The primary function of fertilizer is to supply plants with necessary nutrients for growth, mainly Nitrogen (N), Phosphorus (P), and Potassium (K), often referred to as NPK. Nitrogen promotes leaf development, phosphorus supports root and flower production, and potassium assists with overall plant health. Standard, pure fertilizer, consisting only of these nutrient chemicals, does not function as an insecticide and is not designed to kill insects. The perception that fertilizer kills insects often stems from specialized combination products or unintended consequences of misuse.
Clarifying the Confusion Between Fertilizer and Pesticide
The belief that fertilizer is toxic to pests stems from commercial formulations that blend plant food with pest control chemicals. Many products marketed to homeowners, such as “weed and feed” or “fertilizer with insect control,” are combination products. These mixtures contain plant nutrients (fertilizer) and separate active ingredients (pesticides) designed to kill insects, weeds, or fungi. The added pesticide component is responsible for insect mortality.
Consumers must examine the product label for the “Active Ingredients” section to identify the true source of toxicity. These active ingredients are chemical compounds like pyrethroids or neonicotinoids, which are neurological toxins. Neonicotinoids are systemic insecticides that are absorbed by the plant’s vascular system. When a pest feeds on the treated plant, it ingests the toxin, which leads to paralysis and death.
The fertilizer component provides plant nutrients, while the insecticide component provides pest control. When insects are killed by a “fertilizer” application, it is the result of the deliberately included insecticide. The practice of tank mixing, where growers intentionally combine liquid fertilizer with registered insecticides before application, further contributes to this confusion. This is done to save time and labor by applying both products in a single pass.
Direct Mechanisms of Harm from Fertilizer Overapplication
While pure fertilizer is not a poison, excessive application can directly harm insects and other soil organisms. The primary non-pesticidal way fertilizer causes mortality is through osmotic stress, commonly known as “salt burn.” Fertilizers are composed of various salts. When applied in high concentrations, these salts create a hypertonic environment in the soil or on the plant surface.
This high salt concentration draws water out of soft-bodied invertebrates, a process known as desiccation. Organisms like earthworms, slugs, and the larval stages of many insects, which have permeable skins, are highly susceptible to this effect. This dehydration causes tissue damage and eventual death.
Gross overapplication of high-nitrogen fertilizers can also lead to the release of ammonia gas. This gas is chemically caustic and causes localized chemical burns to insects and their eggs in direct contact or within the immediate application zone. This direct toxicity is solely the result of chemical concentration and improper use, not the intended nutritional function of the fertilizer.
How Fertilizers Indirectly Influence Insect Populations
Beyond direct harm, fertilizer application significantly alters the plant’s chemistry, which indirectly affects insect populations and feeding behaviors. The most pronounced effect comes from high levels of nitrogen fertilization, which dramatically increases the nutritional quality of the plant tissue for many insect herbivores. Nitrogen is a component of amino acids and proteins, which are often the limiting nutrients for insect growth and reproduction.
When plants are given high doses of nitrogen, they develop rapid, succulent growth and accumulate higher concentrations of soluble nitrogen compounds. This makes the plant’s sap and foliage a more palatable and protein-rich food source. This stimulates feeding and boosts the survival and reproductive rates of pests like aphids and spider mites. Plants with higher nitrogen content attract greater numbers of sap-sucking insects.
Furthermore, plants grown in high-nitrogen conditions may produce fewer natural defensive compounds, which typically deter herbivores. This suppression of defenses, combined with improved nutritional value, creates an environment where pest populations can expand. In contrast, organic fertilizers release nitrogen slowly, resulting in lower concentrations of soluble nitrogen in plant tissue and often leading to fewer insect pest problems.