The soybean weevil (Rhyssomatus subtilis), one of the most feared pests in the Northwest Argentina due to its impact on crops, was detected for the first time in northern Córdoba, specifically in the Argentine Malvinas region. Although these are isolated cases, the discovery raises concern and highlights the need to prevent its spread.

A report published by Aapresid's Pest Management Network (REM) reveals that the strongest hypothesis regarding the causes of its arrival is human activity: the movement of agricultural machinery, trucks, or harvesters from affected areas of the Northwest.

R. subtilis is a difficult pest to control for several reasons. It survives the winter as larvae buried up to 15 cm deep. Adults emerge from the soil in staggered bursts after spring rains, which may require multiple interventions for control. It is nocturnal and hides during the day in corn stubble and heads.

In early vegetative stages, they feed on cotyledons and tender shoots, causing the death of seedlings and the fall of the stand. As the crop progresses, attacks on terminal shoots can generate low, branched structures, reducing the number of pods per plant.

In early reproductive stages, they puncture immature pods, which can induce abortion or pod drop. During stages 5 and 6, the females lay eggs inside the pods. The larvae then develop, feeding on the developing grains.

The REM emphasizes that, under high-pressure conditions, fields have been recorded with up to 100% of plants showing symptoms of attack.

Although the problem currently appears to be a temporary problem, specialists recommend not letting our guard down. "Today, it does not represent a regional problem. It could only reappear in the same fields where it was found if soybeans are planted again without rotation," explained Roberto Peralta, a representative of Halcón Monitoreos, in an interview with the REM. Along these lines, the same REM report recommends a three-pronged approach:

1. Monitoring

• Use of a vertical screen to detect adults (although it only allows an estimate of ~60% of the population, as the insect tends to drop to the ground).
• Direct observation of shoots and field edges, especially where they border grasses.
• Accurate identification: damage can be confused with that of the weevil, ants, or rodents.

2. Agronomic management

• Avoid soybean monoculture, which favors its proliferation.
• Include grasses in the rotation: summer grasses such as corn or sorghum are not hosts for the weevil, which interrupts its cycle and reduces populations.
• Thorough cleaning of machinery and equipment moved from areas with confirmed presence.

3. Chemical Control

• Seed treatment with insecticides such as neonicotinoids (thiamethoxam, imidacloprid, acetamiprid) or phenylpyrazoles (fipronil, ethiprole), useful during the first 20–25 days.
• Foliar applications with night-acting products such as pyrethroids, neonicotinoids, sulfoximines, or isoxazolines, especially in R5–R6.

The REM concludes that, while this is not a widespread threat, it sets a precedent for vigilance. “Its spread will depend on the agronomic decisions made today. Rotation and early monitoring will be the main barriers to preventing the weevil from establishing itself in new regions,” the report states.