Aug. 1, 2019
Usted puede encontrar la versión en español de este artículo aquí. 'Monitorar, la clave en el combate al Gusano Cogollero en Brasil'
Brazil recently harvested its second crop of maize, popularly known as "safrinha maize" in its Central and South regions. According to Brazilian Agricultural Research Corporation (Embrapa), the most serious pest this crop faces is the fall armyworm (Spodoptera frugiperda
), which can cause losses of more than 40% in corn production, the main cereal grown in Brazil.
Fall armyworm is also one of the major problems for rice cultivation, and is present in practically the entire Brazilian territory, affecting crops such as chard, watercress, lettuce, alfalfa, cotton, common chicory, alstroemeria, peanuts, rice, roasted rice, oats, potatoes, antirrhinum majus, broccoli, sugarcane, carrots, rye, barley, chicory, coconuts, cauliflower, chrysanthemums, Peace Lily, spinach, beans, gerbera, gypsophila, lisianthus, casserole, cassava, passion fruit, millet, corn, mustard, pastures, pepper, capsicum, cabbage, rose, arugula, rubber, soybeans, sorghum, stevia, all cultures with the occurrence of the biological target, tomatoes, wheat and triticale.
The newly hatched caterpillars of the Spodoptera frugiperda scrape the leaves and lodge in the so-called corn cartridge, where their excrements are found. By destruction of the cartridge, especially in the phase close to flowering, expressive damages can be caused, further accentuated during periods of drought. The damage is greater when the attack occurs in plants with 8 to 10 leaves. This species attacks the cartridge preferentially, but can also be found attacking seedlings, with a habit similar to that of the threaded caterpillar, spikes and, also, piercing the base of the plant, reaching the point of growth and provoking the symptom of "typical dead heart".
The control form of this caterpillar begins with the treatment of seeds for control in the early stages of the crop, and the application of systemic insecticides, which controls the pest when the water supplies are satisfactory. During water shortages, previous treatments should be supplemented with sprays directed to the cartridge region. It is recommended that registered products are used for the crops.
Researcher at Embrapa Maize & Sorghum, Simone Martins Mendes, said that currently, genetically modified maize in which specific genes of Bacillus thuringiensis (Bt) soil with insecticidal activity have been introduced are planted across 5.3 million hectares of Brazil. This represents about 82% of the first crop corn areas, according to a survey carried out by Céleres Consulting.
"However, the use of this technology in a large part of the crops does not deprive the farmer of the task of monitoring pest, mainly due to the reduction of resistance or reduction of efficiency of Bt maize on the carcass caterpillar, a reality in the main maize producing regions of the country. In this scenario, it is essential to maintain vigilance and monitoring in the crops," said Mendes.
According to the researcher, the presence of this pest can reach levels that demand additional measures of control. This can be done through biological control, with the release of beneficial insects in the crop, also known as natural enemies, such as the Trichogramma wasp or the use of bio-insecticides. Or even with chemical control, with the application of insecticides. These control measure should be used when the occurrence of the pest reaches a level that causes damage and is no longer viable for the use of biological control. This can be verified through monitoring.
Basically, two forms of monitoring have been advocated: the first is the use of pheromone traps, and the second is field monitoring. "In the first case, the trap contains a device that exudes the 'scent' of the female moth from the caterpillar to attract the male. In this case, the traps need to be checked frequently, and if three moths are caught, it is a sign that a critical level of infestation has been reached. Here, the producer must take into account the management measures to be adopted. A trap can monitor around five hectares," noted Mendes.
In turn, field-based monitoring focuses on the injuries (shaved or damaged leaves) caused by the carcass caterpillar on maize plants. In this case, he needs to divide the crops into plots and adopt a strategy for monitoring. "It is necessary to be aware of the maximum level of 20% of plants, with injuries to be decided by the use of chemical controls," Mendes said.
From this moment, the plants grow larger and become less vulnerable to pest infestations. "Also, when the caterpillar gets too big, it is difficult to reach it with any measure of control. The damage caused by the carcass caterpillar in the plant can reduce productivity by approximately 40%," said Mendes.
According to Ivênio Rubens de Oliveira, also a researcher at Embrapa Maize and Sorghum, in the last few harvests, there was an increase in spraying with chemical insecticides to strengthen the control of the caterpillar, even in plantations with Bt maize. In Brazil, there are 227 products registered against fall armyworm (See table).
"There is a tendency to schedule this spraying, often to coincide with the application of other products to control other problems, for example, herbicide applications. The problem is not to discuss the compatibility of the products in mixtures (also subject to another opportunity), but the target to be reached, the application technology used, and the ideal positioning of the insecticide at a time when the pest really needs control," said Oliveira.
If the corn plants are still very small, what is the actual dosage of product that will fall on them? "The calculation is done by the area of the crop, and not by the area occupied by the plants. Currently, because of the dozens of host plants for the carcass caterpillar, including the Poaceae, as brachiaria, which are also present in the planting areas during the off-season, this pest can already reach the corn plantation with larger caterpillars and cause unexpected losses, because small plants, especially those with up to two completely developed leaves, are easily destroyed by fall armyworm," he explained.
"In other words, for most corn producing regions in Brazil, we are no longer monitoring the arrival of the pest, but rather detecting the beginning of its attack, since we assume that fall armyworm is somehow already present in the area. The keyword continues to be 'monitoring'," concluded Oliveira.
"Today there are several commercial brands available in the market for the control of the fall armyworm, offering different active ingredients and, consequently, different mechanisms of action. But it becomes important to position the products according to the current moment in which the plants and the pests are at," explained Oliveira.
According to the specialist, it is strategically more interesting that the control is done while the caterpillars are still small (<1.5 cm): "This favors the action of the insecticide and prevents serious damage to the plants. In this case, it is possible to opt for the use of a growth regulating insecticide."
"But if the caterpillars are already larger (> 1.5 cm), there is a risk that these insecticides will not act in time to avoid damage. In this case, neurotoxins, which act on the nervous system of the insects, could kill them in a short time," said Oliveira.
According to Oliveira, another criterion is in the toxicological class and the environmental class of each product. "There is a scale that varies from I to IV, with I being the most toxic to humans and the most harmful to the environment, and IV, the opposite of this. Whenever possible, we must meet these criteria to choose the products."
"This also leads us to an approach on insecticide selectivity. What does that mean? Selectivity of an insecticide is the property that each product has to control the pest, with less impact on the other organisms present. It is clear that biological insecticides are the most selective, but growth regulators and even some neurotoxins, such as the diamines, also do well in this role," added Oliveira.
"If the criteria for the right use of products, especially those recommended by IPM (Integrated Pest Management) are not obeyed, there is a risk of selecting resistant pests. It is the situation in which products are applied, yet there does not seem to be control anymore," warned Oliveira.
When this happens, the [technicians] are often "tempted to increase the dosage to see if the effect improves, but over time the situation only worsens. A situation may occur, when at first, the pest is controlled, but in a short time the pest returns in a much larger population. It's what we call pest resurgence".
"It also occurs when pests that already exist in the crop, but did not cause damages, can change their status and become important. We saw this recently with bedbugs and spittlebugs in corn. One of the causes for this is the misuse of insecticides that eliminate natural enemies and/or do not control the target pests any more," explained Oliveira.
Another factor to be analyzed is the effect of insecticides on the pollinators, important for high levels of productivity. According to Oliveira, there are several studies showcasing the deadly effect of various insecticides on these organisms.
"Of course, there is a positive side to using insecticides. If not, they would not be so popular. The use of insecticides has already been incorporated into the production system for many years. The producer sees the efficiency in the control, a smaller chance to go wrong and the guarantee of financial returns," Oliveira admitted.
He explained that it is usually chemical insecticides that offer quick action to deal with emergencies that should be avoided with monitoring. "They are [pesticides] that allow the farmer to see curative action. The farmer 'likes' to apply the product and see the dead pests, with their 'legs' up!"
"Another issue is that the producer does not like the complexity of dealing with a number of relevant and inherent control situations, such as plague and cropping, population size, the location where the pest is attacking, what equipment is needed, and where it goes. The insecticide is a ready package that is well known. It seems simple, if you do not go deep into the negative points mentioned above," explained Oliveira.
Finally, he said, the producer understands the cost/benefit language very well, and the insecticide easily allows this calculation, as well as the available information. "The path to sustainability is not easy. Nobody said it would be. There is not yet a mechanism to automatically decide what is right or wrong in the case of pest control, especially in large crops such as corn," he pointed out.
“It is necessary to apply the knowledge acquired over decades, always observing social, environmental and economic issues. Thus, the decisions made will be better and the results, increasingly visualized, and what is really good will gain a prominent place in the Brazilian agriculture scenario,” noted Oliveira.
Products registered or launched for the control of fall armyworm in Brazil
(Click to enlarge the image)
This story was initially published in AgroPages '2019 Latin America Focus' magazine. Download the PDF version of the magazine to read more stories.