code because they can potentially harm beneficial insects and other wildlife, and thus should be used sparingly. Although all of the products listed here carry OMRI approval, many fall into the restricted category and should be used only when cultural controls have failed, and always according to label directions.

BT (Bacillus thuringiensis)

Bacillus thuringiensis, often abbreviated as Bt, is a naturally occurring bacteria that makes pests sick when they eat it. There are two strains commonly used as natural pesticides.

Bacillus thuringiensis (commonly referred to as Bt) is a bacterium that targets leaf-eating caterpillars. Within 2 to 3 days of ingestion, the insects will stop feeding and die. Bt quickly degrades in sunlight, so apply in late evening for best results. Photo by www.alisorganics.com.

Bacillus thuringiensis kurstaki (Btk) gives excellent control of leaf-eating caterpillars, such as cabbage worms and tomato hornworms, but has no activity against insects that don’t eat treated leaves. After the insects eat the bacteria, their guts rupture, and they die. Btk is therefore one of the safest natural pesticides you can use in terms of controlling caterpillar pests of vegetables or fruits without harming beneficial insects.

Bacillus thuringiensis israelensis (Bti) can be useful in controlling fungus gnats in greenhouses or houseplants, or for preventing mosquito problems in standing water that can’t be drained or controlled with fish.

Which Pests Does Bt Control? Most caterpillars seen eating leaves can be controlled by Bt when applied at the proper time. In vegetable gardens, armyworms, cabbage worms, diamondback moths, melon worms, corn earworms, green cloverworms, pickleworms, tomato fruitworms, tomato hornworms, grape leafrollers, grapeleaf skeletonizers, salt marsh caterpillars, and various webworms and budworms are candidates for treatment with Bt.

Grapeleaf skeletonizer larvae. Photo by Wikimedia Commons/Alison Hunter.

How to Use. Sunlight degrades Bt after a few hours, so it’s best if you apply it late in the day so it can be consumed during the nightly feeding. Keep in mind that your objective is to place the substance where the caterpillars will eat it. In the case of corn earworms, this means squirting the Bt solution into the tips of young ears of corn. When using Bt to control leaf-eating pests, repeat treatment every 7 to 10 days, or until it’s no longer needed.

Always follow label directions for diluting concentrated products of Bt and other natural pesticides. Some Bt products include genetically modified strains; products listed by OMRI include only naturally occurring forms.

Webworms. Photo by Getty Images/emprised.

How to Store. Mix only as much concentrate as you’ll need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot. Store Bt pesticides in their original containers on a high shelf, out of the reach of children and pets, in a cool place where temperatures will not exceed 100 degrees Fahrenheit. Under good storage conditions, powdered or other dry Bt products may last five years, while liquid products should be replaced after 2 to 3 years.

Diatomaceous Earth

Diatomaceous earth (DE) is a powder made from fossilized prehistoric crustaceans called diatoms. The sharp edges of DE cut into insects’ bodies, causing them to die of dehydration. Diatomaceous earth is most useful in dry situations — for example, puffing it into crevices where cockroaches have been seen. In the first few days after their habitat is treated, cockroaches may become more visible as they desperately search for water, but they’ll die within two weeks. DE becomes less effective when wet, yet still can be used in the garden to make life difficult for newly emerged Japanese beetles or cutworms. In dry weather, DE spread beneath plants will kill slugs.

Diatomaceous earth is made up of tiny fossils with sharp edges that will lacerate soft-bodied insects and cause them to dehydrate and die. In addition to garden use, you can also rub it into your dog or cat’s fur, or add to dust baths for your chickens to control fleas, lice, ticks, and mites. Photo by www.alisorganics.com.

DE makes an excellent filter material and has numerous industrial uses. When buying DE to use around your house, be sure to buy a product listed as “food grade.”

Which Pests Does Diatomaceous Earth Control? Most indoor invaders, including roaches, silverfish, spiders, and even fleas are impacted by DE. Including DE in chickens’ dust bath mixture helps prevent problems with lice. Diatomaceous earth also has many uses around the homestead because it can help control fleas on dogs and reduce parasites in horses, pigs, and other animals.

Whiteflies. Photo by Adobe Stock/Floki.

How to Use. Lightly sprinkle dry DE on the soil’s surface where slugs, newly emerged Japanese beetles, or other unwanted pests will come into direct contact with the dry particles. Renew after rain or heavy dew.

Indoors, use a bulb puffer to blow DE into crevices where bugs are likely to hide. You can also puff DE onto newly hatched larvae of many pests, including squash bugs, Mexican bean beetles, and Colorado potato beetles.

To be effective, the insects must be well-coated with the dust. Honeybees and other beneficial insects have no way to protect themselves from the mechanical effects of DE. When applying DE to plants that bees are likely to visit, cover them with an old sheet after treatment so the DE will target pests and the bees can’t get to the plants. Later, uncover the plants, and then wash off the DE with a fine spray of water.

How to Store. Store DE products in their original containers on a high shelf, out of the reach of children and pets, in a dry place. When stored in an airtight container and kept dry, DE has an indefinite storage life.

Horticultural Oils

When applied directly to pests, horticultural oils interfere with respiration, causing insects to suffocate and die. These oils can also kill beneficial mites and cause leaf injury to some plants, and frequent use can reduce yields, even when the pest is controlled. Best applied in cool weather, horticultural oils are valuable tools in the organic orchard, where you can use them to control pests that overwinter in bark crevices. Oily leaf surfaces also make poor sites for insects to lay eggs, and may deter early outbreaks of mites, aphids, and scale on fruit trees.

When applied directly to pests, horticultural oils cause insects to suffocate and die. Early spring applications to fruit trees will kill insects that overwinter in bark crevices, such as aphids, mites, mealybugs, and scale. Photo by www.alisorganics.com

Some horticultural oils include herbal essential oils, which may repel some pests and suppress some diseases. Neem oil is the only horticultural oil considered to be an active ingredient in pesticides.

Highly refined mineral oils, often called superior oils, evaporate quickly, so they’re less likely to injure foliage compared to heavier oils. Superior oils often control powdery mildew in addition to spider mites, whiteflies, and other difficult pests.

Which Pests Do Horticultural Oils Control? Early spring applications to fruit trees will kill insects that overwinter in bark crevices, such as aphids, mites, mealybugs, and scale, as well as eggs of some caterpillars. In the vegetable garden, you can bring spider mites on cucumber family crops under control with horticultural oil. You can also use these oils to prevent powdery mildew and black spot on roses.

Spider mites. Photo by Getty Images/BobYue.

How to Use. The recommended practice is to spray trees in spring, just before flower buds open. Do this during a period of mild weather when temperatures are expected to stay above 40 degrees. Never apply horticultural oil in very warm weather (above 80 degrees) because heat increases risk of leaf injury. Also, don’t apply on humid evenings, because slow drying of leaf surfaces increases risk of leaf injury. Always follow label directions for diluting concentrated products.

How to Store. Mix only as much concentrate as you’ll need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot, away from water supplies. Store horticultural oils in their original containers on a high shelf, out of the reach of children and pets, in a cool place where temperatures won’t exceed 100 degrees. Under good storage conditions, horticultural oils can last up to five years.

Insecticidal Soap

The fatty acids in insecticidal soaps break down the protective cuticles of soft-bodied pests, such as aphids, so they quickly become dehydrated and die. Insecticidal soaps provide an effective way to control aphids and other soft-bodied insects if no beneficials are present to do the job.

Aphids, mites, and other small sucking insects that don’t have much of an exoskeleton (shell) often can be controlled with two applications of insecticidal soap, 5 to 7 days apart. After it’s applied directly to the offending insects, the fatty acids of the soap will cause the bugs to die through desiccation. Photo by www.alisorganics.com

Which Pests Does Insecticidal Soap Control? Aphids, mealybugs, spider mites, psyllids, thrips, and whiteflies are the most common pests controlled by insecticidal soap. Insecticidal soap may also have some activity against very young earwigs, grasshoppers, harlequin bugs, leafhoppers, sawfly larvae, and squash bugs, provided the larvae become well-soaked with soap solution.

Mealybugs. Photo by Getty Images/kurapy11.

How to Use. Soap sprays have no residual effect and only kill insects that you spray directly. Be sure to thoroughly wet both sides of leaves and all crevices. You may need to repeat applications every 5 to 7 days as new aphids or whiteflies hatch and form colonies. When using insecticidal soap to control spider mites on cucumber family crops, cover the treated plants with an old sheet for a day after treatment to enhance effectiveness while reducing risk of leaf burn. Always follow label directions for diluting insecticidal soap concentrates. Using too much can cause injury to leaves.

You can make soap sprays at home by mixing 1 tablespoon of dishwashing liquid per quart of water. Insecticidal soaps are purer, however, and therefore less likely to injure foliage. In the vegetable garden, tomatoes and peas are the most likely plants to be damaged by soap sprays. Be careful when using any soap on leafy greens, which tend to pick up soapy flavors.

If you have hard water, use bottled water when mixing insecticidal soap, or use an already-diluted product. Minerals in hard water can greatly reduce the effectiveness of insecticidal soap.

How to Store. Mix only as much concentrate as you’ll need. If you don’t use it within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot, far from water sources and storm drains. Store insecticidal soap in its original containers on a high shelf, out of the reach of children and pets, in a cool place where temperatures won’t exceed 100 degrees. Under good storage conditions, insecticidal soap products can last five years or more.

Neem Oil

Derived from the bark, leaves, and fruit of a common Asian evergreen tree, neem or neem oil contains the active ingredient azadirachtin as well as several natural steroids. When applied to insects and the plants they’re eating, neem oil causes many insects to feed less, grow more slowly, molt less, and lose interest in laying eggs.

In addition to the smothering action of neem oil, contact with or ingestion of neem’s active ingredient slows feeding and radically reduces reproduction of squash bugs, Mexican bean beetles, and a few other hard-to-control insects. Photo by www.alisorganics.com

Which Pests Does Neem Control? Neem’s effects are strongest on young insects, particularly those that grow rapidly, such as squash bugs, Colorado potato beetles, and Mexican bean beetles. Neem can also provide good control of aphids and many small, leaf-eating caterpillars.

Aphids. Photo by Adobe Stock/Derrick Neill.

How to Use. It’s best to use neem preventively, before pests become a serious problem. For example, if squash bugs always plague your garden, begin spraying plants with neem every 7 to 10 days as soon as you spot the first adults. Egg laying and overall vigor will be reduced, but you’ll still need to handpick adults and remove egg clusters regularly to get good control. Employ the same strategy when using neem oil to manage leaf-eating beetles, spraying weekly when you see the first adults and egg clusters. It’s best to apply neem in warm weather.

Shake neem products well before using to emulsify the neem oil. Using lukewarm water also helps to get a good mix. Thoroughly wet both sides of leaves as well as places where insects may be hiding. If you expect honeybees to visit the plants, exclude them with an old sheet or piece of row cover for 24 hours after applying neem.

How to Store. Mix only as much concentrate as you’ll need. If not used within a few days, dispose of unused solution by spraying it on grapes, cucurbits, or other plants at risk for powdery mildew (neem has slight activity against this disease). Store neem products in their original containers on a high shelf, out of the reach of children and pets, in a dark place where temperatures won’t go below 40 degrees. Under good storage conditions, neem products last about two years.

Pyrethrum

One of the oldest pesticides known, pyrethrum is also the strongest insecticide allowed under National Organic Standards guidelines. Made from the dried flowers of the Dalmatian daisy (Tanacetum cinerariifolium), pyrethrum insecticides are known for their fast knock-down of unwanted insects. Insects typically become paralyzed as soon as they come into contact with pyrethrum, so it’s often used in wasp sprays. Pyrethrum use in the garden should be undertaken with care and only after cultural methods that might manage a pest have been exhausted. Pyrethrum insecticides are highly toxic to bees, wasps, and other beneficial insects, as well as to fish.

Pyrethrum is the strongest insecticide allowed under National Organic Standards guidelines. Made from the dried flowers of the Dalmation daisy (Tanacetum cinerariifolium), pyrethrum insecticides paralyze insects on contact. Photo by www.alisorganics.com.

Which Pests Does Pyrethrum Control? Aphids, armyworms, cucumber beetles, cutworms, squash bugs, whiteflies, leafhoppers, thrips, and Colorado potato beetles are often brought under control with pyrethrum. Pests that cannot be reached with the spray — for example, corn earworms or leaf miners — should not be treated with pyrethrum products. Additionally, very challenging pests, such as cucumber beetles and squash bugs, are best managed by excluding them with row covers, with pyrethrum used as a late-season remedy should pests get out of control.

How to Use. Pyrethrum degrades quickly in sunlight, but you should still take precautions to protect beneficial insects from exposure. When using pyrethrum to control insects that take flight, such as cucumber beetles, apply pyrethrum early in the morning and then cover the treated plants with row cover or an old sheet to exclude bees and other beneficials for 24 hours. To put a damper on squash bug populations, spray plants as soon as you see the first nymphs, and again one week later.

Don’t use pyrethrum in situations where lady beetles, honeybees, and other beneficials are active. Used carelessly, pyrethrum can wipe out these and other beneficial insects.

Home production of pyrethrum pesticides is practical for the resourceful homesteader. Native to current-day Yugoslavia, the Dalmatian daisy is a cousin of feverfew, which it closely resembles. Hardy to Zone 6, the plants grow as short-lived perennials and often reseed in hospitable spots. If you soak the dried flowers in warm water for three hours, the resulting spray is highly toxic to insects for about 12 hours. People who are allergic to other members of the Aster family may react badly to this daisy’s pollen.

Dalmation daisy (Tanacetum cinerariifolium). Photo by Getty Images/fedsax.

Purchased pyrethrum products require less handling and therefore may be safer to use. Look for the OMRI label when choosing a pyrethrum insecticide because non-listed products often contain piperonyl butoxide, which is considered a possible human carcinogen. Organic pyrethrum products often contain oils or soaps to enhance their effectiveness.

How to Store. Mix only as much concentrate or infusion as you’ll need. If not used within one day, place the container in the sun for a few hours and dispose of unused solution by pouring it out in the sun. Sunlight rapidly degrades pyrethrum, and the half-life of pyrethrum in soil is only 1 to 2 hours. Store pyrethrum products in their original containers on a high shelf, out of the reach of children and pets, in a dark place at cool room temperatures. Under good storage conditions, the shelf life of pyrethrum is about one year. Dried pyrethrum daisies can be stored in the freezer in an airtight container for at least six months.

Spinosad

A biological pesticide, spinosad is based on the bacterium Saccharopolyspora spinosa, which was discovered in 1982 in an old Caribbean rum still. It was soon found that these bacteria produce a substance that works as a neurotoxin in many (but not all) insects. Susceptible insect species that are exposed to spinosad become excited to the point of exhaustion, stop eating immediately, and die within two days.

Spinosad is a biological pesticide based on the bacterium Saccharopolyspora spinosa. Photo by www.alisorganics.com.

Which Pests Does Spinosad Control? Spinosad controls all types of caterpillars and also has good impact on Colorado potato beetle larvae and blister beetles. Spinosad works best on pests that consume a lot of leaf tissue. There are also several fire ant baits based on spinosad, as well as a hair rinse for head lice.

Colorado potato beetle larvae. Photo by Adobe Stock/pavlofox.

How to Use. In the vegetable garden, apply spinosad to dry leaves as soon as a target pest becomes evident. To a limited extent, spinosad is taken up by plant leaves, so one application often lasts for 10 days or more. Shake spinosad products well before using. Thoroughly wet both sides of leaves as well as places where insects may be hiding. Spinosad breaks down in sunlight, so late-day applications will better expose insects to the toxins. If you expect honeybees to visit the plants, exclude them with an old sheet or piece of row cover for 24 hours after applying spinosad. Because spinosad is more persistent than Bt, don’t apply it to leafy greens or vegetables that you plan to harvest within one week.

Cabbage looper caterpillar. Photo by Getty Images/Tpopove.

How to Store. Mix only as much concentrate as you’ll need. If not used within a few days, dispose of unused solution by spraying it on grass-covered soil, far from drainage ditches and water supplies. Store spinosad products in their original containers on a high shelf, out of the reach of children and pets, in a dark place where temperatures won’t go below 40 degrees. Under good storage conditions, spinosad products last about three years.


Possibly the largest citizen science project of its kind, The Big Bug Hunt is using reports from real gardeners to track how bugs and pests spread. The creator of the project, Growing Interactive, is devising a prediction system that will send alerts when pests are heading your way, so you can take preventative action. And with an easy identification guide available at the website, it’s easy to take advantage of this new tool in the fight against garden pests by reporting the insects in your own garden at www.BigBugHunt.com. Photo by Growing Interactive.


Barbara Pleasant has practiced organic vegetable gardening for 30 years and has authored numerous books. She lives in Floyd, Virginia, where she grows vegetables, herbs, fruits, and flowers, and keeps a small flock of chickens.

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