Rape stem weevil

The rape stem weevil (Ceutorhynchus napi) is a small beetle belonging to the Curculionidae family. It is a major pest of oilseed rape, characterized by its dark, ash-grey appearance and distinctive long snout.

Adult beetles typically measure between 3 and 4 mm in length. Their bodies are oval-shaped and covered with fine scales, which help them blend into the soil and plant debris during dormancy.

The larvae are white, legless, and have a brown head capsule. They spend their entire developmental phase inside the host plant's stem, shielded from the external environment.

The pupal stage occurs in the soil, where the insect constructs a small earthen cell. This stage is crucial for the survival of the population and serves as a bottleneck for its annual cycle.

Beneficial insects, particularly parasitoid wasps like Tersilochus fulvipes, are natural enemies that help regulate the population density of these weevils in the ecosystem.

The rape stem weevil specifically targets Brassicaceae crops, with oilseed rape being the primary host. Both adults and larvae cause direct damage to the crop.

Adults consume young leaf tissue, but the primary economic damage is caused by the larvae feeding on the inner tissues of the stems, disrupting the flow of water and nutrients.

This feeding activity causes significant structural damage, leading to bent, twisted, or split stems that make the plant vulnerable to secondary fungal and bacterial infections.

In cases of severe infestation, the stems can break completely, resulting in substantial lodging of the crop and a dramatic reduction in seed yield and oil content.

The economic impact is exacerbated because internal damage is often invisible until the plant has already suffered permanent physiological stress, making detection vital.

Adult weevils emerge from their overwintering sites in the soil during early spring, once the temperature threshold of approximately 10°C is reached.

They migrate to the oilseed rape fields, feeding on young tissues to mature sexually. The timing of this migration is critical for farmers to plan their scouting efforts.

Mating and egg-laying occur in the spring. Females bore small holes into the tender stem tissue to deposit their eggs, after which the plant often heals the wound, leaving a scar.

Larval development takes about four to six weeks. Once mature, the larvae leave the stem, drop to the soil, and bury themselves to form pupae by early summer.

New adults emerge in mid-summer, feed briefly on secondary cruciferous plants, and return to the soil for hibernation, completing one generation per year.

The most diagnostic sign of an infestation is the bending or spiraling of the main stem, commonly referred to as "S-shape" distortion, visible during the growth phase.

Upon closer inspection, you can find small, circular puncture marks on the epidermis where the female laid eggs, often accompanied by longitudinal cracks in the stem.

Cutting open a suspect stem will reveal internal tunnels and larval excrement, which causes the inner pith to darken and rot over time.

Affected plants often show poor vigor, stunted growth, and premature senescence compared to healthy neighboring plants in the same field.

The use of yellow water traps at the field margins provides an early warning system, allowing for accurate monitoring of adult arrival before egg-laying begins.

Integrated Pest Management (IPM) is the most effective approach, focusing on cultural practices to minimize the reliance on prophylactic chemical applications.

• Crop rotation with a break of at least three years between Brassicaceae crops.
• Spatial isolation of new fields from previous year's rape stubble.
• Control of cruciferous weeds that serve as alternative hosts for the weevils.
• Deep tillage after harvest to disturb pupation sites in the soil.
• Targeted insecticide applications focused on the adult migration period.

Chemical control is only effective against adult weevils before they deposit eggs. Once larvae are established within the stems, they are virtually unreachable by conventional foliar insecticides.