Preliminary Evaluation of the Biocontrol Potential of Stethorus punctillum, a Key Natural Enemy of Spider Mites in Northwest China

Abstract

Spider mites are globally distributed pests that cause significant damage to a wide range of crops. The use of predators for the control of pest mites is an effective and environmentally sustainable strategy. Stethorus punctillum Weise (Coleoptera: Coccinellidae), a well-known predator of spider mites, has been widely recognized as the primary natural enemy of pest mites in China. However, its pest control efficacy, particularly under field conditions, is not well known. In this study, we evaluated the biocontrol impact of S. punctillum on a key spider mite pest, Tetranychus urticae Koch (Acarina: Tetranychidae), through a combination of laboratory and field experiments. Laboratory assays showed that the predation rates in relation to the prey numbers were consistent with the Holling-II functional response model. The actually maximum predatory numbers of third-instars of S. punctillum, 3-day-old female adults, and male adults on the pest were 116.67, 181.67, and 166.67 mites per day, respectively, corresponding to the theoretically maximum values of 391.26, 498.07, and 413.95 mites per day individually. Field exclusion experiments demonstrated that both larval and adult stages of S. punctillum significantly suppressed spider mite populations’ growth across three different initial prey densities (80, 110, and 140 individuals for larvae; 100, 150, and 200 individuals for adults) on three economically important crops: maize, cotton, and apples. Within 96 h of their introduction, the pest population growth rate was reduced by 13.2–43.2% by larvae and 25.3–51.5% by adults of S. punctillum compared to predator-free control groups. These findings demonstrate that S. punctillum has a significant control efficacy on spider mite populations under both laboratory and field conditions, highlighting its potential as a promising biocontrol agent for integrated spider mite management in Northwest China.

1. Introduction

Spider mites are global pests that cause severe damage to a wide variety of crops [1,2,3], especially maize, cotton, and fruit trees [4,5,6]. Spider mite feeding causes leaf chlorosis, abscission, and even death of whole plants, resulting in significant yield loss [7]. The two-spotted spider mite (Tetranychus urticae Koch) is emerging as a globally significant pest in agricultural ecosystems [8]. At present, spider mite control is mainly performed through chemical acaricides, which, however, can lead to a series of problems such as pesticide-induced resistance, resurgence, and ecological environment damage [1,9]. Therefore, the deployment of natural enemies for spider mite control can enhance ecological equilibrium through trophic cascade regulation, promoting resource-efficient agroecosystem resilience [10,11]. Stethorus punctillum Weise, a major predator of T. urticae and other spider mites [12], is widely distributed across most temperate regions in North America, Europe, and Asia [13]. Current knowledge of S. punctillum biology predominantly concerns its biological characteristics in laboratory tests [12,14], its seasonal field dynamics [15], methods of field releases [16,17], and its interactions with other natural enemies in greenhouses [18,19]. On the contrary, the field-based biological efficiency of S. punctillum against pest mites in different crops remains poorly understood.

In recent years, the intensification of agriculture in Northwest China has significantly increased [20], and the resulting simplification and homogenization of the agricultural landscape have led to a loss of biodiversity [21]. The trend has adversely impacted the pest control functions provided by natural enemies [22], leading to increased dependence on chemical pesticides because of the disruption of tri-trophic cascades [23]. Frequent pesticide application has intensified a series of well-known disadvantages of chemical pesticides such as pest resurgence, harm to non-target species, and environmental pollution [21,24]. To promote better conservation of biodiversity in local agricultural and natural ecosystems, spider mite management activities need to better conserve natural enemies and improve farmland habitats near crop fields [20,25,26]. This approach has great promise for more sustainable development of the farmland ecological environment in Northwest China [27,28]. Elucidating the biocontrol functions of S. punctillum for spider mites across diverse crops enables landscape-scale ecological network analysis to better assess this predator’s biological control potential [27,29,30].

In this study, we aimed to evaluate (1) the predation ability of S. punctillum (including larvae and adults) on the two-spotted spider mite T. urticae in the laboratory based on predation functional response and (2) the suppressive efficacy of this predator on the target pest population growth under different natural enemy–pest ratios on three main crops in fields, respectively. Our results will provide scientific insight for the conservation and utilization of the ladybeetle S. punctillum to improve the IPM level on spider mites in Northwest China.

2. Materials and Methods

2.1. Test Insect Sources

The population of spider mite T. urticae and ladybeetle S. punctillum was collected from an apple orchard in the Horticultural Farm of Dangzhai Town, Ganzhou District, Zhangye City, Gansu Province (38.84° N, 100.49° E). The indoor rearing of insects was based on an existing method, with significant improvement [12]. T. urticae was reared on Phaseolus vulgaris L. seedlings in the laboratory. Mites were introduced onto bean seedlings when the first compound leaves were fully expanded, and all seedlings were watered once every 5 days. The predator S. punctillum was reared on fresh bean leaves infested with T. urticae, which were replaced daily. Rearing was carried out in an artificial climate chamber (26 ± 1 °C, RH 50 ± 10%, photoperiod 16L: 8D).

2.2. The Functional Response of S. punctillum on T. urticae in Laboratory Trials

In the laboratory, we presented predator life stages with prey in varying densities to assess their functional responses. Before this test, third-instar larvae of S. punctillum were starved for 3 h, while 3-day-old female and male adults were starved for 12 h. Fresh primary leaves of P. vulgaris were placed in Petri dishes (90 mm diameter × 15 mm h). T. urticae adults with a similar body size were transferred on the leaves using a camel-hair brush. Individual predators (one larva or adult of S. punctillum per dish) were introduced, and the dishes were then sealed by parafilm. The dishes were aerated via microperforations in the parafilm made with an insect pin, whereas the mites could not escape. Ladybeetle larval trials employed prey densities of 40, 80, 120, 160, and 200 adult mites/dish, while adult (both female and male) trials used 50, 100, 150, 200, and 250 adult mites/dish. Each prey density and predator combination was replicated three times, and a total of 45 predator–prey density combinations were tested. After 24 h, the number of surviving mites was counted under a stereomicroscope to calculate the predation rate [31].

2.3. Spider Mite Control by S. punctillum on Different Crops in the Field

Regarding crop selection, we selected three widely cultivated crops—maize, cotton, and apple—to assess the biocontrol services of S. puntillum on two-spotted spider mites in Ganzhou District, Zhangye City, in 2024. In the field experiment, both maize (variety “Zhengnong 16-002”) and cotton (variety “Zhongmian 113”) were sown in early May (spring), mulched, and spaced 30 cm apart; each crop covered > 2000 m² and was randomly divided into three (≈667 m²) plots as replications. An apple orchard (variety “Fuji”, >2000 m²) of 20-25 y and row spacing of 4 m × 6 m was also selected. No chemical pesticides were applied during the whole growing period in the cotton and maize fields, and no insecticides were applied 30 days prior to the experiment in the apple orchard.

Regarding the natural enemy–pest ratio design, in each crop field, during the autumn peak period of spider mites (maize, middle August, milky stage; cotton, early September, flowering and bolling stage; and apple, early September, fruit expansion stage), we deployed paired cages with predators present and absent to evaluate their suppressive efficacy on this pest population. For ladybeetle larvae, the enemy–pest ratio was quantized as three prey densities of 80, 110, and 140 adult mites per plant for each predator larva (third-instar). Another three prey densities of 100, 150, and 200 adult mites per plant were assigned to each ladybeetle female adult (3-day-old). Each pair of cages was replicated three times. A total of 36 cages (18 for larvae and 18 for adults of the predator) were deployed in each crop field.

Regarding the cage and sampling survey, in the maize and cotton fields, in each plot, for each cage with a predator present, only one plant with uniform growth and enough naturally occurring spider mites was selected, while another similar plant about 3 m apart in the same transection was selected as the control for the absence of predator. A middle-section leaf with mites was selected for maize, whereas the third unfolded leaf at the top of cotton was selected. The pair of cages with or without predator was deployed with the same prey density gradient. To achieve our desired mite densities, we retained only the desired number of adult mites, selecting ones of uniform body size, and all the remaining mites and any other arthropods on the selected leaves were removed with a small soft brush. Then, the leaves were caged with pre-positioned net bags, which were closed with a nylon cord attached to the opening of the bag. The population of spider mites on the leaves of each treatment was recounted in the field by folding back the net bag daily for 4 days (the third-instar larvae stage lasts about 4 d) after the bags were put in place.

Similarly, in the experimental apple orchard, trees for each paired cage were spaced more than 10 m apart. For each selected tree, branches on the same side were selected for cage treatment, one leaf with a medium level of mite damage was chosen, and the required number of spider mites of uniform body size was retained as described above. All other arthropods on the selected leaves were removed using a small soft brush. After introducing one third-instar larva or one 3-day-old female adult of S. punctillum onto each test leaf, the leaf was enclosed in a net bag, which was then sealed. The control group had no S. punctillum introduced. The population of spider mites on the leaves was recounted daily for the first four days after the bags were put in place.

2.4. Data Analysis

For the laboratory tests, one-way ANOVA with Tukey’s HSD was used to compare the numbers of consumed prey under different prey densities treated with larvae or adults of the predator. Before the ANOVA, the normality and variance homogeneity of the data were tested and the model hypothesis was well fitted. The Holling-II model was fitted to the functional response of predators to different prey density gradients [32]; the equation was the following:

N_a = a∗N₀∗T/(1 + a∗T_h∗N₀),

where Na represents the actual numbers of prey consumed by a predator during T time (set as 1 d, thus T = 1), a represents the instantaneous attack rate of the predator on the prey, N₀ represents the initial prey density, T_h represents the handling time (i.e., the time required to search and eat one prey item), and 1/T_h is the theoretical maximum daily predation rate.

For field caged exclusion experiments on each crop, due to the nonindependence of prey population growth data, generalized linear mixed-model (GLMM) analysis with Poisson distribution was employed to assess the suppressive efficacy of the ladybeetle on the spider mite population under different enemy–pest ratios (corresponding to various prey density gradients). In the model, for each prey density, fixed effects included the treatment (a binary variable, predator present or absent), sampling time (days after caged), and their interaction (treatment ∗ day), while repetitions were the random effect. GLMM analyses were conducted using the “glmmTMB” function in the “glmmTMB” package [33], and the fixed effects of the model were tested using the “Anova” function in the “car” package [34]. All analyses were performed in R 4.2.1 software [35].

For a direct comparison of the pest population growth rate during the 96 h sampling period in the field cage trial, we calculated a population-relative suppressive rate (PRSR), as follows:

PRSR = (N_c − N_t)/N_c∗100%,

where N_c represents the number of spider mites in the control cage without predator and N_t represents the number of spider mites in the paired cage with a predator present. The PRSR values were calculated for the different enemy–pest ratio gradients individually.

3. Results

3.1. The Functional Response of Larvae and Adults of S. punctillum in the Laboratory

The actually maximum predatory capacity by third-instar larvae and 3-day-old adult females and males of S. punctillum on T. urticae were 116.67 ± 2.33, 181.67 ± 2.73, and 166.67 ± 5.93 mites per day, respectively; predatory capacity was significantly increased with an increasing prey density in all insect stages, and the predation of females was a little higher than that of males under the same prey density treatment (

Table 1. The number of prey consumed per predator (larva or adult of Stethorus punctillum) to assess the functional response in the laboratory.

Ladybeetle Life Stage	Prey Density	Consumed Numbers of Prey	ANOVA Statistics
Third-instar larva	40	32 ± 1.53 e	F4,10 = 155.62 p < 0.001
	80	58.33 ± 2.33 d
	120	72.67 ± 3.76 c
	160	93 ± 2.52 b
	200	116.67 ± 2.33 a
Female adult	50	47.33 ± 1.86 e	F4,10 = 294.58 p < 0.001
	100	94.67 ± 0.67 d
	150	129.67 ± 3.84 c
	200	153.67 ± 4.48 b
	250	181.67 ± 2.73 a
Male adult	50	47.33 ± 1.45 d	F4,10 = 194.06 p < 0.001
	100	85.33 ± 2.73 c
	150	125.67 ± 2.60 b
	200	140.67 ± 2.40 b
	250	166.67 ± 5.93 a

Note: The data are means ± SE. Data with different lowercase letters in the same column indicate significant differences with Tukey’s HSD test (p < 0.05).

). The functional responses of third-instar larvae and 3-day-old adult females and males of S. punctillum feeding on T. urticae conformed to Holling’s Type II model (Figure 1,

Table 2. Functional responses of S. punctillum larvae and adults to the spider mite.

Ladybeetle Life Stage	Functional Response Equation	R2	χ2	Instant Attack Rate (a)	Handling Time (Th)/(d)	Maximum Daily Consumption (1/Th)
Third-instar larva	Na = 0.7978N0/(1 + 0.0020N0)	0.972	−0.0497	0.7978	0.0026	391.26
Female adult	Na = 1.1389N0/(1 + 0.0023N0)	0.989	−0.0172	1.1389	0.0020	498.07
Male adult	Na = 1.1120N0/(1 + 0.0027N0)	0.980	−0.0179	1.1120	0.0024	413.95

). The chi-square goodness-of-fit tests demonstrated that there was no significant difference between the observed and theoretical values fitted by the model (R² > 0.97, p ≈ 1).

The instantaneous attack rates of third-instar larvae and 3-day-old adult females and males of the predator were 0.7978, 1.1389, and 1.1120, respectively. The handling time per prey was 0.0026, 0.0020, and 0.0024, respectively. The theoretical maximum predation rates were 391.26, 498.08, and 413.95 prey per day individually (Table 2). In all three cases, the number of prey eaten rose in a decelerating manner with an increasing prey density, and the model-predicted and actual predation values had a good fit, indicating that both larvae (Figure 1A) and adults (Figure 1B,C) have a strong biocontrol effect on the spider mites.

3.2. Impacts of S. punctillum on Caged Cohorts of Spider Mites on Three Crops in the Field

In the field cage experiments of three crops over 4 days, the population abundance of spider mites on the test leaves showed an increasing trend. Notably, predator-inclusive groups demonstrated gradual population growth, whereas the control group which had no predator experienced a sharp exponential increase. Cages with a higher initial prey density induced a higher growth rate, due to a higher intrinsic rate of increase or a shorter population doubling time, especially in the predator-free control group, whereas these increasing trends were significantly suppressed by the predator, both for larvae and adults (

Table 3. Population-relative suppressive rate (PRSR) of spider mite cohorts under different predator–pest abundance ratios on the 4 d after caging in different crop fields.

Ladybeetle Life Stage	Predator–Prey	PRSR of Spider Mites on Day 4
		Maize	Cotton	Apple
larvae	1:80	20.08% ± 1.12%	21.99% ± 2.21%	38.88% ± 1.93%
	1:110	13.16% ± 3.98%	23.74% ± 2.04%	23.48% ± 1.01%
	1:140	26.21% ± 4.52%	43.19% ± 2.04%	21.27% ± 3.53%
adults	1:100	31.99% ± 11.64%	25.28% ± 4.95%	30.86% ± 3.32%
	1:150	38.52% ± 5.66%	38.24% ± 1.18%	38.36% ± 1.78%
	1:200	51.47% ± 1.90%	40.02% ± 2.24%	42.69% ± 0.59%

Note: The data are means ± SE.

, Figure 2, Figure 3 and Figure 4). The population-relative suppressive rate (PRSR) in the maize field ranged 13.16–26.21% for S. punctillum larvae, whereas a higher PRSR ranged 31.99–51.47% for their adults. The PRSR in the cotton fields ranged 21.99–43.19% for S. punctillum larvae, whereas a similar PRSR ranged 25.88–40.02% for their adults. The PRSR in the apple orchard was in the range of 21.27–38.88% for the larvae and 30.86–42.69% for the ladybeetle adults (Table 3).

For the cages with the ladybeetle adult in the maize field, for all prey density treatments, spider mite abundance was significantly increased following the sampling times both in cages with and without a predator present (see

Table 4. Generalized linear mixed-model (GLMM) analysis results of cage experiment in maize field. The fixed effects were treatment (with or without predator), sampling time (Day), and their interaction (Treatment∗Day). The GLMM analysis was performed under different predator stages (larva and adult) and initial prey densities individually.

Ladybeetle Life Stage	Initial Prey Densities	Fixed Effect	χ2	df	p
Larva	80	Treatment	52.30	1	<0.001
		Day	180.62	3	<0.001
		Treatment × Day	11.86	3	0.0079
	110	Treatment	52.12	1	<0.001
		Day	96.63	3	<0.001
		Treatment × Day	10.12	3	0.0176
	140	Treatment	100.38	1	<0.001
		Day	109.05	3	<0.001
		Treatment × Day	2.25	3	0.5225
Adult	100	Treatment	82.87	1	<0.001
		Day	93.19	3	<0.001
		Treatment × Day	5.29	3	0.1517
	150	Treatment	176.71	1	<0.001
		Day	108.84	3	<0.001
		Treatment × Day	7.29	3	0.0632
	200	Treatment	410.40	1	<0.001
		Day	294.69	3	<0.001
		Treatment × Day	44.38	3	<0.001

Note: Values marked in bold are statistically significant (type II Wald chi-square test; p < 0.05).

, fixed effect of “Day”, p < 0.001). For ladybeetle larvae, the population abundance of the control group (without predator) was significantly higher than that in the cages with the predator present during the 4-day sampling time at the prey densities of 80 (χ² = 52.30, df = 1, p < 0.001, Figure 2A), 110 (χ² = 52.12, df = 1, p < 0.001, Figure 2B), and 140 (χ² = 100.38, df = 1, p < 0.001, Figure 2C) individuals per plant (Table 4), except for the low prey density of 80 (Figure 2A, p = 0.3284) and 110 (Figure 2B, p = 0.0840) spider mite individuals per plant after being caged for 1 d.

For the cages with the ladybeetle adults, the pest population growth was significantly suppressed compared to the control cages without predators at the prey densities of 100 (χ² = 82.87, df = 1, p < 0.001, Figure 2D), 150 (χ² = 176.71, df = 1, p < 0.001, Figure 2E), and 200 (χ² = 410.40, df = 1, p < 0.001, Figure 2F) individuals per plant 1–4 days after the caged treatment.

In the cotton field, for all prey density treatments, spider mite abundance was significantly increased followed by the sampling times both in cages with and without a predator present (see

Table 5. Generalized linear mixed-model (GLMM) analysis results of cage experiment in cotton field. The fixed effects were treatment (with or without predator), sampling time (Day), and their interaction (Treatment∗Day). The GLMM analysis was performed under different predator stages (larva and adult) and initial prey densities individually.

Ladybeetle Life Stage	Initial Prey Densities	Fixed Effect	χ2	df	p
Larva	80	Treatment	65.92	1	<0.001
		Day	180.07	3	<0.001
		Treatment × Day	4.65	3	0.1995
	110	Treatment	93.38	1	<0.001
		Day	108.27	3	<0.001
		Treatment × Day	6.80	3	0.0787
	140	Treatment	229.39	1	<0.001
		Day	117.31	3	<0.001
		Treatment × Day	11.35	3	0.0100
Adult	100	Treatment	100.72	1	<0.001
		Day	150.74	3	<0.001
		Treatment × Day	2.14	3	0.5439
	150	Treatment	103.60	1	<0.001
		Day	106.46	3	<0.001
		Treatment × Day	15.05	3	0.0018
	200	Treatment	284.55	1	<0.001
		Day	201.62	3	<0.001
		Treatment × Day	8.17	3	<0.001

Note: Values marked in bold are statistically significant (type II Wald chi-square test; p < 0.05).

, fixed effect of “Day”, p < 0.001). For ladybeetle larvae, the population abundance of the control group (without predator) was significantly higher than in the cages with the predator present during the 4-day sampling time at the prey densities of 80 (χ² = 65.92, df = 1, p < 0.001, Figure 3A), 110 (χ² = 93.38, df = 1, p < 0.001, Figure 3B), and 140 (χ² = 229.39, df = 1, p < 0.001, Figure 3C) individuals per plant (Table 5).

For the cages with the ladybeetle adults, the pest population growth was significantly suppressed compared to the control cages without predators at the prey densities of 100 (χ² = 100.72, df = 1, p < 0.001, Figure 3D), 150 (χ² = 103.60, df = 1, p < 0.001, Figure 3E), and 200 (χ² = 284.55, df = 1, p < 0.001, Figure 3F) individuals per plant 1–4 days after the caged treatment.

In the apple orchard, for all prey density treatments, spider mite abundance was significantly increased with the sampling times both in the cages with and without a predator present (see

Table 6. Generalized linear mixed-model (GLMM) analysis results of cage experiment in apple orchard. The fixed effects were treatment (with or without predator), sampling time (Day), and their interaction (Treatment∗Day). The GLMM analysis was performed under different predator stages (larva and adult) and initial prey densities individually.

Ladybeetle Life Stage	Initial Prey Density	Fixed Effect	χ2	df	p
Larva	80	Treatment	121.98	1	<0.001
		Day	136.81	3	<0.001
		Treatment × Day	3.93	3	0.2689
	110	Treatment	67.47	1	<0.001
		Day	94.89	3	<0.001
		Treatment × Day	0.10	3	0.9924
	140	Treatment	42.11	1	<0.001
		Day	82.15	3	<0.001
		Treatment × Day	0.84	3	0.8395
Adult	100	Treatment	116.91	1	<0.001
		Day	171.15	3	<0.001
		Treatment × Day	1.76	3	0.6231
	150	Treatment	162.74	1	<0.001
		Day	110.98	3	<0.001
		Treatment × Day	7.26	3	0.0640
	200	Treatment	259.31	1	<0.001
		Day	192.40	3	<0.001
		Treatment × Day	24.95	3	<0.001

Note: Values marked in bold are statistically significant (type II Wald chi-square test; p < 0.05).

, fixed effect of “Day”, p < 0.001). For the ladybeetle larvae, the population abundance of the control group (without predator) was significantly higher than that in the cages with the predator present during the 4-day sampling time at the prey densities of 80 (χ² = 121.98, df = 1, p < 0.001, Figure 4A), 110 (χ² = 67.47, df = 1, p < 0.001, Figure 4B), and 140 (χ² = 42.11, df = 1, p < 0.001, Figure 4C) individuals per plant (Table 6).

For the cages with the ladybeetle adults, the pest population growth was significantly suppressed compared to the control cages without predator at the prey densities of 100 (χ² = 116.91, df = 1, p < 0.001, Figure 4D), 150 (χ² = 162.74, df = 1, p < 0.001, Figure 4E), and 200 (χ² = 259.31, df = 1, p < 0.001, Figure 4F) individuals per plant 1–4 days after the caged treatment.

4. Discussion

Spider mites have emerged as economically critical pests affecting staple crops—notably maize, cotton, and fruit trees—in Northwest China, a region experiencing escalating agricultural intensification linked to agroecosystem destabilization [36,37,38]. The anthropogenic landscape’s homogenization is potentially leading to biodiversity erosion and trophic cascade collapse, thereby intensifying reliance on synthetic pesticides [21]. Long-term application of chemical pesticides (especially broad-spectrum pesticides) can trigger a series of agricultural ecological problems, such as a decrease in the population of natural enemies, an increase in the resistance of pests, and the disruption of the ecological balance [39,40].

In this study, the predatory capacity of ladybeetle S. punctillum was significantly increased with an increasing prey density in all insect stages, and the functional response of S. punctillum to two-spotted spider mite prey conformed to Holling’s Type II model, consistent with Matter et al. [41]. The theoretical maximum predation rates of third-instar larvae, 3-day-old adult females, and 3-day-old males of S. punctillum on T. urticae were 391.26, 498.08, and 413.95 preys per day individually, much higher than the maximum daily predation of Amblyseius herbicolus Chant on mites [42]. The attack rates of the third-instar larvae, female adults, and male adults of S. punctillum in our study were 0.7978, 1.1389, and 1.1120, which were similar to the findings of predation of Kampimodromus aberrans (Oudemans) on T. truncatus [43] and Neoseiulus californicus (McGregor) on T. urticae [44], but higher than those of Phytoseiulus persimilis (Athias-Henriot), Galendromus occidentalis (Nesbitt), and Neoseiulus californicus (McGregor) [45]. Based on a large number of previous experiments, three biological replications were performed for third-instar larvae, male adults, and female adults at all prey densities in the present experiment. The results show that our experimental results were stable and the model-predicted and actual predation values had a good fit, demonstrating that S. punctillum has a robust biocontrol effect on pest mites.

In field cage experiments in three important regional crops, we found that the introduction of S. punctillum larvae into cages stocked with different prey densities resulted in significantly fewer mites than the controls after 2 days in the maize field. Regarding the introduction of adult predators, there were significant differences in the population of spider mites between the treatments with predators and the control at each observation time. In the cage experiments with S. punctillum larvae and adults in the cotton field and apple orchard, there were significant differences in the population of spider mites between all treatments with introduced predators and the control at each observation time. Compared with P. persimilis in greenhouse crops [46], both S. punctillum larvae and adults exhibited significant pest control ability against spider mites in all tested plants under low, medium, and high prey densities throughout all observation periods, suggesting the promise of this predator for use as a biological control agent.

Spider mite abundance in all cages with a predator present exhibited a relatively gradual increase, whereas the control groups (particularly the highest-density treatment) showed exponential population growth over 4 days, attributable to the elevated initial prey density and absence of predators. In addition to this, initial prey spawning and residual egg incubation could have also been possible causes. Furthermore, the predation rates demonstrated a decelerating pattern with an increasing prey density, consistent with Type II functional response dynamics. Collectively, predators in the caged environment induced progressively higher relative mite population suppression as the prey density increased, except for larval stages on apple leaves. This anomaly could be attributed to the trichomes on the abaxial surface of apple leaves impeding larval mobility. These findings suggest that S. punctillum can effectively suppress population growth even during spider mite outbreak conditions. However, these semi-controlled field cage observations require validation through large-scale open-field release trials. Discrepancies were observed between the laboratory and field experimental outcomes, with divergent environmental conditions likely serving as a primary contributing factor. Hence, future field releases of natural enemies must account for ambient weather parameters to optimize their efficacy under natural conditions.

Currently, S. punctillum is a known important natural enemy of phytophagous mites, with supplementary predation on aphids and other small arthropod pests [14]. S. punctillum occurs in various crops in Northwest China and shows a seasonally bimodal abundance pattern on widespread landscaping elms (Ulmus pumila L.): populations reach relatively high densities in early spring and resurge in late autumn. This phytophagous refugia suggests that U. pumila may serve as a critical overwintering reservoir, whose strategic cultivation could enhance the conservation of this predatory beetle [26,47] and enhance sustainable spider mite control in the region. Maize, cotton, and apple are widely distributed and critically important staple and cash crops globally, with spider mites being a key pest threatening their yield and quality. Our findings demonstrate that S. punctillum exhibits strong control efficacy against spider mites on these crops and plays a significant role in sustainable spider mite management. Future studies should systematically investigate its application across diverse cropping systems to expand the IPM toolkit for spider mite control.

5. Conclusions

In summary, we verified that ladybeetle S. punctillum is a highly effective predator of leaf mites through indoor and field cage trials. The predation efficiency, functional response, and mite population suppression efficiency on three major economic crops in this study provide a comprehensive understanding of the biocontrol potential of S. punctillum. The results support the notion that the adult stage is more effective than the larval stage and is more suitable for field release to control pest mites. Host plant traits also have an impact on the biological pest control ability of these small ladybeetles. Our findings assist in the implementation of biological mite control strategies and provide information useful for optimizing sustainable management approaches of pesticide-resistant mite populations in global agricultural systems.