Harmfulness and distribution patterns of the European corn borer in Belarus

M. G. Nemkevich, S. V. Boyko

Introduction. Maize is grown as a grain, forage, and industrial crop. In Belarus, the ratio of areas cultivated for grain and green mass changes over the years, with the share of areas cultivated for grain in the total structure of maize acreage constantly increasing. Feed production from maize is the foundation of Belarusian animal husbandry. However, under unfavorable phytosanitary conditions of agroecosystems, there is a risk of reduced crop yield due to the spread of harmful organisms. Long-term data from staff at the Laboratory of Entomology of the RUE "Institute of Plant Protection" indicate that the dominant species having a negative impact on the maize grain harvest is the European corn borer (Lepidoptera: Crambidae: Ostrinia nubilalis Hbn.). The nature of plant damage is the caterpillar gnawing the internal contents of the stem from the top down, and as a result, the stem becomes brittle. Pollination processes of female inflorescences are disrupted when tassels are damaged, which reduces cob grain filling and leads to deformation, with grains arranged irregularly. Damage caused by the pest can cause not only mechanical damage but also impair the sanitary quality of the harvest, provoking the development of pathogenic diseases (fusarium, cob rot, common smut) and viruses.

In Belarus, high numbers and harmfulness of the European corn borer (hereinafter ECB) are observed annually, which is caused by a number of factors, including a significant share of maize in the structure of sown areas, warm winters, optimal air temperatures and rainfall for the development of caterpillars, non-compliance with crop rotation, and the lack of high-clearance sprayers in farms for the application of insecticides during the period of colonization and mass egg-laying by moths; crop protection is hampered by the timing of the pest's appearance.

The harmfulness of the phytophagous insect is manifested to a greater extent in the southern regions of the country, where a high concentration of maize for grain and seed is noted. According to Kravtsov V.I. (2017), the infestation of maize by the pest reached 55.0–75.0%. Boyko S.V. (2023) points out that in the Brest, Gomel, and Grodno regions in 2021, the infestation of maize plants by the European corn borer was 22.0–35.0%. In places with high infestation, the percentage of damaged plants reached 47.6. In 2022, in the Gomel region, plant damage varied from 15.0 to 35.0%, in the Grodno region – 24.6–32.8%, in the Brest region – 32.0–36.0%, and in the Minsk region – 18.0–28.0%. Harvest losses of grain account for 10–30%, while some foreign authors indicate 25–78%.

To obtain high yields of maize grain of proper quality, an important role is assigned to an integrated protection system, which is based on systematic phytosanitary monitoring during the growing season. The main goal of monitoring is to obtain operational and reliable information about the phytosanitary situation in the agrobiocenosis, which allows for the purposeful use of plant protection products, preventing the development of the pest population. At the same time, it is equally important to identify the exact timing of the appearance of their developmental stages that are most vulnerable to protection products. The level of maize crop infestation is controlled by counting pest egg masses, which is carried out on at least 100 plants taken at 20 points in the surveyed area – 5 plants each diagonally or in a checkerboard pattern.

The active ingredient chlorantraniliprole belongs to the chemical class of diamides, which has a new mechanism of action and is classified as a ryanodine receptor modulator; it is mobile in the xylem, which allows the insecticide to move upward through the plant.

It is used for the protection of agricultural crops against a wide range of sucking and leaf-eating insects and is especially effective against lepidopteran pests.

The purpose of these studies was to investigate the biological efficacy of insecticides for the protection of maize against the European corn borer, represented by promising preparations from the diamide class with the active ingredient chlorantraniliprole, which possess sufficient stability at high temperatures and have passed the state registration procedure.

Materials and methods. The studies were carried out during the growing seasons of 2020–2022 in small-plot and production trials of maize at UKSP "Sovkhoz 'Dobrovolec'" in the Klichev district of the Mogilev region and in 2023 at LLC "BelInterGen" in the Slutsk district of the Minsk region.

During the growing season of the crop, the counting of European corn borer egg masses was carried out on 25 stems (5 stems in 5 samples) in each replication of the small-plot experiment and on 100 stems (5 stems in 20 samples) in each replication of the production experiment. To detect larvae (caterpillars) of the pest living inside the maize, the plants were opened. For this purpose, 10 plants were taken from each surveyed area in 10 samples, distributing them evenly over the area. The plants were cut or dug up and then analyzed in the laboratory. Plant damage by the pest was recorded on different cultivars of the crop.

The calculation of biological effectiveness of preparations for reducing plant damage by caterpillars in comparison with the control variant without insecticide application was performed before maize harvesting according to the formula:

(A − B) / A × 100

where E is the effectiveness of reducing plant damage, expressed as a percentage; A is plant damage in the untreated variant, %; B is plant damage in the plot treated with insecticides, %.

How to detect the pest and determine the timing of treatment

Controlling the European corn borer (Ostrinia nubilalis) is complicated by the pest's secretive behavior. The caterpillars feed inside the leaf midribs and tassels, and then penetrate the stems. Therefore, it is critical to perform the treatment at the optimal time — upon the appearance of the first egg masses of the pest.

To determine the growth stages of maize, the BBCH scale (2016) is used. When inspecting crops, examine the leaf axils. Browning and mechanical damage to the bases of the leaf blades signal the entry of caterpillars into the leaf sheath. If such a leaf is pulled back, a powdery residue of processed plant material will be found inside.

To accurately predict the hatching time of the caterpillars, assess the color of the egg masses. Fresh egg masses are flat, oval, milky-white, resemble fish scales, and are covered with a coating similar to stearin drops. Over time, they become cream-colored, and immediately before the caterpillars hatch, their black heads begin to clearly show through the shell. One egg mass can contain from 18 to 70 eggs: according to observations, egg masses of 20 eggs are most common (42.9%), followed by 18 eggs (28.5%), while large egg masses of 40 and 70 eggs account for 14.3% each.

Protection effectiveness of chlorantraniliprole and application rules

Currently, 24 insecticides are registered for the control of the European corn borer, with 16.7% of the preparations containing the active ingredient chlorantraniliprole. One such solution is the combination insecticide Ampligo, MKS (lambda-cyhalothrin, 50 g/L + chlorantraniliprole, 100 g/L), registered for the protection of maize grown for grain, seed, and silage. The preparation, at an application rate of 0.1–0.3 L/ha, possesses stomach-contact and translaminar action, provides a fast "knockdown" effect, and protects crops for up to 3–4 weeks.

• Application rate of Ampligo, MKS — 0.1–0.3 L/ha
• Period of protective action — 3–4 weeks
• Effectiveness against egg masses — 98.7%
• Effectiveness against I–II instar caterpillars — 44.0%
• Economic threshold of harmfulness (ETH) for grain maize — 1.0–2.0 egg masses per 100 plants

Based on the results of multi-year field experiments (2011–2017) in grain maize crops, the average biological effectiveness of the Ampligo preparation in reducing plant damage was 92.8%. The table below presents statistical indicators of stem damage control before harvest. The data confirm the stability of the protective action of the preparation when applied in a timely manner.

In 2021 field trials, applying the insecticide at the vulnerable stage of the pest's development (egg masses) reduced maize damage before harvest by 98.7%. Conversely, treating crops during the period when the population consisted of I and II instar caterpillars reduced damage by only 44.0%, as the pest had already penetrated the plant and become virtually invulnerable. Practical research on the effectiveness of pure insecticides based on chlorantraniliprole with a single active ingredient has been ongoing since 2020.

In 2020 field experiments, protective measures were carried out when the threshold pest density was reached — 2.0 egg masses per 100 plants. In the first ten-day period of July, the average daily air temperature was +19.3 °C, and in the second and third ten-day periods, it decreased to +17.3 °C and +17.9 °C, respectively. July was characterized by a severe drought — only 20.0% of the normal precipitation fell, which negatively affected flowering and grain filling. In August, dry weather persisted: in the first ten-day period at a temperature of +19.8 °C, there was no precipitation at all; in the second ten-day period at +16.8 °C, 5.7 mm fell; and in the third ten-day period at +18.2 °C (2.2 °C higher than the long-term average), 17.6 mm of precipitation fell (77.0% of the norm).

In early July 2021, the average daily air temperature was +21.6 °C, while total precipitation was within the normal range (23.0 mm). During this period, mass egg-laying by the pest was observed. According to counts, 1.0 egg mass was found per 100 plants. The weather conditions that developed subsequently contributed to an increase in the density of egg masses in the agrocenosis — up to 9.0 units/100 plants. August was quite warm (monthly average temperature +17.4 °C, which is 0.2 higher than the long-term average, total precipitation 67.9 mm or 88.2% of the norm), which favorably affected maize development and caterpillar feeding.

In the first ten days of July 2022, with an average daily air temperature of +20.0 °C and total precipitation of 7.0 mm, active flight of adult European corn borer (ECB) was detected. In the second ten days of the month, despite a cold snap (average daily air temperature +15.7 °C, which is 2.6 °C below the norm), mass egg-laying by the pest was observed. According to the counts, 7.0 egg masses/100 plants of European corn borer were found in maize crops at the 10–11 leaf stage (BBCH 20–21). Abundant rainfall (180.0% of the norm) contributed to the active growth of maize plants. In the third ten days of July, the average daily air temperature rose to +18.9 °C, and 25.0 mm of precipitation fell. August was quite hot in terms of temperature. The actual temperature for the month was +20.8 °C, which is 2.9 °C above the norm. At the same time, 57.0% of the normal precipitation fell, which contributed to the development of European corn borer larvae.

The air temperature in the first ten days of July 2023 was +18.6 °C, which is 0.7 °C above the norm, and total precipitation (10.0 mm) was 32.3% of the ten-day norm. Such conditions contributed to the intensive flight of adult pests and plant colonization. In the second ten days of the month, the average daily air temperature (+19.3 °C) was at the level of the long-term average (+18.6 °C), precipitation was 9.8 mm or 37.7% of the norm, and active egg-laying by the pest was observed. According to the counts carried out at the end of the decade (17.07), 1.0 egg mass/100 plants was found in maize crops, and mass flight of European corn borer moths was noted. Based on the results of counts carried out on 20.07, 3.6 egg masses/100 plants were found at the beginning of tassel emergence (BBCH 51), which was facilitated by the prevailing weather conditions: temperature regime was 1.5 °C lower than the long-term average, and precipitation amounted to 129.3% of the norm (it was warm and humid). August was hot in terms of air temperature. The average daily air temperature for the month was +20.3 °C, which is 4.1 °C above the norm, while precipitation reached 120.2% of the norm, which contributed to active feeding and development of European corn borer larvae.

In maize crops of various hybrids, during the growing season at an average daily air temperature of +22.8…+23.3 °C (in the morning hours), the studied insecticides were applied upon reaching the threshold number of pest egg masses. Counts carried out before harvesting the crop showed that in the experimental plots without insecticide application, plant damage by Ostrinia nubilalis larvae ranged from 24.4 to 47.6%.

The use of insecticides with the active ingredient chlorantraniliprole, 200 g/l, at application rates of 0.15 and 0.2 l/ha on experimental variants in 2020–2023 made it possible to reduce this indicator by 75.4 to 92.4%.

Timely and high-quality use of plant protection products ensures the preservation of the maize harvest. Assessment of the economic efficiency of the studied insecticides showed that the reduction in the harmfulness of the European corn borer through the use of Miriad, SC, Coragen, SC, and Rino-A, SC preparations at the studied application rates allowed for maize grain yields from 88.0 to 105.3 dt/ha, while 5.9–15.8 dt/ha or 6.4–21.2% was preserved, respectively, compared to the control variant.

Table 2 – Biological efficiency of insecticides against ECB in maize crops

Application rate Damaged Biological effi-

Experimental variant of preparation, l/ha plants, % ciency, %

Small-plot experiment, UKSP "Sovkhoz Dobrovolets", hybrid LG 3215, 2020, treatment date: 10.07. Variant without treatment – 24.4 – Coragen, SC 0.15 6.0 75.4

0.15 5.5 77.5 Miriad, SC

Small-plot experiment, UKSP "Sovkhoz Dobrovolets", hybrid Belami, 2021, treatment date: 06.07. Variant without treatment – 47.6 – Coragen, SC 0.15 7.5 84.2

0.15 7.0 85.3 Miriad, SC

0.2 5.8 87.8 Small-plot experiment, UKSP "Sovkhoz Dobrovolets", hybrid Luigi SC, 2022, treatment date: 14.07. Variant without treatment – 26.7 –

0.15 4.0 85.0 Coragen, SC

0.15 4.8 82.0 Rino-A, SC

Production experiment, LLC "BelInterGen", hybrid AS 170, 2023, treatment date: 20.07. Variant without treatment – 27.9 –

0.15 4.9 82.4 Coragen, SC

0.15 3.6 87.1 Rino-A, SC

Table 3 – Economic efficiency of insecticides against European corn borer in maize crops

Grain yield, Preserved grain Variant, appl. rate dt/ha dt/ha %

Rino-A, SC, 0.15 – – 98.4 103.7 – – 5.9 11.4 – – 6.4 12.4 Rino-A, SC, 0.2 – – 100.0 105.3 – – 7.5 13.0 – – 8.1 14.1 Coragen, SC, 0.15 88.0 89.6 99.0 100.2 7.2 15.2 6.5 7.9 8.9 20.4 7.0 8.6 Coragen, SC, 0.2 – – 102.7 102.6 – – 10.2 10.3 – – 11.0 11.2 Miriad, SC, 0.15 88.6 89.4 – – 7.8 15.0 – – 9.6 20.1 – – Miriad, SC, 0.2 90.5 90.2 – – 9.7 15.8 – – 12.0 21.2 – – Variant without

80.8 74.4 92.5 92.3 – – – – – – – – treatment LSD05 7.0 3.60 2.53 4.73

Thus, it is advisable to carry out the protection of maize crops against European corn borer with insecticides based on chlorantraniliprole when a threshold or high number of egg masses is detected, mainly on the underside of the leaf, before the larvae penetrate the plant stems. The preparations possess high biological and economic efficiency, as well as thermal stability.

Conclusion. Great damage to maize plants, grown for both green mass and grain, is primarily caused by a polyphagous pest – the European corn borer (Ostrinia nubilalis Hbn.). In some years, damage to maize plants by larvae before harvest reached 47.6%. During the second ten days of June to the second ten days of July, based on the results of counting the number and distribution of the pest, the feasibility and optimal timing of insecticidal treatments are determined depending on the agro-climatic zone. Insecticides based on the active ingredient chlorantraniliprole, 200 g/l, applied in maize agroecosystems at the threshold density of European corn borer egg masses, restrained the harmfulness of the pest larvae and contributed to reducing plant damage before harvest by 75.4–92.4% with grain yield preservation of 5.9–15.8 dt/ha or 6.4–21.2%. Based on the conducted studies, the preparations Coragen, SC, Miriad, SC, and Rino-A, SC with application rates of 0.15–0.2 l/ha have passed state registration and are recommended for use in the system of maize protection against the most common and dangerous pest – the European corn borer.

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M. G. Nemkevich, S. V. Boyko RUE «Institute of Plant Protection», Priluki, Minsk region

PROTECTION OF CORN FROM CORN STALK

BORER (OSTRINIA NUBILALIS HBN.) WITH

INSECTICIDES CONTAINING THE ACTIVE

Annotation. The article presents four years of data (2020–2023) on the effectiveness of insecticides with the active ingredient chlorantraniliprole, 200 g/l for protecting corn from corn borer. It was noted that during the years of research there were favorable weather conditions for the development of the pest, and the damage to plants before harvesting was 24,4–47,6 %. It has been established that it is effective and, most importantly, to protect the crop for a long period of time when using single-component preparations Coragen, KS, Myriad, KS and Rino-A, KS at application rates of 0,15–0,2 l/ha according to the threshold number of ovipositions Ostrinia nubilalis, plant damage decreased by 75,4–92,4 %. The biological effectiveness of the two-component insecticide Ampligo, MKS (lambda-cyhalothrin, 50 g/l + chlorantraniliprole, 100 g/l) averaged 92,8 %. A high preserved grain yield of 5,9– 15,8 c/ha or 6,4–21,2 % was noted.

Key words: corn, corn stem borer, insecticides, diamides, chlorantraniliprole, oviposition, plant damage, efficiency.