Stem weevils and budworms: monitoring and control on 2 fronts - Cloned

Rapeseed stem weevils can cause stem bursts, which penalize yield components, particularly in dry years. Egg-laying disrupts sap flow, making plants sensitive to subsequent stresses and limiting their ability to compensate. Hydric stress or an attack by another pest in spring on an oilseed rape previously weakened by stem weevil are thus more damaging. Control methods are still effective, but the positioning of the intervention is crucial to guarantee the effectiveness of the protection.

Peak of flights possibly reached at the end of February, on rapeseed at sensitivity stage

Southwest

As the weather warmed up at the beginning of the month, the first flights of stem weevils, mainly on cabbage but also on rapeseed, took place in the Occitanie and ex-Aquitaine regions, but at generally low levels on rapeseed crops still in vegetation recovery or early bolting. The drop in temperatures and return of rain over the following days were unfavorable to the intensification of flights and egg-laying activity by females.

The return of clear skies and exceptional temperatures over the last few days have been favorable to significant weevil flights, with the probability of captures and presence in plots currently reaching its peak, on oilseed rape crops in full bolting, mostly at D1-D2 stages (source: BSV Occitanie-Nouvelle Aquitaine), and for some weakened by excess winter water and lack of nitrogen when vegetation resumes.

A sudden and massive arrival

The exceptionally warm weather and sunny spells of the past few days have been very favorable to insect flights, including the meligèthe, which can easily be caught in tanks or observed in plots, particularly on the most advanced rapeseed plants. The beetle is above all a pollinator. Its diet is based on pollen. However, when flowers are still at bud stage, they perforate them to reach the stamens, which can damage the pistil and lead to abortion. The risk of losses is all the greater when the buds are small; but as soon as the flowers are open, the pollen is free to access, and the nuisance generally becomes nil and treatment unnecessary. Females lay eggs in the buds during flowering, but this does not damage the plant.

Visit your plots to assess the condition of your colza and the presence of the beetle

The more vigorous and healthy the crop, the more it can withstand the presence of budworms, even if they are abundant. On the contrary, the more stunted, stressed or constrained the crop, the more susceptible it will be to attack. Observing the condition of the rapeseed is therefore just as important as observing the pest.

Here are a few rules to bear in mind when characterizing the risk of meligèthes on rapeseed:

• Rapeseed fertilization: what to do if the soil is too wet Observe plots from the D1 stage (BBCH50), corresponding to the appearance of clustered buds still hidden under the leaves, to the F1 stage (BBCH60), corresponding to the first flowers opening on half the plants. At the D1 stage, when flower buds are present and still hidden under the terminal leaves, beetles are more difficult to observe. You need to take the time to analyze the area of leaves surrounding the buds. At stage D2 (BBCH53) and E (BBCH57), the buds are fully visible and the larches are easier to spot.
• Counting at the edges or on the tallest plants is not representative of the situation. It is advisable to count on 4 x 5 or 2 x 10 consecutive plants; then calculate an average or a % per plant to be compared with the thresholds mentioned in the table below, taking into account the crop's ability to compensate.
• Avoid trap plants if present.
• Vigilance must now be maintained through regular plant counts, in order to determine where you stand in relation to the thresholds.
• Vigilance is also essential in situations where a tall, very early variety (e.g. ES Alicia or DK Exavance) is mixed. This practice enables us to control certain weak to moderate attacks, but does not rule out monitoring! In the event of heavy attacks, beyond the thresholds indicated below on the plants of interest, control of the populations of meligèthes may be justified.

Control strategy: maintain the population at a tolerable level using appropriate specialties

The aim of the control strategy is to maintain the population at a tolerable level (and not to eradicate it), so that flowering can start without major delays, and so that compensation can be expressed to the full. Rapeseed has considerable capacity for compensation. When the crop is vigorous, it can withstand even heavy attacks by beetles.

Meligethes are resistant to "-ine" pyrethroids (lambda-cyhalothrin, deltamethrin, cypermethrin, etc.). Tau-fluvalinate and etofenprox are 2 pyrethroids that escape rapid metabolism by meligethes and retain their potential efficacy.

Active substances effective on meliger beetles :

• Etofenprox (TREBON,30EC, UPPERCUT 0.2 l/ha)
• Tau-fluvalinate (MAVRIK SMART, TALITA 0.2 l/ha)

In case of concomitant presence of meligethes and rapeseed stem weevils at the intervention threshold, give preference to the use of Etofenprox-based solutions, taking care to respect the conditions of use of the corresponding insecticide specialties.

Recommendations for use

Optimum spray volume of around 200 l/ha: to optimize the effectiveness of insecticide spraying, it is advisable to work at "normal" volumes, avoiding low volumes of less than 100 l/ha.

The context in which spraying is carried out is important: sprayer settings, climatic conditions, characteristics of the product applied.

Protection of bees: Dangerous for bees (phrase SPE8): to protect bees and other pollinating insects, do not apply during flowering and/or exudate production periods, with the exception of uses benefiting from the F, PE, or FPE bee label. In the event of late application (e.g. E stage with first flowers appearing), it is imperative to use effective solutions which benefit from a bee exemption: MAVRIK SMART, TREBON 30EC (use-by stage BBCH61). Please note: these applications are subject to a decree governing application times: 2 hours before sunset and 3 hours after sunset.

Reasonable use of these solutions is essential. Read labels and available documentation carefully, and follow recommendations for use.