Insecticides and topical repellants

Definition

Insecticides and topical repellants are chemical or biological substances (pesticides) used to kill adult or larval-stage insects or prevent troublesome (and often disease-causing) insect behavior, such as biting. They are mainly used to target insects that feed on crops or stored foods or, like mosquitoes and ticks, are vectors of human diseases such as malaria, yellow fever, West Nile virus, Lyme disease, and dengue fever.

Insecticides specifically kill insects; repellents deter or repel insects and may or may not also be insecticides. Many different types exist, from common consumer-oriented bug sprays to powerful formulas used in industrial agriculture. They can also be classified in various ways, including whether or not the effect is residual in the environment; the mode of action; and whether they are naturally or synthetically derived. The main modes of action include stomach poisons (which must be ingested), contact poisons (which kill on contact), and fumigants (which kill upon respiration). Most forms of insecticides and topical repellents are sprayed or dusted onto a designated treatment area.

Insecticides and insect repellents are considered greatly influential in increasing global agricultural output and reducing many forms of disease, thereby contributing to longer human life expectancies and population growth. However, they can also have negative environmental effects, especially when overused. Some insecticides are toxic to non-insect species, including humans, and the toxicity can build up in food chains and disrupt entire ecosystems. Therefore, scientists continue to pursue biotechnology and other methods, including genetic engineering, to develop safer and more effective insecticides and repellents.

Chemical Insecticides

Carbamate insecticides affect the nervous system of insects. These insecticides block the regulation of the neurotransmitter acetylcholine by inhibiting the vital enzyme cholinesterase. Like carbamate insecticides, organophosphate pesticides affect the nervous system of insects by blocking the regulation of the neurotransmitter acetylcholine. Organophosphates are generally not persistent in the environment. Some are poisonous nerve agents.

Organochlorine insecticides, such as dichloro-diphenyl-trichloroethane (DDT), were used in the past. They were withdrawn from the market because of their harmful health effects and their persistence in the environment.

Organosulfer pesticides have very low toxicity in insects and are mainly used to control mites. The chemical structure of organosulfer pesticides is similar to DDT, with sulfur used in place of carbon as the central atom.

Biological Insecticides

Pyrethroid pesticides are synthetic versions of the natural chrysanthemum pesticide pyrethrin. The ground, dried flowers were used in the early nineteenth century to control body lice among soldiers in the Napoleonic wars. The synthetic versions have been modified to increase their stability in sunlight. Pyrethroids are generally effective against most agricultural insect pests when used at very low concentrations of 0.01 to 0.1 pound per acre. Some synthetic pyrethroids are toxic to the insect's nervous system.

The formamidine insecticides are used in the control of organophosphate-resistant and carbamate-resistant pests. Nicotinoid pesticides are chemically similar to nicotine. They are used to treat soil, stored seeds, and the foliage of cotton, rice cereals, peanuts, potatoes, vegetables, fruits, and nuts.

Limonene is extracted from citrus peel. It is effective against the pests of domestic animals (or pets); these pests include fleas, lice, mites, and ticks. Eugenol extracted from cloves and cinnamaldehyde extracted from cinnamon are used on ornamental plants and on many crops to control various insects.

Neem seed oil extracts are chemically similar to limonene. The oil has insecticidal, fungicidal, and bactericidal properties when insects consume or come in contact with it. The oil disrupts molting by inhibiting the juvenile molting hormone.

Spinosad, a product of the soil bacterium Saccharopolyspora spinosa, is effective against various caterpillar pests, which come in contact with or consume crops that have been treated. Crops treated include cotton, vegetables, tree fruits, and ornamentals.

Rotenoids (rotenone) are produced in the roots of beans. Used to treat the foliage of crops, rotenoids are both a stomach and contact insecticide used to control leaf-eating caterpillars.

Larvicides

Larval control can be used when insect breeding sites are within flying range of communities. Frequently, they are used to supplement the effects of other control methods. The control of mosquitoes is the most common use of larvicides. Spores from several serotypes of the bacterium Bacillus thuringiensis are specific to various species of mosquito larvae. The gene for the toxic B. thuringiensis protein has been added to the deoxyribonucleic acid (DNA) of leafy plants through genetic engineering. The plant manufactures the substance that destroys the mosquito larvae.

Larva-eating fish, too, can be used for the control of mosquito larvae. When confined in water containers, larva-eating fish have been used for malaria control by reducing the numbers of malaria-carrying mosquitoes.

Physical Barriers

Floating layers of expanded polystyrene beads prevent mosquito breeding when used in isolated sites such as cesspools and water tanks. The barriers are used in areas where malaria is common.

Chemical Insect Repellants

The mechanisms of action of insect repellents are not well understood. The active ingredients DEET (NN-diethyl metatoluamide), Merck IR3535, and Picaridin are the most commonly available synthetic repellants. Each has efficacy against a broad range of insects.

DEET is used to repel biting pests such as mosquitoes and ticks that carry Lyme disease. Consumer products containing DEET include a variety of liquids, lotions, sprays, and impregnated materials such as wristbands.

Biological Insect Repellents

Produced by wild tomato plants as a natural pesticide is 2-Undecanone. Oil of lemon eucalyptus also has pesticide qualities. The chemically synthesized version, methyl nonyl ketone, is applied to skin or clothing to repel mosquitoes, biting flies, and gnats. Synthetic insect repellent is available as a lotion and as a spray. Oil of citronella comes from dried, cultivated grasses and has a distinctive floral scent that masks the carbon dioxide humans exhale and the lactic acid of human bodies, to which mosquitoes and other biting insects are attracted.

Insect Attractants

Insect attractants are signal-carrying chemicals known as pheromones. Sex pheromones encourage sexual behavior. Thus, a male insect may be attracted to and attempt to copulate with an object covered with sex pheromones; this interrupts the insect's reproductive cycle. Insect pheromone has been used in high concentrations in insect-breeding grounds. As a result, insect populations diminish, making it difficult for insects to locate mates; mating behavior is further affected, and reproduction is further interrupted.

Another type of attractant, the insect bait, is used to control cockroaches and ants and must be eaten by the insects to be effective. The baits are formulated with food or other insect attractant, which is eaten by the ants or cockroaches and then transported back to their respective colonies. Most baits confine the insecticide to a small, secure area, to help lower the risk of human or pet exposure to the chemicals within.

Impact

Insecticides are often considered essential to the success of modern agriculture. Meanwhile, without topical insect repellants, humans would be highly susceptible to infectious diseases transmitted by a huge vector population. However, insecticides and repellents have earned criticism and controversy as well. The poisonous nature of these compounds has the potential to harm animal populations beyond the insects they target, as demonstrated by the example of DDT, which contributed to the critical endangerment of bald eagles and other predator species. Farm workers have also been sickened by exposure to certain insecticides. As scientists have realized the ways in which insecticides can accumulate in the environment, efforts to create more ecologically friendly versions have shown success.

Even widely accepted insecticides and repellents can have unintended negative consequences, most notably the development of resistance in insect populations. This occurs when naturally hardy individual insects survive treatment and spread, making formerly effective insecticides no longer viable. This may force the use of broader-effect types of insecticides, which are more likely to also harm beneficial insects and otherwise disrupt the ecosystem. Indeed, by the 2010s ecologists warned that insecticides were contributing to a serious decline in the global insect population, a downward trend that continued in the 2020s at an estimated 2.5 percent each year. Paradoxically, heavy insecticide use can also cause unnatural growth in certain species if their natural predators are killed off. Due to these concerns, many agricultural and environmental scientists suggest insecticide use should be kept to a minimum in favor of methods such as biological pest control.

Bibliography

“National Report on Human Exposure to Environmental Chemicals.” Centers for Disease Control and Prevention, 15 Dec. 2022, www.cdc.gov/exposurereport. Accessed 6 Mar. 2023. A U.S. government report on the effects of insecticides and other chemicals on human health.

Eaton, Alan T. "How Insecticides Work." University of New Hampshire, extension.unh.edu/resource/how-insecticides-work-fact-sheet. Accessed 6 Mar. 2023.

"Insecticides." National Pesticide Information Center, 26 Aug. 2019, npic.orst.edu/ingred/ptype/insecticide.html. Accessed 6 Mar. 2023.

Veer, Vijay, and Reji Gopalakrishnan. Herbal Insecticides, Repellents, and Biomedicines: Effectiveness and Commercialization. Springer, 2016.

Ware, George W., and David M. Whitacre. The Pesticide Book. 6th ed. MeisterPro Information Resources, 2004. Comprehensive coverage of chemical and biological insecticides and repellents.

World Health Organization. Pesticides and Their Application for the Control of Vectors and Pests of Public Health Importance. 6th ed., 2006. apps.who.int/iris/bitstream/handle/10665/69223/WHO‗CDS‗NTD‗WHOPES‗GCDPP‗2006.1‗eng.pdf. Accessed 5 Mar. 2023. A report of the WHO Pesticide Evaluation Scheme of the Department of Control of Neglected Tropical Diseases.