Understanding crop pest evolution to develop biocontrol strategies

The Egyptian cotton leafworm (Spadoptera littoralis) – a moth species found throughout the Mediterranean Basin, as well as in Africa and the Middle East – is currently a widespread pest in France. Since moth larvae are highly polyphagous, feeding upon a variety of different species, they cause damage to a diversity of crops, including corn, cotton, peppers, legumes, and tomatoes. 

As part of increased efforts to reduce pesticide levels, scientists currently struggle to develop effective biocontrol methods to fight crop pests, such as strategies to disrupt their reproduction or trap them through the use of sex pheromones. However, since pheromone synthesis is expensive, other control strategies may be needed. 

A team of scientists led by the Sorbonne University in Paris has recently investigated the evolution of olfactory receptors in the Egyptian cotton leaf worms. The research, published in the journal Proceedings of the National Academy of Sciences, represents an important step forward toward the development of such strategies.

In 2019, the experts identified OR5, an olfactory receptor in the Egyptian cotton leafworm which plays a critical role in recognizing the main component in the female sex pheromones. In the current study, they explored this receptor’s evolutionary trajectory within Spodoptera to clarify its specificity and functionality. 

By combining a computational approach to model the 3D structure of OR5 with the resurrection of its ancestral receptors in the laboratory, the researchers found that OR5 most likely emerged around seven million years ago, as an essential tool in the reproduction of two Spodoptera species – the Egyptian cotton leafworm and the tobacco cutworm (S. litura). 

The experts also used site-directed mutagenesis – a technique which introduces one or more precise mutations into a specific gene to investigate the functional impacts on the encoded protein. They identified eight amino acids (AAs) that structure the receptor’s specific characteristics. 

Since previous studies suggested that only one or two AA substitutions might be sufficient to change the functionality of ecologically important receptors, the discovery of such a large number of AAs behind OR5 is quite puzzling and requires further investigations.

Clarifying how olfactory receptors emerge and acquire their specificity during evolution can help scientists anticipate the development of resistance to pheromone-based products employed to protect crops. These findings could help devise new biocontrol strategies relying on both agonist molecules (which occupy receptors to the exclusion of the key pheromone compound) and antagonist molecules (which block the receptor from being activated by the key pheromone compound).

More about the Egyptian cotton leafworm

The Egyptian cotton leafworm, also known as the Spodoptera littoralis or the Mediterranean Brocade, is a major pest of cotton and many other crops. Native to Africa, it has spread to parts of Europe and Asia and is particularly problematic in the Mediterranean region.

The adult moth is brown and has a wingspan of 3-4 cm. The larvae, or caterpillars, are the damaging stage of the life cycle. They are initially green but turn brown as they mature, with light and dark stripes along the body.

The larvae feed on the leaves, buds, and bolls of the cotton plant, causing significant damage. They can completely defoliate a plant if the infestation is heavy. After feeding, they pupate in the soil and emerge as adults to repeat the cycle.

The Egyptian cotton leafworm can have multiple generations per year, and its population can increase rapidly under favorable conditions. It can overwinter as pupae in the soil, allowing it to survive in colder climates.

Control methods for the Egyptian cotton leafworm include the use of insecticides, biological control agents such as parasitic wasps and Bacillus thuringiensis (a bacterium that produces toxins harmful to certain insects), and cultivation of cotton varieties that are resistant to the pest. Integrated pest management strategies, which combine multiple methods of control, are often the most effective.

Due to the importance of Egyptian cotton in the global textile industry, the Egyptian cotton leafworm is a significant concern for farmers, scientists, and policymakers. Efforts to manage and control this pest are critical for maintaining the productivity and sustainability of cotton production.

More about crop pests

Crop pests are organisms that cause significant damage to crops in agriculture. They can be insects, birds, mammals, nematodes, fungi, bacteria, viruses, and weeds. The damage caused by these pests can significantly reduce the yield and quality of crops, leading to economic losses for farmers.

Some common types of crop pests include:

Insects

Insects are the most common type of crop pests. Examples include aphids, beetles, caterpillars, and weevils. They can damage crops by feeding on plant tissues, transmitting diseases, or both.

Nematodes

These are tiny, worm-like organisms that live in the soil and feed on the roots of plants, which can stunt growth and reduce yield.

Fungi

Fungi can cause diseases in crops, such as rusts, smuts, and mildews. They can damage crops by producing toxins, causing rot, or disrupting the water and nutrient transport within the plant.

Bacteria and Viruses

These can cause diseases in crops. Bacterial diseases include bacterial wilt, fire blight, and bacterial leaf spot. Viruses can cause diseases like tobacco mosaic virus, potato virus Y, and cucumber mosaic virus.

Birds and Mammals

Larger pests like birds and mammals can also cause significant damage. Birds can peck at fruits and seeds, while mammals like rodents can gnaw on stems and roots. Larger mammals like deer can trample and eat crops.

Weeds

Weeds compete with crops for resources like light, water, and nutrients. They can also harbor pests and diseases, making them harder to control.

Controlling crop pests is a complex task that involves a combination of cultural practices (crop rotation, intercropping, use of resistant varieties), biological control (use of predators, parasites, or pathogens of the pests), chemical control (pesticides), and other methods like the use of genetically modified crops. 

The goal is to keep pest populations below the level at which they would cause economic damage, a concept known as the economic threshold.

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By Andrei Ionescu, Earth.com Staff Writer

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