Household bug sprays are commonly used against spiders, which are arachnids and not insects. The application of an aerosolized liquid must physically reach the spider to begin the process of intoxication. Once the chemicals make contact, they bypass the protective outer layer to reach the central nervous system. This direct neurotoxicity is what causes the rapid paralysis and subsequent death, although the spider’s biology sometimes makes this process less efficient than with true insects.
Active Ingredients in Spider Sprays
The killing power in many over-the-counter pest control sprays is due to a family of compounds called synthetic pyrethroids. These chemicals, which include variants like deltamethrin, cypermethrin, and bifenthrin, are synthesized in a lab to mimic a natural substance. They are chemically similar to pyrethrins, which are naturally occurring insecticides found in the flowers of chrysanthemums.
The synthetic versions are designed to be more stable and potent than the natural extracts, allowing them to remain effective for longer periods. These active ingredients are dissolved in a solvent, often oil-based, and then pressurized for delivery as a fine mist or stream. The spray formulation is engineered to maximize the delivery of the neurotoxin onto the target pest.
Disrupting the Nervous System
The primary mechanism by which these pyrethroid compounds cause death is through neurotoxicity. Nerve impulses are electrical signals transmitted by the rapid opening and closing of voltage-gated sodium channels in the neuron membrane. This action allows sodium ions to rush into the cell, which creates the electrical signal.
Pyrethroids bind to these sodium channels and prevent them from closing properly once the nerve impulse has passed. This results in a prolonged influx of sodium ions, causing the neuron to remain in a constant state of excitement. The nerve cell is unable to reset, leading to continuous, uncontrolled firing of impulses.
This sustained depolarization of the nerve cells causes a state of hyperexcitation throughout the spider’s nervous system. The unceasing nerve signals lead to severe tremors, uncoordinated movement, and eventually complete muscle paralysis.
How the Spray Makes Contact
The chemicals must enter the spider’s body to reach the nervous system. The most common method is direct contact kill, where the spray physically coats the spider’s body. The toxins are then absorbed directly through the thin, waxy outer layer of the exoskeleton or penetrate the softer membrane tissue at the joints of the legs and abdomen.
The second entry point is through the respiratory system, although spiders do not actively breathe like mammals. Spiders possess specialized organs called book lungs, which have small external openings called spiracles. When the spray is aerosolized, the microscopic droplets can enter through these spiracles and be absorbed into the spider’s internal circulatory fluid. This respiratory uptake allows the toxins to be distributed systemically, often resulting in a faster onset of paralysis.
Why Spiders Are Not Always Easy to Kill
Spiders can sometimes be more difficult to kill with a quick spray compared to many household insects. This reduced efficacy is partly because most household sprays are formulated specifically to target insect biology. Spiders, being arachnids, possess a different physiology and a larger body mass than typical insect pests, often requiring a higher or more concentrated dose of the chemical to be lethal.
Spiders also minimize contact with residual chemicals on surfaces because their long, elevated legs keep their bodies lifted away from the ground. Some spiders have an inherent biological advantage, as their venom contains neurotoxins that act on sodium channels, which can confer a degree of natural tolerance to the pyrethroid chemicals. The immediate effect seen after spraying is frequently a “knockdown” that temporarily paralyzes the spider, and if the dose is not sufficient, the spider may metabolize the toxin and recover.