Synthetic chemicals for use as insecticides enabled the green revolution and an explosion in agricultural productivity.  Widespread use of these chemicals, however, was accompanied by negative environmental and human health impacts. Furthermore, insects develop resistance to chemicals with the same mode of action (MoA). Stricter regulations on pesticide use, health and safety concerns, and the demands of organic agriculture have put terpenes in the spotlight as potential insecticide alternatives with distinct MoAs. [1-3]
MoA describes the biological response of an organism when it comes into contact with a chemical compound. As of 2015, there were 580 insect species resistant to at least one insecticide.  Rotating insecticides with different MoAs is one strategy to reduce the development of biological resistance among insects.
Secondary plant metabolites, including terpenes, are often part of a plant’s overall defense mechanisms.  Various terpenes have been integrated into organic insecticide products due to demonstrated scientific viability. [1-3, 5] The Environmental Protection Agency considers D-limonene, α-terpinene, and p-cymene acceptable for use in insecticide products with an exemption for residue tolerance due to a high safety profiles in human beings.
D-limonene is the most established application of a terpene in insecticide products. According to the 1994 EPA evaluation of D-Limonene, this terpene is used in products to control fleas, ticks, flies, and mosquito larva. More recently, aqueous solutions of 1% D-limonene controlled at least 99% of whiteflies and 69-100% of mealybugs and scale insects.  Against three species of beetle (including the rice weevil), D-limonene fumigation at 2.14 mg/cm2 effectively halted egg laying and subsequent survival.  The MoA has not yet been elucidated, but some scientists speculate that certain terpenes interfere with the octopaminergic system of insects, which has neurotransmitter and neuromodulatory functions.  At high concentrations, D-limonene may cause skin irritation. It may also remove the waxy cuticle of certain plants. 
Research involving the isolation of specific terpenes as novel insecticides is emerging. One study in 2018 found that β-ocimene activated the defense response of Chinese cabbage against peach aphids.  Researchers have also demonstrated that essential oil blends of terpenes confer additive efficacy since insects oxidize the major terpene component, giving the lesser terpene component the ability to act as the toxicant.  Terpenes such as γ‐terpinene may also synergize with synthetic insecticides to amplify their effect. 
The Insecticide Resistance Action Committee (IRAC) was established in 1984 to address insecticide resistance and develop the MoA classification system. IRAC classifies 32 groups of MoAs and several groups for compounds with unknown MoAs. Currently, the IRAC does not recognize isolated terpenes, but it does include Chenopodium ambrosioides (Mexican tea).  D-limonene and trans-pinocarveol are the major components of the C. ambrosioides essential oil. 
The volatility of terpenes is one challenge associated with their use due to rapid oxidization with air exposure. Encapsulating essential oils in microemulsions with nanotechnology is a promising commercial solution to this challenge. [1, 3]
Natural, novel alternatives to synthetic insecticides is a pressing issue of the 21st century. Insecticides formulated with terpenes not only offer new MoAs suitable for organic cultivation and environmental sustainability, but also ensure the continuation of our way of life.
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- Kang, Zhi-Wei, et al. “Volatile β-Ocimene Can Regulate Developmental Performance of Peach Aphid Myzus Persicae Through Activation of Defense Responses in Chinese Cabbage Brassica Pekinensis.” Frontiers in Plant Science, vol. 9, 2018, p. 708, https://www.frontiersin.org/article/10.3389/fpls.2018.00708. Journal Impact Factor = 4.298, Times Cited = 2 (PubMed)
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