While natural pest control measures should always be your first port of call, organic pesticides can be used as a last resort. But pesticides are potent, so it’s important to choose the right active ingredient for the pest in question, and take steps to avoid unwanted casualties among beneficial insects.
It is not a complicated process, because there are only a few choices among organic pesticides with clear records of success: Spinosad, pyrethrin, and neem. I am also adding homemade hot pepper sprays to the list, though these and other aroma-based sprays are best used preventatively, as deterrents, and have limited value for insect emergencies.
What is Spinosad?
Made from a soil-dwelling bacterium, spinosad controls a long list of leaf-eating pests, including beetles such as flea beetles. Spinosad has a negligible effect on honeybees and other pollinators, because it is most potent when eaten by the target pest, and bees don’t eat leaves. Ingestion of spinosad by leaf-eating pests causes paralysis and death.
Spinosad is the only organic pesticide I keep on hand, and I use it exclusively to keep cabbage white caterpillars and armyworms from taking over my autumn broccoli, cabbage, and kale. Spraying every 10 days keeps these veggies free of these persistent pests. After spraying, I cover the plants with horticultural fleece or tulle (wedding net) for a couple of days to exclude yellow jacket wasps, the only beneficials likely to visit the plants.
Organic pyrethrin products are made from the flowers of Chrysanthemum cinerariifolium, commonly known as Dalmation daisy. While pyrethrins are considered non-toxic to mammals, they cause immediate convulsions among insects upon contact. Most insects quickly die after being sprayed with pyrethrins, so it’s important to avoid unwanted casualties among beneficials by covering treated plants with horticultural fleece or tulle for a day after pyrethrins are used. Sunlight neutralises pyrethrins within a few hours.
Certified organic growers are allowed to use pyrethrins only as a last resort, and the same standard should apply in the garden. Pyrethrins are simply too potent for everyday use in a garden that includes vegetables and flowers. That said, because of pyrethrin’s ability to knock down pests quickly, it can be useful for treating colonies of hugely invasive insects.
Best Uses for Neem
Neem and neem oil products are made from the seeds of the neem tree (Azadirachta indica), native to Southeast Asia. Neem includes a buffet of chemicals that impact insects, including the most common active ingredient, azadirachtin. Products available to gardeners are mostly neem oils made from whole neem seeds. Neem oil also suppresses several common leafspot diseases when used preventatively, so neem oil products are often called combination garden sprays.
Neem oil impacts insects by reducing feeding, egg laying, and interfering with the maturation process, and it is most effective against little soft-bodied insects including aphids, spider mites and whiteflies. When used to manage outbreaks of larger insects, adults often escape unscathed, though immature individuals may be seriously set back.
Hot pepper sprays also can deter feeding by spider mites, aphids, and pesky animals, but have limited impact on larger garden insects. The same tiny insects are impacted by neem, so sprays that contain both capsaicin and neem can be extremely useful against aphids, whiteflies and spider mites in a greenhouse, where natural predators are in short supply.
I am amused and confused by products that claim to be “pesticide free” yet are designed to kill insects using essential oils or chemical salts. Research on the effectiveness of various essential oil pesticides is spotty, but the future looks promising. Recently, researchers found that lavender, jasmine and mustard essential oils may help control spider mites in aubergines, and that a similar mixture restores susceptibility of bed bugs to traditional pesticides. We still have much still to learn about interactions between pest insects, beneficial insects and essential oils. Stay tuned.