Seasonal Flight Dynamics of Grapholita funebrana in a ‘Stanley’ Plum Orchard in Relation to Temperature and Growing Degree-Days

1. Introduction

European plum (Prunus domestica L.) is an important fruit crop cultivated throughout temperate regions, with fruit production intended for both fresh consumption and processing. However, fruit yield and quality can be affected by numerous biotic factors, among which fruit-feeding insects are particularly important because of the direct damage they cause to fruit and the associated economic losses. Among these pests, Grapholita funebrana (Treitschke) (Lepidoptera: Tortricidae), commonly known as the plum fruit moth, is considered an important pest of plum and other stone fruit species within the family Rosaceae [1,2]. The species is widely distributed across fruit-growing regions of Europe, Asia, and North Africa, and climatic conditions are an important factor influencing its potential distribution and population activity.

Damage is caused by the larvae, which penetrate the fruit and feed internally, thereby affecting fruit development and marketable quality. The severity of infestation may vary according to cultivar and fruit characteristics. Rizzo et al. [13] comparing four plum cultivars, including ‘Stanley’, demonstrated that specific morphological and physicochemical fruit traits influence susceptibility to G. funebrana. In that study, ‘Stanley’ was among the less susceptible cultivars, indicating that pest–host plant interactions should be considered when interpreting infestation levels and developing integrated pest management strategies.

The seasonal dynamics of G. funebrana show considerable variation depending on climatic conditions and geographical region. Temperature influences the development of ectothermic insects and, consequently, the timing of adult emergence, the duration of developmental stages, and the succession of generations. Classical studies on the phenology of G. funebrana demonstrated a relationship between development and thermal accumulation, and a lower developmental threshold of 10 °C was proposed for estimating development based on daily mean temperatures [4]. From this perspective, thermal accumulation may provide more informative phenological indicators than calendar dates alone, particularly when comparing seasons characterized by different thermal regimes.

Monitoring adults using pheromone traps is an important tool for assessing the seasonal dynamics of tortricid moths and identifying periods of increased flight activity. Sex pheromones enable the detection of males and the characterization of seasonal flight curves, thereby providing information that can contribute to optimizing the timing of pest management interventions. The importance of the olfactory system in the response of G. funebrana to pheromone components is also supported by recent studies on proteins involved in the perception of chemical signals [9]. Nevertheless, pheromone-trap captures should be interpreted as indicators of male flight activity rather than as a direct measure of fruit infestation. Studies on G. funebrana management have shown that changes in the number of males captured are not always accompanied by proportional changes in fruit damage [10].

In Romania, information on the seasonal dynamics of G. funebrana has also been obtained from studies conducted in plum orchards in Bistrița. Jakab-Ilyefalvi [7] reported interannual variation in male flight curves and highlighted the relationship between climatic conditions and pheromone-trap captures. Similar patterns of seasonal variability were reported by Shevchuk [16] based on multi-year monitoring of G. funebrana populations. These findings indicate that identifying periods of pest risk solely on the basis of calendar dates may be insufficient and support the integration of direct monitoring with meteorological and phenological indicators.

The importance of climatic factors is further supported by recent studies investigating the potential distribution of the species. Using distribution models based on bioclimatic variables, Yang et al. [19] showed that climatic factors contribute substantially to determining areas potentially suitable for G. funebrana. In parallel, the recent development of genomic resources for this species reflects increasing interest in improving our understanding of its biology, ecology, and adaptation [1]. These authors also emphasized that, compared with other fruit-feeding pests, research specifically focused on G. funebrana remains relatively limited.

Nevertheless, information integrating pheromone-trap captures, thermal conditions during the days preceding monitoring, and the progression of seasonal captures in relation to thermal accumulation remains limited, particularly under the fruit-growing conditions of northeastern Romania. Furthermore, the stability of these relationships across years characterized by different thermal regimes requires local and multi-year evaluation. Considering temperature only on the day of trap assessment may not fully capture the biological response of adults to preceding thermal conditions. Therefore, investigating short-term thermal windows may provide a more accurate characterization of the relationship between temperature and adult flight activity.

Against this background, the present study aimed to evaluate the seasonal flight dynamics of adult G. funebrana in a ‘Stanley’ plum orchard in northeastern Romania over two consecutive growing seasons, 2025 and 2026. Specifically, the objectives were to (a) characterize and compare seasonal flight patterns under two orchard management systems; (b) evaluate the relationships between pheromone-trap captures and temperature on the monitoring date, as well as mean temperatures calculated over 3-, 5-, and 7-day thermal windows; and (c) determine GDD₁₀ accumulations corresponding to the progression of cumulative seasonal captures. By integrating pheromone monitoring with short-term thermal indicators and growing degree-day accumulation, this study aims to provide an experimental basis for characterizing the local phenology of G. funebrana and for the future development of forecasting and warning tools adapted to plum-growing conditions in northeastern Romania.

2. Materials and Methods

2.1. Study Site and Experimental Organization

The study was conducted during 2025–2026 in a European plum (Prunus domestica L.) orchard located at the Didactic Farm of the “Ion Ionescu de la Brad” Iași University of Life Sciences (IULS), Romania. The plant material used in the study consisted of the ‘Stanley’ cultivar, which was the same in both study years and under both management systems.

Trees were planted at a spacing of 5 m between rows and 4 m between trees within rows, corresponding to a theoretical planting density of approximately 500 trees ha⁻¹.

The study was designed to evaluate the seasonal flight dynamics of adult Grapholita funebrana (Treitschke) (Lepidoptera: Tortricidae) under two different orchard management strategies:

V1—conventional management: conventional phytosanitary treatments combined with monitoring using AtraFUN pheromone traps;

V2—no-chemical-treatment management: no chemical treatments were applied, and monitoring was performed using AtraFUN pheromone traps.

The use of the same plum cultivar and the same monitoring method under both management systems allowed seasonal capture patterns to be compared in relation to the plant protection strategy applied.

Three AtraFUN pheromone traps, designated C1, C2, and C3, were installed within each management system and used to monitor the flight activity of adult G. funebrana.

2.2. Characterization of the Management Systems

The two management systems differed in the plant protection strategy applied in the ‘Stanley’ plum orchard during the 2025 and 2026 growing seasons.

Under V1—conventional management, plant protection was carried out according to a conventional treatment program. The commercial insecticides Laser, Vantex, Mospilan, and Decis EXP were used for pest control at the application rates specified in the phytosanitary program. The same treatment scheme was applied in both study years, with application rates of 0.90–0.96 L ha⁻¹ for Laser, 0.20–0.25 L ha⁻¹ for Vantex, 0.45 kg ha⁻¹ for Mospilan, and 0.15 L ha⁻¹ for Decis EXP.

Under V2—no-chemical-treatment management, no chemical products were applied for pest control. The Grapholita funebrana population was monitored exclusively using AtraFUN pheromone traps, under the same monitoring conditions as those used in the conventional management system. The traps were used specifically to monitor male flight activity.

This experimental organization allowed a descriptive comparison of the seasonal flight dynamics of adult G. funebrana between an orchard area managed according to a conventional plant protection program and an orchard area in which no chemical treatments were applied, under the same orchard conditions, using the same cultivar and covering the same study periods.

Table 1. Characterization of the management systems used for monitoring Grapholita funebrana in the ‘Stanley’ plum orchard.

Variant	Management system	Pest control	G. funebranamonitoring
V1	Conventional management	Laser, Vantex, Mospilan, Decis EXP	3 AtraFUN pheromone traps (C1–C3)
V2	No chemical treatment	No insecticides applied	3 AtraFUN pheromone traps (C1–C3)

Table 1. Characterization of the management systems used for monitoring Grapholita funebrana in the ‘Stanley’ plum orchard.

Variant	Management system	Pest control	G. funebranamonitoring
V1	Conventional management	Laser, Vantex, Mospilan, Decis EXP	3 AtraFUN pheromone traps (C1–C3)
V2	No chemical treatment	No insecticides applied	3 AtraFUN pheromone traps (C1–C3)

2.3. Monitoring of the Grapholita funebrana Population

Adult G. funebrana were monitored using AtraFUN pheromone traps equipped with a species-specific sex attractant. The traps were used to detect males and to characterize the seasonal dynamics of adult flight.

Three traps (C1, C2, and C3) were used for each management system. At each monitoring date, captured adults were counted separately for each trap, and the values were recorded individually. This approach allowed both the total number of adults captured at each assessment and the variation among the three traps within the same management system to be determined.

In 2025, monitoring was conducted from 15 May to 19 September. In 2026, monitoring began on 20 April and continued until 9 September under conventional management and until 31 August under the no-chemical-treatment management system. The difference in the starting dates of monitoring between the two study years was taken into account when interpreting the onset of observed flight activity and the associated thermal accumulations.

For each monitoring date, the total number of adults captured was calculated according to the following equation:

N_i = C1_i + C2_i + C3_i

where N_i represents the total number of adults captured on monitoring date i, while C1_i, C2_i, and C3_i represent the numbers of adults captured in the three individual traps.

Because each management system was represented by a single orchard plot, the three traps within each variant were considered monitoring subsamples rather than independent experimental replicates of the management system. Consequently, comparisons between V1 and V2 were used to describe differences in seasonal flight dynamics, without making causal statistical inferences regarding the effect of the management system.

To characterize population dynamics, the following parameters were determined: the total number of adults captured at each monitoring date, cumulative captures, timing of the first observed capture, date and magnitude of the seasonal flight peak, and seasonal distribution of captures. These variables corresponded to the parameters established in the study analysis protocol.

2.4. Meteorological Data and Characterization of Thermal Conditions

To evaluate the relationship between thermal conditions and the flight activity of adult Grapholita funebrana, entomological data were analyzed in relation to daily temperature series corresponding to the monitoring periods in 2025 and 2026.

Meteorological data were obtained from the Meteostat database using records from the Iași weather station [11], located at 47.1667° N, 27.6333° E, at an altitude of 102 m. Station 15090 is also identified within the Romanian national meteorological network as the Iași weather station.

Daily temperature data were synchronized with the pheromone-trap assessment dates to allow analysis of the relationship between thermal conditions and the intensity of adult flight activity.

The daily mean temperature corresponding to each trap assessment date was defined as T0. To evaluate potential delayed associations between temperature and adult flight activity, mean temperatures were also calculated over 3-, 5-, and 7-day thermal windows (T3, T5, and T7, respectively) ending on each monitoring date, with the monitoring date included in each thermal window.

This approach allowed the relationship between trap captures and thermal conditions on the monitoring date to be evaluated together with the associations between adult flight activity and thermal conditions during the immediately preceding period.

2.5. Calculation of Thermal Accumulation and Cumulative Captures

To characterize the relationship between thermal accumulation and the seasonal flight dynamics of adult Grapholita funebrana, growing degree-days (GDD) were calculated using a lower developmental threshold (Tbase) of 10 °C, according to the phenological model described for this species by Charmillot et al. [4].

Daily growing degree-days were calculated based on the daily mean air temperature according to the following equation:

GDD10,i=max(Tmed,i−10,0)

where GDD10,i represents the thermal accumulation for day i, and Tmed,i represents the mean air temperature on that day. For days when the mean temperature was ≤10 °C, the contribution to thermal accumulation was set to zero.

Cumulative thermal accumulation up to a given date was calculated by successively summing the daily GDD values according to the following equation:

$${\displaystyle \sum GD{D}_{10}={\displaystyle \sum _{i=1}^n\max \left(T_{med,i}-10,0\right)}}$$

In both study years, GDD₁₀ accumulation was calculated starting from 1 January of the respective year, ensuring that the thermal values associated with trap captures were comparable between seasons and reproducible using the same calculation criteria.

For each monitoring date, the total number of adults captured in the three traps within the same management system was calculated according to the following equation:

N_i = C1_i + C2_i + C3_i

where N_i represents the total number of adults captured on monitoring date i, while C1_i, C2_i, and C3_i represent the numbers of adults captured in the three individual traps.

Captures were converted into cumulative percentages of the total seasonal captures for each year × management system combination. The cumulative percentage at monitoring date i was calculated as follows:

$$C_i\left(\%\right)={\displaystyle \frac{{\displaystyle \sum _{j=1}^i{N}_j}}{N_{total}}}\times 100$$

where C_i (%) represents the cumulative percentage of captures up to monitoring date i; N_j is the number of adults captured at each monitoring date j; represents the total number of adults captured from the beginning of the monitoring period up to monitoring date i; and Ntotal is the total number of adults captured throughout the entire monitoring season.

Based on the cumulative capture curves, thermal accumulations corresponding to the first observed capture and to the attainment of 10%, 25%, 50%, 75%, and 90% of the total seasonal captures were determined.

Because monitoring began on different dates in the two study years, the thermal accumulation associated with the first capture was interpreted as the value corresponding to the first capture observed within the monitoring program, rather than as a biological threshold for the onset of flight. This distinction was necessary because monitoring began on 15 May in 2025, whereas in 2026 it began on 20 April.

2.6. Statistical Analysis

Statistical analyses were performed to evaluate the relationship between air temperature and the flight activity of adult Grapholita funebrana. Analyses were conducted separately for each study year and management system.

The association between the number of adults captured and thermal conditions was evaluated using Pearson’s correlation coefficient (r) and the non-parametric Spearman’s rank correlation coefficient (ρ). Relationships were tested separately between trap captures and the mean temperature on the monitoring date (T0), as well as the mean temperatures calculated over the 3-, 5-, and 7-day thermal windows (T3, T5, and T7, respectively).

For linear relationships, the coefficient of determination (R²) was calculated to express the proportion of variation in trap captures associated with variation in the thermal parameter analyzed. The statistical significance of the correlation coefficients was assessed at p < 0.05.

Cumulative captures in relation to GDD₁₀ were analyzed to characterize the phenological progression of seasonal flight and to identify the thermal accumulations corresponding to 10%, 25%, 50%, 75%, and 90% of the total seasonal captures.

Data were graphically represented using seasonal flight curves and cumulative capture percentage curves plotted against GDD₁₀ accumulation.

3. Results

3.1. Seasonal Flight Dynamics of Grapholita funebrana

Monitoring of Grapholita funebrana using AtraFUN pheromone traps revealed marked differences in adult abundance between the two study years and between the management systems. Seasonal capture patterns were characterized by alternating periods of low activity and pronounced increases in adult flight, with the major flight peaks occurring during the first half of June.

In 2025, total captures were higher under the no-chemical-treatment management system, with 1,027 adults captured throughout the monitoring period, compared with 929 adults under conventional management. The seasonal maximum under the no-chemical-treatment management system was recorded on 11 June (170 adults), whereas under conventional management the maximum was reached on 13 June (113 adults). Following the main June peak, captures generally declined, although a secondary increase in flight activity was observed in early July, followed by a progressive decrease toward the end of the monitoring season.

In 2026, a different quantitative pattern was observed. Total captures were higher under conventional management (710 adults) than under the no-chemical-treatment management system (447 adults). Nevertheless, the timing of the main flight peaks was very similar between the two management systems: 9 June under conventional management (96 adults) and 10 June under the no-chemical-treatment management system (101 adults). A second period of increased adult flight activity was observed from late June to early July.

Comparison between the two study years revealed substantial interannual variability. The total number of adults captured decreased from 1,956 individuals in 2025 to 1,157 individuals in 2026, corresponding to an overall reduction of approximately 40.8%. The decrease was more pronounced under the no-chemical-treatment management system (56.5%) than under conventional management (23.6%).

Despite the differences in adult abundance, the timing of the major flight peaks remained relatively consistent. Under both management systems and in both study years, maximum adult flight activity occurred during the first half of June. This temporal consistency identifies the first half of June as a recurrent period of high G. funebrana flight activity under the conditions of the ‘Stanley’ plum orchard studied. However, the marked differences in capture magnitude between 2025 and 2026 highlight the need to examine seasonal population dynamics in relation to variation in meteorological conditions.

Table 2. Summary of Grapholita funebrana captures in the ‘Stanley’ plum orchard during 2025–2026.

Year	Management system	Total adults captured	Maximum capture	Date of maximum capture
2025	Conventional management	929	113	13 June 2025
2025	No chemical treatment	1,027	170	11 June 2025
2026	Conventional management	710	96	9 June 2026
2026	No chemical treatment	447	101	10 June 2026

Table 2. Summary of Grapholita funebrana captures in the ‘Stanley’ plum orchard during 2025–2026.

Year	Management system	Total adults captured	Maximum capture	Date of maximum capture
2025	Conventional management	929	113	13 June 2025
2025	No chemical treatment	1,027	170	11 June 2025
2026	Conventional management	710	96	9 June 2026
2026	No chemical treatment	447	101	10 June 2026

Total 2025–2026: 3,113 adults of G. funebrana, of which 1,639 were captured under conventional management and 1,474 under the no-chemical-treatment management system.

Figure 1. Seasonal flight dynamics of Grapholita funebrana under conventional and no-chemical-treatment management in the ‘Stanley’ plum orchard during 2025 and 2026.

Figure 1. Seasonal flight dynamics of Grapholita funebrana under conventional and no-chemical-treatment management in the ‘Stanley’ plum orchard during 2025 and 2026.

3.2. Flight Dynamics in Relation to Thermal Conditions

Analysis of thermal conditions during the monitoring periods revealed differences between 2025 and 2026, accompanied by variation in the abundance and seasonal distribution of G. funebrana captures.

In 2025, monitoring began in mid-May, when the daily mean temperature was 14.7 °C on 15 May. In early June, daily mean temperatures exceeded 24 °C, reaching 24.2 °C on 3 June and 24.3 °C on 5 June. The highest captures of the season were subsequently recorded during the first half of June. Under the no-chemical-treatment management system, the seasonal maximum of 170 adults was recorded on 11 June, when the daily mean temperature was 19.4 °C, whereas under conventional management, the maximum of 113 adults was recorded on 13 June, at a daily mean temperature of 17.5 °C.

In 2026, thermal conditions and capture dynamics showed a different pattern compared with the previous year. The seasonal maximum under conventional management was recorded on 9 June, with 96 adults captured, whereas under the no-chemical-treatment management system, the maximum of 101 adults was recorded on 10 June. A second period of increased flight activity was observed from late June to early July.

Comparison between the two study years showed that the total number of adults captured decreased from 1,956 individuals in 2025 to 1,157 individuals in 2026, corresponding to an overall reduction of approximately 40.8%. Captures decreased by 23.6% under conventional management and by 56.5% under the no-chemical-treatment management system. Nevertheless, under both management systems and in both study years, the main seasonal capture peaks occurred during the first half of June.

3.3. Statistical Analysis of the Relationship Between Temperature and Capture Dynamics

To quantify the relationship between thermal conditions and the flight activity of adult G. funebrana, the number of individuals captured at each monitoring date was analyzed in relation to the mean temperature on the monitoring date (T0), as well as the mean temperatures calculated over the 3-, 5-, and 7-day thermal windows associated with each monitoring date (T3, T5, and T7, respectively). Relationships were evaluated using Pearson’s correlation coefficient (r) and Spearman’s rank correlation coefficient (ρ), while the explanatory capacity of the linear relationships was expressed by the coefficient of determination (R²). Statistical significance was assessed at p < 0.05.

Separate analyses for each study year revealed marked differences in the association between captures and thermal conditions. In 2025, no statistically significant correlations were detected between capture levels and temperature on the monitoring date or the mean temperatures calculated for the 3-, 5-, and 7-day thermal windows, irrespective of the management system (p > 0.05). Pearson’s correlation coefficients were very low, ranging from −0.181 to 0.002 under conventional management and from −0.188 to −0.040 under the no-chemical-treatment management system. The R² values were also very low (<0.04), indicating that variation in captures during 2025 was not adequately explained by a simple linear relationship with temperature.

In contrast, in 2026, positive and statistically significant correlations were detected between temperature and the number of adults captured. Under conventional management, Pearson’s correlation coefficient increased from r = 0.598 for temperature on the monitoring date (p = 0.003) to r = 0.649 for the 3-day thermal window (p = 0.001), reaching its highest value for the 5-day thermal window (r = 0.673; p < 0.001; R² = 0.452). For the 7-day thermal window, the relationship remained similar in magnitude (r = 0.670; p < 0.001; R² = 0.449).

A similar pattern was observed under the no-chemical-treatment management system in 2026. The Pearson correlation between captures and temperature on the monitoring date was moderate and statistically significant (r = 0.541; p = 0.011), while the use of mean temperatures calculated over the thermal windows resulted in somewhat stronger relationships. For the 5-day thermal window, r = 0.590 (p = 0.005; R² = 0.348), whereas for the 7-day thermal window, r = 0.592 (p = 0.005; R² = 0.351).

Spearman’s rank correlation analysis confirmed and further emphasized these relationships. In 2026, the strongest association was identified for the 5-day thermal window, both under conventional management (ρ = 0.765; p < 0.001) and under the no-chemical-treatment management system (ρ = 0.774; p < 0.001). The concordance between the Pearson and Spearman analyses indicates a consistent positive association between thermal conditions during the period immediately preceding monitoring and the intensity of adult flight activity.

Table 3. Relationship between air temperature and Grapholita funebrana captures during 2025–2026.

Year / Management system	Thermal window	Pearson r	p	Spearman ρ	p	R2
2025 Conventional management	T0	−0.181	0.431	−0.126	0.588	0.033
	T3	−0.037	0.873	0.021	0.929	0.001
	T5	0.002	0.994	−0.027	0.909	<0.001
	T7	−0.055	0.812	−0.139	0.548	0.003
2025 No chemical treatment	T0	−0.188	0.416	−0.157	0.496	0.035
	T3	−0.069	0.768	−0.031	0.896	0.005
	T5	−0.040	0.864	−0.111	0.634	0.002
	T7	−0.085	0.716	−0.236	0.303	0.007
2026 Conventional management	T0	0.598	0.003	0.711	<0.001	0.357
	T3	0.649	0.001	0.737	<0.001	0.422
	T5	0.673	<0.001	0.765	<0.001	0.452
	T7	0.670	<0.001	0.760	<0.001	0.449
2026 No chemical treatment	T0	0.541	0.011	0.739	<0.001	0.292
	T3	0.580	0.006	0.747	<0.001	0.336
	T5	0.590	0.005	0.774	<0.001	0.348
	T7	0.592	0.005	0.764	<0.001	0.351

* T0, mean temperature on the monitoring date; T3, T5, and T7, mean temperatures calculated over the 3-, 5-, and 7-day thermal windows ending on and including the monitoring date, respectively. Significant correlations (p < 0.05) are indicated in bold.

Table 3. Relationship between air temperature and Grapholita funebrana captures during 2025–2026.

Year / Management system	Thermal window	Pearson r	p	Spearman ρ	p	R2
2025 Conventional management	T0	−0.181	0.431	−0.126	0.588	0.033
	T3	−0.037	0.873	0.021	0.929	0.001
	T5	0.002	0.994	−0.027	0.909	<0.001
	T7	−0.055	0.812	−0.139	0.548	0.003
2025 No chemical treatment	T0	−0.188	0.416	−0.157	0.496	0.035
	T3	−0.069	0.768	−0.031	0.896	0.005
	T5	−0.040	0.864	−0.111	0.634	0.002
	T7	−0.085	0.716	−0.236	0.303	0.007
2026 Conventional management	T0	0.598	0.003	0.711	<0.001	0.357
	T3	0.649	0.001	0.737	<0.001	0.422
	T5	0.673	<0.001	0.765	<0.001	0.452
	T7	0.670	<0.001	0.760	<0.001	0.449
2026 No chemical treatment	T0	0.541	0.011	0.739	<0.001	0.292
	T3	0.580	0.006	0.747	<0.001	0.336
	T5	0.590	0.005	0.774	<0.001	0.348
	T7	0.592	0.005	0.764	<0.001	0.351

* T0, mean temperature on the monitoring date; T3, T5, and T7, mean temperatures calculated over the 3-, 5-, and 7-day thermal windows ending on and including the monitoring date, respectively. Significant correlations (p < 0.05) are indicated in bold.

3.4. Flight Dynamics in Relation to Thermal Accumulation (GDD)

To further characterize the phenology of G. funebrana, seasonal flight dynamics were analyzed in relation to thermal accumulation expressed as growing degree-days (GDD). Calculations were based on a lower developmental threshold of 10 °C (GDD₁₀), and trap captures were expressed as cumulative percentages of the total seasonal captures. Thermal accumulations corresponding to the first observed capture and to the attainment of 10%, 25%, 50%, 75%, and 90% of cumulative seasonal captures were determined.

Table 4. Thermal accumulations (GDD₁₀) associated with cumulative capture thresholds of Grapholita funebrana in the ‘Stanley’ plum orchard.

Year	Management system	First observed capture	10%	25%	50%	75%	90%
2025	Conventional management	192.3	208.2	321.1	392.7	561.4	712.6
	No chemical treatment	192.3	224.0	295.2	392.7	509.2	712.6
2026	Conventional management	22.7	129.9	203.0	464.5	824.5	1,337.5
	No chemical treatment	22.7	179.5	305.3	317.4	664.7	898.4

* GDD₁₀, cumulative growing degree-days calculated using 10 °C as the lower developmental threshold. Values represent thermal accumulations corresponding to the first observed capture and to the indicated percentages of total seasonal captures.

Table 4. Thermal accumulations (GDD₁₀) associated with cumulative capture thresholds of Grapholita funebrana in the ‘Stanley’ plum orchard.

Year	Management system	First observed capture	10%	25%	50%	75%	90%
2025	Conventional management	192.3	208.2	321.1	392.7	561.4	712.6
	No chemical treatment	192.3	224.0	295.2	392.7	509.2	712.6
2026	Conventional management	22.7	129.9	203.0	464.5	824.5	1,337.5
	No chemical treatment	22.7	179.5	305.3	317.4	664.7	898.4

* GDD₁₀, cumulative growing degree-days calculated using 10 °C as the lower developmental threshold. Values represent thermal accumulations corresponding to the first observed capture and to the indicated percentages of total seasonal captures.

In 2025, the thermal progression of captures was remarkably similar between the two management systems. Although the initial 10% and 25% cumulative capture thresholds showed moderate differences, 50% of the seasonal captures were reached at 392.7 GDD₁₀ under both management systems, while the 90% threshold corresponded to 712.6 GDD₁₀. This agreement indicates a high degree of synchronization in the seasonal progression of flight activity in 2025, despite differences in total adult abundance between the two management systems.

In 2026, the distribution of captures in relation to thermal accumulation was more differentiated between the two management systems. The 50% cumulative capture threshold was reached at 464.5 GDD₁₀ under conventional management and at 317.4 GDD₁₀ under the no-chemical-treatment management system, whereas 90% of seasonal captures were accumulated at 1,337.5 and 898.4 GDD₁₀, respectively. The higher values recorded under conventional management were associated with a more prolonged distribution of adult flight activity throughout the monitoring season. Direct comparison of the thermal accumulation associated with the first observed capture between the two years should be interpreted with caution. In 2026, monitoring began on 20 April, whereas in 2025 the first available monitoring date was 15 May. Therefore, the value of 192.3 GDD₁₀ recorded in 2025 cannot be interpreted as a thermal threshold for the biological onset of flight; rather, it represents the thermal accumulation reached at the time of the first capture observed within the monitoring program.

Overall, expressing captures in relation to thermal accumulation provides a clearer representation of the phenological progression of the population than the exclusive use of calendar dates. Early cumulative capture thresholds, particularly 10% and 25%, may be of potential interest for forecasting and warning purposes because they precede the period of maximum adult flight activity.

However, the variability observed between the two study years indicates that the GDD₁₀ values obtained during 2025–2026 should be regarded as local experimental values rather than definitive phenological thresholds. Their validation over a longer multi-year period, together with pheromone monitoring and meteorological data, could subsequently allow the establishment of reference thermal ranges for forecasting G. funebrana flight activity in plum orchards in northeastern Romania.

Figure 2. Cumulative percentage of Grapholita funebrana captures in relation to growing degree-day accumulation (GDD₁₀) under conventional and no-chemical-treatment management in the ‘Stanley’ plum orchard during 2025 and 2026. Horizontal lines indicate the 10%, 25%, 50%, 75%, and 90% thresholds of cumulative seasonal captures.

Figure 2. Cumulative percentage of Grapholita funebrana captures in relation to growing degree-day accumulation (GDD₁₀) under conventional and no-chemical-treatment management in the ‘Stanley’ plum orchard during 2025 and 2026. Horizontal lines indicate the 10%, 25%, 50%, 75%, and 90% thresholds of cumulative seasonal captures.

4. Discussion

The results obtained during 2025–2026 revealed pronounced seasonal and interannual variability in the flight activity of adult Grapholita funebrana in the ‘Stanley’ plum orchard at the Didactic Farm of IULS Iași. Although total capture levels differed between years and between the two management systems, the main flight peaks were concentrated in the first half of June. This relative temporal stability, in contrast to the quantitative variation in captures, suggests that the seasonal phenology of the species is closely related to climatic conditions, whereas adult abundance may be influenced by a more complex set of factors.

The dynamics observed in Iași are comparable with those reported from other plum orchards in Romania. At the Fruit Research and Development Station Bistrița [7], in a study conducted during 2016–2018 that also included the ‘Stanley’ cultivar, recorded adult emergence during May and flight peaks that varied among years, occurring from mid-June to July. The author also reported reduced flight activity during periods characterized by lower temperatures and high precipitation. Interannual variability was also documented in the multi-year study conducted by Shevchuk [16] who distinguished low, normal, and high levels of G. funebrana flight intensity and highlighted the contribution of meteorological conditions to changes in the abundance of captured adults. These findings are consistent with our observations, in which the timing of the main flight peaks was relatively similar between management systems, whereas capture magnitude varied considerably between 2025 and 2026.

Comparison between V1—conventional management and V2—no chemical treatment revealed different patterns between the two study years. In 2025, captures were higher in V2, whereas in 2026 the relationship between the two management systems was reversed. Consequently, the observed differences between V1 and V2 cannot be attributed exclusively to the application of phytosanitary treatments. This distinction is important because pheromone traps primarily reflect male flight activity and do not directly measure fruit infestation levels. Lo Verde [10], in a study that also included the ‘Stanley’ cultivar, found that changes in male captures in pheromone traps were not always proportionally reflected in economic damage, while cultivar susceptibility was an important factor influencing infestation levels. Therefore, our results should be interpreted as differences in seasonal flight activity between the two management systems, without directly equating the number of males captured with the intensity of fruit damage. Furthermore, the spatial analysis of pheromone-trap captures conducted by Sciarretta [15] showed that capture distribution may vary within an orchard, supporting the interpretation of individual traps as spatial monitoring points.

Analysis of the relationship between temperature and captures revealed marked differences between the two study years. In 2025, the relationships were generally weak, whereas in 2026 the association became more evident, particularly when temperatures during the days preceding each monitoring date were considered. The results obtained for the 5-day thermal window (T5), with similar Spearman correlation coefficients in V1 and V2 (ρ = 0.765 and 0.774, respectively; p < 0.001), suggest that flight activity may be more closely associated with thermal conditions integrated over a short period than with temperature recorded only on the monitoring date. This finding is consistent with the temperature-dependent development of G. funebrana. Published phenological models for this species use temperature as a major developmental factor, and the model derived from the research of Charmillot [4] uses a lower developmental threshold of 10 °C.

However, the differences between 2025 and 2026 indicate that temperature should not be considered the sole factor determining flight intensity. Studies conducted in Romania and other European regions have shown that seasonal meteorological conditions can affect both the intensity and timing of flight peaks. Phenological models developed for G. funebrana confirm the usefulness of thermal accumulation for forecasting, but require calibration and validation under local conditions [2]. This variability supports the integration of meteorological data with direct pheromone-trap monitoring rather than relying on fixed calendar dates to forecast pest activity.

The analysis of GDD₁₀ accumulation further supports this interpretation. The lower developmental threshold of 10 °C used in the present study is consistent with the phenological model proposed by Charmillot et al. [4], in which development from egg to adult requires approximately 410 degree-days, whereas the duration of one generation, expressed from egg to egg, requires approximately 420 degree-days. These values are not directly equivalent to the cumulative capture thresholds determined in the present study because they describe the biological development of the insect, whereas the GDD₁₀ values calculated for Iași characterize the seasonal progression of male flight activity. Nevertheless, the agreement observed in 2025, when 50% and 90% of cumulative captures were reached at the same thermal accumulations in V1 and V2, indicates the potential of GDD as a complementary tool to pheromone-based monitoring.

However, the more pronounced differences between the two management systems in 2026 indicate that the GDD₁₀ values obtained over only two years should not be considered definitive biological thresholds for local G. funebrana populations. Models developed in a particular region require validation under local climatic and population conditions before being used for forecasting purposes. In this context, continued monitoring over multiple years would allow the stability of the identified cumulative capture thresholds and the T5 thermal window to be assessed. Integrating these indicators with precipitation data and, particularly, with direct assessments of fruit infestation could contribute to the development of a more robust local model for forecasting and warning of G. funebrana activity in plum orchards in northeastern Romania.

The results suggest that the response of adult G. funebrana to temperature is not exclusively immediate. In 2026, the stronger associations observed for mean temperatures calculated over the 3–7-day thermal windows, compared with temperature on the monitoring date, indicate that thermal conditions during the preceding period may be more strongly associated with flight intensity. Among the thermal windows analyzed, the 5-day mean temperature (T5) showed the most consistent association with capture levels, particularly according to Spearman’s rank correlation coefficient.

However, the absence of a similar relationship in 2025 highlights the complexity of population dynamics. In that year, although total adult abundance was considerably higher, temperature alone did not explain the temporal distribution of captures. This interannual difference suggests that flight activity results from the interaction among thermal conditions, population phenology, the succession of generations, and other biotic and abiotic factors.

Therefore, temperature can be considered an important factor in the synchronization of flight activity, but not an independent and universal predictor of G. funebrana abundance. For the development of a local forecasting system, our results indicate the potential value of using short-term thermal windows, particularly a 5-day window, in combination with data obtained from pheromone-trap monitoring.

An important limitation of the present study is the experimental design, as each management system was represented by a single orchard plot. The three pheromone traps placed within each plot constituted monitoring subsamples rather than independent experimental replicates. Consequently, the differences observed between V1 and V2 should be interpreted descriptively and cannot be attributed exclusively to the effect of the management system. Nevertheless, monitoring over two consecutive seasons allowed a comparative characterization of temporal flight dynamics and, particularly, an assessment of the relationships among G. funebrana captures, short-term thermal conditions, and GDD₁₀ accumulation. From this perspective, the comparison between the two management systems should be regarded as exploratory, whereas the main contribution of the study lies in characterizing the local thermal phenology of the species and identifying potential indicators that can subsequently be validated through multi-year monitoring and replicated field designs.

Overall, comparison between the two study years and the two management systems showed that the timing of the main flight activity was more stable than capture magnitude, whereas the relationship with temperature and the progression of captures in relation to GDD₁₀ showed substantial interannual variability. These findings support the combined use of pheromone-based monitoring and thermal indicators, while cautioning against interpreting either trap captures or thermal accumulations as independent and invariant indicators of infestation risk.

5. Conclusions

Pheromone-trap monitoring conducted during 2025–2026 revealed substantial interannual variation in the abundance of Grapholita funebrana in the ‘Stanley’ plum orchard, whereas the principal flight peaks remained consistently concentrated in the first half of June. The relationship between temperature and trap captures also differed between years, with no significant associations detected in 2025 and markedly stronger relationships in 2026. Among the short-term thermal windows evaluated, the 5-day mean temperature showed the most consistent association with adult captures in 2026. The GDD₁₀ analysis further demonstrated that thermal accumulation can provide useful information on seasonal flight progression, although considerable interannual variation was observed. Therefore, the thermal values obtained in this two-year study should be regarded as local experimental benchmarks rather than definitive forecasting thresholds. Continued multi-year monitoring integrating pheromone captures, weather variables and direct fruit-infestation assessments will be necessary to validate these relationships and develop a reliable local forecasting system for G. funebrana in northeastern Romania.