Commercial products that contain various plant extracts such as garlic, hot pepper wax, and cinnamon are available and registered for use on some crops and ornamentals. However, there is very little scientific research that demonstrates the effectiveness of these products, and their modes of action are not understood. Studies are needed to show direct mortality or measurable negative effects from specific botanicals on specific insect pests to make sound pest management decisions (Cloyd et al. 2009). If you are interested in experimenting with essential oil products, contact your county Extension office for guidance.
In addition to its categorization as a botanical, neem is also a plant-derived horticultural oil. The neem tree is native to India and is the source of hundreds of products, including insecticides made from the extracts of the seeds and bark. The primary insecticidal extract is azadirachtin. When azadirachtin is used for pest management, it can act as an insect repellant, an anti-feedant (interferes with feeding), and growth regulator (interferes with molting and growth) (Schmutterer 1990). When neem oil or neem soap is used, it poisons upon contact much like other soaps and oils. In some cases, neem can also be a systemic insecticide (when applied to the soil, the active ingredients are absorbed into the plant and transported to the growing tips and leaves).
Neem insecticides are effective against many caterpillars, flies, whitefly, and scales, and are somewhat effective against aphids. Neem may not show signs of efficacy for 3–7 days, and it can degrade within 3–4 days. Multiple applications are generally needed to obtain good management of the targeted pests. Neem is regarded as nontoxic to vertebrate animals and has been shown to minimally affect many beneficial insects such as bees, spiders, and ladybugs.
Pyrethrum, also known as pyrethrins, is extracted from the seed of Chrysanthemum cinerariaefolium and has been used as an insecticide for over 100 years. Today these plants are grown primarily in Kenya. Pyrethrum is effective against a wide range of soft-bodied garden pests such as scales, whitefly, mealybugs, and thrips, but will not control mites. Pyrethrins are neurotoxins that attack an insect’s nervous system and cause repeated and extended nerve firings. They may also have a repellant effect. Pyrethrins are easily broken down by stomach acids in mammals, so toxicity to humans and pets is very low.
However, toxicity can occur when significantly more product is applied than specified on the label. Do not spray pyrethrins around ponds or other bodies of water, as they can kill fish. Pyrethrum is a broad-spectrum insecticide that is toxic to beneficial insects. Pyrethrum can paralyze susceptible insects upon exposure, but also degrades in sunlight within hours (ExToxNet n.d.). To get adequate management of some pests, repeated applications are needed. Pyrethrum products frequently contain a low hazard activator or synergist such as piperonyl butoxide or piperonyl cyclonene that substantially increases the effectiveness of the pyrethrum and reduces its cost (Pedigo and Rice 2008). Depending on the way these synergists have been manufactured, some pyrethrum products containing synergists may be allowed for use in organic agriculture.
Some microbes can be fermented to produce an insecticide such as abermectins, a fermented product of Streptomyces avermitilis (Dybas 1989) used in baits for household insect pests. The best known home gardening product of this type is spinosad. Metabolites of Saccharopolyspora spinosa, a soil-inhabiting bacteria that is fermented, are the basis for this new class of insecticide. The fermentation process has been industrialized to produce commercial insecticides.
Spinosad is composed of spinosyns A and D. The fermented product is very toxic to caterpillar pests such as cabbageworm, cabbage looper, diamondback moth, armyworm, and cutworm, as well as fruit flies such as spotted wing drosophila. Spinosad can act on a susceptible insect’s stomach and nervous system.
It is primarily ingested by feeding insects but can have some efficacy when sprayed directly on insects. Affected pests cease feeding and undergo partial paralysis within minutes upon exposure to spinosad, but it may take up to two days for the insects to die (Salgado et al. 1998).
Spinosad is systemic in some plants. Depending on the fermentation process and formulation, some spinosad insecticides are considered organic. Spinosad has low toxicity to many beneficial insects that prey on pests, and is nontoxic to mammals and other vertebrates, with the exception of some fish (e.g., slightly toxic to trout). Spinosad is toxic to bees for three hours after application, so do not apply to blooming plants during the day.
Because it is selectively toxic for many pest species and relatively safe to nontarget species, spinosad has become highly desirable as an organic insecticide. However, its popularity raises concerns about the development of pest resistance. Therefore, alternate the use of spinosad with other products.
Horticultural oils were used for insect control as early as 1763 (Olkowski et al. 1993) and are still popular today. Such control agents are often petroleum-based; however, plant-based oils considered acceptable in organic farming are also available. Horticultural oils work by disrupting insect feeding and egg laying when the pest is entirely coated. Eggs covered with oils are prevented from gas exchange, which suffocates the developing pest.
Horticultural oils have minimal phytotoxic (poisonous) effects on plants when used properly. Application timing, plant species, temperature, and oil type all contribute to the level of effectiveness and risk of phytotoxicity. Phytotoxic effects are easily noticed by the browning or “burning” of the leaves or new growth on the stems.
Dormant and Summer Oils.
Dormant and summer horticultural oils can control egg, nymph, larva, and adult stages of overwintering leafrollers, aphids, mites, and scales (Nielsen 1990). Dormant oils are effective at controlling overwintering eggs and soft-bodied insects and can be used in the early spring before active plant growth begins.
Only use dormant oils on woody trees and shrubs in dormant or delayed-dormant stages to avoid severely burning the foliage. Do not apply either type of oil during freezing weather because it will reduce the effectiveness of the oil properties and coverage of the application.
Summer oils can be applied to some woody plants (see the label for specific plants) during the growing season. Some horticultural oils can be applied in either summer or winter; however, the concentration used in summer is far lower than in the winter. To use summer or dormant oils, first dilute with water. (Commercial oil products contain emulsifying agents that allow them to mix easily with water.)
Pests rarely develop resistance to oil sprays, and the products cause little or no harm to most beneficial insects (Raupp et al. 1992). When oils are used correctly (as directed on the label), they are not hazardous to human health.