Chapter 12

Insect Pest Management for Organic Crops

Minerals for Controlling Insect Pests

Mineral-based pesticides include sulfur, copper products, diatomaceous earth, and kaolin clay. These must be used with caution, even though they are allowed. Sulfur can reduce the populations of some beneficial insects and may burn plants if used during hot weather. Diatomaceous earth can cause respiratory problems and itching in the farmworkers who apply it. Copper can accumulate in soils, so it is allowed with restrictions. The organic regulations state “copper-based materials must be used in a manner that minimizes accumulation in the soil…” Certifiers may require soil testing to verify that copper is not accumulating in the soil. Certain highly toxic minerals, including arsenic and lead, are specifically prohibited.

Sulfur

Both sulfur and lime sulfur (aka calcium polysulfide) are considered to be allowed, synthetic materials; their use is regulated in certified organic production. Formulations of elemental sulfur include dry flowable, water-dispersible granules, soluble concentrates, and wettable powders. Lime sulfur is a liquid. It is used as a dormant application to fruit trees to control mites, and scale insects. 

Application

Elemental sulfur may cause phytotoxicity at high temperatures; it should not be used if temperatures are forecasted to be above 90 degrees F (32°C) within three days following application. It should not be applied within two weeks of an oil application.

Kaolin Clay

A naturally occurring aluminosilicate clay mineral that is processed into uniformly sized particles for use as a plant protectant. Commercial formulations are applied in a water suspension. After the water evaporates, a dry white particle film layer is left on plant surfaces. Several modes of action have been suggested for insect control, including: 1) direct death by interfering with insect feeding by masking the color of the plant tissue; 2) physically blocking insects from reaching vulnerable plant tissue; 3) repelling or deterring insects by creating an unsuitable surface for feeding or egg-laying such as apple maggot, codling moth, and leafhoppers; 4) disrupting host-finding capability by masking plant tissue color and reflecting light; and 5) acting as an irritant to the insect, triggering an excessive grooming response that distracts the pest.

Application

Kaolin clay is available as a wettable powder to be mixed with water. Application can be made with most commercially available spray equipment, but large amounts of water are required. The material must be well mixed prior to and during application, and inhalation of dust during mixing and application can cause lung damage. To prevent caking, it is suggested that the material either be added during mechanical agitation or to first completely mix the necessary amount in a small amount of water before filling up the tank to the recommended volume.

Diatomaceous Earth

Diatomaceous earth is comprised of the calcium carbonate skeletons of diatoms that developed in water bodies. Deposits of these are mined and used for various purposes. Diatoms are small, usually single-celled phytoplankton commonly found in aquatic or moist environments. Diatoms are encased inside a cell wall made of silica, the same compound used to make glass.

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