Introduction
The black scale Saissetia oleae (Olivier) (Hemiptera: Coccidae) is a widely distributed pest that primarily affects olive (Olea europaea L.) and citrus (Citrus spp.) trees (Tena et al., Reference Tena, Soto, Vercher and Garcia-Marí2007). This pest is known to infest around 150 plant species, including trees, shrubs, and herbaceous plants from diverse families, such as Nerium oleander L., Amygdalus spp., and Solanum spp. (Tzanakakis, Reference Tzanakakis2003; Abd-Rabou, Reference Abd-Rabou2005; Ouguas and Chemseddine, Reference Ouguas and Chemseddine2011). Thought to have originated in South Africa (De Lotto, Reference De Lotto1976), S. oleae has expanded its range to tropical and some subtropical regions (Hill, Reference Hill1983). In Morocco, it was first reported on olive trees in the 1970s (Elhormiti and Laraichi, Reference Elhormiti and Laraichi1979). During the period from 1971 to 1974, severe infestations in the provinces of Kenitra, Fez, Meknes, and Marrakech caused an estimated loss of over 118,000 tons of olives (Elhormiti and Laraichi, Reference Elhormiti and Laraichi1979). The pest feeds by sucking sap from the phloem, which may lead to deformation or abscission of young shoots, especially under high infestation levels (Tzanakakis, Reference Tzanakakis2003). However, the most damaging effect results from the honeydew excreted by the insects, which promotes the growth of sooty mold fungi (Ben-Dov and Hodgson, Reference Ben-Dov, Hodgson, Ben-Dov and Hodgson1997). This black fungal coating reduces photosynthesis and respiration, ultimately causing early leaf and fruit drop, branch dieback, and substantial yield losses (Tena et al., Reference Tena, Soto, Vercher and Garcia-Marí2007).
After hatching, S. oleae crawlers (first instars) move to preferred feeding sites, mainly on leaves, and the immature stages undergo three molts, with the later instars feeding on small twigs (Tena et al., Reference Tena, Soto, Vercher and Garcia-Marí2007). Adult females reproduce parthenogenetically, laying between a few hundred and 2,500 eggs depending on host conditions and temperature (Morillo, Reference Morillo1977; El Aalaoui et al., Reference El Aalaoui, Mokrini and Sbaghi2025). This high fecundity can trigger pest outbreaks under favourable environmental conditions (Tena et al., Reference Tena, Soto, Vercher and Garcia-Marí2007). The number of generations per year varies geographically, typically one to two, influenced by climate, host species, and agricultural practices (Blumberg et al., Reference Blumberg, Swirski and Greenberg1975). In Morocco, coastal humid zones may support two generations, while arid interior regions usually have one (El Aalaoui et al., Reference El Aalaoui, Mokrini and Sbaghi2025). Similar patterns occur in California, where irrigation and canopy density can promote partial second generations (Daane and Caltagirone, Reference Daane and Caltagirone1989). In eastern Spain, the life cycle remains unclear, with one to two generations and crawler emergence reported in March, June, and October (Panis, Reference Panis1977; Noguera et al., Reference Noguera, Verdú, Gómez-Cadenas and Jaques2003). Temperature extremes in winter and summer cause high mortality, leading to population fluctuations and variable economic damage (Fernández et al., Reference Fernández, Mendivil and Almagro1979; Katsoyannos, Reference Katsoyannos1996). Effective management of S. oleae requires understanding its seasonal biology. Insecticides are most effective against the first and second instars (Flint et al., Reference Flint, Kobbe, Clark, Dreistadt, Pehrson, Flaherty, O’Connell, Phillips and Morse1991), and predator or parasitoid releases should target these stages (Schweizer et al., Reference Schweizer, Morse and Luck2003). Therefore, integrated pest management (IPM) that combines biological, cultural, and chemical methods is essential (Shoemaker et al., Reference Shoemaker, Huffaker and Kennett1979; Flint et al., Reference Flint, Kobbe, Clark, Dreistadt, Pehrson, Flaherty, O’Connell, Phillips and Morse1991).
Currently, control mainly depends on insecticides such as chlorfenvinphos, carbaryl, methidathion, and azinphos-methyl (Alonso, Reference Alonso2001). However, their continued use poses environmental and health risks (Boller et al., Reference Boller, Avilla, Joerg, Malavolta, Wijnands and Esbjerg2004) and may lead to pest resistance (Hawkes et al., Reference Hawkes, Janes, Hemingway and Vontas2005). Reducing reliance on these chemicals by enhancing the role of natural enemies is therefore necessary. Native or exotic biological control agents of S. oleae include spiders, hemipteroids, chrysopids, coccinellids, and ants that together can greatly reduce the reproductive potential of the S. oleae population by more than 80% (Santos et al., Reference Santos, Pereira, da Conceição Rodrigues, Torres, Pereira and Nogueira2009; Delrio and Foxi, Reference Delrio and Foxi2010). Two exotic parasitoids of S. oleae, Metaphycus bartletti Annecke & Mynhardt and Metaphycus helvolus (Compere) (Hymenoptera: Encyrtidae), were introduced from France to Cyprus in the early 1990s; after limited releases, both established successfully, reducing S. oleae populations from outbreak levels to near absence (Orphanides, Reference Orphanides1993). In Egypt, augmentative releases of native parasitoids – including M. bartletti, M. helvolus, Coccophagus lycimnia (Walker), and Scutellista caerulea (Fonscolombe) – were effective, with M. bartletti reaching parasitism rates up to 83% (Abd-Rabou, Reference Abd-Rabou2001). The success of Metaphycus spp. has also been reported in France, Turkey, and California, helping reduce pesticide use (Panis, Reference Panis1977; Delrio and Foxi, Reference Delrio and Foxi2010). In Brazil, 16 parasitoids were recorded on olive trees, with Coccophagus caridei (Brèthes) (Hymenoptera: Aphelinidae), Diversinervus elegans Silvestri (Hymenoptera: Encyrtidae), and Mesopeltita truncatipennis (Waterston) (Hymenoptera: Pteromalidae) being the most common; parasitism ranged from 3% to 31%, peaking in summer and autumn (Prado et al., Reference Prado, Alvarenga and Santa-Cecília2015). Predators such as Chilocorus bipustulatus (L.), Exochomus nigromaculatus (Goeze), Pharoscymnus pharoides Marseul, Propylea quatuordecimpunctata (L.), Pullus sp., and Scymnus sp. (Coleoptera: Coccinellidae) also contribute to S. oleae control (Abd-Rabou, Reference Abd-Rabou2001; Moghaddam et al., Reference Moghaddam, Abdollahipour and Fathipour2021).
In Morocco, despite the presence of natural enemies of black scale, chemical control remains the primary method. Therefore, a more proactive and augmentative biological control strategy is needed, starting with identifying additional natural enemies of S. oleae. One unexplored option is using predatory phytoseiid mites (Acari: Phytoseiidae), which are effective biological control agents against many pests. These mites feed on mites, thrips, whiteflies, and armored scales, as well as pollen and honeydew (Juan-Blasco et al., Reference Juan-Blasco, Qureshi, Urbaneja and Stansly2012; Xiao et al., Reference Xiao, Avery, Chen, McKenzie and Osborne2012; Barghout et al., Reference Barghout, Ibrahim and El Saiedy2022). Twenty-nine species from 15 genera of phytoseiid mites have been recorded in Morocco, but only a few have been studied in detail (Zahidi et al., Reference Zahidi, Akchour, Kreiter, Tixier, Msanda and El Mousadik2023). To date, there are no reports of native or exotic phytoseiid mites attacking the eggs, nymphs, or adults of S. oleae on Moroccan olives.
Amblyseius swirskii Athias-Henriot (Acari: Phytoseiidae) is an effective biological control agent against several pests, including Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae), and Phenacoccus solenopsis Tinsley (Hemiptera: Pseudococcidae) and phytophagous mites such as Polyphagotarsonemus latus (Banks) (Acari: Tarsonemidae) and Tetranychus urticae Koch (Acari: Tetranychidae) (Stansly and Castillo, Reference Stansly and Castillo2010; Calvo et al., Reference Calvo, Bolckmans and Belda2011; Barghout et al., Reference Barghout, Ibrahim and El Saiedy2022). The eggs and nymphs (crawlers) of S. oleae are similar in size and shape to those of B. tabaci. Saissetia oleae lays eggs on newly developing unfolded leaves, where eggs and crawlers are protected from larger predators but remain accessible to predatory mites. Due to this, and because A. swirskii is commercially available and proven effective against whiteflies and other pests in greenhouses and open fields (Stansly and Castillo, Reference Stansly and Castillo2010; Calvo et al., Reference Calvo, Bolckmans and Belda2011), it was selected for preliminary testing against S. oleae. Our objectives were to investigate whether A. swirskii mites prey on the eggs and nymphs of S. oleae, and to evaluate if these mites can reduce S. oleae populations on olive plants under controlled conditions.
Materials and methods
Insects rearing
The S. oleae individuals used in this study originated from a laboratory colony maintained at the insectarium of the National Institute of Agricultural Research (INRA) in Zemamra, Morocco (latitude 32°37′48″ N, longitude 8°42′0″ W, elevation 165 m). This colony was initially established using infested olive branches, stems, and leaves collected from local olive orchards in the Zemamra zone. These infested materials were introduced into entomological cages (80 × 80 × 80 cm), constructed with a wooden frame and mesh-covered sides for airflow, and containing healthy, uninfested olive plants of the ‘Picholine marocaine’ variety. The scale insects were allowed to colonize and propagate on these plants. The colony was maintained under controlled environmental conditions in a growth chamber set at 25 ± 1 °C, with a relative humidity of 60 ± 10% and a photoperiod of 16 h light and 8 h dark. To ensure uniformity in age and to enhance colony growth, newly emerged first-instar nymphs (less than 24 h old) were isolated in a separate cage under the same conditions to complete their development. Prior to the experimental phase, the colony was reared on olive plants for at least one complete generation. Specimens of S. oleae from this colony were identified based on the keys provided by De Lotto (Reference De Lotto1965, Reference De Lotto1971), and voucher samples were preserved in the INRA insectarium. Individuals at different developmental stages used in the experiments were collected from this colony.
The predatory mite A. swirskii used in this research was sourced from a laboratory colony maintained at the INRA insectarium in Zemamra, Morocco. This colony was established and continuously reared on a combination of developmental stages of Dactylopius opuntiae (Cockerell) (Hemiptera: Dactylopiidae) on Opuntia ficus-indica (L.) Mill., and various life stages of P. solenopsis infesting branches, stems, and leaves of purslane (Portulaca oleracea L., Portulacaceae). Rearing took place in plastic containers (11 × 7 × 3 cm) (El Aalaoui et al., Reference El Aalaoui, Mokrini and Sbaghi2025). The mites were kept in a controlled environment with a temperature of 25 ± 2 °C, relative humidity of 65 ± 5%, and a 16-h light to 8-h dark photoperiod. Before beginning the experiments, the colony had been maintained for at least 10 successive generations on the same mixed-prey diet. Voucher specimens, identified and validated by the research team, are preserved at the INRA insectarium in Zemamra (El Aalaoui et al., Reference El Aalaoui, Mokrini and Sbaghi2025). The experimental work was carried out between December 2024 and March 2025.
Laboratory trials
Two separate laboratory trials were conducted to evaluate the predatory potential of A. swirskii on different developmental stages of S. oleae. Trial 1 focused on egg predation, while Trial 2 assessed predation on early nymphal stages. Early stages – eggs and young nymphs – were selected because they are less mobile and more vulnerable to predation, making them critical targets for effective biological control. The trials were carried out in transparent plastic containers (11 × 7 × 3 cm) with tightly fitting lids. Each lid was cut in the centre to form a rectangular opening (7 × 5 cm), which was then covered with fine mesh to allow proper ventilation. Inside each container, a moistened filter paper was placed to maintain appropriate humidity, along with one fresh, untreated olive leaf (O. europaea, ‘Picholine marocaine’ variety) laid on a thin layer of agar to prevent desiccation. In each experimental unit, one A. swirskii adult (10 days old) starved for 24 h prior to the trial (Xu and Enkegaard, Reference Xu and Enkegaard2010) was introduced. For Trial 1, 30 S. oleae eggs were gently transferred onto the olive leaves using a fine camel-hair brush (No. 000, CAMLIN, USA). In Trial 2, 30 nymphs (10 days old) were used as prey. Each Trial was replicated seven times using a predator sex ratio of 1:1. Control groups included: (i) units without predators to assess natural mortality (n = 7) and (ii) units containing 30 A. swirskii adults per container, maintained without any prey or food source (n = 7), to assess predator survival in the absence of food. The containers were placed in a climate-controlled chamber set at 25 ± 1 °C, with 60 ± 10% relative humidity and a 16:8 h light:dark photoperiod. Survival of A. swirskii adults, the number of consumed S. oleae eggs, and eventually nymphs were recorded using a stereoscopic microscope at 1, 2, 4, and 6 days without replacing prey. Saissetia oleae eggs and nymphs were considered dead when observed to be empty or desiccated. All A. swirskii and S. oleae individuals used in these trials originated from laboratory colonies maintained at the INRA insectarium in Zemamra. All experiments were independently replicated trice at different time intervals.
Screehouse trials
The ability of A. swirskii to reduce populations of S. oleae on isolated olive plants (Picholine Marocaine variety) was evaluated in a climate-controlled screenhouse (11 m × 7 m × 3 m), maintained at 25 ± 4 °C and 60 ± 10% relative humidity under natural light. Olive plants were grown in plastic pots (33 cm diameter, 12 cm height) filled with a substrate mixture of fine sand (2/3) and peat (1/3), and maintained in the same screenhouse for 3 months prior to the experiment. Plants were artificially infested with S. oleae eggs obtained from a laboratory colony. Each plant received 200 eggs, counted using a magnifying headset (2.75× magnification). After infestation, each plant was enclosed in a ventilated cylindrical cage made of sealed transparent plastic film, with the top covered by coarse mesh organdy fabric to allow airflow. Twenty days after infestation, most S. oleae eggs had hatched (typically 10–14 days at 25 ± 4 °C; De Lotto, Reference De Lotto1965; El Aalaoui et al., Reference El Aalaoui, Mokrini and Sbaghi2025), and early nymphs were present on the plants. At this time, A. swirskii mites (10 days old, with a 1:1 sex ratio) were introduced at four different predator–prey ratios, forming the experimental treatments: T0 (Control): S. oleae eggs only, without predatory mites, T1: 1:2 ratio (A. swirskii: S. oleae eggs) – 100 adults per plant, T2: 1:4 ratio – 50 adults per plant, T3: 1:8 ratio – 25 adults per plant, and T4: 1:10 ratio – 20 adults per plant. Mites were released using 30 mL plastic cups attached to the plant branches with wire. Each treatment was replicated 20 times in a completely randomised block design. The number of S. oleae individuals (eggs and motile stages: nymphs and adults) was recorded 21 days after infestation, prior to the application of treatments, to determine the initial pest density across all treatments using a hand lens. Subsequently, the numbers of A. swirskii eggs and motile stages (larvae, nymphs, and adults), as well as S. oleae eggs and motile stages, were recorded weekly for 10 consecutive weeks starting 1 week after the release of predatory mites, using a magnifying headset (2.75×). Predation was assessed on the combined male and female adults, and differences in prey consumption between sexes were not analysed. Phytoseiid larvae were distinguished from nymphs and adults by having three pairs of legs, whereas nymphs and adults had four pairs. Nymphs progressed through protonymph and deutonymph stages before reaching adulthood, leaving behind identifiable exuviae (Abad-Moyano et al., Reference Abad-Moyano, Pina, Dembilio, Ferragut and Urbaneja2009). These exuviae were removed after each observation to avoid double-counting. Regular irrigation was provided throughout the experiment to ensure optimal plant growth under screenhouse conditions. The entire trial was replicated twice at different time intervals. Additionally, plant damage caused by S. oleae was evaluated weekly after treatment using a visual scale from 0 to 10, where 0 = dead plant, 5 = moderate quality with acceptable color and form and minimal chlorosis or necrosis, and 10 = excellent condition with healthy appearance, ideal color, and form. This scale, originally described by Gettys et al. (Reference Gettys, Thayer and Sigmon2021) for assessing visual plant quality, was adapted for use on olive plants. Similar methods have been applied to assess the effects of herbicides, salt stress, and other stressors on plant health (Gettys and Haller, Reference Gettys and Haller2012; Tootoonchi et al., Reference Tootoonchi, Gettys, Thayer, Markovich, Sigmon and Sadeghibaniani2020).
Statistical analysis
All statistical analyses were conducted using R software version 4.3.2. Before analysis, data were checked for normality using the Shapiro–Wilk test and for equal variances using Levene’s test. When assumptions were not met, data were log-transformed to meet these requirements. For the laboratory trials, the effect of time (0, 2, 4, and 6 days) on S. oleae (eggs and nymphs) mortality, as well as on the survival of both prey (S. oleae eggs and nymphs) and predatory mites in control groups, was analysed using one-way repeated measures ANOVA. Post hoc comparisons were conducted with the Student–Newman–Keuls (SNK) test. At each time point, independent t-tests were used to compare the number of prey consumed (eggs vs. nymphs) and the effect of predator presence (predator vs. control). For the screenhouse trials, two one-way ANOVAs were used to compare the densities of S. oleae and the predatory mite A. swirskii among treatments. Post hoc comparisons for significant differences were performed using the SNK multiple range test. Plant visual quality, scored on a scale from 0 to 10, was also analysed using one-way ANOVA followed by SNK post hoc tests to evaluate treatment effects on plant health. Results are presented as means ± standard error (SE) and visualised to clearly show the impact of treatments on pest and predator densities as well as plant visual quality. Significance was set at α = 0.05 for all analyses.
Results
Laboratory experiment
Under controlled laboratory conditions, the predatory mite A. swirskii exhibited significant predation on both eggs and first-instar nymphs of S. oleae compared to the control over the 6-day observation period (Table 1).
Mean (±SE) number of dead Saissetia oleae eggs and nymphs per day following release of the predatory mite Amblyseius swirskii under laboratory conditions (25 ± 1 °C, 60 ± 10% RH, and 16:8 h L:D photoperiod)
Within each column, means followed by the same uppercase letter are not significantly different among sampling days (Student–Newman–Keuls test, P < 0.05). Within each row, means followed by the same lowercase letter are not significantly different between prey stages (S. oleae eggs vs. nymphs) according to the t-test (α = 0.05). An asterisk
* indicates a significant difference between predator and control treatments (t-test, α = 0.05).
The number of dead eggs and nymphs in the presence of A. swirskii increased significantly over time (at day 6: eggs: F₃,₈₀ = 329.9, P < 2.0 × 10−16; nymphs: F₃,₈₀ = 198.6, P < 2.0 × 10−16). By the end of the experiment (day 6), mortality had reached 17.9 eggs and 10.2 nymphs, compared to only 1.5 and 2.0, respectively, in the control. The number of dead eggs was significantly higher than nymph mortality at each time point (at day 6: t = 14.37, df = 31.6, P = 2.1 × 10−15), suggesting that A. swirskii exhibited a higher predation rate on S. oleae eggs than on the mobile nymphs. In the control treatment without A. swirskii, the mean numbers of dead S. oleae eggs and nymphs remained significantly lower compared to the predator treatments throughout the experiment (eggs: t = 33.494, df = 23.985, P < 2.2 × 10−16; nymphs: t = 26.631, df = 33.14, P < 2.2 × 10−16), indicating that natural mortality or other factors in the control had minimal impact on prey death. This significant difference confirms that the higher mortality observed in predator treatments is due to the predation activity of A. swirskii. In the predator survival control (no prey provided), A. swirskii survived up to 6 days, with the number of dead predators gradually increasing from 0.0 on day 1 to 1.3 individuals by day 6, suggesting that S. oleae may support predator survival.
Screehouse experiment
Efficacy of A. swirskii on S. oleae eggs
Before the application of A. swirskii, no significant differences in egg numbers were observed among the treatments (F₄,₉₅ = 0.016, P = 0.999), with mean egg numbers averaging approximately 139.5 across all groups (fig. 1A). From week 1 onwards, all treated groups showed significant reductions in egg numbers compared to the control (T0), with the greatest reductions consistently recorded in the T1 (1:2 ratio of A. swirskii to S. oleae eggs) treatment. At week 1, significant differences among treatments were detected (F₄,₉₅ = 2537.0, P < 2 × 10−16), where the control maintained the highest egg number (128.9 eggs), while T1 (1:2 ratio of A. swirskii to S. oleae eggs) exhibited the lowest (10.0 eggs). This pattern persisted in subsequent weeks, with treatments T2 (1:4 ratio), T3 (1:8 ratio), and T4 (1:10 ratio) also showing significantly lower egg numbers than the control throughout the observation period. During weeks 6, 7, and 8, egg numbers remained significantly lower in treated groups compared to the control; however, no significant difference was observed between T1 and T2 (week 6: F₄,₉₅ = 1483.0, P < 2 × 10−16; week 7: F₄,₉₅ = 1076.0, P < 2 × 10−16; week 8: F₄,₉₅ = 1043.0, P < 2 × 10−16). Similarly, treatments T3 and T4 maintained intermediate reductions. In weeks 9 and 10, egg numbers among the predatory mite treatments were higher and did not differ significantly from each other, while the control consistently showed the highest egg numbers (week 9: F₄,₉₅ = 746.8, P < 2 × 10−16; week 10: F₄,₉₅ = 437.2, P < 2 × 10−16). Across the entire 10-week observation period, exposure time had a significant effect on egg numbers (P < 0.001), with egg numbers steadily decreasing in the treated groups and gradually declining in the control group from 139.5 eggs at baseline to 30.0 eggs at week 10. Treatment T1 reached zero egg number by week 9, confirming its high efficacy; T2 and T3 reached zero egg counts by week 10. In T4, the egg number steadily decreased from 44.8 prior to treatment application to one egg at week 10. These results demonstrate that all treatments significantly suppressed egg production compared to the untreated control, with T1 showing the most consistent and profound reductions throughout the study period.
Mean (±SE) densities of Saissetia oleae eggs (A) and motile stages (B) per olive plant (Picholine marocaine variety) under screenhouse conditions (25 ± 4 °C, 60 ± 10% RH, natural light), following exposure or non-exposure to adult Amblyseius swirskii (10 days old, 1:1 sex ratio). Treatments were T0 (Control): S. oleae eggs only, without predatory mites, T1: 1:2 ratio (A. swirskii: S. oleae eggs) – 100 adults per plant, T2: 1:4 ratio – 50 adults per plant, T3: 1:8 ratio – 25 adults per plant, and T4: 1:10 ratio – 20 adults per plant. Different letters above the bars indicate statistically significant differences between treatments within the same sampling week (Student–Newman–Keuls test, α = 0.05).
Efficacy of A. swirskii on S. oleae motile stages
Before treatment application, there were no significant differences in the number of motile stages among the treatments (F₄,₉₅ = 0.072, P = 0.99), with mean numbers ranging from 60.4 to 61.3 across all groups (fig. 1B), confirming similar baseline infestation levels. From week 1 onwards, significant differences among treatments were observed (week 1: F₄,₉₅ = 42.1, P < 2 × 10−16). The untreated control (T0) and T4 (1:10 ratio) had the highest numbers of motile stages (∼85.0), while treatments T1 (1:2 ratio) and T2 (1:4 ratio) showed the greatest reductions (∼70.0), and T3 (1:8 ratio) showed an intermediate reduction (74.2). This trend of significant suppression in the treated groups compared to the control continued throughout the 10-week period. During weeks 5, 6, 7, and 8, T1 consistently showed the lowest numbers of motile stages, significantly lower than T2 and T3, which were not significantly different from each other but both were lower than T4 (week 5: F₄,₉₅ = 444.9, P < 2 × 10−16; week 6: F₄,₉₅ = 1002.0, P < 2 × 10−16; week 7: F₄,₉₅ = 1437.0, P < 2 × 10−16, week 8: F₄,₉₅ = 2509.0, P < 2 × 10−16). In weeks 9 and 10, no significant differences were recorded among T1, T2, and T3, all of which had significantly lower numbers than T4. All treatments significantly reduced the number of motile stages compared to the control, which maintained high levels throughout (week 9: F₄,₉₅ = 3278.0, P < 2 × 10−16; week 10: F₄,₉₅ = 3709.0, P < 2 × 10−16). Within each treatment, the number of motile stages declined significantly over time, confirming the progressive efficacy of A. swirskii. In the control group (T0), numbers remained high and stable, increasing from 60.5 before treatment to 110.0 by week 10. In contrast, T1 numbers dropped sharply from 61.3 to 6.0 by week 10. Similar patterns were observed in T2, T3, and T4, with reductions from 70.0, 74.2, and 84.8 before treatment to 8.0, 8.0, and 12.0, respectively, by week 10.
Egg numbers of A. swirskii
From week 1 onwards, the number of A. swirskii eggs per plant increased significantly across treatments (fig. 2A). The highest oviposition rate was consistently recorded in T1 (1:2 ratio; 100 predators per plant), followed by T2 (1:4 ratio; 50 predators per plant), T3 (1:8 ratio; 25 predators per plant), and T4 (1:10 ratio; 20 predators per plant), reflecting density-dependent reproduction (week 1: F₃,₇₆ = 1079.0, P < 2 × 10−16). This pattern remained consistent throughout the trial. During weeks 2–5, oviposition continued to be significantly influenced by treatment, with T1 maintaining a high reproductive rate, while T3 and T4 exhibited marked reductions in egg numbers (week 2: F₃,₇₆ = 962.9, P < 2 × 10−16; week 3: F₃,₇₆ = 991.1, P < 2 × 10−16; week 4: F₃,₇₆ = 825.7, P < 2 × 10−16; week 5: F₃,₇₆ = 1006.0, P < 2 × 10−16). Between weeks 6 and 9, although overall egg numbers declined – likely due to reduced prey availability or age-related declines in fecundity – the relative pattern across treatments remained unchanged (week 6: F₃,₇₆ = 997.9, P < 2 × 10−16; week 7: F₃,₇₆ = 756.8, P < 2 × 10−16; week 8: F₃,₇₆ = 687.9, P < 2 × 10−16; week 9: F₃,₇₆ = 607.0, P < 2 × 10−16). At week 10, T1 still had the highest mean egg number (9.95 eggs/plant), while T4 recorded the lowest (1.50 eggs/plant) (week 10: F₃,₇₆ = 591.4, P < 2 × 10−16). Within each treatment, egg counts declined significantly over time, confirming the progressive effect of the predators: in T1, counts dropped from 119.5 to 10.0; in T2, from 83.8 to 8.0; in T3, from 59.8 to 4.5; and in T4, from 30.0 to 1.5 eggs between weeks 1 and 10.
Mean (±SE) densities of Amblyseius swirskii eggs (A) and motile stages (B) per olive plant (Picholine marocaine variety) following the release of 10-day-old adults (1:1 sex ratio) to control Saissetia oleae, under screenhouse conditions (25 ± 4 °C and 60 ± 10% relative humidity under natural light). Treatments were T1: 1:2 ratio (A. swirskii: S. oleae eggs) – 100 adults per plant, T2: 1:4 ratio – 50 adults per plant, T3: 1:8 ratio – 25 adults per plant, and T4: 1:10 ratio – 20 adults per plant. Different letters above the bars indicate statistically significant differences between treatments within the same sampling week (Student–Newman–Keuls test, α = 0.05).
Weekly dynamics of A. swirskii motile stages
The results show highly significant differences in the number of motile stages of A. swirskii among treatments at each week after release (fig. 2B). In week 1, treatment T1 had the highest number of motile stages (approximately 169), followed by T2 and T3, while T4 had the lowest number (around 40.0) (week 1: F₃,₇₆ = 1282.0, P < 2 × 10−16). This pattern remained consistent through week 10, with all treatments showing a decline in numbers of motile stages over time, but T1 consistently maintaining the highest numbers and T4 the lowest (week 2: F₃,₇₆ = 1236.0, P < 2 × 10−16; week 3: F₃,₇₆ = 1172.0, P < 2 × 10−16; week 4: F₃,₇₆ = 1060.0, P < 2 × 10−16; week 5: F₃,₇₆ = 1268.0, P < 2 × 10−16; week 6: F₃,₇₆ = 1645.0, P < 2 × 10−16; week 7: F₃,₇₆ = 2222.0, P < 2 × 10−16; week 8: F₃,₇₆ = 2139.0, P < 2 × 10−16; week 9: F₃,₇₆ = 2566.0, P < 2 × 10−16; week 10: F₃,₇₆ = 2403.0, P < 2 × 10−16). Within each treatment, significant differences were also observed across weeks, with a clear and progressive decrease in the number of motile stages from week 1 to week 10. In T1, numbers decreased from about 169.0 to 40.0 motile stages over this period, with significant differences between each week. In T2, numbers dropped from 100.0 to 10.0; in T3, from 74.5 to 7.0; and in T4, from 40.0 to 2.5 motile stages between weeks 1 and 10.
Effect of treatments on treated plant visual quality
The results reveal highly significant differences in plant visual quality among treatments at each week following treatment application (fig. 3). Before treatment, the untreated control showed the highest visual quality score (6.0), significantly greater than all other treatments, which had similar scores around 5.5 to 5.5 (F₄,₉₅ = 5.05, P = 9.9 × 10−4). From week 1 onward, treatment T1 consistently maintained the highest plant visual quality scores, starting at 6.0 and progressively increasing to 9.9 by week 10. In contrast, T0 (untreated control) showed the lowest scores throughout, declining from 4.5 at week 1 to only 0.9 by week 10. Treatments T2, T3, and T4 exhibited intermediate values, generally grouped below T1 but above T0 (week 1: F₄,₉₅ = 30.3, P = 3.0 × 10−16; week 2: F₄,₉₅ = 66.5, P < 2 × 10−16; week 3: F₄,₉₅ = 146.4, P < 2 × 10−16; week 4: F₄,₉₅ = 143.7, P < 2 × 10−16; week 5: F₄,₉₅ = 539.2, P < 2 × 10−16; week 6: F₄,₉₅ = 341.4, P < 2 × 10−16; week 7: F₄,₉₅ = 707.8, P < 2 × 10−16; week 8: F₄,₉₅ = 1162.0, P < 2 × 10−16; week 9: F₄,₉₅ = 1286.0, P < 2 × 10−16; week 10: F₄,₉₅ = 1484.0, P < 2 × 10−16). Specifically, at week 2, T1 and T2 had the highest visual quality scores (6.5), significantly surpassing T3 (6.0), T4 (5.5), and T0 (4.5). This pattern continued with T1 holding the highest quality scores through all subsequent weeks (weeks 3–10), while T0 remained significantly lower. Within each treatment, the treated plant visual quality score changed significantly over time (P < 2 × 10−16 for all), with T1 showing a steady improvement from 6.0 to 9.9, while T0 exhibited a progressive decline from 6.0 before treatment to 0.9 at week 10. Treatments T2, T3, and T4 showed moderate increases, all reaching around 9.5 by the end of the experiment.
Mean (±SE) visual quality scores of treated olive plants (Picholine marocaine variety) following exposure or non-exposure to Amblyseius swirskii adults (10 days old, 1:1 sex ratio) released to control Saissetia oleae, under screenhouse conditions (25 ± 4 °C and 60 ± 10% relative humidity under natural light). Treatments were T0 (Control): S. oleae eggs only, without predatory mites, T1: 1:2 ratio (A. swirskii: S. oleae eggs) – 100 adults per plant, T2: 1:4 ratio – 50 adults per plant, T3: 1:8 ratio – 25 adults per plant, and T4: 1:10 ratio – 20 adults per plant. Different letters above the bars indicate statistically significant differences between treatments within the same sampling week (Student–Newman–Keuls test, α = 0.05).
Discussion
Our laboratory trials demonstrated that adult A. swirskii effectively preyed on S. oleae eggs and early nymphal stages under controlled conditions. Predation rates on eggs were significantly higher than on nymphs, which is consistent with the immobility and greater vulnerability of the eggs compared to the motile nymphs. Additionally, the lower consumption of nymphs relative to eggs is likely due to their larger body size, as prey size plays a crucial role in predator predation potential (Sabelis, Reference Sabelis and Crawley1992). Khan et al. (Reference Khan, Sayyed, Akram, Razald and Ali2012) also reported that prey developmental stages significantly influence predator consumption rates, development, and fecundity. To our knowledge, this is the first study reporting predation of S. oleae eggs and immature stages by A. swirskii. In the control treatment without A. swirskii, the mean numbers of dead S. oleae eggs and nymphs remained significantly lower than in the predator treatments throughout the experiment, although some mortality occurred. This mortality in the control may be due to declining quality of olive leaves over time, which likely reduced the nutritional support for the insects, as reflected by decreased honeydew secretion and increased mortality. Moreover, the removal or deterioration of olive leaves might have limited nutrient flow to the sites where eggs were laid and nymphs fed (Juan-Blasco et al., Reference Juan-Blasco, Qureshi, Urbaneja and Stansly2012). Nevertheless, the high mortality of S. oleae eggs and nymphs in the presence of A. swirskii highlights the mite’s potential as a biological control agent targeting early life stages of scale insects. The low natural mortality observed in control containers confirms that predation was the primary factor driving pest population reductions. However, the survival of A. swirskii adults was limited without prey or alternative food sources, indicating that continuous access to suitable food is necessary for predator establishment under field conditions.
Our screenhouse experiment further supported the predator’s capacity to suppress S. oleae populations on olive plants, with predator–prey ratios of 1:2 and 1:4 resulting in substantial reductions in scale insect density compared to control plants and other tested predator–prey ratios. These results are consistent with other studies where A. swirskii effectively suppressed pest populations on various crops. It has been reported to control Rhyncaphytoptus fucifoliae Keifer (Acari: Eriophyoidea) (Abou-Awad et al., Reference Abou-Awad, El-Sawaf and Abdel-Khalek1999), B. tabaci (Gennadius) (Hemiptera: Aleyrodidae) (Nomikou et al., Reference Nomikou, Janssen, Schraag and Sabelis2001), Kampimodromus aberrans (Oudemans) (Acari: Phytoseiidae) (Pappas et al., Reference Pappas, Broufas and Koveos2007), Carpoglyphus lactis (Linnaeus) (Acari: Carpoglyphidae) (Nguyen et al., Reference Nguyen, Vangansbeke, X and De Clercq2013), Anagasta kuhniella (Zeller) (Lepidoptera: Phycitidae) (Al-Shemmary, Reference Al-Shemmary2018), Eotetranychus frosti (McGregor) (Acari: Tetranychidae) and Cenopalpus irani Dosse (Acari: Tenuipalpidae) (Bazgir et al., Reference Bazgir, Shakarami and Jafari2018), Phyllocoptruta oleivora Ashmead (Acari: Eriophyidae) and Eutetranychus orientalis (Klein) (Acari: Tetranychidae) (Al-Azzazy and Alhewairini, Reference Al-Azzazy and Alhewairini2020), and Oligonychus afrasiaticus (McGregor) (Acari: Tetranychidae) (Al-Azzazy and Alhewairini, Reference Al-Azzazy and Alhewairini2024). Amblyseius swirskii has shown potential as a biological control agent against Diaphorina citri Kuwayama (Hemiptera: Psyllidae), with possible use as a preventive measure to reduce psyllid egg populations on young plants in nurseries and fields, although further studies are needed to confirm its effectiveness in management strategies (Juan-Blasco et al., Reference Juan-Blasco, Qureshi, Urbaneja and Stansly2012). Notably, the predation by A. swirskii on S. oleae eggs and young nymphs is ecologically significant because these stages are less mobile and more exposed on plant surfaces, making them accessible targets for predation. This contrasts with later nymphal and adult stages that often have protective waxy coverings or reside in cryptic locations, limiting predator access (McMurtry and Croft, Reference McMurtry and Croft1997). Furthermore, early suppression of S. oleae populations can prevent heavy infestations that cause substantial damage to olive plants, as evidenced by the improved plant health scores in treated plants.
In our study, A. swirskii sustained its population solely by preying on S. oleae, as no pollen or alternative food sources were provided. This indicates that S. oleae alone can support the reproduction and persistence of A. swirskii under screenhouse conditions. However, previous research has shown that supplemental foods – particularly pollen from Typha angustifolia (Typhaceae), Typha latifolia (Typhaceae), and Castanea sativa (Fagaceae) – significantly improve the survival, development, and fecundity of phytoseiid mites, including A. swirskii (Nomikou et al., Reference Nomikou, Janssen and Sabelis2003; González-Fernández et al., Reference González-Fernández, De la Peña, Hormaza, Boyero, Vela, Wong, Trigo and Montserrat2009). Alternative food sources such as pollen and honeydew are considered important for sustaining phytoseiid populations in field conditions (Villanueva and Childers, Reference Villanueva and Childers2004). Although S. oleae produces honeydew, its nutritional value for A. swirskii remains uncertain. Ragusa and Swirski (Reference Ragusa and Swirski1977) found that adding honeydew from S. oleae or Saissetia coffeae (Walker) (Hemiptera: Coccidea) slightly improved the development rate of A. swirskii and that honeydew from S. oleae or Pseudococcus longispinus (Targioni Tozzetti) (Pseudococcidae: Hemiptera) increased the fecundity of Tetranychus cinnabarinus (Boisduval) (Acari: Tetranychidae). However, the quality of honeydew varies by source. Honeydew from B. tabaci has been shown to be more nutritious than that from coccids, supporting better survival and reproduction in A. swirskii and Euseius scutalis (Athias-Henriot) (Acarina: Phytoseiidae) (Swirski et al., Reference Swirski, Amitai and Dorzia1967; Nomikou et al., Reference Nomikou, Janssen and Sabelis2003). While A. swirskii adults can survive on either pollen or B. tabaci honeydew, oviposition tends to be higher with pollen (Nomikou et al., Reference Nomikou, Janssen and Sabelis2003), although some studies suggest greater longevity on B. tabaci honeydew (Ragusa and Swirski, Reference Ragusa and Swirski1977; Momen and El-Saway, Reference Momen and El-Saway1993). Other sources, including psyllid honeydew and plant sap, have also been shown to support the survival and development of phytoseiid mites (McMurtry and Croft, Reference McMurtry and Croft1997). In citrus systems, pollen increases mite abundance, and mixed diets of pollen and prey often enhance reproduction more than single food types (Villanueva and Childers, Reference Villanueva and Childers2004; Messelink et al., Reference Messelink, Van Maanen, Van Steenpaal and Janssen2008). Further studies are needed to assess whether A. swirskii can effectively utilise S. oleae honeydew and whether supplemental food sources like pollen could improve predator establishment and efficacy in olive orchards.
Our study demonstrates the potential of A. swirskii as a biological control agent against S. oleae, particularly when applied preventively in newly infested plants. However, several important questions remain unanswered. These include the optimal release rates and frequencies under field conditions, the predator’s prey preferences when other olive pests or beneficial arthropods are present, and possible interactions with existing natural enemies. Although A. swirskii can feed on various prey and non-prey food sources, its compatibility with other biological control agents – such as parasitoids and predators already established in olive ecosystems – needs further investigation to develop effective IPM strategies. Future research should investigate the life history parameters of A. swirskii when preying on S. oleae, including development time, reproduction, longevity, and population growth. In addition, it is important to evaluate key factors such as suitable host plants (e.g. different olive varieties), prey preference, release strategies (rates and frequency), density-dependent effects, and the overall impact on S. oleae populations in the field. Such studies will provide critical insights into the predator’s long-term efficacy, population dynamics, and its potential role in IPM programmes under field conditions. Successful biological control examples include the release of the coccinellid predator Exochomus quadripustulatus (Linnaeus) (Coleoptera: Coccinellidae) against S. oleae and other olive scale pests in Greece (Katsoyannos, Reference Katsoyannos1984). In Morocco, although natural enemies of black scale are present, chemical control remains the primary method. In contrast, the parasitoid Metaphycus spp. has proven effective in France, Turkey, and California, helping to reduce pesticide use (Delrio and Foxi, Reference Delrio and Foxi2010). If field trials confirm its effectiveness, A. swirskii could complement existing control measures by targeting eggs and first-instar nymphs of S. oleae, which are protected inside newly developing, unopened leaves and are difficult for most predators to reach. Additionally, these early immature stages are not attacked by nymphal parasitoids, which prefer later instars (Chu and Chien, Reference Chu, Chien, Kiritani, Hj and Chu1991). It is important to evaluate the predatory role of native mites on S. oleae and their interactions with A. swirskii to better understand the potential for combined biological control.
Conclusion
In conclusion, A. swirskii demonstrates considerable potential for controlling S. oleae eggs and early nymphs under laboratory and field conditions, offering an environmentally friendly alternative to chemical insecticides. Future field trials should focus on evaluating its effectiveness under diverse environmental conditions, determining long-term impacts on pest populations and olive plant health, and optimising release strategies to maximise biological control outcomes.
Acknowledgements
The research of this study was supported by the National Institute of Agricultural Research (INRA-Morocco) and MCRDV project.
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
The data that support the findings of this study are available from the corresponding author, Mohamed El Aalaoui, upon reasonable request.
