Cascudinho is a cause for concern in the Midwest

The soybean mealworm (Myochrous armatus), a polyphagous insect belonging to the Chrysomelidae family, has caused successive losses to crops in the Brazilian Midwest. The most critical cases occur in the north and center-west of Mato Grosso do Sul and in the south and east of Mato Grosso.

In the last harvest, reported losses reached 30% in infested areas.

The increase in the pest population worries producers and requires integrated and assertive actions for management and control, explains post-doctorate in entomology Andressa Lima de Brida, researcher at Fundação MS.

Biology and behavior

The adults of M. armatus They measure approximately 5 mm in length by 3 mm in width and are matte black in color, with variations from brown to grayish due to the adhesion of soil particles to the body.

With little flight capacity, they exhibit peculiar defensive behavior, known as thanatosis: when threatened, they throw themselves to the ground and remain motionless or bury themselves in the ground.

The larvae live in the soil, where they feed on organic matter and roots of various host plants, including brachiaria, corn, beans and wild peanuts.

Adults attack soybean seedlings, causing severe damage such as cut tips, curled leaves and damaged main stems.

Crop losses

The damage caused by M. armatus are greater in the initial stages of soybean development, from emergence to phenological stage V5 (25 days), teaches Andressa.

The pest reduces the vigor of young plants, causing yellowing, wilting and death. In severe cases, the damage causes gaps in the stand and requires replanting, directly impacting productivity and production costs.

The intense attack forms swarms, which are distributed irregularly throughout the crops, making control difficult. During the day, the insects hide under clods or in straw, avoiding contact with pesticides and making conventional spraying less effective.

In areas without straw, often during the hottest times of the day, adults of M. armatus end up lodging themselves in the plant's trefoil, a fact that can contribute to the application technology. However, the time is not suitable for insecticide applications.

Management and control

the management of M. armatus requires integrated strategies. Among the most promising are:

• Seed treatment: protects seedlings during the initial phase, considered the most critical.

• Initial sprays: Nighttime applications, when the insect is more exposed, increase effectiveness.

• Use of chemical and biological products: Active ingredients such as fipronil, chlorpyrifos, thiamethoxam and acephate have shown good results in controlling the pest. Recent studies also indicate the effectiveness of essential oils -- such as orange, garlic and citronella oil -- which act as repellents, neurotoxicants and insecticide enhancers.

• Constant monitoring: Tools such as the beating cloth help in early detection and assessment of the level of infestation, allowing quick decisions on management.

• Rotation of active ingredients: prevents insect population resistance and resilience.

Challenges and prospects

Although current strategies show good results, the management of M. armatus still faces challenges due to the insect's habit of hiding in straw and under the soil.

This reinforces the need to integrate techniques such as monitoring, chemical and biological management and the development of new solutions to increase control effectiveness.

Furthermore, the approach must consider the management of the agricultural system as a whole, since the soybean beetle is polyphagous and finds shelter in host plants such as corn and tigueras, says Andressa.

Crop rotation and the adoption of practices that reduce populations between harvests are essential.

The entomologist also explains that the MS Foundation, in its unit in São Gabriel do Oeste (MS), has intensified research aimed at controlling this pest.

New tests are being developed with new technologies and combinations of control strategies to bring solutions to rural producers.