When to Spray Rapeseed – Growth Stages and Treatment Timing
Rapeseed is one of the most profitable arable crops in Bulgaria, but also one of the most demanding in terms of crop protection. The question when to spray rapeseed requires a thorough answer, because this crop is treated both in autumn and spring, and the windows for effective application are narrow and dependent on the specific growth stage.
In this article we will examine in detail all stages of rapeseed crop protection – from autumn herbicide and insecticide treatment, through winter care, to spring fungicide and insecticide treatments, adapted to the climatic conditions in Bulgaria.
Why Rapeseed Requires So Much Attention
Rapeseed is a crop with a long growing season – from sowing in September to harvesting in June–July. During these 9–10 months it is exposed to numerous threats:
- Autumn: weeds, cabbage stem flea beetle (Psylliodes chrysocephala), slugs
- Winter: frost damage with a weak rosette, snow mould
- Spring: pollen beetle (Brassicogethes aeneus), stem canker, phoma, Sclerotinia
- Late spring: pod midge, seed weevil
Each of these risks requires timely intervention. Skipping even one critical treatment can cost 30–50% of the yield.
Autumn Treatments of Rapeseed
Herbicide Treatment – September/October
Weed control in rapeseed starts from the moment of sowing. Rapeseed is a slowly developing crop and weeds can easily smother it.
Pre-emergence herbicides:
- Applied immediately after sowing
- Active substances: metazachlor, clomazone, napropamide
- Period: immediately after sowing (usually the first half of September)
Post-emergence herbicides:
- Applied at the 2–4 leaf stage of rapeseed
- Against grass weeds: graminicides (quizalofop, fluazifop, clethodim)
- Against broad-leaved weeds: metazachlor-based products
- Period: October, usually 3–4 weeks after emergence
Important: Autumn herbicide treatment is more effective than spring treatment, because weeds are in early stages and competition has not yet caused damage.
Insecticide Treatment against Cabbage Stem Flea Beetle – September/October
The cabbage stem flea beetle (Psylliodes chrysocephala) is one of the most dangerous autumn pests of rapeseed in Bulgaria. Adult beetles gnaw cotyledons and young leaves, while larvae penetrate into petioles and the stem.
When to spray:
- When the first beetles appear on young plants
- Usually end of September – October
- Economic threshold: 1–2 beetles per plant at cotyledon stage
What to use:
- Pyrethroid insecticides (lambda-cyhalothrin, alpha-cypermethrin)
- Neonicotinoid insecticides (for more serious attacks)
Timely insecticide treatment is critical – if the beetles damage the growing point, the plant dies or is catastrophically delayed.
Autumn Growth Regulator – October/November
In years with a warm autumn and abundant rainfall, rapeseed can outgrow – the rosette becomes too large and the stem elongates. This increases the risk of frost damage.
When to apply:
- Stage BBCH 14–18 (4–8 true leaves)
- Usually mid-October – beginning of November
- Active substances: metconazole, tebuconazole (simultaneous fungicidal and regulatory action)
The regulator slows vertical growth and stimulates root system development, which improves winter survival.
Winter Period – Monitoring
During winter, direct treatments are generally not carried out. However the period is not passive:
- Observe for slugs in warm and wet winters
- Watch for snow mould with prolonged snow cover
- Assess crop condition after thawing – percentage of surviving plants
If losses exceed 40–50%, reseeding or a crop change may be necessary.
Spring Treatments of Rapeseed
First Spring Treatment – Growth Regulator + Fungicide (March)
With the resumption of vegetation, rapeseed rapidly enters the stem elongation phase. This is the moment for a combined treatment.
When:
- Stage BBCH 30–33 (beginning of stem elongation)
- Usually the second half of March for Southern Bulgaria, beginning of April for the North
What is done:
- Growth regulator (metconazole, mepiquat chloride) – controls height and strengthens the stem
- Fungicide – prophylaxis against stem canker (Phoma lingam) and Cylindrosporium
- Foliar fertilisation with boron – a critical microelement for rapeseed
Why boron is important: Rapeseed has twice the boron requirements compared to wheat. Deficiency leads to poor pollination and empty pods.
Insecticide Treatment against Pollen Beetle – March/April
The pollen beetle (Brassicogethes aeneus, also known as Meligethes aeneus) is the most dangerous spring pest of rapeseed. Beetles enter flower buds and feed on pollen; in mass infestation they can destroy a significant proportion of flowers.
When to spray:
- Upon reaching economic threshold: 2–4 beetles per plant at green bud stage (BBCH 51–59)
- Usually end of March – mid-April
- Do not spray during flowering – to protect bees!
What to use:
- Pyrethroids (for susceptible populations)
- Organophosphates or oxadiazines (for resistant populations – an increasingly common problem in Europe)
Warning: Resistance of pollen beetle to pyrethroids is a growing problem. Rotate mechanisms of action.
Second Fungicide Treatment – April/May
The main fungicide treatment in rapeseed is directed against Sclerotinia (white stem rot) – a disease that can cause losses of up to 40% in wet springs.
When:
- Stage BBCH 65–69 (full flowering to end of flowering)
- Usually mid-April – beginning of May
- Spray when the petals of the first flowers fall (they create an entry point for infection)
What to use:
- Boscalid, dimoxystrobin, azoxystrobin – systemic fungicides with protective and curative action
This is the treatment with the highest return on investment in rapeseed. Even under moderate disease pressure, protection against Sclerotinia typically pays for itself.
Insecticide Treatment against Pod Midge and Seed Weevil – May
When:
- Upon formation of the first pods (BBCH 71–75)
- Usually May
- Economic threshold: 1 weevil per plant
These pests pierce the pods, larvae feed on the seeds inside and cause direct yield losses.
Practical Rapeseed Spraying Calendar for Bulgaria
| Period | Growth stage | Treatment | Priority | | ---------------- | ---------------- | ---------------------------------------------------- | ----------- | | September | After sowing | Pre-emergence herbicide | High | | October | 2–4 leaves | Post-emergence herbicide + insecticide (flea beetle) | High | | October–November | 4–8 leaves | Growth regulator + fungicide | Medium–high | | March | Start elongation | Regulator + fungicide + boron | High | | March–April | Green bud | Insecticide (pollen beetle) | Very high | | April–May | Flowering | Fungicide (Sclerotinia) | Very high | | May | Pods | Insecticide (pod midge/seed weevil) | Medium |
Note: timing for Southern Bulgaria is 1–3 weeks earlier than for the North.
The Drone as Solution for Rapeseed Spraying
Rapeseed is a crop where drone spraying proves extremely effective for several reasons:
Spring Spraying on Tall Crops
At the time of the Sclerotinia fungicide treatment rapeseed is 100–150 cm tall and in full flower. Entry of ground machinery:
- Breaks stems and severs flowers
- Creates tracks that hinder harvesting
- Can damage up to 8–10% of plants in the tracks
The DJI Agras T50 flies at 2–3 metres above the crop and applies the preparation without any contact with the plants. Learn more about rapeseed drone spraying.
Protecting Bees
Rapeseed is a major nectar crop and beekeepers position hives near rapeseed fields. The drone can spray early in the morning or late in the evening, when bees are not flying, with precise application and no droplet drift. This reduces the risk to pollinators.
Speed within a Narrow Window
Insecticide treatment against pollen beetles must be done within a few days, and fields may be hundreds of decares. The DJI Agras T50 treats up to 200 decares per hour, enabling treatment of the entire area on time.
For a full list of drone spraying services and information about drones for crop protection, visit our website. See also the advantages of drones in agriculture.
Common Mistakes in Rapeseed Spraying
- Skipping the autumn insecticide – the flea beetle can destroy the stand before winter.
- Spraying insecticide during flowering – kills bees and is legally restricted in Bulgaria.
- Lack of boron in foliar fertilisation – leads to poor pollination and low yields.
- Skipping fungicide against Sclerotinia – "I saw no symptoms" is not an argument, because the treatment is prophylactic.
- Using only pyrethroids against pollen beetle – with resistant populations the effect is zero, while the pest continues to destroy buds.
Specifics for Bulgarian Conditions
- Dobrudzha: Main rapeseed region. Relatively later vegetation. Attention to winter frost damage.
- Thracian Plain: Earlier start of spring treatments. Higher pollen beetle pressure due to warmer climate.
- Northwestern Bulgaria: Wetter, more problems with Sclerotinia and Phoma.
Climate changes in recent years lead to warmer autumns (rapeseed outgrows), milder winters (more surviving pests) and more intense spring droughts. Adapting the crop protection programme is mandatory.
Frequently Asked Questions
What is the most important treatment in rapeseed?
The fungicide treatment against Sclerotinia (white stem rot) during flowering (BBCH 65–69) is the treatment with the highest return on investment in rapeseed. Even under moderate disease pressure, this treatment typically pays for itself. Skipping it in a wet spring can lead to losses of up to 40% of yield.
Can rapeseed be sprayed during flowering?
Fungicide treatment during flowering is not only permissible but mandatory in wet conditions. Insecticide spraying, however, is prohibited during flowering to protect bees and pollinators. The drone enables precise fungicide treatment early in the morning or late in the evening, when bees are not flying.
What are the main rapeseed pests in Bulgaria?
The main pests are: cabbage stem flea beetle (autumn, September–October), pollen beetle (spring, March–April), pod midge and seed weevil (May). In Dobrudzha and the Thracian Plain the pollen beetle is especially aggressive due to the warmer climate. Monitor economic thresholds and apply insecticides on time.
When is growth regulator applied to rapeseed?
The regulator is applied twice: in autumn (BBCH 14–18, October–November) to prevent outgrowth and improve winter survival, and in spring (BBCH 30–33, March) to control height and strengthen the stem. The spring application is usually combined with a fungicide and boron fertilisation.
Is drone spraying of rapeseed worthwhile?
Yes, especially for spring treatments when rapeseed is 100–150 cm tall. The drone does not break stems, does not create tracks and can work in wet soil. For the Sclerotinia fungicide spraying, the drone is the only solution that does not cause mechanical damage to the stand. See rapeseed drone spraying.
Conclusion
Knowing when to spray rapeseed is of decisive importance for the profitable growing of this valuable crop. The key moments are: autumn control of flea beetle and weeds, spring protection against pollen beetle before flowering and fungicide treatment against Sclerotinia during flowering. Each of these treatments has a narrow time window in which it must be carried out.
The AirAgro team has DJI Agras T50 drones ready to treat your rapeseed areas quickly, precisely and without crop damage – even in the most critical phases.
📞 Request a free consultation for crop protection of your rapeseed areas. We offer:
- Free farm inspection with assessment of the crop's condition
- Drone demonstration of the DJI Agras T50 on your field
- Personalised treatment plan for autumn and spring treatments
We serve farms in Dobrudzha, the Thracian Plain and all of Bulgaria. Contact us at airagro.bg or call us – early booking guarantees priority service during peak periods!
