Thadomal Shahani Engineering College
Plants, unlike most other organisms, are stationary. They cannot move around to defend themselves or to protect themselves from being consumed by predatory pests (phytophagous insects). To overcome this, many plants have developed various defense mechanisms. These mechanisms could be physical, like the presence of thorns/ needle-like short hair on the plant body and leaves that harm the predators; or also chemical, like a deterring smell or the production of toxins.
When insects consume such plants, the toxic compounds produced by them render the predators incapable of performing some vital functions or in case of extreme toxicity, can even lead to their death. The toxins produced are majorly grouped into five categories as follows:
(1) Nitrogen compounds (primarily alkaloids) – Alkaloids in plants lead to feeding deterrence in insects, the bitter taste paired with the physiological discomfort experienced post-ingestion makes plants with such compounds unappealing to its predators. This deterrence increases with the increase in the concentration of alkaloid present.
(2) Terpenoids – Plants with terpenes can repel insects by preventing them from laying eggs, i.e, oviposition. This is the direct method. The plant protects itself indirectly by attracting natural predators of herbivores.
(3) Phenolics (flavonoids, tannins, etc.) – Phenolics have an anti-nutritive effect on the growth and development of insects and their larvae.
(4) Proteinase inhibitors – Proteinase inhibitors are capable of interfering with insect protein digestion by binding to digestive proteases of phytophagous insects, resulting in an amino acid deficiency, affecting insect growth and development, fertility and thus survival.
(5) Growth Regulators – Insect Growth Regulators (IGRs) produced by plants affect insects by disrupting the growth of immature insects, preventing them from growing into healthy adults. Because of these compounds, insect eggs may not hatch, the emerging adult’s wings or other body parts may be deformed, transformation might become faulty, and adults may become sterile.
However, herbivores have evolved counter-strategies to avoid eating these plants and thus save themselves from its poison. This is achieved mainly by aversive conditioning, also called as conditioned taste/food aversion. This is based on the concept that if a particular food ingested by an organism causes it discomfort, then the consumer tends to remember the effect and refrains from eating it in the future. The intensity of this aversion depends upon and is directly proportional to factors like the uniqueness and intensity of the flavour, the extent of discomfort caused. The duration between consumption and the onset of discomfort also plays a role. This leads to the insect registering and remembering the experience and the specific plant being avoided by it, a trait which is then passed on to its future generations, which in turn leads to adaptive avoidance of the poisonous plants.
This happens because avoiding certain compounds while eating food involves constant learning. This quality of learning is universal to all organisms, including insects. Insects have been known to learn using a variety of sensory stimuli, including visual, olfactory, gustatory, tactile, auditory and temperature signals. The major learnings are reinforced in one of the following ways:
1. Non-Associative Learning: In this type, the insects develop sensitization (enhancing of response) or habituation (waning of response) to a sensory cue. This happens due to repeated or prolonged exposure.
2. Associative Learning: This happens when a response to a particular stimulus is recognized by the insect.
3. Learning by trial-and-error: Insects and animals often sample their food before eating in a new environment. This way, they can identify and avoid poisonous plants and remember the same for future encounters.
4. Social cues: Taking in account the behaviour/eating pattern of other members of the species’ social group and modifying one’s own diet accordingly have been noted in many insects.
Studying this in detail can help with the planning of Integrated Pest Management systems and the creation of organic insecticides which, instead of killing the pests will condition them to avoid the target object(s).
Some plants with insect repelling properties include Neem, Menthol, Common foxglove and Varuna. These plants contain the toxin terpene. Cassava, Cherry and Lima bean contain glycosides. Wild Tomato, Rosary pea, Garlic, Chrysanthemum, etc. are some other plants with insect repellent properties.