The influence of abiotic factors on the development of the apple leaf midge

The threat of the apple leaf midge and how to detect it

The apple leaf midge (Dasineura mali Kieffer) did not have significant economic importance for a long time. However, since the 1990s, the damage caused by this phytophage has increased sharply, and today it colonizes commercial orchards everywhere. The larvae damage young leaves and shoots, stunting their development and disrupting photosynthesis, which directly reduces apple yield in the long term. Apple trees account for 95% of the area of all fruit and berry plantations, making the control of this pest critical.

Controlling the midge is difficult due to the cryptic lifestyle of the larvae. After hatching, they congregate on the upper side of young leaves along the edges, causing the leaves to curl tightly and turn red. Upon completion of feeding, the leaves become brown, stiff, and brittle, forming characteristic galls. The first damage in the orchard can be noticed immediately after the apple tree flowering.

• Number of larvae in one gall — from 9 to 130 pcs.
• Full development cycle of the pest — 20–25 days
• Period from egg to the end of the second larval stage — from 6 to 27 days
• Installation height of sticky traps — 0.5–1.0 m
• Depth of pupae in the soil — 5–7 cm

Depending on the region, the pest produces a different number of generations. In the conditions of Europe and Northern Canada, 3–4 generations develop, in New Zealand — 4–5. Previously, only 2 generations of the midge were recorded in our latitudes, but due to climate warming and changes in cultivar composition, these data have changed. All generations overlap, so during the season, one can encounter the pest at different stages of development simultaneously in the orchard.

Pest monitoring and factors initiating adult emergence

For orchard protection, it is important to accurately determine the emergence time of adults from overwintering sites. The pest overwinters in the upper layers of the soil, so its awakening is influenced by a complex of soil and climatic factors. The agronomist needs to track the minimum, maximum, and average air temperature, soil temperature and humidity, as well as the difference between air and soil surface temperature (delta T).

To assess the overwintering stock of the midge and forecast the timing of pest emergence on a specific farm, the following monitoring scheme is used:

1. In the early spring period, soil samples are taken along the tree canopy projection at a depth of 5–7 cm. Special attention is paid to areas where shoot colonization reached 60% last season. In the experiments, samples were taken from the Alesya, Verbnoye, and Syabryna cultivars.
2. Under laboratory conditions, the soil is washed through sieves of different diameters using the elutriation method to isolate the pupae.
3. Pupae are placed along with the soil in cloth cages directly in the orchard to accurately record the moment adults emerge.
4. Yellow sticky traps are hung in the orchard at a height of 0.5–1.0 m from the ground to monitor the beginning of flight under natural conditions.

Results and discussion. To determine the abiotic factors influencing the beginning of the flight of the first-generation adults of the apple leaf midge at permanent plots of the commercial orchard of RUE "Tolochin Cannery," yellow sticky traps were hung out during the early spring period from 2021 to 2024. In 2021, traps were hung on May 4, and just 2 days later, on May 6, during the beginning of the bud stage (BBCH 55-56), the first phytophage specimens were noted.

In 2022, also at the beginning of the apple bud stage, during the red bud phenophase on May 5 (BBCH 57), yellow sticky traps were hung. In the count conducted a week later (May 12), the beginning of pest emergence was already noted with an average adult count of 0.3 per trap.

In 2023, traps were hung during the green cluster apple phenophase (April 18), and the first midge specimens were caught on a yellow sticky trap on April 23 during the beginning of bud exposure (BBCH 55).

In the challenging weather conditions of the spring of 2024, observations on the emergence dates of the apple leaf midge continued under production conditions in the orchard of RUE "Tolochin Cannery." At the end of February, the daytime air temperature remained around +10 °C for 3 days (with a positive anomaly of +7.3 °C). The transition of the average daily air temperature through +5 °C upwards (the onset of the growing season) was noted at the beginning of the 3rd decade of March, 1.5–2 weeks earlier than its usual dates. On March 30, the daytime temperature in

Tolochin reached +18 °C, and the soil warmed up to +8 °C during the day. In this regard, yellow sticky traps were hung on April 1 (maximum daytime temperatures reached: air +24 °C, soil +11.2 °C), however, a subsequent cold snap delayed the emergence of the gall midge. The first individuals of the pest were noted only on April 9 during the beginning of bud break in apple trees – the green cone phenophase or 53 BBCH at an average air temperature of +17.3 °C and soil temperature of +11.3 °C.

Figure – Dynamics of average daily air and soil temperatures in the first ten days of April 2024 (data from the iMetos weather station, RUE "Tolochin Cannery", Vitebsk region)

Summarizing the data obtained over 4 years, it was established that the most constant factors are the average air temperature from +13.2 °C to +14 °C and the average soil temperature from +11.2 °C to +11.8 °C, while other indicators varied significantly by year, including soil moisture – from 21.1 % to 48.5 %. Table 1 – Influence of abiotic factors on the emergence of the first generation adults of the apple leaf midge (field trial, RUE "Tolochin Cannery", Vitebsk region)

2021 2022 2023 2024 Beginning of adult flight 06.05 12.05 23.04 09.04 average 13.4 14 13.2 17.3 Air temperature, °C max 21.1 17.2 19.7 25.4 min 1.7 9.8 8.2 7.7 Air temperature delta, °C 3 5 4 4 Soil temperature, °C 11.2 11.8 11.2 11.3 Soil moisture, % 21.1 48.5 43.8 44.5

Even in the complex weather conditions of the spring of 2024, when the flight period of the apple leaf midge was 13 days earlier than in 2023 and a month earlier compared to 2021 and 2022, the average soil temperature, as the determining abiotic factor for the start of the midge flight, corresponded to the indicators of previous years (+11.3 °C).

With the aim of a comparative assessment of the influence of abiotic factors on the emergence of the first generation adults in different orchard locations under the conditions of 2022–2023, we established a laboratory-field trial at the experimental orchard of the RUE "Institute of Plant Protection" and the RUE "Tolochin Cannery"

The flight periods of the apple leaf midge in the experimental orchard coincided with the periods in the commercial orchard – May 12 in 2022 and April 23 in 2023. The determining factor for the start of the apple leaf midge flight in the orchard of the RUE "Institute of Plant Protection" was also the sustained warming of the soil above +11 °C. During two years of observations on the influence of abiotic factors on the emergence of the first generation adults of the apple leaf midge, it was also proven that the determining indicators are the average daily air temperature (+13...+14 °C) and soil temperature above +11 °C (+11.2...+ 14.0 °C). Moreover, under the conditions of 2023, these indicators did not differ in the Minsk and Vitebsk regions. Soil moisture fluctuated from 13 % to 48.5 %. Table 2 – Influence of abiotic factors on the emergence of the first generation adults of the apple leaf midge (laboratory-field trial).

Experimental orchard of RUE "Tolochin

Indicators RUE "IPP" Cannery"

2022 2023 2022 2023 Beginning of adult flight 11.05 22.04 12.05 23.04 average 13.8 13.1 14 13.2 Air temperature, °C max 21.3 19.4 17.2 19.7 min 2.5 5.3 9.8 8.2 Air temperature delta, °C 6 5 5 4 Soil temperature, °C 14.0 11.2 11.8 11.2 Soil moisture, % 13.0 21.1 48.5 43.8

Conclusions. Studies conducted in 2021–2024 showed that the emergence of the first generation adults of the apple leaf midge for 3 years occurred during the budding period (BBCH 55–57), and in the anomalous conditions of the spring of 2024 – during the bud break period (BBCH 53). The determining abiotic factors affecting the flight dates of the pest are an average soil temperature above +11 °C (+11.2...+11.8 °C) and an average air temperature above +13 °C (+13.1...+17 °C).

The results obtained will be used in the future to forecast the beginning of pest flight.

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The research was carried out with the financial support of a grant for research work by doctoral students, postgraduate students, and applicants of the National Academy of Sciences of Belarus on the topic "Bioecological features of the development of the apple leaf midge Dasineura mali Kieffer (Diptera: Cecidomyiidae) in industrial orchards of Belarus and substantiation of measures to limit its harmfulness" in 2022–2023.

O. V. Dichkovskaya RUE «Institute of Plant Protection», Priluki, Minsk region

ABIOTIC FACTORS AFFECTING FLYING OF APPLE

Annotation. As a result of long-term observations of the beginning of flying of apple leaf midge imago made in industrial and experimental gardens with installed iMetos digital weather stations it was established that determining abiotic factors affecting the flight of the phytophage were the average soil temperature above +11 °C (+11.2...+11.8 °C), as well as the average air temperature above +13 °C (+13.2...+17.3 °C).

Key words: apple leaf midge, apple tree, gardens, yellow sticky trap, abiotic factors, digital weather station.