Insecticides developed from elemental (mineral) sources mined from the earth are classified as natural products and often cost less than other processed or harvested insecticides. The toxicity of mineral-based insecticides depends on the chemical properties of the mined ele- ments. Some mineral insecticides such as sulfur are regis- tered for organic use and have relatively low toxic effects on people and nontarget organisms. In contrast, lead arsenate is a natural mineral product that was cancelled as a pesticide in 1988 due to its toxicity and persistence in the environment.
Diatomaceous earth is a fine particle dust comprised of fossilized diatoms that is effective against slugs and soil-dwelling insects. Diatoms are small, usually single-celled phytoplankton commonly found in aquatic or moist environments. Diatoms are encased in- side a cell wall made of silica, the same compound used to make glass. Diatomaceous earth works as a fine abrasive that disrupts the exoskeleton cuticle of a slug or insect and causes it to desiccate (dry out).
Use diatomaceous earth only in landscape areas that do not contain edible plants (e.g., ornamental gardens) ;To create an effective barrier for slugs, apply diatomaceous earth in a 3-inch wide,1-inch thick band around the habitats that slugs use. Repeat applications after periods of rain. Note, however, that diatomaceous earth can also be toxic to beneficial insects such as predatory ground beetles and is highly toxic to bees if applied to blooms.
Elemental sulfur is a finely ground powder that can be applied either as a dust or a spray. This mineral is one of the oldest pesticides known, and reported pest resistance is rare. Sulfur acts as a metabolic disruptor (interferes with a chemical reaction, digestion, or the transport of substances into or between cells) to in- sects such as aphids, thrips, and spider mites. Most sulfur formulations have low toxicity to people but can be an eye and skin irritant. Sulfur is highly toxic to fish, so it is important to keep it away from water (ExToxNet n.d.).
Do not use sulfur on a crop just before harvest if you plan to preserve it; sulfur can produce off-flavors in canned products, and sulfur dioxide can form, which may cause containers to explode. In addition, sulfur is phytotoxic to most crops if applied two weeks before or after the application of a horticultural oil.
Iron phosphate is very effective at managing slugs and snails when combined with bait. Baited iron phosphate usually comes in pellet form. Scatter the product around the crop in need of protection and areas where slugs seek refuge, such as garden bed borders and rocks. Liquid formulations are also available. Follow label suggestions for subsequent applications.
Slugs that feed on iron phosphate will stop eating, usu- ally seek a hiding place, and then die of starvation. Iron phosphate is considered relatively nontoxic and does not affect insects, birds, or mammals when applied in the recommended amount. Avoid over-application, as there is some evidence that iron phosphate baits can negatively affect earthworms (Edwards et al. 2009). Because iron phosphate is nontoxic only in the labeled ap- plication amounts, be sure to store it in a safe place away from pets and children. Most brands of iron phosphate are approved for organic production by the National Organic Program.
Kaolin is a fine clay that is sprayed on plant foliage or fruit to deter feeding and egg laying of insect pests such as apple maggot, codling moth, and leafhop- pers. It can also have some repellant properties that cause irritation to insects upon contact (Stanley 1998).
The effectiveness only lasts as long as the clay film cov- ers the fruit or foliage to mask its chemical, visual, and tactile cues. Reapplication is necessary if rain washes the product off. Kaolin’s toxicity to pests is additionally dependent on the insect being on the fruit or foliage during the entire time of pest susceptibility. You will need to monitor insect activity to be sure that plants are protected during the required times. Kaolin is an organi- cally-approved material.
Natural soaps are derived from plants (coconut, olive, palm, cotton) or animal fat (whale oil, fish oil, or lard) and have been used since the 1700s to control certain soft-bodied insects such as aphids (Olkowski et al. 1993). Soaps are fatty acids that can degrade or dissolve the protective layers of the insect cuticle, causing the insect to desiccate. Insecticidal soaps are considered nontoxic to humans and many beneficial insects, but selectively kill certain pest insects. Some soaps are approved for use in organic agriculture.
Insecticidal soaps are very effec- tive for managing soft-bodied insects like aphids, scales, whitefly, mealybugs, thrips, and spider mites. The soap must contact the insect’s outer skeleton to be effective. Leaf-feeding insects are often found on the undersides of leaves, so be sure to fully cover plant foliage. Results from the application of soap are usually seen in 1–3 days. Multiple applications are often needed to be effective. Insecticidal soaps are usually diluted with water before applying.
Do not use household soaps as insecticides. Household soaps vary tremendously in composition, purity, and effectiveness, and thus have the potential to harm crops.
For example, household soaps can be phytotoxic to some plants, resulting in leaf burn. Only use soaps that are specifically registered and sold for use as insecticides. Be sure to read the product label for known phytotoxic effects and always test the product on a small portion of the plant to see if leaf burn occurs. Leaf burn symptoms usually develop within two days.