Nepal | July 08, 2020

Host plant resistance: Way to avoid agrochemicals

Yubak Dhoj GC
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In recent days, the host plant resistance mechanism – where the plant defends itself from pests – has proved most effective. It requires no skill in applying the technique, and no cash investment is required from the farmers

Indiscriminate use of pesticides has led to serious concerns regarding their adverse effects on non-target organisms, pesticide residue on food and food products, pest resurgence, development of resistance in insects to insecticides, toxic effects on human beings, and environmental pollution. The insects and pests, against which farmers are compelled to use chemicals, are persistent creatures, with high reproductive rates, a fast generation turnover, wide genetic diversity across locations, and an ability to withstand, metabolise and avoid toxic chemicals.

As a result, it has become difficult to control several insect pests through the currently available pesticides. Therefore, it is important that we adopt pest control strategies that are ecologically sound, economically practical and socially acceptable.

Pest traps, no pesticides

Illustration: Ratna Sagar Shrestha/ THT

Prevention is the first tool in pest management because it is the most effective, least expensive and the most environment-friendly solution. Choosing a healthy plant that thrives in the desired location with the available light, water and nutrients prevents stress and minimises pest problems. Stressed plants can attract pests; hence, growing healthy crops with proper care is the first principle of integrated pest management.

The second most important tool in pest management is early intervention. Being present and observant in the field ensures early detection. Reacting to problems quickly before they have time to multiply requires a less dramatic intervention. Many safe, practical, non-chemical methods of plant protection may reduce or eliminate the need to spray chemicals.

In recent days, the host plant resistance (HPR) mechanism – where the plant defends itself from pests either through antixenosis, antibiosis or tolerance – has become one of the most effective, economical and environmentally friendly alternatives to chemical pesticides.

The most attractive feature of HPR is that farmers need virtually no skill in applying the technique, and there is no cash investment required from resource poor farmers.

Genes from the wild relatives of crops, and novel genes, such as those from Bacillus thuringiensis can be deployed in different crops to make HPR an effective defense against insect pests. HPR will not only cause a major reduction in pesticide use and slow down the rate of development of resistance to insecticides in insect populations, but also lead to increased activity of beneficial organisms and reduction in pesticide residues in food and food products. Furthermore, here in Nepal, we have a number of other options in integrating such plants in pest prevention plantation practices (P4 plantation techniques), which also reduces the need of spray.

Some plants avoid pest insects using their non-preference (antixenosis) mechanisms, making themselves unattractive to the insect for feeding, laying eggs or shelter as much as possible. Plants do this with their morphological characteristics or the presence of allelochemicals, which make them less preferred. Similarly, there are other virtues like antibiosis, which adversely affects the insects (survival, development or reproduction).

The most commonly observed symptoms in insects include larval death in the first few instars and abnormal growth rate. These symptoms may appear due to various physiological processes, viz. presence of toxic substances, nutrient imbalances and presence of anti metabolites and enzymes. In the same way, plants have some tolerance mechanisms, which produce good yields in comparison to other varieties with the same level of pest burden.

There are other attributes, such as the colour of plants, which serve as a defense mechanism. Insects can often be affected by the wavelengths of light reflecting off of plant surfaces, such as the Spanish white deters thrips from colonising the plant. High density of hairs on leaves may also serve to deter the laying of eggs by small insects. Hooked hairs on certain bean varieties have been shown to trap landing aphids and leafhoppers. Plants’ waxiness is another virtue, which makes leaves resistant to flea beetles, but susceptible to aphids and the white fly. The cotton variety has narrow twisted bracts and no leaf nectaries, making them resistant to the Heliothis moth.

Certain plant varieties protect themselves against wounding by exuding gum, latex and resin, such as in the conifer, pine (against pine shoot moth) and some legumes (against young bruchid larvae). Phenols produced by plants are found to be effective against root lesion nematode. Similarly, maize varieties have high levels of quinoneDimboa, which reduces the survival rate of corn borer larvae.

Then we have Pheromones, intra-specific chemical messengers that attract insects, which can be used in a lure and kill approach. Allomones, such as repellents, suppressants, deterrents and antibiotics, are deleterious to the receiving organisms whereas kairomones, excitants (biting, piercing or oviposition), feeding stimulants (promote continuous feeding) are beneficial to the receiving organisms. These compounds serve to hinder insect activities in one way or the other. The Nepal Government has given due priority to the utilisation of host plant resistance in pest management.

GC is Secretary, Ministry of Agriculture and Livestock Development

A version of this article appears in print on August 26, 2019 of The Himalayan Times.

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