The growing challenge of caterpillars in the cotton production system

Brazil, in the 2023/2024 harvest, had one of the largest areas sown in recent years, 1.944,7 thousand hectares (Conab 2024), and there is an expectation of high production of the main global fiber, making the country reach world leadership in export.

Pests have been limiting factors in Brazil for several years. Some of them almost extinguished the culture in certain regions, due to their high occurrence and the damage they caused. And caterpillars can cause great damage to cotton crops, as well as adjacent crops in the Central West region, such as soybeans and even corn.

These pests have gained a great ally in management, with the entry of transgenic Bt technology (Bacillus thuringiensis), mainly in corn, cotton and soybeans. The integrated pest management was of great value, with a reduction in applications in crops by around 20% overall, and in the caterpillar complex by around 80%, in the first years of adoption. In soybeans, the decrease in certain areas was in the order of 90% in the number of applications.

However, some genera of caterpillars, such as spodoptera sp., stood out, due to technologies that often do not cover them, or even the process of resistance that, due to various conditions, increased the frequency.

This last point is very dynamic in the Brazilian Cerrado, due to the agroecosystem used in the Central West region, an environment favorable to the multiplication of pests, as a production system prevails in which soybeans are the main crop to be established in the vast majority of areas, which can be rotated or not, with corn and cotton, and, after harvest, a subsequent crop is established (2nd harvest) or even a cover crop, and, in some areas, in “fallow”. In this cultivation system, pests have been adapting for several years, even before the entry of bTS. Factors such as favorable climatic conditions, high temperatures and mild winters, become ideal for the multiplication of insects.

Efficiency of management methods

The implementation of crops with Bt technology (Bacillus thuringiensis) marked a significant advance in agriculture, offering more efficient, “smooth” and sustainable production. However, the presence and adaptation of caterpillars in these plantations challenges producers to look for innovative management methods - efficient and cost-effective.

The economic effect of resistant caterpillars on Bt crops is significant, negatively impacting productivity and profitability. Some data show losses of around 30% in productivity when uncontrolled (Graph 1), in addition to some cases of 100% when the screwworm habit occurs - eliminating the stand. The consumption of leaves, stems, flower buds, flowers and ears by these pests drastically reduces crop yields. Additionally, inefficient management of these pests can result in increased use of chemical insecticides, contradicting the environmental objective of Bt crops and increasing production costs.

Species like Spodoptera frugiperda (popularly known as fall armyworm) and helicoverpa sp. are among the most destructive, causing considerable damage to Bt crops of soybeans, corn and cotton, due to their adaptability and lower susceptibility to Bt toxins.

A S. frugiperda It presents great polyphagy, passing through soybean, cotton and corn crops, becoming the first or second most important pest. It can be referred to as a plague in the production system. In soybean cultivation, it is a pest that in the first instars tends to attack the leaves, and with development it can attack different parts of the plants, including the pods and the forming grains. Caterpillars can cut young plants at the base of the stem and lead to smaller plant stands. Some invasive plants, such as viola string, trapoeraba, sour grass, crow's foot grass, as well as cover crops such as millet, brachiaria and forage radish, behave as good hosts, enabling development and permanence of the complex spodoptera in cultivation areas.

The largest emergence of Spodoptera eridania

Among other caterpillars of the genus spodoptera spp., the S. eridania It is a species that has increased its frequency in crops. The adults are brown in color, with silvery spots on the wings and a rectangular spot on the first pair of wings. The female also lays eggs in masses, being able to lay up to two thousand eggs during the cycle. The caterpillars are green, brown to black in color with a yellow longitudinal stripe that often connects to the inverted Y on the head. In addition to two lines on the side, with the presence of white triangles. They can reach 50 mm in length. This species today in soybeans and Bt cotton has occurred in most areas, and its control is being carried out. It is worth mentioning that it is often noticed in the field, when in more advanced stages of development, which makes it difficult to control.

Factors such as polyphagy are a problem in the production system, in addition to weeds, corn tigueras, as suppliers of populations for the beginning of infestations.

Field sampling

Given the facts and all the changes, the foundation of integrated pest management (IPM) is good field sampling - knowledge -, where the monitor must carry it out following some important points, such as sampling frequency, intervals of three to seven days, a good number of points, which represents the crop, in addition to good accuracy in relation to pests. The number of points in a plot varies depending on the operational nature, but the issue of good representation of these samples cannot be ignored. Not knowing who the pest is, how many there are and where they are can lead to the failure of any strategy adopted.

In cases such as caterpillars of the genus spodoptera, which have an uneven distribution (i.e., occurrence in patches in the plots), greater care is needed in sampling. With the advent of bTS, some details in the observations become important, with greater care in the initial phase, new structures (leaves, buds and flowers), and the technician must pay attention, due to changes in recent years, such as oviposition, in addition to changes regarding the productive potential of different materials, and understanding the level of control.

Monitoring is often not well applied, leading to control failures, accelerating processes such as resistance, especially in the environment Bt. Failures in cotton plants, due to failure to observe the caterpillars on the needles and subsequent decline, mean that the pest is no longer controlled, especially in areas with hot periods (years such as El Niño) that accelerate its cycle.

It can be said that the “peace of mind” provided by technology caused some to become careless in sampling, leading to late decision-making for control. The attack on positions such as soybean flowers, cotton pointers, style and stigma requires accuracy to avoid errors.

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