Chapter 11

Insect Pest Management for Organic Crops

Botanical Insecticides

Insecticides that are that derived from unmodified (not genetically engineered) plants or plant parts are commonly referred to as “botanicals.” Botanicals have been used in agriculture for centuries. Most botanicals are less toxic to humans, wildlife and the environment, and they degrade more rapidly than synthetic-organic insecticides into harmless components in the field. For these reasons, many botanicals are allowed in organic crop production. Because botanicals generally break down quickly after application, they may also be of use near harvest when insect pest control is needed but other materials should not be applied because of pre-harvest interval (PHI) restrictions. Rapid degradation also means that botanicals are less likely to cause environmental problems. However, botanical insecticides are not without concerns. They are usually broad-spectrum poisons that can harm beneficial insects and some botanicals (e.g., rotenone) are acutely and chronically toxic to humans and other mammals. Moreover, the fact that botanicals break down rapidly in the environment also means that they provide very short-term pest control, so that sprays must be timed precisely to coincide with insect pest events, or be applied at lower pest populations, or be applied more frequently.

Neem

Neem products are derived from the neem tree, Azadirachta indica. Neem products are broad-spectrum insecticides, which work by contact or ingestion. These products may contain neem oil (sometimes formulated as a soap) or the purified active ingredient, azadirachtin.

Types of Insect Pests Controlled

Neem extracts have been shown to affect a broad range of insects, but efficacy varies among species. On fruit crops, neem has shown some efficacy against aphids, including rosy apple aphid, woolly apple aphid, tarnished plant bug, some leafhoppers, pear psylla, and spotted tentiform leafminer.

Application

Frequent applications are more effective than single sprays because neem does not persist well on plant surfaces. Like most other botanically derived materials, it can be rapidly broken down by sunlight and washed away by rain. Neem may not show signs of efficacy for three to seven days, and it can degrade within three to four days.

Pyrethrum

Pyrethrin is a natural insecticide, derived from the crude extract of the Chrysanthemum flower (Pyrethrum). Pyrethrins are the active chemicals in pyrethrum. “Pyrethrum” and “pyrethrin” are often used interchangeably. Pyrethrum is a fast-acting contact poison that “knocks down” susceptible insects. Pyrethrum induces a toxic effect in insects when it penetrates the cuticle and reaches the nervous system leaving the insects paralyzed. Pyrethrum may also have a repellant effect. Since pyrethrum is a contact poison, the target pest must be present and hit by the spray.

Types of Insect Pests Controlled

Used correctly, pyrethrum is moderately to highly effective against aphids, apple maggot, European apple sawfly, caterpillars, leafhoppers, whiteflies, Lepidoptera larvae (including codling moth), mealybugs, pear psylla, plum curculio, many of the true bugs (Hemiptera), and flower thrips. Within these groups, pests may have a greater or lesser susceptibility to pyrethrum products. Some insects may be able to recover after the initial knockdown if the dose is too low. Frequent repeat applications of pyrethrum are required because of its rapid photo-degradation and short residual activity in the field.

Application

Pyrethrins (the active chemicals) are rapidly broken-down by sunlight. Therefore, it is recommended that pyrethrum be applied before dawn or in late evening when the target insect pests are active and present in the field, and UV light is minimal. Use of UV-inhibiting adjuvants may allow for a longer period of residual activity.

Ryania

Ryania is extracted from the stems of a woody South American plant, Ryania speciosa. Ryania is a slow-acting stomach poison. Although a slow-acting stomach poison, it causes insects to stop feeding soon after ingestion.

Sabadilla

Sabadilla comes from the ripe seeds of the tropical lily Schoenocaulon officinale. Sabadilla is a broad-spectrum contact poison, but has some activity as a stomach poison. The alkaloids in Sabadilla affect insect nerve cells, causing loss of nerve function, paralysis, and insect death.

Rotenone

Although the Organic Materials Review Institute (OMRI) Generic Materials List identifies rotenone as “Allowed with Restrictions” on organic farms, the material is only legally allowed under certain conditions. The National Organic Program (NOP) is currently exploring possible changes to the regulations, potentially limiting or prohibiting the use of rotenone. In recent studies, rotenone has been linked to neurotoxic symptoms similar to Parkinson's disease. For these reasons, U.S. organic growers have discontinued use of rotenone.

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