Increase in the soybean mealworm population: Myochrous armatus in the Midwest of MS

Soy (Glycines max L.) is the most cultivated legume in the world. In Brazil, production continues to grow. This fact is attributed to the solidification of the culture as a key point for the trade of products from the soy agro-industrial complex in the world. Such magnitude of planted area provides a source of food and shelter for different types of insect pests adapted to the crop.

During the soybean crop development cycle, some problems can compromise productivity, such as the occurrence of pests and weeds, as well as others, such as those resulting from adverse weather conditions. Pest insects stand out for causing extremely significant damage. Among them are defoliating caterpillars and stink bugs, causing a reduction in yield and compromising final productivity.

O Myochrous spp. Baly, 1865 (Coleoptera: Chrysomelidae), called soybean mealworm, is a polyphagous insect that feeds on different cultivated plants and weeds. This pest causes a reduction in crop yield and hinders development due to the damage it can cause in the initial stages of the crop.

Myochrous armatus has caused serious damage to many producers in the Center-West of Mato Grosso do Sul (MS), detected in the 2017/2018 harvest in the North of MS, in the regions of Coxim and Sonora. In the 2021/2022 harvest, the occurrence of this pest was high in many agricultural areas of MS, causing damage in the initial stages of the crop. And many areas were subjected to new replanting, delaying the harvest and causing economic losses to producers. In addition to MS, the high incidence of mealworms was found in areas in the South and East regions of Mato Grosso.

In the 22/23 harvest, it was observed that some regions that did not yet have so much incidence of this pest began to suffer severe attacks (Campo Grande, Chapadão do Sul, Sidrolândia, Bandeirantes, Camapuã, Jaraguari and Rio Negro).

The soybean mealworm, when found in the fields of São Gabriel do Oeste (MS), generated questions and doubts regarding its identification. It was compared to another insect, known as torrãozinho, which is Aracanthus morei. After collection, the specimens were sent for identification and evaluated by a specialized taxonomist, together with researchers from CropSolutions. This is the species M. armatus and not A. morei. The species are morphologically distinct, belonging to two families: Chrysomelidae and Curculionidae.

The larvae of the soybean mealworm, M. armatus, are yellowish and live in the soil. They feed on organic matter and roots of different species. Adults measure approximately 5 mm in length and 3 mm in width, and are matte black in color with variations from brown to greyish, depending on the type of soil in which they live (soil particles are adhered to their body). The lateral margin of the anterior part of the adult thorax has teeth. And the body is covered with short, robust scales. In observations, it is clear that the soybean mealworm has little ability to fly, it presents thanatosis when it feels cornered: it pretends to be dead, remaining motionless. And it often hides in the soil, close to the plant’s stem.

The damage and injuries caused by M. armatus, in soybeans, occur in the adult stage of the insect. The attack begins shortly after the emergence of the soybean crop in (VE), coinciding with the period when the rains begin, being an important pest until the V5 phenological stage. During drought, the mealworm remains in the soil, in the pupa stage, remaining in this phase for approximately seven months, until it once again has suitable conditions for the emergence of adults. These insects feed on seedlings, stems, stems and petioles. The damage caused is greater the younger the soybean plant. When the pest attacks, the development of the crop may become unfeasible, resulting in stand failure. Added to this, the attacked plants show reduced initial development, followed by yellowing, wilting and death, and these symptoms can occur in large patches distributed irregularly across crops.

Indications that an insect infestation is occurring are the finding of seedlings without their pointer, severed main stem, leaflets hanging from the plant and the presence of entire leaves fallen to the ground, which serve as shelter for the adults. In plants in the flowering stage, some adults can be observed. However, to date, it has not been detected whether they are capable of causing harm. But they are being studied in these phases, mainly with regard to the ability to cut flowers or even pods at the knife stage, which means constant observation and monitoring of this pest in the field.

Pest monitoring is generally carried out based on the principles of Integrated Pest Management (IPM), which indicates the level of control action according to the number of insects identified in the monitoring. For the mealworm, the level of control is still under study. However, the beating cloth is considered an essential tool for monitoring this pest.

The adoption of soybean mealworm control strategies in agricultural crops presents a great challenge, since this insect shows quite peculiar behavior, with a habit of sheltering in straw or under clods of soil during the day, being protected against direct contact between insecticides during spraying, invalidating some applications. Currently, producers have invested in seed treatment, as it offers initial protection for seedlings in the phase considered most critical of insect attack. Added to this, it is essential to use chemical management and/or the association of biological products in initial sprays when the plant has its first pair of leaves, always rotate the active ingredients and, if possible, carry out nighttime applications, when the insect is more exposed.

Most commercially available insecticides do not provide effective control. As a result, many producers end up adapting doses and mixtures. However, some active ingredients have demonstrated good results in reducing the population of these insects, such as fipronil, chlorpyrifos and thiamethoxan, imidacloprid, in addition to biological products, which have contributed to the pest management scenario.

Many biological products based on entomopathogenic bacteria and fungi have demonstrated efficacy under laboratory conditions. Likewise, for many agricultural pests, presenting great potential when associated with chemical insecticides.

With the objective of evaluating microbiological insecticides on pathogenicity and chemical insecticides on adult mortality of M. armatus, an experiment was developed in the Entomology Laboratory of CropSolutions – Pesquisa, Tecnologia e Inovação Agropecuária – Ltda., based in São Gabriel do Oeste, Mato Grosso do Sul.

The EUs experimental units consisted of seven treatments: T1 – Control (2 ml of distilled water); T2 - beauveria bassiana IBCB 66 (200 g/ha); T3 – Metarhizium anisopliae IBCB 425 (200 g/ha); T4 - Bacillus thuringiensis isolated 1641 and 1644; T5 – Fipronil 800 (80 g/ha); T6 – Chlorpyrifos (800ml/ha) and T7 – Etiprole (2,5 L/ha). The experimental units consisted of Petri dishes (9 cm in diameter and 1,5 cm in height), containing one soybean trefoil per dish and five adults of M. armatus. After spraying the treatments (manual sprayer), the Petri dishes were sealed and stored in a B.O.D air-conditioned chamber at 25°C. After insect mortality, treatments with fungi and bacteria were isolated in Petri dishes containing filter paper and stored in BOD at 25°C to confirm the cause of death.

Treatments T2 - Beauveria bassiana IBCB 66 (200 g/ha), T3 – Metarhizium anisopliae IBCB 425 (200 g/ha) and T4 - Bacillus thuringiensis (isolates 1641 and 1644) were pathogenic to adults of M. armatus, with mortality rates ranging from 58% to 86% after 24 hours of application. Treatments T5 – Fipronil 800 (80 g/ha), T6 – Chlorpyrifos (800 ml/ha) and T7 – Etiprole (2,5 L/ha + 2,5% gold) showed 72% to 100% mortality after 24 hours from application. After 72 hours of applying the treatments, the mortality rate varied from 2% to 42% (Graph 1). The integration of different forms of management to control pests, such as the use of insecticides associated with biodefenses, is a powerful weapon against M. armatus.

Microbiological insecticides based on microorganisms (fungi, viruses, bacteria and nematodes) are capable of infecting, incapacitating and killing insects.

The mechanisms of action are varied and highly specific for certain species of insects, so that each application can result in the control of only a portion of the pest complex present in the crop. Insects, when infected by the fungus, lose mobility and color, presenting a rigid and brittle body, and may sometimes be covered by mycelium and spores with a typical appearance and color of the associated entomopathogen.

In the case of bacteria, the transmission of Bacillus thuringiensis (Bt) occurs through spores: dormant bacterial cells capable of resisting heat, desiccation and even radiation. These spores contain genetic material, cytoplasm and all the substances necessary for the bacteria to survive, in a form of suspended animation. Furthermore, they remain encapsulated within crystals, which facilitate infection of host insects. After being digested by the susceptible insect, the crystals dissolve, releasing toxic proteins that destroy the mesenteric membrane (middle intestine) and allow the proliferation of bacterial cells, leading the insect to death through a process of generalized infection called septicemia. Bioinsecticides based on B. bassiana, M. anisopliae e Bt They are among the most used commercial products in agricultural pest control programs. Given the effectiveness of insecticides

of the organophosphate and phenylpyrazole chemical groups tested in the present work, the association of chemical and biological insecticides can present a synergistic effect and significantly increase control efficiency M. armatus. There is still a lot to be researched and tested in different situations, but this association has shown promising results.

An important point to be highlighted is that the management of M. armatus is not only in the control of the pest itself, but in the management of the system as a whole, since the soybean mealworm is a polyphagous insect and feeds on several species of plants, such as brachiaria, fedegoso, wild peanut, beans and corn. Therefore, the management of invasive and corn tiguera plants is very important, since the soybean mealworm can shelter and feed on these plants. This was confirmed in this 22/23 harvest. The highest incidence of mealworm occurred in the soybean/brachiaria and soybean/corn planting system due to the volume of straw, which serves as shelter for adults.

Little by little, research is beginning to identify the behavior of this pest, including its biology and ecology. Studies are being carried out to control this pest. Whether via seed treatment or spraying with different products, in addition to application technology, which is fundamental due to the complexity of the target, a combination of strategies will be needed to manage this pest. Carrying out correct identification, mapping behavior, defining the best times for application and adopting techniques recommended by integrated pest management are ways that can help producers deal with soybean mealworm.

By Andressa Lima de Brida (CropSolutions); Marlon Augusto Luft (CEO Luft & Libman); It is Juliano Deimiros Ribeiro (CropSolutions)

Article published in issue 285 of Cultivar Grandes Culturas magazine