How effective is azadirachtin as an insecticide, really?
Neem - nature's insecticide. Neem has insecticidal and sterilizing properties and has long been used as a herbal medicine. But do you know its effects on insects, the residual activity of azadirachtin, the systemic action of azadirachtin, the efficacy of azadirachtin products, and so on? If not, let's find out together.
Long ago, Indian farmers had already discovered that neem could withstand periodic locust plagues on their seeds and leaves, and it has always been a traditional insecticide used by local people to control pests. In the early 19th century, Indian scientists began conducting related research, but it was not until 1959, when German entomologist Heinrich Schmuterer personally witnessed thousands of locusts passing through in Sudan and found that neem was the only green plant still standing, that people began to take notice and wanted to explore its value for pest control.
In addition to its antifeedant effect, subsequent research found that azadirachtin also strongly interferes with the transmitters involved in the synthesis and release of insect juvenile hormone and ecdysone, affecting molting and reproduction. Electrophysiological studies on antifeeding showed that the site of action in the cabbage white butterfly is the maxillary styloconic sensilla on the larval mouthparts. Therefore, azadirachtin has both a preventive crop-protection effect (antifeeding) and the property of hindering normal insect growth (lethal). From the perspective of pest control, this is a great advantage. More than 200 species of insects, belonging to 8 orders within the class Insecta (Coleoptera, Diptera, Hemiptera, Homoptera, Hymenoptera, Lepidoptera, Orthoptera, and Thysanoptera), are known to be susceptible to azadirachtin. These insect species account for more than 90% of all insect species tested so far, showing that its action covers a very broad range. Here we introduce the effects of azadirachtin on insects, the residual activity of azadirachtin, the systemic action of azadirachtin, the efficacy of azadirachtin products, and so on.
(1) Effects of neem on insects
The ways in which neem products act on various insects are as follows:
1. Inhibit the development of eggs, larvae, or pupae.
2. Hinder molting in larvae or adults.
3. Block communication or mating between males and females.
4. Repel larvae or adults.
5. Prevent females from laying eggs.
6. Make adults infertile.
7. Poison larvae or adults.
8. Prevent feeding.
9. Inhibit intestinal peristalsis and cause loss of the ability to swallow.
10. Transmit erroneous physiological signals at each developmental stage, interfering with normal metamorphosis.
11. Inhibit chitin formation.
(2) Residual activity of azadirachtin
Most of neem's pest-control effect is achieved after treatment of plants, through insect feeding or the production of a repellent effect, so the length of its residual activity greatly affects its pest-control performance. Schmutterer pointed out that under field conditions in tropical regions, the residual activity of neem can last an average of 5 days. Purified and isolated azadirachtin decomposes under ultraviolet light, but when azadirachtin contained in neem oil was subjected to simulated sunlight tests, it was found to decompose much more slowly, indicating that neem oil contains certain factors that can maintain and enhance the biological activity of azadirachtin. This can serve as one of the research directions for improving neem insecticide formulations.
(3) Systemic action of azadirachtin
Existing test results show that azadirachtin applied to plants does not translocate, but soaking the roots of potted plants in seed extract can effectively prevent leaf-eating insects from feeding. For example, soaking potted cabbage in water-soluble seed extract causes the cabbage white butterfly to avoid feeding, and soaking the cut end of cut chrysanthemum stems in the extract has the same effect against the American serpentine leafminer, but newly grown leaves cannot be protected. If rice seeds are soaked in neem seed extract before sowing, the mortality of green rice leafhoppers feeding on the resulting seedlings increases. These examples show that neem extract has a systemic action.