Efficacy of insecticides against the pollen beetle in winter rapeseed crops

Winter oilseed rape is a valuable oil and forage crop that allows for high yields of quality vegetable oil and protein. In addition, oilseed rape serves as an excellent predecessor in crop rotation: after it, the yield of cereal crops increases by 4–5 centners/ha. However, growing this crop requires significant expenditure on plant protection, as without proper control, losses from pests, diseases, and weeds can deprive the farm of nearly half of the harvest.

• Potential seed yield — from 30–40 centners/ha
• Crude fat content in seeds — 47–49%
• Oil content in seeds — 45–48%
• Protein content in seeds — 22–25% (in green mass — 3–4%)
• Crop losses without crop protection — 30–45%
• Economic threshold of the pollen beetle — 3–5 adults per plant

The threat of resistance: why conventional insecticides are losing effectiveness

The main pest of spring and winter oilseed rape remains the pollen beetle (Meligethes aeneus F.). During periods of mass colonization, the pest infests up to 100% of plants, and yield losses from adult damage exceed 50%. To contain the number of pollen beetles, agronomists have to carry out 1 to 3 treatments per season. Such intensive chemical pressure inevitably leads to the formation of resistance in insects.

The growth dynamics in the number of resistant pest species clearly shows the scale of this problem. To prevent resistance, it is important to regularly evaluate the effectiveness of preparations and rotate insecticides from different chemical groups with different modes of action.

Experimental design and conditions for using preparations

To find effective protection schemes, field trials were conducted in 2021–2023. The experiments were established on sod-podzolic medium loamy soil with the following indicators: pH (KCl) — 6.1, humus — 1.7%, content of P2O5 — 269 mg/kg, K2O — 256 mg/kg. The predecessor of winter oilseed rape in the experiment was spring barley.

The trials evaluated insecticides with different modes of action that are most in demand in agricultural production. The treatment of plots was carried out with a manual sprayer at an application rate of the working solution of 200 l/ha. The effectiveness of the preparations was calculated according to standard methodologies, and the economic assessment was performed in 2024 prices.

• Variant 1: Control (without the use of insecticides).
• Variant 2: Systemic insecticide (thiacloprid, 480 g/l).
• Variant 3: Combined insecticide (imidacloprid, 150 g/l + lambda-cyhalothrin, 50 g/l).
• Variant 4: Combined insecticide (cypermethrin, 50 g/l + chlorpyrifos, 500 g/l).
• Variant 5: Contact insecticide (alpha-cypermethrin, 100 g/l).

Weather conditions of the 2021 growing season significantly influenced plant development and pest dynamics. April was temperature-contrasting: in the first ten days, the average temperature was 0.8 °C below normal, in the second it exceeded the norm by 2.0 °C, and in the third, it fell 4.4 °C below normal. The total precipitation for April was 39.9 mm (94.3% of the norm).

May turned out to be cool with an average daily temperature 1.7 °C below normal; at the beginning of the first ten days, night frosts down to –2 °C were recorded. Precipitation in May was unevenly distributed: an excess in the first and second ten days (154.5–308.7% of the norm) was replaced by a deficit in the third (43.2% of the norm). June and July were hot — 2.8–3.5 °C warmer than normal, while in June the amount of precipitation was close to the norm, and in July a severe deficit was observed.

How weather affects pest development

The timing of winter oilseed rape colonization by the pollen beetle directly depends on the hydrothermal conditions of the season. Rising temperatures stimulate the emergence of adults and accelerate reaching the economic threshold during the budding phase. For effective crop protection, it is important to monitor weather dynamics, which determine the rate of increase in the pest population.

In April 2022, the air temperature was close to the multi-year average, with an increase of 1.5 °C in the third ten days. Precipitation for the month was 7.0 mm, or 15.7% of the norm. Air temperature in May and June was 2.4 and 2.5 °C below normal, respectively. Precipitation was excessive — 119.4 and 87.0 mm, which amounted to 282.3 and 127.9% of the multi-year average levels. Heat distribution in July was uneven: in the first ten days, the average daily temperature exceeded the norm by 1.8 °C, and in the second and third ten days, it decreased by 0.7–2.9 °C. Total precipitation in July was close to the norm — 91.2 mm, or 102.5% of the norm.

The hydrothermal conditions of the 2023 growing season were inconsistent. The average air temperature in May was +13.3 °C (within the normal range) with a precipitation deficit — 10.5% of the norm. In June, the temperature exceeded the long-term average by 2.0 °C, and precipitation amounted to 56.4%. In July, the temperature and soil moisture were in line with long-term average values, but varied significantly by decade: in the first decade, the air temperature was 1.8 °C above the long-term average, while in the second and third, it was 2.9 °C and 0.7 °C lower, respectively. Total precipitation by decade was 91.9%, 162.0%, and 50.4% of the norm.

• Economic threshold in 2021 — middle of the 2nd decade of May (full budding phase)
• Economic threshold in 2022 — end of the 2nd decade of May (budding phase)
• Population density before treatment in 2022 — 3.0–3.2 adults/plant

Results of insecticide application

For reliable control of the pollen beetle, it is critically important to correctly select the active ingredient. Experience with the use of preparations has shown that systemic neonicotinoids and combined insecticides outperform pure pyrethroids in efficacy. Treating crops with preparations based on thiacloprid, a mixture of imidacloprid with lambda-cyhalothrin, or cypermethrin with chlorpyrifos ensures high mortality of the pest by the third day.

In the 2021 trials, the first treatment during the full budding phase showed high efficacy for thiacloprid, 480 g/L (0.15 L/ha), the preparation imidacloprid, 150 g/L + lambda-cyhalothrin, 50 g/L (0.2 L/ha), and a mixture of cypermethrin, 50 g/L + chlorpyrifos, 500 g/L (1.0 L/ha) — adult mortality reached 88.4–90.3%. This is 6.9–8.8% higher than the performance of the synthetic pyrethroid alpha-cypermethrin, 100 g/L (0.15 L/ha). However, by the seventh day, due to rising temperatures at the beginning of the third decade of May and a lack of precipitation, efficacy decreased to 29.4–34.7%, which necessitated a repeat treatment. After the second application, the efficacy on the third day was 90.4–91.8%, and on the seventh day — 69.8–77.1%.

* Note: for the no-insecticide variant, the table shows the actual number of individuals/plant.

In the 2022 trials, on the third day after the first treatment, the pest population decreased by 87.0–91.5% (5.7–10.2% more effective than alpha-cypermethrin). However, by the fifth day, due to warming, the threshold population of the beetle recovered in all variants, which forced a repeat spraying. After this, the efficacy on the third day was 86.0–89.7%, and on the seventh day — 50.3–64.2% (in the variant with alpha-cypermethrin — 50.3%).

* Note: for the no-insecticide variant, the table shows the actual number of individuals/plant.

• Combined preparations and systemic neonicotinoids provide initial efficacy against the beetle at the level of 87.0–91.8%, significantly outperforming pure pyrethroids.
• In hot and dry weather, the protective effect of treatments is reduced to 5 days, which creates a need for repeated spraying.
• The second treatment allows for the reliable protection of crops until the beginning of rape flowering, after which the pest damage subsides.

Pest colonization dynamics and treatment efficacy

The economic threshold for the pollen beetle in winter rape fields is reached at the beginning of the first decade of May, when the crop enters the budding phase. Before the start of treatments, the pest population in the field reaches 3.0–3.2 adults per plant. In this phase, it is extremely important to perform the first treatment in a timely manner, as the pest quickly damages the buds and reduces the yield potential.

In field trials, double application of modern insecticides showed high biological efficacy. On the third day after the second application, products based on thiacloprid, a mixture of imidacloprid with lambda-cyhalothrin, and cypermethrin with chlorpyrifos reduced the pest population by 86.2–89.0%, which is 2.6–5.4% more effective than the alpha-cypermethrin regimen. On the seventh day after the second spraying, the efficacy was 60.8–68.8% relative to the control. On average over the years of observation, the efficacy of the double application on the third day was 86.0–89.7%, and on the seventh day, it remained at the level of 60.8–64.2%. Further counts of the pollen beetle population were not conducted due to the onset of the full flowering phase of the rapeseed.

* Note: the no-treatment variant indicates the actual number of individuals per plant.

Crop yield and economic profitability of protection schemes

Double protection of crops during the budding period guarantees the preservation of the oilseed harvest. The use of modern insecticides allowed for the additional preservation of 6.8 to 7.9 dt/ha of rapeseed on average over the years of field trials. Treatment with the standard pyrethroid alpha-cypermethrin also provides an increase (4.6 dt/ha), but this result is 2.2–3.3 dt/ha lower than that of modern combined products.

Economic calculation confirms the feasibility of using combined and systemic products. The highest conditional net income in the experiments was obtained when treating with a tank mixture of imidacloprid and lambda-cyhalothrin—it amounted to 717.3 rub/ha with the value of the preserved harvest at 860.5 rub/ha. The application of thiacloprid and the mixture of cypermethrin with chlorpyrifos brought 611.9–650.6 rub/ha of net income. Treatment with alpha-cypermethrin showed the minimum economic return due to the low level of preserved seed harvest.

• Preserved harvest with two treatments — 6.8–7.9 dt/ha
• Maximum net income — 717.3 rub/ha
• Pest population reduction — up to 81.5–91.5%

When spraying crops with insecticides based on thiacloprid, 480 g/l (0.15 l/ha), imidacloprid, 150 g/l + lambda-cyhalothrin, 50 g/l (0.2 l/ha), and cypermethrin, 50 g/l + chlorpyrifos, 500 g/l (1.0 l/ha), the highest significant preserved harvest was obtained – 6.8–7.9 dt/ha, as well as conditional net income – 611.9–717.3 rub/ha, which is 2.2–3.3 dt/ha and 175.8–281.2 rub/ha higher, respectively, than in the alpha-cypermethrin, 100 g/l (0.15 l/ha) variant.

To avoid the manifestation of resistance in pollen beetle adults in winter rapeseed crops during multiple spraying in the protection system, it is recommended to alternate treatments with insecticides having different active ingredients and mechanisms of action.

System of winter rapeseed protection against pests, diseases, and weeds / A. A. Zaprudsky [et al.] // Integrated technologies for protecting agricultural crops from pests, diseases, and weeds / Natl. Acad. of Sciences of Belarus, Institute of Plant Protection; ed.: S. V. Soroka, A. G. Zhukovsky, E. A. Yakimovich. – Minsk, 2019. – P.45–53.

Methodological guidelines for registration trials of insecticides, acaricides, molluscicides, rodenticides, and pheromones in agriculture / Sci.-Pract. Center of the NAS of Belarus for Agriculture, Institute of Plant Protection; ed. by L. I. Trepashko. – Priluki, Minsk district: [s. n.], 2009. – 320 p.

A. A. Zaprudsky 1, D. F. Privalov 1, S. A. Gaidarova 1, E. V. Strelkova 2 RUE «Institute of Plant Protection», Priluki, Minsk region Belarusian National Technical University, Minsk

EFFICIENCY OF APPLICATION OF RAPE BEETLE

INSECTICIDES TO WINTER RAPE

Annotation. The paper presents the results of the research on evaluating the efficiency of the insecticides from different chemical groups applied to winter rape against rape beetle. It’s established that double treatment with the preparations based on thiacloprid, 480 g/l (0.15 l/ha), imidacloprid, 150 g/l + lambda-cyhalothrin, 50 g/l (0.2 l/ha) and cypermethrin, 50 g/l + chlorpyrifos, 500 g/l (1.0 l/ha) ensures a reduction in the number of rape beetle up to 81.5–91.5 %, obtaining the highest saved yield – 6.8–7.9 c/ha and net income – 611.9–717.3 rubles/ha.

Key words: winter rape, rape beetle, insecticides, efficiency, saved yield.