Spinosad is a bio-insecticide derived from a naturally occurring soil bacterium, Saccharopolyspora spinosa. This substance is a mixture of two chemicals, Spinosyn A and Spinosyn D, which are fermentation products of the organism. Used widely in both organic and conventional agriculture, Spinosad is recognized for its effectiveness against a broad range of insect pests. This article will explore its specific action against aphids and detail the necessary precautions for its safe and effective use in the garden.
How Spinosad Targets Aphids
Spinosad is effective because it possesses a dual mode of action, killing pests through both contact and ingestion. When an insect eats or comes into contact with the treated foliage, the substance affects the nervous system. Specifically, Spinosad causes a unique excitation of the insect’s nicotinic acetylcholine receptors. This overstimulation leads to involuntary muscle contractions, tremors, and eventual paralysis.
The insect stops feeding almost immediately, and death typically occurs within one to two days after exposure. However, aphids are “sucking” insects that feed on plant sap, which presents a challenge for effectiveness compared to chewing pests. Because aphids do not consume large amounts of the treated leaf surface, Spinosad’s performance against them can be inconsistent.
For reliable control, the product must either directly contact the aphid or move through the leaf surface, a process known as translaminar movement, to reach the pests. Since Spinosad does not move systemically throughout the entire plant after a foliar application, thorough coverage is important for control. A successful application requires the product to physically coat the aphids or the leaf tissue they are actively feeding on.
Best Practices for Application
Before mixing, always read the product label to determine the correct dilution rate for the specific concentration you have purchased. While rates vary by product, a general guideline for many liquid concentrates is typically a few tablespoons per gallon of water. Mix only the amount of solution needed for the immediate treatment session, as the effectiveness of the prepared solution degrades over time.
The most important factor for success is achieving complete coverage of the plant foliage, especially the undersides of leaves where aphids congregate. Aphids tend to hide on new growth and the lower surfaces of leaves, requiring the spray to be directed upward. Ensure the spray thoroughly wets all plant parts without excessive runoff.
To protect foraging insects, apply the spray in the late evening or very early morning when pollinator activity is minimal. This timing allows the product to dry completely before bees become active. If the infestation persists, a reapplication can be made, but do not apply the product more often than the label-specified interval, which is typically not less than seven days.
When applying any pesticide, including organic options, wear basic personal protective equipment like gloves and eye protection. After handling, wash your hands and any contaminated clothing thoroughly with soap and water. Keep the concentrate and the mixed solution away from children and pets, and follow all label instructions regarding safe storage.
Impact on Beneficial Insects
Spinosad is generally considered to have low toxicity to mammals, birds, and fish, especially when compared to many conventional synthetic insecticides. However, its toxicity to insects is not limited only to pests; it can pose a significant risk to beneficial insects, particularly honey bees. The substance is highly toxic to bees while it is in the wet, liquid state.
This toxicity is why timing the application is important; once the spray has dried, usually within three hours, the residue is far less harmful to adult bees. Growers should never apply Spinosad to plants that are in bloom or when bees are actively foraging in the area. Using it in the evening mitigates the risk to pollinators by allowing it to dry overnight.
For established beneficial predators, such as adult ladybugs, lacewings, and parasitic wasps, Spinosad is generally considered safe once the residue has dried on the plant. This selective toxicity profile allows beneficial populations to survive and continue aiding in natural pest control. Some research suggests that chronic exposure to even low concentrations may cause adverse neurological effects in insects, underscoring the need for careful, targeted application.