Secondary Plants Improve the Settlement of Orius laevigatus in Greenhouses

Abstract

In greenhouse systems, secondary plants are used to attract and support the multiplication of beneficial arthropods, thereby improving biological control. Three plants were selected for this study: alyssum (Lobularia maritima (L.) Desv.), yarrow (Achillea millefolium L.), and dill (Anethum graveolens L.). This study was performed in two years, 2021 and 2025, and focused on Orius laevigatus (Fieber) (Hemiptera, Anthocoridae), one of the most important predators of Thysanoptera pests in greenhouse crops. Four ornamental crops (carnation, sweet William, statice, and gerbera daisy) were included to analyse the movement and installation of the predator. Alyssum and yarrow housed O. laevigatus in both years (total mean values per sampling date of 3.0 ± 1.3 and 2.7 ± 1.0 on alyssum and 7.0 ± 2.8 and 1.8 ± 0.8 on yarrow in 2021 and 2025, respectively), increasing its population in the greenhouse. Dill was unsuitable for sustaining predator populations and attracted additional potential pests. Its short flowering period and rapid decline further limited its usefulness. Orius laevigatus adults did not show great mobility during the study and had small populations among the ornamental crops in the greenhouse. Ornamental plant statice (Limonium sinuatum (L.) Mill.) had the highest predator population. The interest of the secondary plants is discussed, highlighting their potential for biological control in greenhouses.

1. Introduction

Pest control strategies increasingly rely on the use of biological control, particularly in greenhouses [1]. The classical strategy for implementing biological control in greenhouses is to introduce beneficial arthropods directly into the crop, with or without the presence of the pest, but other strategies can be used [2], in particular, the use of plants that can harbour (or attract) beneficial arthropods, even before planting the crop, with the aim of increasing the biological control efficiency [3]. The general term “secondary plants” has been used [3], which includes different roles these plants can play in the agroecosystem (companion plants, repellent plants, barrier plants, and others) [3]. This strategy is part of a conservation biological control approach that aims to improve the presence and impact of natural enemies by manipulating plant-based resources in the landscape [4]. A meta-analysis has shown a general increase in the number of natural enemies and a reduction in pest damage in crops due to diversification strategies [5]. This diversification strategy requires the selection of plants that provide resources, such as shelter, nectar, alternative prey/hosts, and pollen, and the establishment of these plants or plant communities in the managed system [6,7,8].

Amoabeng et al. [9] highlighted that most plants used in habitat management belong to three families, Apiaceae, Asteraceae, and Lamiaceae, but two other families, Brassicaceae and Fabaceae, also contain several species of interest in habitat manipulation studies. Several reviews have reported the screening of plants useful for habitat manipulation, which have generally identified the associated arthropod fauna [4,10,11], and a meta-analysis review has identified several flowering secondary plants (and plant families) that improve the biological fitness of important predators and parasitoids [12]. Plants native to a region may be equally or even more attractive to local natural enemies than commonly recommended exotic plants [4], and native plants attractive to important natural enemies have been determined in several crop production systems in Spain [13,14,15,16].

The plants investigated in the present study are included in the category of “insectary plants” following the definition by Parolin et al. [3] because they have flowers that provide nectar and pollen to the natural enemies we are interested in. They can also be considered “banker plants”, as they can have several types of arthropods that serve as prey (or host) to different predators (or parasitoids) [2,3,17]. In the management of banker plants, biological control agents are released onto them, and they spread into the rest of the greenhouse as they reproduce and increase in number. This represents a mini-rearing system for natural enemies and can serve as a substitute for the current augmentative release of natural enemies in greenhouses [2,3] because the production and release of natural enemies can be continuous.

Lobularia maritima (L.) Desv. (Brassicaceae), known as alyssum, has been the object of many studies, highlighting its attractiveness. Syrphidae is the most prominent Diptera family associated with alyssum. Previous studies have classified it as one of the most attractive plants for these dipterans [2,10,18,19], whose adults feed on its nectar and pollen, and larvae can be voracious predators of aphids. Another family found on alyssum is the Anthocoridae (Hemiptera), especially those of the genus Orius, as they are found in greater numbers on alyssum than on other plants [2,10,11,20,21]. Alyssum represents a vital resource for a wide variety of Hymenoptera parasitoids, increasing the longevity and fertility of species in the Braconidae, Ichneumonidae, Eulophidae, and Trichogrammatidae families [18,22,23].

Achillea millefolium L. (Asteraceae), known as yarrow, has received less attention as a secondary plant than alyssum. However, previous studies have highlighted its effectiveness in attracting Hymenoptera parasitoids compared with other plants or spontaneous vegetation [24,25]. Other arthropod fauna of interest, including Syrphidae, have been studied on yarrow, but the results have been inconsistent. For example, Colley and Luna [19] reported that syrphids prefer yarrow. However, other studies [24] have shown that there is not a particular interest of syrphids for yarrow when other plants were present. The same authors [24] indicated that anthocorids were attracted to yarrow.

Anethum graveolens L. (Apiaceae), known as dill, has also attracted interest in secondary-plant research. However, it is much less important than alyssum or yarrow. Sirphydae adults are strongly attracted to dill and feed on its nectar and pollen [26]. Chrysopidae (Neuroptera) species visit and use dill resources, enabling their survival and reproduction [6,27]. Even Coccinellidae (Coleoptera) species visit dill flowers [6], but the scarce information available indicates that Anthocoridae are not common visitors of dill [28].

Thysanoptera (Insecta) is one of the most important pest groups in greenhouses. Frankliniella occidentalis Pergande (Thysanoptera, Thripidae) and Thrips tabaci Lindeman (Thysanoptera, Thripidae) are the most important pest species, particularly in greenhouses in southern Spain [29,30], although damage caused by them has been recorded worldwide [31]. They are very polyphagous; have been reported in many horticultural, fruit, and ornamental crops, causing damage to flowers, fruit, and leaves; and can be vectors of different viruses [32].

Thysanoptera have several natural enemies that help control their populations, and these natural enemies are widely used in augmentative biological control strategies. Predators such as phytoseiids (Acari: Mesostigmata, Phytoseiidae) [33] and anthocorids (Insecta: Hemiptera, Anthocoridae) [34,35,36] are particularly important among natural enemies. In turn, among Anthocoridae species, Orius laevigatus (Fieber) is commonly used in agriculture to control insect pests, mainly thrips and whiteflies [1,37,38].

Studies on secondary plants, mainly in their role as insectary plants, have been conducted mainly in open-field crops or in outdoor locations, and very few have been conducted in a greenhouse [36]. The present study used three secondary plants in a greenhouse environment over two years and tested two hypotheses: (a) the three species were able to equally house and foster populations of O. laevigatus, and (b) O. laevigatus moved to and colonised to the same degree different ornamental crops present in the greenhouse. The objectives were to study the presence of O. laevigatus (and other arthropods of interest) on the secondary plants and the movement of O. laevigatus from the secondary plants to the ornamental plants and to compare the total population of O. laevigatus present on both plant types (secondary and ornamental) during the sampling period. As a result of this study, O. laevigatus settled and reproduced on alyssum and yarrow, whereas dill was unsuitable for it. In addition, movement to and colonisation of the ornamental crops by O. laevigatus in the same greenhouse was very limited, although differences were found between them.

2. Materials and Methods

2.1. Location

Trials were conducted in a 240 m² experimental greenhouse at Escuela Técnica Superior de Ingeniería Agronómica (ETSIA), Universidad de Sevilla (Seville, Spain). The geographical coordinates of this location are 37°35′19.84″ N and −5°93′81.65″ S. The experiment was conducted in two different years, 2021 and 2025.

2.2. Experimental Design

Three parallel growing beds were set up in one-quarter of the greenhouse to cultivate ornamental plants. Each bed was 10 m long and 1 m wide, with an elevation of 10 cm above the aisles, which were 1 m wide (Figure 1).

Alyssum, yarrow, and dill were the secondary plants investigated in this study, which were obtained from seeds (Cantueso Natural Seeds, Villarrubia (Córdoba), Spain; https://cantuesoseeds.com/, accessed on 1 May 2020). In both experiments, the seeds of the three species were planted in a seed bed in mid-September 2020 and 2024, and when they reached approximately 10 cm (about 1 month later), they were transplanted into 1 L pots containing a mix of horticultural substrate and sand (3:1) and kept in another greenhouse until its definite location.

In the 2021 experiment, each growing bed was divided into three zones and randomly assigned to three carnation (Dianthus caryophyllus L.) cultivars. Gerbera daisy (Gerbera sp. L.), another ornamental plant, was planted in 15 L pots (with the same previously described substrate mixture and 4 g of Osmocote^®, ICL Specialty Fertilizers, Tel Aviv (Israel)) in one-quarter of the greenhouse (Figure 1a). Both ornamental plant species were transplanted into the greenhouse at the end of October 2020. The beds and pots were drip-irrigated. The secondary plants were transplanted from the 1 L pots to the ground at the beginning of November 2020 and placed in the beds (Figure 1a).

In the 2025 experiment, the same three growing beds were planted with three ornamental plants: carnation, sweet William (Dianthus barbatus L.), and statice (Limonium sinuatum (L.) Mill.) (Figure 1b). Gerbera daisy was planted in another quarter of the greenhouse, with some in 15 L pots (with the same mixture as in 2021) and some in three rows in the ground (Figure 1b). Ornamental plant species were transplanted into the greenhouse (in beds, the ground, and pots) in mid-October 2024. At the beginning of December 2024, secondary plants were transplanted from 1 L pots to five 15 L pots (the same mixtures as used for gerbera daisy) per secondary plant and distributed randomly around the beds and near the gerbera daisies (Figure 1b). Secondary and ornamental plants cultivated in the ground were not fertilised due the high presence of nutrients in the soil.

2.3. Release of Orius laevigatus

Biosur (Vícar (Almería), Spain; https://www.biosur.es, accessed on 15 January 2021) provided the insects, commercially available as Orius Biosur product. There were two releases in 2021, on 21 February and 30 April, with 1000 adults distributed homogenously over the secondary plants on each date. In 2025, there were also two releases, on 26 February and 30 April, with 1500 adults distributed homogenously over the secondary plants on each date.

2.4. Sampling Procedure

The secondary plants were sampled on 9 dates in the 2021 experiment: 18 February; 2, 9, and 23 March; 13 April; 4, 11, and 21 May; and 22 June. In the 2025 experiment, 11 sampling dates were used: January 31, February 13 and 27, March 13 and 27, April 9 and 24, May 6 and 22, and June 6 and 18. The phenology of the secondary plants was recorded on each sampling date and is presented in Appendix A (

Table A1. Phenological state of the secondary plants during the two years of the study.

	Alyssum	Yarrow	Dill
2021
18-February	Flowering	Vegetative	Vegetative—Initial flowering
2-March	Flowering	Vegetative—First inflorescence stems	Flowering
9-March	Flowering	Initial flowering	Flowering
23-March	Flowering	Flowering	Flowering + Fructification
13-April	Flowering	Flowering	Final flowering and fructification stages
4-May	Flowering + Fructification	Flowering	Senescence
11-May	Flowering + Fructification	Flowering	Senescence
21-May	Flowering + Fructification	Flowering + Fructification	Senescence
22-Jun	Flowering + Fructification	Few flowers + Fructification	Senescence
2025
31-January	Flowering	Vegetative	Initial flowering
13-February	Flowering	Vegetative	Flowering
27-February	Flowering	Vegetative	Flowering
13-March	Flowering	Vegetative	Flowering
27-March	Flowering + Fructification	Vegetative—First inflorescence stems	Flowering
9-April	Flowering + Fructification	Vegetative + Initial flowering	Flowering + Fructification
24-April	Flowering + Fructification	Initial flowering	Flowering + Fructification
6-May	Flowering + Fructification	Flowering	Few flowers + Fructification
22-May	Flowering + Fructification	Flowering	Senescence
6-Jun	Flowering + Fructification	Flowering	Senescence
18-Jun	Few flowers + Fructification	Few flowers + Fructification	Senescence

).

The sampling unit in 2021 was a bunch of flowers with a diameter of approximately 15–20 cm for alyssum, a similar bunch of flowers of 15–20 cm for yarrow, and 2 or 3 inflorescences for dill. On each sampling date, three sampling units from each secondary-plant species were randomly selected from the three beds. The sampling unit was vigorously agitated/tapped three times on a 50 cm × 30 cm white pan, and the arthropods removed from the plant were counted in situ. During this period, specimens of Thysanoptera were collected in vials with 70% ethanol to determine the species in the laboratory using an optical microscope and specific keys for thrips identification [39,40].

The sampling unit in 2025 was similar for alyssum. There were no yarrow flowers during part of the sampling period, and a 15–20 cm bunch of leaves was collected as the sampling unit until flowers were available; then one inflorescence per pot was selected. The sampling unit for dill was one inflorescence. Three sampling units per secondary-plant species were taken on each sampling date, one per three randomly selected pots. The extraction process of the arthropods was the same as that in 2021. During the 2 years, specimens of Thysanoptera were collected in vials with 70% ethanol to determine the species in the laboratory using an optical microscope and specific keys for thrips identification [39,40].

Ornamental plants were also sampled in 2021; between three and six flowers of each carnation cultivar were collected and opened in the laboratory under a stereo binocular (45×) to count thrips and other arthropods, especially O. laevigatus. The sampling of ornamentals in 2025 was carried out in the greenhouse using the same methodology as that used for secondary plants. A sampling unit (one open flower in carnation, one inflorescence in sweet William, a plant inflorescence in statice, and one flower in gerbera daisy) was selected and tapped against a white pan, and the arthropods were counted in situ. Six sampling units per ornamental species were collected on each sampling date.

In 2025, the number of sampling units available in each pot of the secondary plants selected for sampling was also counted. The quantities of sampling units were also obtained for the ornamental plants by counting the flowers and inflorescences on each sampling date. The absolute arthropod population present on the secondary plants on each date was estimated using the expression mean value of arthropods per sampling unit × average sampled units per pot × five pots, and for ornamental plants with the expression mean value of arthropods per sampling unit × the number of sampled units observed. The numbers of sampling units for secondary and ornamental plants are given in Appendix A (

Table A2. Number of sampling units of secondary plants and ornamental crops on each sampling date in 2025, used to estimate the absolute populations of Orius laevigatus.

	31/1/2025	13/2/2025	27/2/2025	13/3/2025	27/3/2025	9/4/2025	24/4/2025	6/5/2025	22/5/2025	6/6/2025	18/6/2025
Secondary plants 1
Alyssum	7.7	6.3	6.0	5.3	6.3	5.3	4.3	4.7	6.7	6.3	9.7
Yarrow	5.0	3.0	3.0	3.3	5.3	4.3	5.3	4.3	4.0	1.3	2.2
Dill	13.0	1.7	2.3	3.7	3.7	5.3	3.7	2.7	-3	-	-
Ornamentals 2
Carnation	0	0	4	3	10	19	23	67	33	15	-
Sweet William	80	45	55	40	120	75	75	90	15	10	-
Gerbera daisy	20	20	20	20	20	20	20	20	20	13	24
Statice	50	50	50	55	70	65	100	120	73	65	60

¹ Figures are the average number of sample units per pot (three pots were sampled on each date). Average density of O. laevigatus in the sample unit on each date is then multiplied by the factor in the table and by 5 (the number of pots with secondary plants) to estimate the absolute population of the predator. ² Figures are the number of sample units on each date. Average density of O. laevigatus in the sample unit on each date is then multiplied by the factor in the table to estimate the absolute population of the predator. ³ No sampling units available.

).

Additionally, four yellow sticky traps were placed in the half of the greenhouse where the study was conducted (Figure 1a,b). They were changed after one week, and all specimens trapped in the laboratory were identified using a stereo binocular (45×) and a general key [41]. Fourteen samples were collected from these traps in 2021 (from October 2020 to May 2021), and nine samples were collected in 2025 (from February to June).

2.5. Statistical Analyses

Data obtained from the sampling dates were analysed separately by year with a repeated-measures analysis of variance (ANOVA) (SPSSv15.0 for Windows), with the factors of date and plant species. Statistical analyses were performed in two steps: first, with the complete set of data for alyssum and yarrow (9 sampling dates in 2021 and 11 sampling dates in 2025), and then, using the three secondary plants (including dill) but only on the sampling dates on which all three plants were available (5 sampling dates in 2021 and 8 sampling dates in 2025). Data were not transformed.

3. Results

Alyssum and yarrow showed the ability to harbour and promote the presence of O. laevigatus over the two years of this study, whereas dill did not (

Table 1. Number of arthropods observed in the secondary plants in the two years.

	Thysanoptera	Orius Total	Orius Adults	Orius Nymphs	Aphididae	Formicidae	Hymenoptera Parasitoids	Collembola	Araneae	Acari	Total 1
2021
Alyssum	345	81	17	64	32	56	3	532	44	101	1226
Yarrow	557	188	97	91	10	10	4	376	39	9	1199
Dill	170	0	0	0	2660	0	1	24	6	0	2863
Total	1072	269	114	155	2702	66	8	932	89	110	5288
2025
Alyssum	163	88	44	44	0	43	4	242	23	9	649
Yarrow	270	59	20	39	3	5	10	248	32	10	657
Dill	287	1	1	0	0	6	0	20	9	1	333
Total	720	148	65	83	3	54	14	510	64	20	1639
Total (two years)	1792	417	179	238	2705	120	22	1442	153	130	6927

¹ All the arthropods observed in each plant are included.

). Orius laevigatus showed a significant presence on the flowers of alyssum (81 individuals in 2021 and 88 individuals in 2025) and yarrow (188 individuals in 2021 and 59 individuals in 2025). The most frequent arthropod group consistently found in the flowers of the three secondary plants was Thysanoptera, with a total population of 1792 individuals (1072 and 720 individuals in 2021 and 2025, respectively; Table 1). The species identified in the two seasons was almost exclusively Thrips tabaci, with 69 individuals in 2021 (only 1 individual was of another species) and 46 individuals in 2025 examined in the laboratory.

Another abundant arthropod group in the secondary plants was Collembola, with 1442 individuals, which occurred frequently in alyssum (532 and 242 individuals in 2021 and 2025, respectively) and yarrow (376 and 248 individuals in 2021 and 2025, respectively) but was almost absent from dill (Table 1). Other arthropods present on the secondary plants included Aphididae, Formicidae, Hymenoptera parasitoids, Araneae, and Acari. Aphididae were very abundant in dill in 2021 (2660 individuals were counted), but almost no presence was observed on the three secondary species in 2025 (Table 1).

Captures with the yellow traps indicated the high presence of certain groups (

Table 2. Number of arthropods captured in the yellow sticky traps in the two years.

	Thysanoptera	Orius	Aphididae	Aleyrodidae	Diptera	Coleoptera	Hymenoptera	Araneae	Total 1
2021	2591	12	4146	109	837	22	482	43	8371
2025	6607	7	129	0	5531	35	41	82	12,500
Total	9198	19	4275	109	6368	57	523	125	20,871

¹ All the arthropods observed are included.

), particularly Thysanoptera in both years (2591 and 6607 individuals in 2021 and 2025, respectively), with other groups that varied between years. Aphididae were the most frequent in the traps in 2021 (4146 individuals) but not in 2025 (129 individuals), and Diptera were very abundant in 2025 (55,331 individuals) but not in 2021 (837 individuals). Few O. laevigatus were captured in both years, with 12 individuals in 2021 and 7 individuals in 2025.

Significant differences were detected in the adult population of O. laevigatus in 2021 between alyssum and yarrow (0.6 ± 0.2 and 3.6 ± 1.4 individuals per sampled unit and date (ISUD), respectively; F_1,4 = 200.0, p < 0.001;

Table 3. Mean values (with standard error) per sampling date and sampling unit of the principal arthropods found in the secondary plants in 2021 and 2025. Repeated-measures analysis of variance was used to compare two plant species (alyssum and yarrow) and three plant species (alyssum, yarrow, and dill).

	Plant Species						Comparison of Two Plant Species 1				Comparison of Three Plant Species 2
	Alyssum		Yarrow		Dill		Plant Species		Date × Plant		Plant Species		Date × Plant
	Mean 3	s.e.	Mean 3	s.e.	Mean 4	s.e.	F 5	p	F 6	p	F 7	p	F 6	p
2021
Thysanoptera	12.8	3.2	20.6	10.4	11.3	2.2	8.0	0.048 (*)	8.0	0.066	5.1	0.052	8.4	0.006 (*)
Orius total	3.0	1.3	7.0	2.8	0.0	0.0	44.7	0.003 (*)	4.0	0.056	11.2	0.009 (*)	19.4	<0.001(*)
Orius adults	0.6	0.2	3.6	1.4	0.0	0.0	200	<0.001 (*)	6.0	0.027 (*)	97.0	<0.001 (*)	38.1	<0.001 (*)
Orius nymphs	2.4	1.1	3.4	1.6	0.0	0.0	4.1	0.112	4.6	0.028 (*)	2.6	0.152	2.9	0.087
Aphididae	1.2	0.5	0.4	0.2	177.3	124.9	1.1	0.349	0.7	0.483	9.6	0.014 (*)	3.0	0.124
Formicidae	2.1	0.5	0.4	0.3	0.0	0.0	4.1	0.114	0.8	0.470	2.4	0.170	0.7	0.540
Hymenoptera	0.1	0.1	0.1	0.1	0.1	0.1	0.0	0.851	1.2	0.359	0.7	0.533	1.1	0.403
Collembola	19.7	8.4	13.9	5.4	1.6	0.8	4.0	0.115	0.9	0.423	31.3	0.001 (*)	2.1	0.174
Araneae	1.6	0.6	1.4	0.6	0.4	0.1	0.3	0.597	2.9	0.107	25.8	0.001 (*)	1.3	0.331
Acari	3.7	2.3	0.3	0.2	0.0	0.0	9.2	0.038 (*)	1.1	0.347	10.2	0.012 (*)	0.9	0.452
Total arthropods	45.4	8.0	44.4	9.6	190.9	122.4	0.0	0.871	2.7	0.130	5.2	0.050	2.7	0.142
2025
Thysanoptera	4.9	1.2	8.2	4.0	12.0	4.2	6.7	0.060	3.8	0.106	7.6	0.02 (*)	2.9	0.114
Orius total	2.7	1.0	1.8	0.8	0.0	0.0	2.6	0.182	1.5	0.273	14.1	0.005 (*)	7.6	<0.001 (*)
Orius adults	1.3	0.5	0.6	0.3	0.0	0.0	3.0	0.157	1.0	0.417	8.2	0.019 (*)	2.6	0.009 (*)
Orius nymphs	1.3	0.6	1.2	0.7	0.0	0.0	0.1	0.782	1.8	0.217	3.6	0.095	3.7	<0.001 (*)
Aphididae	0.0	0.0	0.1	0.1	0.0	0.0	-	-	-	-	-	-	-	-
Formicidae	1.3	0.4	0.2	0.1	0.3	0.2	32.9	0.005 (*)	0.6	0.577	15.2	0.004 (*)	0.4	0.788
Hymenoptera	0.1	0.1	0.3	0.2	0.0	0.0	3.3	0.145	0.9	0.412	2.4	0.174	1.3	0.335
Collembola	7.3	1.8	7.5	2.8	0.8	0.8	0.0	0.910	2.6	0.123	17.8	0.003 (*)	2.5	0.011 (*)
Araneae	0.7	0.2	1.0	0.3	0.4	0.1	1.4	0.295	1.4	0.299	0.9	0.465	1.0	0.466
Acari	0.3	0.2	0.3	0.2	0.0	0.0	0.0	0.904	1.8	0.223	1.2	0.370	1.9	0.182
Total arthropods	19.7	2.3	19.9	5.7	13.9	4.7	0.0	0.919	4.3	0.045 (*)	5.7	0.042 (*)	3.5	0.001 (*)

¹ Statistical comparisons made with alyssum and yarrow using the total sampling dates in each year: 9 sampling dates in 2021 and 11 sampling dates in 2025. ² Statistical comparisons made with alyssum, yarrow, and dill using the sampling dates in which the three plants were present simultaneously: 5 sampling dates in 2021 and 8 sampling dates in 2025. ³ Mean values obtained with the total number of sampling dates made in 2021 (9 sampling dates) and in 2025 (11 sampling dates). ⁴ Mean values obtained using the number of sampling dates available in 2021 (5 sampling dates) and in 2025 (8 sampling dates). ⁵ Degrees of freedom of the F test: 1 and 4. ⁶ Degrees of freedom varied in each year and also depending on whether Mauchly’s sphericity test was significant, in which case the Greenhouse–Geisser correction was used. ⁷ Degrees of freedom of the F test: 2 and 6. (*) Significant, with p < 0.05.

), but no differences were found in the nymphs (2.4 ± 1.1 and 3.4 ± 1.6 ISUD, respectively; F_1,4 = 4.1, p = 0.121; Table 3). In contrast, no significant differences were found in 2025 between the alyssum and yarrow populations of either adults or nymphs of O. laevigatus found in their flowers, with p > 0.05 in all cases (Table 3). The total O. laevigatus population was very similar in alyssum in both years (3.0 ± 1.3 and 2.7 ± 1.0 ISUD in 2021 and 2025, respectively), but there was great variability between the years in yarrow (7.0 ± 2.8 and 1.8 ± 0.8 ISUD in 2021 and 2025, respectively), possibly due to the different phenology of the plant in each year (Table A1), which could have been induced by the differences in the cultivation methods.

Thysanoptera were present in significant densities on the three insectary plants, ranging from 4.9 ± 1.2 to 20.6 ± 10.4 ISUD (Table 3). In general, the densities in the three plants showed non-significant differences in 2021 (F_2,6 = 5.1, p = 0.052; Table 3) and significant differences in 2025 (F_2,6 = 7.6, p = 0.023; Table 3), when dill had the highest density. For the two plants that lasted longer in the experiments, namely, alyssum and yarrow, the 2021 results showed significant differences (F_1,4 = 8.0, p = 0.048; Table 3) and similar results in 2025 (F_1,4 = 6.7, p = 0.060; Table 3). In both years, yarrow showed higher densities but with high variability (20.6 ± 10.4 and 8.2 ± 4.0 ISUD in 2021 and 2025, respectively; Table 3).

Collembola was the second most abundant arthropod group, with remarkably similar densities in alyssum and yarrow in each year (19.7 ± 8.4 and 13.9 ± 5.4 ISUD, respectively, in 2021; 7.3 ± 1.8 and 7.5 ± 2.8 ISUD, respectively, in 2025; Table 3) and no statistical differences when they were compared (F_1,4 = 4.0, p = 0.115 in 2021; F_1,4 = 0.0, p = 0.910 in 2025; Table 3). In contrast, dill had very low densities in both years (1.6 ± 0.8 and 0.8 ± 0.8 ISUD in 2021 and 2025, respectively), and significant differences were observed when comparing the three plants in both years (F_2,6 = 31.3, p = 0.001 in 2021; F_2,6 = 17.8, p = 0.003 in 2025; Table 3).

Aphididae were only present in 2021 with very high densities on dill (177.3 ± 124.9 ISUD) but only on a few dates. It was almost absent from the two other plants (alyssum and yarrow). Therefore, the population comparison with the three plants was significant (F_2,6 = 9.6, p = 0.014; Table 3), but the comparison between alyssum and yarrow was not significant (F_1,4 = 1.1, p = 0.349; Table 3). In 2025, almost no Aphididae were found in the three secondary plants (Table 3).

In general, the mean densities in the other arthropod groups were very low, with significant differences between the plants for Araneae and Acari in 2021 and Formicidae in 2025 (Table 3).

The total number of arthropods registered in each year (Table 3) showed similarity between alyssum and yarrow in 2021 (45.4 ± 8.0 and 44.4 ± 9.6 ISUD, respectively) and 2025 (19.7 ± 2.3 and 19.9 ± 5.7 ISUD, respectively); thus, there were no significant differences in the statistical comparison (p > 0.05 in both years; Table 3). Dill had a very high presence of Aphididae in 2021, which increased the mean number of arthropods (190.9 ± 122.4 ISUD), but no significant differences were observed among the three plants (F_2,6 = 5.2, p = 0.050; Table 3). In 2025, dill had a more similar population (13.9 ± 4.7 ISUD) to the other two plants, but a significant difference was found between them in this year (F_2,6 = 5.7, p = 0.042; Table 3).

Seasonal changes in O. laevigatus density (Figure 2) indicate that its presence on the two secondary plants (alyssum and yarrow) that harboured it were generally low after the first release in the two years of the study (at the end of February). A low number of adults were detected in both years immediately after release, but very few nymphs were observed after a period of 2 months (Figure 2). After the second release (April 30th in both years), the number of adults and, especially, nymphs increased in 2021, with significant values in the date × species effect for adults and nymphs (F_2.0,7.8 = 6.0, p = 0.027 and F_2.8,11.0 = 4.6, p = 0.028, respectively; Table 3). Similarly, this occurred in 2025, but the date × species effect was not significant (F_2.4,9.5 = 1.0, p = 0.417 and F_2.7,10.7 = 1.8, p = 0.217 for adults and nymphs, respectively; Table 3). The number of nymphs and adults remained high for several weeks and declined quickly at the end of June 2021 (plants started to senesce) and moderately declined in 2025. Yarrow and alyssum showed a similar number of nymphs (Figure 2b), with a peak of approximately 10 individuals per sampling unit (ISU) in May 2021 and a peak of 4–6 ISU in 2025 (Figure 2d). In 2021 (Figure 2a), yarrow had a significantly higher population of O. laevigatus adults than alyssum, with a peak of approximately 10 ISU at the beginning of May, whereas that for alyssum was near 2 ISU. This pattern was not observed in 2025, when adults were less abundant (reaching 2–3 ISU), but similar results were obtained for both plant species (Figure 2c).

Thysanoptera were present on the three secondary plants in the two years, with peaks in April (2021; Figure 3a) and May (2025; Figure 3c), particularly in yarrow in 2021, when it reached a peak of 100 ISU. Although dill showed a quick decline in both years, it reached densities similar to those of yarrow in 2025 (Figure 3c), and alyssum showed intermediate densities in general. The date × species effect was not significant in either year (Table 3).

Collembola was present on alyssum and yarrow in 2021 and 2025 and almost absent from dill (Figure 3b,d). It was more abundant in 2021, with peaks of approximately 80 ISU in alyssum, but its density declined quickly in April and May. In contrast, in 2025, the density was not as high (peaks of 20–30 ISU) but declined equally towards the end of the sampling period. The date × species effect was not significant in either year (Table 3).

The absolute population of O. laevigatus estimated in 2025 (Figure 4) showed the potential of the secondary plants to house the predator and the presence of the predator on ornamental plants in the same greenhouse. The results from 2021 are not presented because carnation (with three cultivars) was the only ornamental plant used, and no O. laevigatus individuals were detected in the flowers during this study. Alyssum had the highest potential in 2025 because of its blooming, which occurred over a long period of time, even when the plants were in pots and not in the ground (Table A1 and Table A2). At the end of the sampling period, an estimated population of around 200 adults and 150 nymphs were present on alyssum (Figure 4a,c), much more than that in yarrow, which could not house a high number of predators due to its scarce blooming (Table A1 and Table A2). In general, the O. laevigatus population observed in the ornamental plants (Figure 4b,d) was not correlated with the population estimated in the secondary plants, especially when considering adults. Only statice presented a very low estimated adult population during this period, but a higher estimated nymph population (around 100 individuals) was observed after the first release of the predator (26 February), after the second release (30 April), and at the end of the sampling period (50–60 individuals). This presence could be related to the ornamental plant’s abundant and permanent blossom and the constant presence of thrips (Figure 4f). Gerbera daisy also had a low O. laevigatus (nymph) population at the end of the sampling period, but it was negligible in carnation and sweet William.

4. Discussion

Two of the secondary plants studied, namely, alyssum and yarrow, showed a capability to house and promote the populations of the predator O. laevigatus in a greenhouse environment in two seasons, even with the differences in cultivation in each year. In contrast, the other secondary plant, dill, was unable to house this insect in either year, confirming the poor records of its presence on this plant [28]. The similarities in the results in two separate years with cultivation particularities highlight the robustness of the results regarding the use of these plants in a greenhouse.

The two years of study showed some differences, as there was a greater number of O. laevigatus in 2021 than in 2025. Yarrow had a higher O. laevigatus population than alyssum in 2021, whereas the opposite was observed in 2025. This difference between years can be attributed to the different cultivation methods used in each year. In 2021, the plants were in the soil and grew very vigorously with a great production of blooms in both plants, whereas in 2025, they were in pots, growth was not as vigorous, and they produced less flowers, particularly yarrow.

Alyssum is the most studied of the three secondary-plant species used in this research study. Hogg et al. [10] found that alyssum had the highest population of Anthocoridae (in which Orius spp. are included) compared with nine other plants, and it was also the most abundant Hemiptera group on this plant, together with Lygaeidae. They found no Thysanoptera on alyssum in their study, but this might be due to the use of vacuum sampling. Other studies have also highlighted the presence of anthocorids on this plant [2,4,11], and our results confirm that alyssum is also a suitable plant for O. laevigatus in greenhouses.

The capacity of alyssum to house and promote the presence of different groups of natural enemies of pests (as indicated in the Introduction) has led to its implementation in different crops to improve the effectiveness of biological control in vineyards [23], strawberry [20], peach [24], apple [25], brassicas [42,43], and lettuce [11].

The other plant that harboured O. laevigatus in this study was yarrow, which housed this predator in similar or even higher densities than alyssum in the two years of the study, with the stable presence of adults and nymphs. Although yarrow has been less studied than alyssum, Aparicio et al. [24] also found that yarrow was able to house anthocorids with numbers very similar to alyssum. The results presented in our work confirm this aspect and make yarrow another plant of interest for O. laevigatus in greenhouses.

Thysanoptera were generally observed on the three secondary plants in our study. The main (and almost only) identified species was T. tabaci, an important pest in greenhouses and outdoor crops. Its presence can be a threat to crops because of its ability to colonise many plants. Although this can be a drawback to using this type of plants in greenhouses, the presence of thrips can be a great advantage for using predators on some of these plants, as occurred in this study with alyssum and yarrow, where O. laevigatus and other anthocorids can find shelter, prey (thrips), and alternative food (pollen) and have been utilised with success [44]. In contrast, dill housed thrips (a result highlighted also in [28]) but not anthocorids, which is another reason why this plant would not be suitable for use in greenhouses.

Aphididae were very scarce on alyssum and yarrow in both years, which is interesting because aphids (with different species) are important pests in greenhouse crops. Dill showed a different pattern, at least in 2021, when it had a heavy aphid infestation (also observed in the results of [28]), although the species was not identified. Another abundant group of arthropods in the secondary plants was Collembola, whose populations in alyssum and yarrow were similar in both plants in both years but significantly lower in dill in both years. Other arthropod groups were less abundant, although their presence could be of interest, as with Araneae.

The flowering period duration is important for ensuring that natural enemies have long-term access to resources. Of the three plant species in our study, alyssum showed the longest flowering period, from the start (and normally before) of the sampling period in February until the last sampling date, in the middle of June (but could be longer), with abundant and continuous flower production. This long flowering period has also been highlighted in another study [45], as it is one of the reasons for including this plant in this type of study and explains its attraction to different insects (syrphids, hymenopteran parasitoids, and others) [10,11,21].

In our study, yarrow presented a more reduced flowering period, although it was dependent on the year of study because it produced abundant flowers in 2021. In general, its flowering period is shorter than that of alyssum, starting middle–end of March (or later), which is synchronised with the second release of the predator (end of April), and producing abundant flowers until the end of June (and even later). Some studies have included yarrow for its late-season flowering outdoors [19]. Dill flowered early in the greenhouse, but only during a short period, and at the beginning of May, the plants had no flowers and died soon after. Compared with the other two plants, the short flowering period observed in our study, which other authors have confirmed as being as short as seven days [26], limits its usefulness as secondary/insectary plant inside a greenhouse.

In different species of the genus Orius, short photoperiods and low temperatures induce reproductive diapause [46,47]. Orius laevigatus showed a reduced response to daylight but was more affected by temperature [48], which can affect its reproductive performance. The temperatures during the first release (end of February in both years) were lower than those during the second release (end of April in both years). This could have caused difficulties in reproduction, as observed in both seasons, with almost no nymphs present in the secondary plants after the first release. This contrasts with the second release, when the presence of nymphs was clear and, in some cases, abundant on the sample day after the release (as in 2021).

Orius laevigatus was evident in two of the secondary plants in the last part of the sampling period in both years. However, its presence in ornamental plants was not as evident, particularly in carnation, in which almost no O. laevigatus adults or nymphs were observed in the flowers during the two years of the study, which may be explained by low attraction to this crop, as observed by other authors [49]. In contrast, the ornamental plant statice presented a low density of O. laevigatus on its flowers in 2025 but, with its abundant blooming, housed a relatively important population, which was probably attracted by the constant presence of thrips. Gerbera daisy also had low densities of O. laevigatus (mainly nymphs), but its population was low due to the low number of flowers present throughout the study. Sweet William also had very low O. laevigatus densities throughout the study. The movement of predators from the secondary plants to the crop plants (ornamental species in our case) was low, as observed by other authors [11].

Our results showed very low captures of O. laevigatus in the yellow traps, indicating low mobility of adults, or that they are not very attracted to yellow. Other groups of beneficial insects, such as hymenopteran parasitoids and lacewings (Neuroptera, Chrysopidae), are attracted by yellow sticky traps [50,51], but this could be a drawback for its performance on outdoor and indoor crops.

The implementation of these secondary plants in greenhouse crops to improve the installation of O. laevigatus has been performed with success in other types of crops, such as sweet pepper [37], which is normally planted in August in greenhouses in the south of Spain. Although the low movement of predators from secondary plants to crops could be an obstacle to using such plants [11], their presence has shown a good effect in the increment in parasitism of different pests by hymenopterans [8,22,52,53,54] due to different mechanisms [2], which could be enough of a reason to consider its utilisation for both outdoor and indoor crops.

5. Conclusions

Alyssum and yarrow housed O. laevigatus adults and nymphs, increasing its population in a greenhouse. This result was confirmed after two years. Alyssum presented long flowering and vegetation periods, which are crucial for secondary/insectary plants. Yarrow showed a more variable phenology between the two years of the study, and the cultivation method should be considered. Dill was neither attractive to O. laevigatus nor useful as a secondary/insectary plant in the greenhouse, mainly because of its limited flowering and vegetative periods, which was consistent in both years. In addition, dill was attractive to thrips and a reservoir of a great number of Aphididae, at least in one year of the study. Colonisation of the ornamental plants cultivated in the greenhouse by O. laevigatus from the secondary plants was not remarkable, being almost null in the case of carnation (D. carophyllus), very low in gerbera daisy (Gerbera sp.) and sweet William (D. barbatus), and of some importance in statice (L. sinuatum). These results provide valuable information on the use of the tested secondary plants for the development of biological control inside greenhouses, but they also consider the suitability of the crop cultivated for beneficial arthropods.