Plants and harmful organisms

In agricultural ecosystems there are a number of ecological limitations among which the presence of harmful organisms such as insect pests can be found, that is why there are alternatives of pest control known technically to reduce the presence of these unwanted agents. It is also important to mention that plants have their regulatory mechanisms to avoid being affected by these organisms and of which we will describe based on those mentioned by Kogan 1994, in his article plant resistance in pest management.

Let's start by pointing out that the population of a harmful organism is regulated by many factors, the most important being the articulation with the host plant, a host plant is one that is affected by a harmful organism and from which it obtains the nutrients it needs to complete its life cycle, the inhibition of the growth of the population of these organisms is generally derived from the morphological and biochemical characteristics of the plant, which can affect the behavior or metabolism of the harmful organism.

It seems that phytophagous insects (they feed on plants) were originally polyphages, that is, they ingested a great diversity of plants, Some of those plants according to certain specialists evolved to produce and concentrate certain secondary metabolites with adverse effects on the insects that consumed them. This defense mechanism on the part of the plant prevents these insects from feeding on them, but at present it has been described in some articles that certain species of these pathogenic insects have developed mechanisms of being able to feed on these plants. That is to say that over time the metabolites became excitants of feeding so that, what at first was a defense of the plant became an intimate bond between the insect and the plant. For example, cucurbitacin (produced by cucurbits), which is avoided by phytophages due to its bitter taste, acts as an exciting agent of the appetite of some species of the genus Diabrotica.

On the other hand, it is important to mention that there are also physical and chemical components of the plant involved in the selection of the host by the harmful organism, firstly, among the possible physical factors are the firmness of the tissues, presence of hairs and thorns, color among others that can act as barriers to establishment, feeding or oviposition, there are also some chemical factors, it is said that the external environment of the plant is dominated by compounds of the secondary metabolism, that have been exuded from the outer layers of the tissues. These compounds generate the olfactory stimulus that lead to the recognition and finding of the host, however, internally, the plant contains a complex mixture of compounds, some that serve as a nutrient for insects and others that act as inhibitors or excitants of feeding, toxic, while others are indifferent.

The primary metabolites serve as nutrients to harmful organisms, while the secondary ones can act by causing an attached stimulus, but they lack nutritional value. We can also talk about induced resistance caused by external factors such as nutrition, irrigation or the action of pathogens on the plant. for example, the plant in the presence of infectious agents can produce phenolic compounds such as phytoalexins and it has been proven that, in addition to being a response mechanism against pathogen attack, phytoalexins can act as feeding inhibitors.

Similarly, there has been talk of genetic resistance, which, although it can be influenced by environmental factors, is determined by the genetic content of the plant. Three fundamental mechanisms are distinguished: Antixenosis when the plant is refractory to harmful organisms that intend to colonize it, antibiosis which refers to all the adverse physiological effects, temporary or permanent that the harmful organism suffers when feeding on the plant. The symptoms can be lethal or acute, mild or sub-chronic and tolerance which is the ability of the plant to tolerate the damage caused by harmful organisms.

Finally, dear readers, according to Koga (1994), the most common symptoms that occur in insects are: Death of the larva in the early stages, abnormal growth rates in the larva, failure to reach the pupal stage, failure in the emergence of adults from the pupae, malformed adults or not reaching the normal size among others and the possible explanations for these effects may be the following: presence of toxic metabolites, absence or insufficiency of essential nutrients, unbalanced proportions of nutrients, presence of antimetabolites that prevent essential nutrients from reaching, presence of enzymes that inhibit normal digestion processes.