Impact of Shelterbelts on the Diversity and Dynamics of Natural Enemies in Wheat Agroecosystems

Abstract

Biological and integrated pest management strategies have recently been widely adopted in crop protection, with one of the key approaches involving the use of natural enemies (predators and parasites). In order to identify and monitor beneficial arthropod species associated with winter wheat crops, an experiment was conducted between 2022 and 2024 in two locations in the Transylvanian Plateau: Turda, within an open-field agroecosystem, and Bolduţ, within an agroecosystem with protective agroforestry shelterbelts. The research aimed to evaluate the diversity of beneficial arthropod fauna in two agroecosystems, as well as the impact of insecticide treatments on the natural enemies of wheat pests. According to our findings, it can be stated that the beneficial arthropods identified in the two agroecosystems belonging to the same groups, but the abundance in all three years is higher in the agroecosystem with agroforestry shelterbelts. Among all the analyzed arthropods, the order Araneae was characterized by the highest abundance, recording 995 individuals in an insecticide-free variant in 2022. The treated variants with insecticide showed a decrease in both abundance and diversity in agroecosystems with and without shelterbelts during all three experimental years, compared to the variant without insecticides. Regarding abundance, the largest difference was recorded in 2024 in the shelterbelt agroecosystem with a reduction of 781 individuals. In terms of diversity, the lowest value was observed for the Shannon_H index in open-field agroecosystem in 2024 for the variant treated with insecticides (1.426), compared to the untreated variant, where the index reached a value of 1.841. The application of insecticide treatments caused an increase in the mortality of beneficial arthropods, reaching its highest level in 2024.

1. Introduction

Wheat is one of the most widely cultivated and important cereal crops worldwide [1]. It serves as a staple food for billions, providing a major source of nutrition and calories [2,3]. Due to its high nutritional value and diverse uses, wheat is essential to global food security, economies, and cultures [3,4]. Wheat, being the main host and preferred food source for many cereal crop pests, both influences and is, in turn, influenced by the ecological conditions in the field that drive the evolution of these pests [5,6].

Biological control of arthropod pests is an important ecosystem service that reduces plant damage and the need for pesticide application [7,8,9]. Predatory arthropods play a crucial role in regulating herbivore populations within agricultural agroecosystems, contributing significantly to natural pest control [6,10]. Beneficial arthropods regulate agricultural pest populations through predation and parasitism, thereby contributing significantly to pest suppression and reducing reliance on synthetic pesticides within integrated pest management frameworks [7,11,12].

The efficacy of predatory arthropods is shaped by multiple factors, such as habitat structural complexity, the presence of alternative prey sources, and disturbances caused by the use of broad-spectrum insecticides [13,14]. Spiders, for instance, are effective polyphagous predators capable of capturing a substantial number of prey throughout their lives, making them valuable biological control agents in pest management [15,16]. Enhanced spider biodiversity improves pest management effectiveness, as different species exhibit distinct hunting behaviors that contribute to the suppression of a broader range of insect pests [15,17,18,19]. Understanding the ecological interactions and environmental factors that influence predatory insect populations is essential for optimizing their contribution to sustainable agriculture [20]. Several studies in Romania have focused on the biodiversity of beneficial arthropods [21,22,23]. These studies highlight the important role that beneficial arthropods play in agricultural ecosystems, particularly in pest regulation. Understanding the diversity and dynamics of these natural enemies is crucial for developing sustainable pest management strategies in cereal crops such as wheat.

Climate change is affecting our planet on a large scale and represents one of the greatest threats to ecosystems and biodiversity [24]. The effectiveness of forest shelterbelts is well recognized in combating drought and other climate and terrain-related challenges such as storms, torrents, blizzards, and landslides, as well as in preventing and mitigating large-scale soil degradation processes [25]. Moreover, agroforestry shelterbelts play an essential role in protecting and supporting the development of beneficial arthropod populations [26,27,28,29]. Several studies have been conducted on beneficial arthropods in wheat crops in our study area, the Transylvanian Plateau [30,31,32,33], which have shown that the presence of agroforestry shelterbelts positively influences the abundance of beneficial arthropods. The authors observed a higher number of beneficial arthropods in fields with shelterbelts compared to those without, as well as the negative impact of insecticide treatments on these useful arthropods.

Agroforestry, the intentional integration of trees and shrubs into agricultural systems, presents a multifaceted approach to land management that enhances ecological function and promotes sustainable agricultural practices [34]. Agroforestry shelterbelts, a specific application of this approach, involve establishing linear plantings of trees and shrubs along field edges or within agricultural landscapes to serve a variety of purposes, including providing shelter for beneficial arthropods [35,36]. The strategic placement of agroforestry shelterbelts can significantly influence the microclimate within agricultural fields, affecting temperature, humidity, and wind speed, which in turn impacts insect behavior and distribution [37]. By creating a more diverse and heterogeneous environment, agroforestry shelterbelts can support a greater abundance and diversity of insect species, including pollinators, predators, and parasitoids that contribute to natural pest control and overall ecosystem health [38]. Furthermore, the selection of plant species for agroforestry curtains is crucial for maximizing their effectiveness as insect shelters [39,40]. Native trees and shrubs that provide pollen, nectar, and other resources for beneficial arthropods should be prioritized, and the planting design should consider the specific habitat requirements of target insect species [41].

The aim of this study is to investigate the community of natural enemies of crop pests in winter wheat (Triticum aestivum) fields within the landscape of the Transylvanian Plateau, and to evaluate the effects of insecticide applications on these beneficial species in climatic condition of three experimental years (2022–2024). Through this study, we aim to resume research on the abundance and diversity of beneficial arthropods, as well as the influence of agroforestry shelterbelts on them, in the context of climate change and the evolving technologies applied to wheat cultivation (e.g., increased number of treatments and diversification of active substances used). The objectives were to: (i) identify and quantify beneficial arthropod species associated with winter wheat, (ii) assess their diversity within the shelterbelt and open-field contexts, (iii) and quantify mortality resulting from insecticide treatments. Moreover, considering the essential role of beneficial arthropods as natural biological control agents in wheat crop, we emphasize the need to reduce pesticide use in order to limit the accumulation of chemical residues in the environment and agricultural products. In this context, agroforestry shelterbelts play an important role in protecting and supporting populations of beneficial arthropods, thereby contributing to the development of integrated, sustainable strategies adapted to current agroclimatic conditions.

2. Materials and Methods

2.1. Study Site

Given the important role of beneficial arthropods in controlling wheat pests, their populations were monitored in winter wheat crops between 2022 and 2024 at two locations within the Agricultural Research and Development Station (ARDS) Turda, Cluj County, Romania. In addition to its own land and experimental fields, ARDS Turda also manages a field crop farm designed with an anti-erosion system and bordered by agroforestry shelterbelts (Bolduț Farm, Cluj County, Romania). These are composed of mixtures of over 36 tree and shrub species. The outer rows consist of fruit tree species and fruit-bearing shrubs: Cerasus avium, Malus sylvestris, Pirus piraster, Prunus spinosa, Crataegus pentagyna, Rosa canina, Vaccinium vitisidaea, Corylus avellana, Ligustrum vulgare, Staphylaea pinnata, and elderberry (Sambucus nigra). The inner rows of the shelterbelts are formed by forest species: Quercus robur, Ulmus sp., Robinia pseudoacacia, Acer platanoides, Acer pseudoplatanus, Fraxinus excelsior, Tilia cordata, and Salix alba.

The two sites are located relatively close to each other, approximately 14 km apart (Figure 1).

The study was conducted in two wheat agroecosystems from Transylvanian Plateau: one located at Turda (46°35′12.3″ N 23°48′40.7″ E) under open-field conditions (without agroforestry shelterbelts), and the other at Bolduț (46°36′13.5″ N 23°56′29.1″ E), in fields protected by agroforestry shelterbelts (Figure 2). The arrangement of the agroforestry shelterbelts is oriented both north–south and east–west, and the experimental plots are surrounded by these. The relief is hilly, and the land cover classes are arable land and grassland. Both areas are characterized by chernozem soils with a neutral pH reaction.

The experiment included, for each location, two experimental plots (0.5 hectares) with three replications each, separated by a 10 m buffer zone between replications, and a 25 m buffer zone between the variants with and without insecticides.

The winter wheat (T. aestivum) variety Andrada was used as the biological material in the experiment. This cultivar was created at the Agricultural Research and Development Station (ARDS) Turda and is best adapted to the agroecological conditions of the central and northern regions of Romania.

In the first variant, no insecticide treatments were applied. In the second variant, insecticides were applied at two key stages: first, at the end of the tillering phase, simultaneously with herbicide application; and second, during the phenological stage ranging from booting to ear emergence. Phenological growth stages of wheat are strongly genetically determined but are also influenced by climatic conditions. Consequently, the timing of crop protection treatments varies from year to year. Thus, in the first two years, 2022 and 2023, the treatments were applied on similar dates, with the first treatment in the last decade of April and the second treatment in the last decade of May. However, in 2024, the first treatment was advanced compared to previous years and was applied in the first decade of April, specifically on 1 April 2024 (at both locations), due to the warm spring of 2024 and the early appearance of pests in the crop, and the second treatment was applied in the second decade of May in the open-field agroecosystem and in the first decade of May in the shelterbelt agroecosystem.

2.2. Pesticides Used in the Experiment

In both agroecosystems of the winter wheat crops with and without agroforestry shelterbelts, various chemical inputs were used, including mineral and foliar fertilizers, herbicides, fungicides, and insecticides. The technology applied in the two wheat agroecosystems is that used in the conventional system [42,43]. The products applied during the three experimental years were the same in both agroecosystems and are presented in

Table 1. Chemicals used in the three experimental years.

	2022	2023	2024
Fertilizers administered	NPK 1 (16:16:16) 300 kg/ha−1 Nitrocalcar 3 (27% s.a) 200 kg/ha−1	NPK (20:20:0) 300 kg/ha−1 UREE 4 (46% s.a) 100 kg/ha−1	NP 2 20:20:13 Sulfur −200 kg/ha−1 Nitrocalcar (27% s.a) 200 kg/ha−1
Foliar fertilizer	Tonivit 5 (1 L/ha−1) [TA]	Tonivit (1 L/ha−1) [TA]	Maxigrow 6 (600 mL/ha−1) [TA]
Herbicides	Sekator 7 (100 g/amidosulfuron 25 g/L iodosulfuron-metil-Na 250 g/L mefenpyr dietil) 0.15 L/ha−1 [TA] Amino 600 SL 9 (600 g/L acid 2.4-D) 1 L/ha−1 [TA]	Brodwaystar 8 (piroxsulam 70.8 g + florasulam 14.2 g + cloquintocet mexil (safener) 70.8 g) 250 g/ha−1 [TA] Amino 600 SL (600 g/L acid 2.4-D) 1 L/ha−1 [TA]	Brodwaystar (piroxsulam 70.8 g + florasulam 14.2 g + cloquintocet mexil (safener) 70.8 g) 250 g/ha−1 [TA] Amino 600 SL (600 g/L acid 2.4-D) 1 L/ha−1 [TA]
Fungicides	Falcon PRO 10 (53 g/L protioconazol 224 g/L spiroxamină 148 g/L tebuconazol) 0.8 L/ha−1 [TA] Nativo 12 (175 g/L protioconazol 150 g/L trifloxistrobin) 0.8 L/ha−1 [TB]	Falcon PRO (53 g/L protioconazol 224 g/L spiroxamină 148 g/L tebuconazol) 0.8 L/ha−1 [TA] Nativo 175 g/L protioconazol 150 g/L trifloxistrobin 0.8 L/ha−1 [TB]	Flexity 300 SC 11 (300 g/L Metrafenonă) 0.25 L/ha−1 [TA] Mizona 13 (200 g/L piraclostrobin, 30 g/L fluxapiroxad) 0.5 L/ha−1 [TA] Revycare 14 (100 g/L mefentrifluconazol, 100 g/L piraclostrobin 1 L/ha−1 [TB]
Insecticides	APIS 200 SE 15 (200 g/L acetamiprid) 0.15 L/ha−1 [TA] Evure (tau-fluvalinat 240 g/L) 0.2 L/ha−1 [TB]	APIS 200 SE (200 g/L acetamiprid) 0.15 L/ha−1 [TA] Evure (tau-fluvalinat 240 g/L) 0.2 L/ha−1 [TB]	APIS 200 SE (200 g/L acetamiprid) 0.15 L/ha−1 [TA] Mavrik (tau-fluvalinat 240 g/L) 0.2 L/ha−1 [TB]
Adjuvant	Trend 16 (1 L/ha−1) [TA]; [TB]	Dassoil 17 (0.6 L/ha−1) [TA] Vital 18 (100 mL in 100 L water) [TB]	Dassoil (0.6 L/ha−1) [TA] Inex 19 0.25 L/ha−1 [TB]

¹ NPK; ² NP; ³ Nitrocalcar; ⁴ Uree—AMEROPA Group Represented by AZOMURES, Mureș County, România; ⁵ Tonivit—UPL, Bucharest, Romania; ⁶ MaxiGrow; ¹⁹ Inex–Agroleg Varo SRL, Mureș County, Romania; ⁷ Sekator; ¹⁰ Falcon PRO; ¹² Nativo–Bayer Crop Science SRL Bucharest, Romania; ⁹ Amino 600 SL, Evure, Mavrik–SC Adama Agricultural Solutions SRL, Bucharest, Romania; ⁸ Brodwaystar; ¹⁷ Dassoil–Pioneer Hi-Bred Romania SRL, Bucharest, Romania; ¹¹ Flexity 300 SC; ¹³ Mizona; ¹⁴ Revycare–BASF Romania, Bucharest, Romania; ¹⁵ Apis 200 SE–Innvigo Better Chemistry, Cluj, Romania; ¹⁶ Trend–FMC Agro Operational Romania SRL, Bucharest, Romania; ¹⁸ Vital–Alchimex S.A., Bucharest, Romania.

. At both experimental sites, two treatments were applied annually during 2022–2024: the first treatment (TA) and the second treatment (TB). The application of chemical treatments was performed using a MET 1500 sprayer mounted on a John Deere 6620 SE tractor, ensuring uniform coverage across all experimental plots. Fertilization was applied in autumn as follows: 2021—NPK (16:16:16); 2022—NPK (20:20:0); and 2023—NP (20:20:13 + S). Additional nitrogen sources were applied in spring as follows: calcium ammonium nitrate in 2022 and 2024 and urea in 2023.

2.3. Natural Enemies—Sampling

The monitoring of useful arthropods was carried out by collecting specimens using a standard entomological sweep net. The diameter of the net was 30 cm and net handle length was 50 cm. One hundred double sweeps per sample were performed every ten days, starting in the spring months and continuing until harvest (collections were made only on days without rain or wind).

After collection, the samples were refrigerated, and a few days later, the species of beneficial arthropods present in the two agroecosystems were identified (Figures S1 and S2 in Supplementary Material).

Biodiversity indices, applied in this study, were used to quantify and compare the diversity and structural composition of beneficial arthropods communities in the two agroecosystems [36] between 2022 and 2024 period. These included the species richness (Taxa_S), total number of individuals, Shannon-Wiener diversity index (Shannon-H), the Simpson diversity index (Simpson_1-D), and the dominance index (D), which together provide a comprehensive picture of species richness and dominance relationships. Community structure was further characterized through measures of distribution and balance, using Evenness (Evenness_e), Equitability (Equitability_J) indices and the Chao-1 estimator. Such multivariate ecological approaches are commonly employed in agroecological studies to assess insect biodiversity under varying management regimes [44,45].

Shannon–Weaver Diversity Index (Shannon-H):

$${\displaystyle \mathrm{H}=\sum _{\mathrm{i}=1}^{\mathrm{n}}\mathrm{ni}/\mathrm{N}\times \ln \times \mathrm{ni}/\mathrm{N}}$$

(1)

where

H = diversity

n_i = number of individuals of each species

i = 1, 2, 3, …, n species

N = total number of individuals

Simpson’s Diversity Index (Simpson_1-D):

Is = N (N − 1)/∑ n (n − 1)

(2)

where

n = number of individuals of each species

N = total number of individuals

Dominance (D):

D_A = N_A/N × 100

(3)

where

D_A = dominance of species A

N_A = number of individuals of species A found in the samples analyzed

N = total number of individuals of all species present in the analyzed samples

Evenness (Evenness_e):

E% = H/lnS

(4)

where

H = diversity

S = total number of species in the examined crop

Jaccard Similarity Coefficient (Equitability_J):

J = c/a + b − c × 100

(5)

where

a = number of species in ecosystem 1

b = number of species in ecosystem 2

c = number of species common to both ecosystems

Species richness was estimated based on the number of observed taxa per sampling locality, and the Chao 1 estimator was used to account for unseen species and provide a more robust estimate of total species richness.

The mortality of beneficial arthropods was calculated using the Henderson-Tilton formula:

Effectiveness (%) = [1 − (Nt after treatment/Nt before treatment)/(Nc after treatment/Nc before treatment)] × 100

(6)

where

Nt = the number of insects in the treated area

Nc = the number of insects in the control (untreated) area

The use of the Henderson-Tilton formula is essential in field experiments, as it allows for an accurate estimation of the efficacy of plant protection treatments by adjusting the data according to natural population variations in the control plots [46,47].

For the statistical interpretation of the results, the programs used was: Past version 4.03 freeware license on Windows and ANOVA (USAMV, Cluj Napoca, Romania) for abundance (using SD (5%)) and Microsoft Excel program (mortality, arthropods number and Pearson correlation).

2.4. Climate Conditions During the Experimental Period

Climatic data were obtained from the Turda Meteorological Station, which operates under the jurisdiction of the Northern Transylvania Regional Meteorological Center, a subdivision of the Romanian National Meteorological Administration. The station is situated within the Agricultural Research and Development Station (ARDS) Turda, in the Transylvanian Plain (Romania), at an altitude of 427 m, with coordinates 46°35′ N latitude and 23°47′ E longitude.

3. Results

3.1. Results on Natural Enemies Abundance

Among the individuals collected in the open-field and shelterbelt agroecosystems during 2022–2024 (

Table 2. Abundance of natural enemy species in wheat crop across open-field agroecosystem atTurda and shelterbelt field at Bolduț, comparing insecticide-treated and insecticide-free plots.

Families/Species	Open-Field Agroecosystem						Shelterbelt Field Agroecosystem
	Insecticides-Free Plot			Insecticides Plot			Insecticides-Free Plot			Insecticides Plot
	2022	2023	2024	2022	2023	2024	2022	2023	2024	2022	2023	2024
Coccinella 7—punctata L.	10	9	33	5	7	1	23	24	34	13	12	13
Propylaea 14—punctata L.	4	2	4	3	2	0	10	9	8	8	2	3
Other Coccinellidae Species	0	0	0	0	0	0	11	6	1	5	5	0
Cantharis fusca L.	36	38	29	19	24	3	38	59	31	30	24	12
Other Cantharidae Species	31	0	0	18	0	0	109	87	0	18	37	0
Malachius bipustulatus L.	10	12	15	5	6	3	18	14	23	6	8	6
Nabis ferus L.	59	75	271	30	21	71	85	82	289	66	52	101
Other Nabidae Species	0	0	0	0	0	0	0	10	0	0	29	36
Orius spp.	1	0	2	1	0	0	9	3	4	5	2	2
Anthocoris spp.	0	1	0	1	0	1	6	3	5	4	3	3
Adelphocoris spp.	0	0	1	0	0	1	1	2	4	1	1	3
Tachyporus hypnorum F.	0	0	0	0	0	3	0	0	7	0	0	2
Staphylinus spp.	0	0	5	0	0	1	8	0	6	7	0	1
Chrysopa carnea S.	20	21	11	9	11	8	20	46	5	18	33	4
Episyrphus balteatus DeGeer	0	3	16	0	1	5	1	3	9	0	2	2
Other Syrphidae Species	83	93	17	40	49	3	133	132	77	49	57	7
Tachinidae spp.	0	0	0	0	0	0	9	0	0	0	0	0
Platypalpus spp.	139	157	85	60	73	29	177	152	244	130	71	109
Other Empididae Species	0	0	0	0	0	0	0	138	0	0	55	0
Collyria coxator Villers	2	5	8	0	2	2	8	9	2	8	2	1
Aphydius avenae Haliday	4	37	14	1	21	0	49	48	15	29	26	5
Other Parasitic Hymenoptera Species	69	96	207	53	59	74	151	101	213	264	61	111
Order Araneae	483	353	337	345	231	233	995	746	444	698	501	219
No. of total individuals	951	902	1055	590	507	438	1861	1674	1421	1359	983	640

), 10 individual species were identified, along with 23 groups or taxa at the genus/family level. These belong to 13 different orders of natural predators and parasitoids, including Coleoptera, Hemiptera, Diptera, Hymenoptera, and Araneae. The abundance of natural predators and parasitoids was consistently higher in plots without insecticide treatment compared to treated plots, in both agroecosystems and across all three study years. For example, in the shelterbelt agroecosystem the order Araneae reached its highest numbers in insecticide-free plots: 995 individuals in 2022 compared to 698 in treated plots with insecticide; 746 in 2023 compared to 501 with insecticide; and 444 in 2024 compared to 219 in insecticides treated plots. Other important groups, such as Cantharidae, Coccinellidae, and the genus Platypalpus, also showed higher abundances under conditions without insecticides (e.g., In 2024, within the shelterbelt agroecosystem, Platypalpus spp. populations reached 244 individuals in the insecticide-free plot, compared with 109 in the insecticide-treated plot). Parasitoids like Aphidius spp. and other parasitic Hymenoptera followed the same pattern, reaching maximum numbers in 2022–2023 in the shelterbelt agroecosystem with 49 individuals in insecticides–free plot compared to 29 in the one with insecticides in 2022. The total number of individuals clearly reflects the negative effect of insecticides on beneficial populations: in 2022 in the shelterbelt agroecosystem, 1861 individuals were recorded in untreated plot compared to 1359 in treated plot, with a similar trend observed in subsequent years.

3.2. Results on Diversity and Evenness Indices

The collected data reveal differences between the two variants, with and without insecticides (

Table 3. Diversity indices of predatory and parasitic arthropods in open-field and shelterbelt agroecosystems (2022–2024), from insecticide-free and insecticide-treated plots, including Taxa Richness, Total Individuals, Dominance, Simpson and Shannon Indices, Evenness, Equitability, and Chao-1 Estimator.

	Open-Field Agroecosystem						Shelterbelt Field Agroecosystem
	Insecticides-Free Plot			Insecticides Plot			Insecticides-Free Plot			Insecticides Plot
	2022	2023	2024	2022	2023	2024	2022	2023	2024	2022	2023	2024
Taxa_S	14	14	16	14	13	13	20	20	19	18	20	19
Individuals	951	902	1055	590	507	436	1861	1674	1421	1360	983	640
Dominance_D	0.2992	0.2166	0.2158	0.3699	0.2577	0.346	0.3137	0.2318	0.1955	0.3154	0.2831	0.2054
Simpson_1-D	0.7008	0.7834	0.7842	0.6301	0.7423	0.654	0.6863	0.7682	0.8045	0.6846	0.7169	0.7946
Shannon_H	1.68	1.875	1.841	1.518	1.772	1.426	1.765	2.023	1.918	1.654	1.911	1.892
Evenness_e^H/S	0.3831	0.4659	0.3938	0.3258	0.4526	0.3201	0.292	0.3782	0.3581	0.2905	0.3379	0.349
Equitability_J	0.6364	0.7106	0.6639	0.5751	0.6909	0.5559	0.589	0.6754	0.6513	0.5723	0.6378	0.6425
Chao-1	14	14	16	17	13	13.5	21	20	19	18	20	19.25

). Consistently, higher species richness values were recorded under insecticide-free conditions, both in terms of the actual number of species (Taxa_S) and the estimated values (Chao-1). For example, in 2024, 16 species were recorded in the variant without insecticides in open-field conditions compared to only 13 in the variant with insecticides, with Chao-1 estimating 16 and 13.5, respectively. These data suggest that the applied insecticide treatment reduces both the actual and potential diversity of the community.

The total number of individuals was consistently higher in the insecticides-free variant across both locations and all study years. In the shelterbelt field, in 2024, the insecticide-free plot recorded 1421 individuals, more than double the 640 observed in the insecticide plots. This marked difference indicates a potential inhibitory effect of the insecticide treatment on the development or survival of the monitored organisms.

The Shannon index (H’), which combines species richness and numerical evenness, had slightly higher values in insecticides-free plots (e.g., 2.023 in the agroecosystem with shelterbelts in 2023 compared to 1.911 in the treated variant). The lowest value was observed for the Shannon_H index under open-field conditions in 2024 in the insecticide plot (1.426), compared to the untreated plot, where the index reached a value of 1.841. A similar pattern is observed in the Simpson index (1–D), suggesting that community diversity is greater in the absence of insecticide treatment (Table 3).

Regarding the dominance of a single species (Dominance_D), lower values in the insecticide-free variant confirm a more balanced distribution of individuals among species. In the insecticide variants, higher values suggest a disturbance that favors dominant species at the expense of more sensitive ones.

The evenness index (Evenness_e^H/S) and equitability (J) were slightly higher in the insecticide-free plots, particularly in the open-field agroecosystem, where the J index increased from 0.5559 (insecticide variant) to 0.6639 (insecticide-free variant) in 2024. These values indicate a more evenly distributed and stable biological community in the absence of insecticide treatment intervention.

The comparative analysis of ecological diversity indices highlights a clear effect of the insecticide treatment on local ecosystems. Insecticide plots exhibited a decrease in diversity and abundance, as well as a tendency toward imbalance in community structure, reflected by increased dominance and a decreased evenness e^H/S index.

The abundance of beneficial arthropods was significant in the variant that was not treated with insecticides (χ² = 184.96; df = 6; p < 0.001). The abundance of arthropods in the agroecosystem with shelterbelts was significant, with an annual mean of 1632 ± 145.9 (mean ± SE), compared to the open-field agroecosystem, which had a mean of 969 ± 45.2 (mean ± SE) in the insecticides-free variant. In the one with insecticides, the trend was similar to that in the untreated variant, increasing from 511.7 ± 43.9 (mean ± SE) in the open-field agroecosystem to 994 ± 207.6 (mean ± SE) in the shelterbelt agroecosystem (Figure 3).

3.3. Results on Natural Enemies Mortality

Figure 4 illustrates the mortality (%) of arthropods, calculated using the Henderson–Tilton formula, in two agroecosystems with and without shelterbelts, over a three-year period (2022–2024), according to the number of days after the first insecticide treatment (DATA) and to the number of days after the second insecticide treatment (DATB).

In both agroecosystems, the first insecticide treatment with acetamiprid, applied during the stem elongation phenophase (27 April 2022 in Turda; 1 May 2022 in the shelterbelt agroecosystem), led to a 100% reduction in beneficial arthropods in the open-field agroecosystem and a 54.5% reduction in the one with shelterbelts. The second treatment with the insecticide tau-fluvalinat (22 May 2022, in both locations, with an interval of 24 days between treatments in open-field conditions and 21 days in the shelterbelt field), applied during the booting phenological stage, reduced beneficial arthropods by 41.8% in open-field conditions and 73.1% in the shelterbelt field (Figure 4). However, after this treatment, the arthropod population recovered more rapidly in the agroecosystem with agroforestry shelterbelts compared to the one without shelterbelts. The major impact in reducing beneficial arthropods occurred in open-field conditions after the first treatment with a neonicotinoid insecticide, where the beneficial arthropods were completely eliminated from the wheat crop.

In 2023, in the open-field agroecosystem, the first insecticide treatment was applied on April 22, and the second on May 20, with an interval of approximately 28 days between the two applications. In the shelterbelt agroecosystem, the first insecticide treatment was applied on April 23, and the second on May 23, resulting in an interval of approximately 30 days between treatments. The insecticides applied were the same as in the previous year, and the dynamics of natural enemy populations following their application followed the same trend. Notably, in this year, the insecticide acetamiprid reduced the arthropod populations in the shelterbelt agroecosystem by nearly 100%, while in the one without shelterbelts, the reduction reached nearly 80%. In both locations, approximately 60 days after the first insecticide treatment and 30 days after the second, the number of beneficial arthropods no longer declined, remaining present in the crop until harvest in both years, 2022 and 2023. In 2024, the first insecticide treatment was applied on April 6 in the open-field agroecosystem and April 4 in the shelterbelt field, followed by the second treatment on May 15 and May 7, respectively, resulting in intervals of 39 and 33 days. As can be seen in Figure 4, the highest mortality of natural enemies occurred in the year 2024.

Figure 5 indicates the number of arthropods before the first insecticide treatment (BTA) and before the second insecticide treatment (BTB) over the three experimental years (2022, 2023, 2024), in the both agroecosystems. A recovery of the arthropod population was observed after the first insecticide treatment in both experimental variants; however, in most years, population levels remained consistently lower in the treated plot.

The abundance of beneficial arthropods was significantly higher in the shelterbelt agroecosystem throughout the three years studied (χ² = 98.26; df = 5; p < 0.001) compared to the open-field agroecosystem (Figure 6). The abundance of useful arthropods was significantly higher in 2022, with a mean of 1610 ± 251 (mean ± SE), compared to 2023, which had a mean of 1328.5 ± 345.5 (mean ± SE), while in 2024, a mean of 1000.5 ± 360.5 (mean ± SE) was recorded in the shelterbelt agroecosystem. Similar results were recorded in the open-field agroecosystem, with the abundance of useful arthropods being higher in 2022, with a mean of 770 ± 180 (mean ± SE), compared to 2023, with a mean of 704.5 ± 197.5 (mean ± SE), and in 2024, the arthropods had a mean of 746.5 ± 308.5 (mean ± SE).

An analysis of the correlation between temperature and the number of beneficial arthropods is essential for understanding the ecological mechanisms that regulate biological balance in agroecosystems. The data presented in Figure 7 show us that in the open-field agroecosystem, the relationship between decadal temperature deviations and the abundance of beneficial arthropods varied markedly among the years. In 2023, a strong and highly significant positive correlation was observed (r = 0.84, p < 0.01; R² = 0.7148), indicating that higher temperatures were closely associated with increased beneficial arthropod numbers. In 2022, the correlation was positive but weaker and not statistically significant (r = 0.60; R² = 0.3623), while in 2024, no meaningful relationship was detected (r = 0.04; R² = 0.0016). These findings suggest that the temperature–beneficial arthropods abundance relationship is strongly context-dependent, likely influenced by additional abiotic and biotic factors varying between years.

The abundance of natural enemies in the agroecosystem with agroforestry shelterbelts showed pronounced interannual variability in its relationship with decadal temperature deviations (Figure 8). In 2022, the beneficial arthropods abundance was strongly positively correlated with temperature deviations (r = 0.73*, p < 0.05, R² = 0.531), whereas in 2023 the correlation was weaker and not statistically significant (r = 0.42), and in 2024 it was effectively absent (r = 0.14). These results indicate that the influence of temperature on beneficial arthropods populations is context-dependent and may be modulated by additional ecological factors, such as resource availability, interspecific interactions, and microclimatic conditions created by shelterbelts.

4. Discussion

The data presented in Table S1 (Supplementary Material) indicate that the autumn months were warm, with the autumn of 2021 being the closest to the multiannual average (based on a 65-year average). In the other years analyzed, autumn temperatures showed an increasing trend. A similar upward trend was also observed during the winter months, with temperature deviations from the average recorded in all three years. These higher winter temperatures have a significant impact on insect populations, as they increase their survival rate [43]. The highest deviation was recorded in February 2024, when the multiannual average was exceeded by 7 °C. The coolest springs occurred in 2022 and 2023, while in 2024, temperatures were higher than the 65-year average for this period, leading to an earlier appearance of arthropods in wheat fields. Summer temperatures were high in all three years of the study. The smallest deviations were recorded in 2023, while the largest were observed in 2024, with differences from the multiannual average reaching up to 4.2 °C in July. According to the Romanian National Meteorological Administration [44], 2024 was the warmest year on record in Romania, with an annual average temperature of 12.9 °C.

Climate change has a significant impact on agricultural insect pests, as well as the natural enemies that help reduce their populations [40]. These poikilothermic organisms, whose body temperature depends on the ambient temperature, are particularly sensitive to environmental changes [40]. As temperatures rise and precipitation patterns shift due to climate change, we can expect to see changes in the geographic distributions, population dynamics, and overall abundance of both insect pests and their natural enemies [41]. Global warming is a global climatic process that is also evident in Romania, where the average annual temperature has increased by 2 °C over the past 100 years, with the last 20 years being among the warmest on record. Against the background of rising average temperatures, significant changes in the precipitation regime have also been observed. These alterations include increased variability, a higher frequency of extreme weather events (such as droughts or intense rainfall), and shifts in seasonal distribution patterns [42]. Such climate dynamics have direct implications for agricultural productivity, soil moisture availability, and the phenology and distribution of both pest and beneficial insect populations.

To better understand the impact of rising temperatures, an analysis of diurnal temperature extremes is necessary. Data presented in Table S2 (Supplementary Material) indicate an increase in extreme daytime temperatures, with heat waves becoming more frequent. These conditions can have both positive and negative effects on pest populations as well as beneficial arthropods.

Numerous studies have demonstrated that higher temperatures tend to accelerate insect feeding, growth, and mobility, which can influence population dynamics by affecting fertility, survival, generation time, population size, and geographical distribution [45,46,47]. Organisms that are unable to adapt and evolve in response to rising temperatures typically struggle to maintain their populations, whereas those capable of thriving in warmer conditions may proliferate rapidly [48].

Changes in the amount, intensity, and frequency of precipitation are essential indicators of climate change [49], and observations show that while precipitation frequency has decreased, its intensity has increased, leading to a rise in the incidence of droughts and floods [50]. These changes directly affect insect species that overwinter in the soil, as fluctuating rainfall patterns influence their survival environment [40]. In particular, heavy rains can cause flooding and prolonged waterlogging, thereby threatening both insect survival and their diapause process [51]. Moreover, intense precipitation and floods can wash away eggs, larvae, and small pests such as aphids, mites, leafhoppers, and whiteflies, which can negatively impact the populations of these arthropods [52]. Therefore, variability in precipitation can have a significant impact on insect populations [43].

The years 2022 and 2024 were the driest years in recent decades. In two of the three years of study, the maximum daily temperatures recorded very high values compared to normal daily temperatures for this area (Table S2 in Supplementary Material). Water deficit associated with temperatures exceeding the multiannual mean was observed throughout the two experimental years (2022 and 2024) [53]. The year 2023 experienced slightly increased precipitation; however, its distribution was irregular, with periods of drought as well as episodes of excessive rainfall (Table S3 in Supplementary Material).

Ecological research on agroforestry systems has primarily focused on the biodiversity benefits provided by trees, which offer food resources such as flowers, fruits, and organic matter—as well as shelter [54]. Our results indicate that the abundance of natural enemies of pests is higher in wheat fields surrounded by agroforestry shelterbelts compared to those without such arrangements. These results are consistent with the observations of other authors, who have demonstrated that linear vegetation elements in the landscape, such as shelterbelts or hedgerows, play an essential role in the conservation and enhancement of the effectiveness of beneficial arthropods [36,54]. Compared to previous studies conducted at the same location [30], our research revealed a significant increase in the abundance of natural predators. For example, in the case of Araneae order, 146 individuals were recorded in 2015 in the insecticide-free variant from shelterbelt agroecosystem, whereas our study documented 995 Araneae individuals in 2023. This notable rise highlights potential changes in the ecosystem dynamics and the effectiveness of current management practices.

In a study conducted by Staton et al. [36] on the population dynamics of aphid pests in wheat crops, it was observed that the parasitism rate of aphids was significantly higher in wheat plots sown between agroforestry shelterbelts, reaching 12.8%, compared to only 7.6% in plots without such protective belts. This difference indicates a higher abundance of natural parasitoids in the near agroforestry shelterbelts, confirming their positive role in supporting beneficial arthropods. Staton’s findings align closely with the results of our study, reinforcing the idea that agroforestry shelterbelts can significantly enhance the effectiveness of biological pest control by providing favorable habitats for natural enemies within cereal-based agroecosystems.

Within the study, the predatory Coleoptera identified belonged to the families Staphylinidae, Coccinellidae, Malachidae, and Cantharidae. The Cantharidae family exhibited the highest abundance during the first two years of observation, whereas in 2024, the Coccinellidae family became numerically dominant. These two families are natural allies in agroecosystems, rapidly reducing aphid colonies [55,56].

Our results confirm the important role of landscape elements, such as agroforestry shelterbelts and semi-natural habitats, in supporting beneficial arthropod communities. In particular, spiders were found in higher densities near these structures, suggesting that they serve as both refuges and sources for colonization of adjacent agricultural fields. Spiders are key natural enemies of agricultural pests, including those affecting cereal crops [57,58]. Similarly to many other beneficial arthropods, their abundance is strongly influenced by the surrounding landscape context [59]. In this regard, recent studies highlight that continuous hedgerows—without major gaps—can support stable communities of predator spiders, contributing to the natural control of aphids and other pests [60]. Our observations are consistent with these findings and support the integration of structural elements into the design of sustainable agroecosystems. In addition to spiders, parasitoid Hymenoptera were found in high number in open-field wheat fields, but not higher than their number recorded in shelterbelt agroecosystem during the study period. These aphid parasitoids play a crucial role in biological control by regulating aphid populations in cereal crops [61,62], Colyria coxator and Aphidius spp. being recognized for their significant contribution to pest control. Their abundance underlines the importance of habitat management practices that promote overwintering sites and floral resources for adult parasitoids throughout the cropping season.

The Simpson and Shannon diversity indices are widely used to evaluate species diversity as they account for both species richness (the number of species) and evenness (the relative abundance of each species). While both indices capture overall diversity, the Simpson index is more strongly influenced by dominant species in a community, whereas the Shannon index gives more weight to rare species and evenness in the distribution of individuals among species [44,63]. In our study, we evaluated the diversity of beneficial arthropods in two agroecosystems, and the results showed that both the Shannon Index and the Simpson Index had higher values in the agroecosystem with shelterbelts compared to the one without. Vizitiu et al. [64], in their study investigating the biodiversity of beneficial arthropods in a vineyard, obtained similar results. They observed consistently higher diversity index values in vineyards adjacent to forests compared to those lacking nearby forested areas, across all three years of the study.

Insecticides are widely recognized as harmful to insects, often causing direct mortality, with non-target species being particularly vulnerable compared to others [65]. The results obtained in our research indicate that the insecticides 200 g/L acetamiprid and tau-fluvalinat 240 g/L caused significant mortality among useful arthropods, with varying effects depending on the timing of application and local conditions. This confirms observations from the scientific literature stating that most broad-spectrum insecticides, especially those based on pyrethroids or neonicotinoids, can negatively affect beneficial arthropods [66,67].

Despite the recognized benefits of agroforestry shelterbelts in supporting natural enemies, our findings—consistent with previous studies [68,69]—indicate that insecticide applications can significantly limit these advantages. In insecticide-treated wheat fields, biological control of pests was found to be considerably weakened, with predator and parasitoid populations recovering slowly after spraying.

5. Conclusions

The comparative analysis of biodiversity indices (Shannon, Simpson, Evenness, and Jaccard) revealed a marginally higher diversity within the shelterbelt agroecosystem. These findings underscore the ecological value of agroforestry shelterbelts, suggesting that such landscape features contribute positively to the conservation of natural enemies and should be considered an integral component of sustainable agricultural systems.

The significant annual fluctuations in the abundance of beneficial arthropods within the same agroecosystem, both in plots treated with insecticides and untreated plots, highlight the dynamic role of environmental factors in the recovery and maintenance of beneficial insect populations. The relationship between temperature and the abundance of natural predators in agroecosystems is highly context-dependent, varying significantly between years and across the two types of agroecosystems. These findings highlight the importance of the long-term monitoring of climatic conditions and agroecosystem structures for the sustainable management of predator populations and the maintenance of biological balance.

These variations emphasize the importance of consistent annual monitoring of natural enemies as a fundamental tool for making informed decisions within integrated pest management (IPM) strategies.