Eco-Friendly Pest Control Methods for German Chamomile (Matricaria chamomilla): Effects on Pest Infestation, Yield, Soil Microbial Activity, and Beneficial Insects

The field experiment was conducted over a six-month duration from March to August 2024 in Kirkuk, Iraq (Latitude: 35°28′ N, Longitude: 44°19′ E; Altitude: 330 m above sea level). The total experimental area was 450 m², arranged in four blocks with 1.0 m alleyways between blocks to minimize spray drift and cross-contamination. The experimental site featured a moderately fertile loam soil. Detailed physicochemical analysis of the topsoil layer (0-30 cm depth) demonstrated a precise particle size distribution of approximately 42% sand, 38% silt, and 20% clay. The chemical properties characterized the environment as having an alkaline soil pH of 7.8, an organic carbon content of 1.15%, and an electrical conductivity (EC) of 1.4 dS/m. The experiment was performed in a field plot where Matricaria chamomilla L. (German chamomile) was cultivated under normal agricultural practices, and synthetic insecticides were not applied throughout the study period. Before they got to transplanting, the team created a moderately fertile loam soil by adding a constant base amount of fully rotted farmyard manure. The study employed a spacing method, where seedlings were set at 30 cm between rows and 20 cm between plants, to obtain the desired field density required for chamomile biomass production and pest control research.

The researchers repeated a randomized complete block experimental design of five treatments four times for a total of 20 experimental units. Each block had one plot for each treatment, and the plots within each block were randomized to reduce spatial bias. Plots were 2.0 m by 1.5 m with 0.5 m between plots and 1.0 m between blocks. The experimental treatments were T1, the untreated control; T2, which used Azadirachta indica neem oil at 5% v/v; T3, which applied Beauveria bassiana at 1 × 10⁸ conidia/mL; T4, which released Chrysoperla carnea at 2 eggs per plant; and T5, which used an IPM package that combined neem oil at 2.5% v/v, B. bassiana at 5 × 10⁷ conidia/mL, and C. carnea at 1 egg per plant. The IPM treatment was designed to exert a reduced-dose complementary control pressure while being compatible with beneficial fauna.

2.1 Pest identification and monitoring

Aphids were identified as Aphis fabae Scopoli based on the presence of paired black cornicles, a short finger-like cauda, and transverse dark abdominal bands visible under a stereo-microscope at 40× magnification. Spider mites were identified as Tetranychus urticae Koch by dense silk webbing on leaf undersides, two dark pigmented spots on the idiosoma, and tarsal claw morphology examined on slide-mounted specimens. Identification was performed by a staff taxonomist at Kirkuk University and cross-checked against reference specimens. Voucher specimens (aphids: UOK-ENT-2024-17; mites: UOK-ENT-2024-18) were preserved in 70% ethanol and deposited in the College entomological collection. For assessment, 10 plants per plot were randomly selected at each weekly interval. On each plant, three leaves (upper, middle, and lower canopy) were inspected. A plant was scored as aphid-infested if it hosted at least one colony of > 5 aphids on any examined leaf, and as mite-infested if it showed active webbing or >10 motile stages per leaf. The infestation rate per plot was calculated as (infested plants / 10 sampled plants) × 100. Aphid and mite rates were recorded separately; the combined pest infestation rate reported in the initial analysis was the arithmetic mean of the two species-specific rates. Assessments began one week after the first treatment and continued weekly for eight weeks.

2.2 Application of treatments

Neem oil was applied as a fine foliar spray using a knapsack sprayer to run off on the scheduled application dates. The cold-pressed Azadirachta indica seed oil contained 0.30% azadirachtin (w/w). For T2, it was applied as a 5% (v/v) aqueous emulsion in 500 L/ha total spray volume (25 L/ha neem oil), with Tween-20 added at 0.1% (v/v) as surfactant. For T5 (IPM), the neem rate was reduced to 2.5% (v/v) (12.5 L/ha). Three neem applications were made at 14-day intervals starting at the 4-leaf stage. B. bassiana conidial suspension (strain UOK-Bb01, >95% viability confirmed by 24-h PDA germination count) was prepared freshly and applied in the evening (18:00-19:30 h) at 1 × 10⁸ conidia/mL in 1000 L/ha water (T3) or 5 × 10⁷ conidia/mL (T5) to reduce UV degradation and desiccation losses. Three fungal applications were made at 14-day intervals, synchronized with neem sprays. Chrysoperla carnea eggs (<24 h old) supplied on cardboard strips were obtained from a commercial agricultural supplier in Kirkuk, Iraq, and were released uniformly over the crop canopy at 08:00 h by stapling egg cards to lower leaf surfaces. The supplier specializes in the production and distribution of biological control agents for IPM programs and provides high-quality beneficial insects for agricultural research and sustainable crop protection practices. T4 received 2 eggs/plant and T5 received 1 egg/plant; three releases were performed at 10-day intervals. Larval establishment was checked 48 h after each release by looking at 10 plants per plot for neonate lacewings; establishment averaged 65-70%. All treatments were reapplied at equivalent phenological stages to maintain temporal consistency. T1 (untreated control) received no spray or release; no water-only carrier control was included, which is acceptable for an ecological comparison of bio-control tactics, but means we cannot partition mechanical spray effects from active-agent effects.

2.3 Data collection

The researchers recorded pest infestation rate (%) by counting infested plants per plot at each observation interval and then calculating percentage values before averaging across all assessment dates. The researchers measured plant yield at harvest by recording dry aerial biomass (g/plant) after oven-drying until the weight became constant. The researchers measured soil microbial activity through microbial respiration (mg CO₂/g soil/day), which they assessed using a standard CO₂ evolution method described by Anderson and Domsch [20] on composite soil samples taken from the rhizosphere after the main treatment period. Sampling occurred two days after the final treatment application. Five rhizosphere soil cores (0–15 cm depth) were collected per plot and composited; visible roots were shaken off, and the adhering soil was passed through a 2-mm sieve. A 50 g subsample was incubated at 25 ℃ for 24 h in a sealed jar with 10 mL of 0.5 M NaOH to trap evolved CO₂. Microbial respiration was expressed as mg CO₂ evolved per g dry soil per day according to the standard soil respiration procedure [20].

2.4 Beneficial insect assessment

Beneficial insects were recorded by direct visual observation and sweep-net sampling during the morning activity window (08:00-10:00 h) on the same days as pest assessments. Observed taxa were grouped as: (1) pollinators — Apis mellifera and non-Apis bees (Halictidae, Megachilidae); (2) predators — Coccinellidae (adults and larvae), Syrphidae, Chrysopidae, Orius spp., and Araneae; and (3) parasitoids — Braconidae (aphid mummies) and Chalcidoidea. On each sampling date, a 3-minute stationary visual count was conducted on each of the 10 plants per plot (30 min/plot), followed by 20 sweep-net strokes per plot with a 38-cm-diameter net. The total beneficial insect count per plot represents the mean of the eight weekly samplings. No killing traps were used during sampling, and all beneficial insects were recorded using non-destructive visual observation and sweep-net techniques.

2.5 Statistical analysis

The researchers analyzed all data through one-way analysis of variance (ANOVA) in SPSS version 28 to evaluate treatment effects on each response variable. Means were separated using Tukey’s Honestly Significant Difference (HSD) test at p ≤ 0.05. The report presents treatment differences as mean values with standard deviation, while Tukey groupings are displayed using lowercase letters.

The study demonstrated that the IPM package outperformed single-tactic treatments because it integrated three complementary mechanisms: neem oil for rapid antifeedant and growth-disrupting effects, Beauveria bassiana for delayed pathogenic suppression, and Chrysoperla carnea for direct predation of soft-bodied pests. This finding is consistent with sustainability-oriented IPM approaches that emphasize the advantages of combining multiple compatible control tactics to achieve more durable and environmentally sound pest management outcomes [19]. Furthermore, recent studies have highlighted the importance of reducing pesticide inputs and preserving soil microbial functions and agroecosystem biodiversity through biologically based pest management strategies [22-26]. To our knowledge, this is the first field study conducted on German chamomile cultivation in Iraq that integrates botanical, fungal, and predator-based tactics within a single IPM package against Aphis fabae and Tetranychus urticae [27-30].

The decrease in infestation, together with the resulting yield increase, supports recent studies that demonstrate that pesticide residues persist as a primary quality problem in medicinal crops, thus proving the benefits of pest management systems that do not leave harmful pesticide traces [18]. The control treatment showed high pest pressure, which decreased both photosynthetic area and assimilate allocation, thus leading to low biomass production. The IPM plots maintained their canopy structure until it became necessary for faster dry matter growth to take place. The control showed no effects from neem, but the neem treatment failed to reach the performance level of the fungal and predator-based methods, which operate at field strength, because botanical treatments deliver quick results that fail to maintain effectiveness during field conditions. The review by Adusei and Azupio [1] and the azadirachtin synthesis study by Kilani-Morakchi et al. [7] demonstrate that neem has minimal environmental impact and serves as an effective insecticide solution, but not as a complete treatment method. The current findings show that neem causes a significant decrease in pest numbers, but B. bassiana and C. carnea treatments produce better results because the botanical component functions more effectively when used with IPM systems, according to studies [1-7]. The use of B. bassiana reduced pest numbers more effectively than neem while increasing both crop yield and soil microorganism activity, according to recent studies, which show that submerged spores at 1 × 10⁸ SS/mL cause high mortality rates in Tetranychus urticae and B. bassiana metabolites create strong larvicidal effects on various insect pests, according to Basso et al. [4] and Vivekanandhan et al. [15]. Recent reviews demonstrate that entomopathogenic fungi function as insecticides, which also offer biocontrol solutions to plants through endophytic benefits under specific environmental and product application conditions, according to the findings of studies [9, 16, 17]. The fungal and IPM treatments produced higher soil microbial activity because their lower chemical input requirements created less environmental damage while chemical applications protected vital plant root activities [22, 31-38].

The yield response — a 133% increase in the IPM package over the untreated control — is large but not unprecedented for heavily infested medicinal herbs. In the control, combined aphid and mite incidence exceeded 68%, which would have led to severe reductions in photosynthetic leaf area and assimilate allocation. Extension records from local unprotected chamomile in northern Iraq show yield losses of 60-70% under similar pest pressure, suggesting our control performance is representative of the growers’ experience. Tight standard deviations reflect consistent loam texture and transplant establishment and are not artificially constrained variability.

The IPM treatment showed 31.75 mg CO₂ g⁻¹ day⁻¹ in soil microbial respiration, which was 74% higher than the control. While this appears to be large, similar increases (50-100%) have been reported in Mediterranean herbaceous systems following cessation of broad-spectrum pesticides [21]. This likely acts by decreasing chemical disruption of the rhizosphere microbiome and increasing root exudation from healthier plants, providing more labile carbon substrates for microbial turnover. However, we recognize that respiration is a single proxy of soil biological activity, and we did not measure microbial diversity, enzyme activities, or nutrient cycling rates; thus, our inference about “soil health” is necessarily narrow [23-26, 39, 40].

The present experiment demonstrated that the best performance level of Chrysoperla carnea was due to its classification as a generalist predator working efficiently in situations of biological control. Recent field and greenhouse studies demonstrate that C. carnea larvae can effectively control greenhouse pests through their predation abilities while they function as conservation biological control agents under suitable habitat and attractant conditions [5-14, 41-43].

The lacewing and IPM plots showed higher numbers of beneficial insects, demonstrating that the management program maintained compatibility with non-target species. Coccinellidae and Syrphidae were the dominant free-living predators, while Apis mellifera accounted for the majority of pollinator observations. The absence of broad-spectrum insecticides likely allowed these populations to persist, in contrast to the pest-dominated control canopies where heavy honeydew and mite webbing created unsuitable microclimates for natural enemies.

The recent studies about pollinators have established that pesticides cause decreases in population numbers and changes in behavior and survival rates for non-Apis bees and other beneficial insects [3, 11]. The present results show that biologically based pest control successfully transformed pest-dominated conditions into resilient agroecosystems. The IPM package demonstrated ecological validity, establishing an inverse relationship where reduced pest pressure directly maximized yield, microbial activity, and beneficial insect biodiversity. The treatment protects underground processes and aboveground biocontrol services while controlling pest populations. The integrated result of current soil-health research shows that pesticide pressure disrupts microbial activities that protect plants, but selective management methods maintain ecological functions. The research results demonstrate that an IPM package that combines botanical elements with microbial biopesticides and augmentative predators provides a strong solution for producing medicinal plants [31-33, 44].

The research shows that an ecologically designed pest control system can effectively reduce aphid and spider mite populations while enhancing German chamomile agricultural results. The IPM package reduced combined pest infestation by 82% compared to the control (from 68.8% to 12.5%) and increased dry aerial yield by 133% (from 18.10 to 42.12 g/plant). It also produced the greatest soil microbial activity (31.75 mg CO₂/g soil/day) and the largest population of beneficial insects (19.75 individuals per plot) across all four measured variables. The combined use of neem oil, Beauveria bassiana, and Chrysoperla carnea produced better results than each individual component because their different modes of action created effective synergies in actual farming environments. For growers, this suggests that biologically based management can achieve marketable yields while avoiding the negative effects typically associated with broad-spectrum chemical insecticides — particularly important for medicinal crops where residue limits are strict, and consumer acceptance depends on clean raw material. However, we did not directly measure residues, essential oil quality, or production costs, so the practical recommendation is framed as a promising implication rather than a proven economic certainty. The evidence is especially relevant in regions where crop quality assessment considers yield, phytochemical purity, biodiversity preservation, and supply-chain safety together.