Plants That Kill: Neem Tree

Adapted from page 217 of Plants That Kill:

Neem, also known as Indian neem, is grown across the tropics and subtropics as a shade tree, for reforestation programmes and in plantations for production of azadirachtin, but is considered invasive in parts of Africa, the Middle East and Australia, where it has become naturalized. The seeds of Philippine neem (Azadirachta excelsa), which is native to Indonesia, Malaysia, Philippines, Papua New Guinea and Vietnam, and has naturalized in Singapore and Thailand, are also a source of neem insecticides. However, even though neem-based pesticides are a good biological alternative to synthetic compounds, accidental ingestion of neem products or seeds has resulted in a number of deaths, especially in children.

While the effectiveness of neem insecticides is directly associated with azadirachtin content, the biological activity of many of the other compounds present in the neem tree (most of which are also triterpenoids of the limonoid group) add to its effect. Used in their natural combination, they may be helpful in mitigating the development of pesticide resistance.

The neem tree (Azadirachta indica) can reach 10–20 m (30–65 ft) in height. It has compound leaves with several pairs of leaflets and heads of 150–250 small white flowers. Photo credit: QpicImages, Alamy Stock Photos

The antifeedant activity of azadirachtin and some of the other neem compounds is through their stimulation of specific ‘deterrent’ cells on the insect mouthparts, while blocking other receptor cells that normally stimulate feeding, resulting in starvation and death of the insect. Insects vary considerably in their behavioural responses to azadirachtin. Studies on the desert locust have shown that it has a particularly high sensitivity to azadirachtin as an antifeedant, being deterred from feeding at concentrations of 0.04 parts per million. Interestingly, North American grasshoppers, including the American grasshopper (Schistocerca americana), which is in the same genus as the desert locust, are insensitive to azadirachtin at such low concentrations. 

Insects that are not deterred from feeding on azadirachtin do not die immediately, but soon stop eating due to the action of the compound on a number of physiological pathways. It interferes with moulting and growth, for example, by blocking production and release of moulting hormones, causing moulting defects, and it disrupts reproduction by reducing the number of viable eggs and live progeny. 

Plants That Kill: A Natural History of the World’s Most Poisonous Plants
By Elizabeth A. Dauncey & Sonny Larsson

This richly illustrated book provides an in-depth natural history of the most poisonous plants on earth, covering everything from the lethal effects of hemlock and deadly nightshade to the uses of such plants in medicine, ritual, and chemical warfare.

Featuring hundreds of color photos and diagrams throughout, Plants That Kill explains how certain plants evolved toxicity to deter herbivores and other threats and sheds light on their physiology and the biochemistry involved in the production of their toxins. It discusses the interactions of poisonous plants with other organisms–particularly humans—and explores the various ways plant toxins can target the normal functioning of bodily systems in mammals, from the effects of wolfsbane on the heart to toxins that cause a skin reaction when combined with the sun’s rays. This intriguing book also looks at plants that can harm you only if your exposure to them is prolonged, the ethnobotany of poisons throughout human history, and much more.

A must for experts and armchair botanists alike, Plants That Kill is the essential illustrated compendium to these deadly and intriguing plants.

  • Provides an authoritative natural history of the most poisonous plants on earth
  • Features hundreds of color illustrations throughout
  • Looks at how and why plants produce toxins
  • Describes the effects of numerous poisonous plants, from hemlock and deadly nightshade to poppies and tobacco
  • Explains poisonous plants’ evolution, survival strategies, physiology, and biochemistry
  • Discusses the uses of poisonous plants in medicine, rituals, warfare, and more


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