Going organic starts with the right plant in the right place with the right care. But when things go awry, here’s a guide to natural pest control.
By Carol O’Meara
A garden is a cheerful way to accessorize your home, wrapping your house in colorful flowers and fresh vegetables. Growing a garden takes patience, time and a bit of energy. What it doesn’t need to take are a lot of chemicals with names you can’t pronounce. Armed with the right information, you can have a glorious garden that’s completely “au naturel.”
The key to successful gardening is putting the right plant in the right place and giving it the proper care. Like our immune systems, plants have ways to defend themselves, but they need to stay healthy to manage it. When you buy a plant, check the nursery tag to learn what exposure, water and nutritional needs it prefers. Then learn which pests can attack it and how to repel them.
A mixed planting is healthier than a monoculture, as it better attracts beneficial birds, bugs and toads. Beneficials patrol and devour pests that can wreak havoc on plants. Plants with shallow flowers, like alyssum, yarrow and mint (control mint by containerizing it), attract beneficial insects, as do many others, including natives like penstemon and agastache.
If your plants fall prey to bugs and disease, many products are available to restore them to health. Natural gardening products are a rapidly growing industry and the choices can be dizzying. To help you pick the right product, here’s a 411 for organic pest control:
Compost teas are water-based solutions that contain compost microbes, in addition to compost nutrients. Soaking or steeping compost in water makes the tea. Some teas help suppress disease, but it depends on the compost they’re made from; different ingredients can target specific diseases, while others don’t help at all. Compost and its parent ingredients vary widely, and we’re not often told what they are; the compost could be made from yard, brewery or farm waste. For this reason compost teas are best used as nutrient enhancers, rather than for con-trolling diseases.
Soaps & Oils
Insecticidal soaps are virtually nontoxic to humans, pets and other mammals. These soaps work by damaging the waxy outer tissue of soft-bodied insects, causing them to dehydrate. Soaps are relatively safe to beneficial insects, since their action occurs on soft-bodied pests like aphids, mealybugs, thrips, whiteflies, mites and scales.
Insects and arthropods don’t breathe the way we do. Instead, tubes in their bodies allow air to move in and out of their shells. Horticultural oils smother these openings so the insect can’t breathe. They also smother fungal spores, preventing disease. Horticultural oils are used to control spider mites, aphids, scales and fungal diseases. Two types of oils are available, summer and dormant, and are used at different times of the year.
Dormant oils are applied to dormant plants to control overwintering insects and their eggs, including aphids, spider mites and scales. Heavier in formulation, they’ll harm leaf tissues if applied in summer. Typically, dormant oils are applied at bud swell, just before the leaves emerge. Summer oils are lighter and are used from bud swell through summer, provided the temperature is below 90˚ F.
Both soaps and oils work on contact, so coat the upper and lower leaf surfaces as well as the stems for best results.
Botanical insecticides are derived from plants. They’re made from flowers, seeds, roots and fruits, and come from a variety of plant families like aster, citrus and peppers. Well-known botanicals include pyrethrins, neem, capsaicin and garlic. These are fast-acting and break down quickly in the environment.
Most have low to moderate toxicity to mammals. But some, like nicotine and rotenone, are quite toxic and should be handled carefully. Home-brewed concoctions can be very dangerous, so for safety, purchase these products and carefully follow the label instructions. Timing applications for pest outbreaks is crucial when using botanicals, since they don’t last long in the garden.
Neem oil, from neem-tree seeds, controls whiteflies, thrips, caterpillars, aphids, mealybugs, spider mites, scale crawlers and beetles. It’s also effective against powdery mildew. Azadirachtin, also derived from neem, stops insect feeding and acts as a growth regulator that prevents young insects from molting.
Garlic and capsaicin, derived from hot peppers, are repellents; their strong chemicals encourage bugs to leave. Both are used on ornamentals to control aphids, spider mites, thrips, whitefly, lace bugs, leafhoppers, and other pests. But their benefit has a drawback: they repel many bugs, including beneficials.
Pyrethrum and pyrethrin are made from daisy flowers. They shouldn’t be confused with pyrethroids, which are synthetic. Pyrethrum paralyzes bugs on contact, including beneficial insects like bees, so be very careful when applying it. Use only the minimum recommended dose and don’t apply it when flowers are blooming and bees are foraging, or on windy days.
Tiny microbes of viruses, bacteria, fungi, protozoa and nematodes pack powerful punches by killing bugs while leaving other creatures alone. Most microbes are host specific, meaning they only target certain bugs. Some, like nematodes, are tricky to use in our dry soils, so talk to your garden center about the ordering and timing for nematodes. They’re usually applied in mid-August to combat Japanese beetles.
Bacillus thuringiensis is a popular microbial insecticide derived from a soil bacterium. Known as Bt, it has different strains that work against different groups of insects. Bt must be eaten in order to be effective, but it doesn’t kill immediately. The bug stops feeding, then dies over the next few days.
Bacillus thuringiensis var. kurstaki (Btk) acts on the larvae of butterflies and moths. Use it to control leaf-feeding caterpillars like tomato hornworms, cabbage loopers and European corn borers. Bacillus thuringiensis var. tenebrionis (Btt) attacks beetles, including Mexican bean beetle, cucumber beetle and elm leaf beetles. Not all beetles are susceptible, though, so check the label. Bacillus thuringiensis var. israelensis (Bti) kills mosquito and fungus gnat larvae. It’s the active ingredient in mosquito dunks for water features.
Spinosad is a fermented by-product of a soil microbe that kills thrips, whiteflies, aphids, leaf miners, scales and spider mites after they eat plants sprayed with it. But be careful: Spinosad is toxic to bees, too, and should never be applied to plants in flower or on windy days.
Diatomaceous earth is the crushed, fossilized remains of single-celled diatoms. With razor-sharp edges, diatoms give bugs the itch to leave. Use diatomaceous earth to repel slugs, sowbugs, flea beetles and soft-bodied insects like aphids.
Sulfur is used primarily for disease control, but it can be useful for tomato psyllids and thrips. It also controls fungi, including powdery mildew, rusts and leaf spots, by changing the plant surface’s pH, which interrupts the fungi’s protein formation. Use caution when applying sulfur in hot, dry weather, as it can burn plants. Also avoid mixing it with oils or other pesticides.
Kaolin clay is sprayed on vegetable plants to form a white barrier that irritates insects and disguises the host plant. Kaolin controls cabbage aphids, thrips, leafhoppers, cucumber beetles and Colorado potato beetles. But it has to be applied early, before the bugs arrive, to camouflage plants.
Shooing Peter Cottontail and his friends takes more than a scarecrow or a hose disguised as a snake. Rabbits, voles and squirrels quickly figure out these ruses.
A short-term solution is spraying with garlic or mint oil to keep creatures out of the garden bed, but those scents are strong to the human nose and must be reapplied after every rain or irrigation.
The long-term fix is to remove any cover these animals use for hiding, such as brush or rock piles, low–growing shrubs, and access to areas under decks and sheds. Mow grass low around fruit trees to discourage voles, and install 1-inch mesh fencing around the garden, 3 feet tall and sunk 6 inches into the ground, to exclude rabbits. Just be sure all the rabbits are gone before you install your fencing, so you don’t keep them in instead of out.
Bees and other beneficial insects need our help to stay safe. Whenever you pick a product for use in your garden, read the label before using it. Glitzy names and happy catchphrases often hide dangerous products, so read the label’s “Environmental Hazards” section to look for harmful side effects to birds, bees, fish and mammals.
According to the Xerces Society, neonicotinoid pesticides are not directly linked to honeybee colony collapse disorder, but can contribute to bees being more susceptible to parasites and pathogens. Too often labels don’t mention this risk. If you want to avoid neonics, look for their active ingredients on the label—imidacloprid, clothianidin, thiamethoxam, acetamiprid and dinotefuran. If the label contains any of these chemicals, avoid using that product.
Bees are vulnerable whenever products are applied to flowers, so never apply any pesticide to a plant in bloom.
Carol O’Meara is an entomologist and a local gardening enthusiast. Read her blog at gardening-afterfive.wordpress.com.