Interspecific Competition Between Eotetranychus sexmaculatus Riley and Oligonychus biharensis Hirst (Acari: Tetranychidae)

Simple Summary

Eotetranychus sexmaculatus and Oligonychus biharensis are devastating mite pests of the valuable tropical crop, rubber trees. Outbreaks of both spider mite species are associated with high temperatures and drought stresses. To evaluate both intra- and interspecific competition and the effects of temperature, we analyzed the population dynamics of E. sexmaculatus and O. biharensis under different temperature treatments. The results reveal the presence of both types of competition and demonstrate that O. biharensis is always more competitive than E. sexmaculatus, regardless of the temperature and the initial ratio. Therefore, O. biharensis has the potential to become a dominant pest of rubber trees. These findings provide a theoretical basis for early detection and effective management strategies for spider mites on rubber trees.

Abstract

Eotetranychus sexmaculatus has long been recognized as an important spider mite pest of rubber trees. Recently, increasing damage from Oligonychus biharensis has elevated its importance as a key spider mite pest. These two species share highly overlapping ecological niches, with outbreaks strongly associated with high temperatures and drought stresses. However, little is known regarding the potential competition between these spider mite species and the role of temperature in shaping these interactions. This study investigates the development and reproduction of E. sexmaculatus and O. biharensis at varying population densities, and evaluates their dynamics at 27 °C, 30 °C, and 33 °C. Five initial population densities of E. sexmaculatus and O. biharensis were evaluated in mixed populations: 0:30, 10:20, 15:15, 20:10, and 30:0. The results demonstrate both intra- and interspecific competition between E. sexmaculatus and O. biharensis. At all three intraspecific densities, the survival rate and lifespan of both species declined as density increased, with fecundity also decreasing at higher densities. Single-species populations of each mite were larger in number when compared with mixed populations. Across all treatments, the mean and peak values of the intrinsic rate of increase (r_m) were greater in O. biharensis than in E. sexmaculatus. Additionally, increasing temperatures accelerated the displacement of E. sexmaculatus by O. biharensis, regardless of the initial population density. When the mixed populations of E. sexmaculatus and O. biharensis were at densities of 10:20, the highest interspecific competition coefficients were obtained at 33 °C, with values of 0.6591. In conclusion, O. biharensis consistently outcompeted E. sexmaculatus at all tested temperatures, irrespective of initial densities, providing valuable insights into the competitive dynamics of dominant rubber tree pests.

1. Introduction

Natural rubber is a highly important tropical crop and an essential industrial material [1,2]. Belonging to the mite family Tetranychidae, Eotetranychus sexmaculatus (Figure 1) is a widely distributed phytophagous mite that has constituted a critical pest problem for rubber tree (Hevea brasiliensis) plantations since the 1980s [1,3]. Our recent field survey of rubber trees revealed the presence of the spider mite Oligonychus biharensis (Figure 2), which belongs to the Tetranychidae family. Since 2016, O. biharensis has increasingly damaged rubber tree leaves, especially in high-cultivation regions such as Yunnan and Hainan. Unlike E. sexmaculatus, O. biharensis has a shorter history of rubber tree destruction and a limited geographic distribution. While these species may occur independently or simultaneously within a field (Figure 3), most severe rubber tree damage in China is attributed to E. sexmaculatus. Recently, reports of O. biharensis infestations have increased, indicating its rise as an important rubber tree pest [2]. Eotetranychus sexmaculatus and O. biharensis share a highly overlapping ecological niche, with both inhabiting and damaging the leaves and occasionally petioles of rubber trees. Moreover, both species are distributed across Hainan, Yunnan, and Guangdong and can be active in the spring, summer, and fall. This overlap likely results in interspecific competition between the two mites, which may be affected by environmental factors such as temperature. Interspecific competition occurs when different species at the same trophic level compete for the same resources to establish their populations [4,5,6]. Competitive interactions are generally asymmetric, leading to fluctuations in population sizes or even competitive displacement [7]. Therefore, further exploration is needed to determine if O. biharensis could replace E. sexmaculatus as the dominant rubber tree pest or whether they will form a stable coexistence.

Interspecific competition is influenced by various complex ecological factors, including temperature, host, and initial population size [5,8]. Intense interspecific competition has been documented within the same ecological niches in different temperature ranges [9]. For example, competition among the booklice species Liposcelis bostrychophila, Liposcelis decolor, and Liposcelis paeta has been reported at 25 °C and 30 °C [9]. As temperature adaptations vary across insect or mite species, environmental changes often have asymmetric effects on their competition. While the population dynamics of mites and insects are highly dependent on environmental temperature [10,11,12], they are also strongly influenced by the initial population size. For instance, when L. bostrychophila or L. decolor have larger populations, L. bostrychophila has a competitive advantage [9]. Competitive displacement is the most brutal result of interspecific competition and arises when two groups cannot coexist in the same environment. This phenomenon is related to ecological niche competition, host adaptability, and high-temperature adaptability [13]. For instance, the B biotype of Bemisia tabaci disrupts the mating of other biotypes, decreasing the proportion of females and contributing to population displacement.

Previous studies of E. sexmaculatus and O. biharensis have primarily focused on their biological characteristics, resistance mechanisms, and genetic functions [14,15,16,17]. However, their capacity for intra- and interspecific competition remains a mystery. Both E. sexmaculatus and O. biharensis are strongly adaptable on the rubber variety RRIM600. While E. sexmaculatus mainly damages the tender leaves of rubber plants [18], O. biharensis consumes rubber trees as well as longan and litchi [19]. The population adaptability of E. sexmaculatus and O. biharensis was assessed at 21–33 °C. At 27 °C, the developmental duration of E. sexmaculatus is shorter than O. biharensis, while the lifespan of female adult mites is longer than O. biharensis. The fecundity of spider mites is relatively high at 24–33 °C. Studies have demonstrated that a temperature of 27–30 °C benefits E. sexmaculatus population growth, whereas O. biharensis is more suited for growth at 33 °C [20]. The overlap in these habitats indicates the potential for competition, although it is unclear if this has led to a change in the pattern of mite pest occurrence. Additionally, the role of environmental effects like temperature on these changes is unknown. This study examines the dynamics of E. sexmaculatus and O. biharensis in both single and mixed populations in moderate (27 °C) and high (30 °C and 33 °C) temperatures. Moreover, we calculated the intrinsic increase rate of the two spider mites and their interspecific competition coefficients to clarify the extent of the competition. This study provides a theoretical basis for pest forecasting and the effective control of rubber tree pests. By understanding the occurrence of spider mites, early monitoring and timely prevention and control can be achieved to limit the loss of glue production.

2. Materials and Methods

2.1. Mite Collection and Rearing

E. sexmaculatus and O. biharensis populations were collected from the rubber plantation in the Chinese Academy of Tropical Agriculture experimental field in Danzhou, Hainan Province (longitude: 109°28′9′′ E, latitude: 19°32′2′′ N). The mites were continuously reared on the leaves of the rubber tree variety Reyan 73397 for several generations in a laboratory setting (27 ± 1 °C, 70 ± 5% RH, L:D = 14:10). All experimental mites were maintained on Reyan 73397 leaves. E. sexmaculatus and O. biharensis used during these experiments were offspring produced by parents for continuous breeding, without combining with wild-type individuals.

2.2. Investigations on the Damage Caused by Spider Mites on Rubber Trees

Four rubber plantations in Huanglian Mountain, Mengla County, Yunnan Province were randomly selected and surveyed for E. sexmaculatus and O. biharensis presence and damage using the inter-row continuous plant method (longitude and latitude: 101°41′4′′ E and 21°82′7′′ N, 101°41′5′′ E and 21°83′3′′ N, 101°41′6′′ E and 21°83′0′′ N, 101°41′7′′ E and 21°83′2′′ N. From each plantation, 20 branches were collected from individual trees, with five leaves per branch analyzed for damage caused by E. sexmaculatus and O. biharensis. Twenty trees were selected from each rubber plantation, representing a total of eighty trees. A total of 400 rubber tree leaves were assessed. Both single-species and mixed populations were quantified, and the rate of leaf injury was calculated. No pesticides were applied to the field prior to the survey.

2.3. Mite Development and Reproduction in Single and Mixed Populations

Single population: The eggs of E. sexmaculatus or O. biharensis were obtained from the laboratory colonies and placed on leaf discs in groups of 5, 10, and 20. Each population was monitored once every 24 h and all treatments were replicated six times. Leaf discs (1.5 × 1.5 cm) were replenished every three days, and all experiments were maintained in an artificial climate box (27 ± 1 °C, 75 ± 5% RH, L:D = 14:10). The development duration and survival rate of each mite species were recorded during the juvenile stage, while fecundity and lifespan were observed during the adult stage.

Mixed population: Interspecific competition responses were measured by placing the eggs of both E. sexmaculatus and O. biharensis on the same leaf discs. Treatments consisted of E. sexmaculatus and O. biharensis eggs ratios of 5:5, 5:15, 10:10, and 15:5. Populations were monitored every 24 h and replicated six times. Leaf discs were replenished every three days, and all experiments were maintained in an artificial climate box (27 ± 1 °C, 75% ± 5% RH, L:D = 14:10). The developmental duration and survival rate of each mite species were recorded during the juvenile stage, while fecundity and lifespan were documented during the adult stage.

2.4. Interspecific Competition Under Various Temperatures

To investigate the role of temperature on interspecific competition, E. sexmaculatus and O. biharensis were assessed at five initial densities (0:30, 10:20, 15:15, 20:10, and 30:0), with a female-to-male ratio of 1:1, in artificial climatic chambers set to 27 °C, 30 °C, and 33 °C (75 ± 5% RH, L:D = 14:10), 30 pairs of spider mites were utilized, totaling 60 individuals. Adult virgin male and female mites were selected and transferred onto whole rubber tree leaves (96 cm²), with each treatment replicated three times. Leaves were refreshed every three days. The number of eggs, larvae, nymphs, and adult females and males of each species were recorded every five days. Each mixed population experiment concluded when one species was eliminated. Single-population experiments were concluded after all mixed-population experiments were completed.

2.5. Data Analysis

Each species’ data were analyzed independently using a three-way ANOVA, with temperature, initial density, and rearing time as the primary effects, and population size as the response variable. All analyses were conducted using Excel (version 2010) and SPSS (version 26.0), employing a one-way ANOVA and Tukey’s multiple comparisons. Graphs were generated using Graphpad Prism (version 9.4). The results are presented as the mean ± standard error. The specific calculation formula for the relevant indicator parameters is as follows:

(1)

R = $X_i$/T × 100%, where R is the rate of leaf injury, X represents the total number of leaves with mite hazards, i is (1) the total number of leaves damaged by E. sexmaculatus alone, (2) the total number of leaves damaged by E. sexmaculatus alone and in combination, (3) the total number of leaves damaged by O. biharensis alone, and (4) the total number of leaves damaged by O. biharensis alone and in combination, respectively. T denotes the total number of leaves assessed [21].

(2)

$\overline{D}$ = $\sum (x_i\times n_i)/n$, where D represents the development duration, x is the number of developmental days, i represents egg, larvae, protonymph instar, deutonymph instar and egg–adult, respectively (the same below). Similarly, n denotes the total number of spider mites at different developmental stages. Generation time represents the total developmental time from egg to adult mite [5].

(3)

$\overline{S}$ = C_i/N × 100%, where S is the survival rate, C represents the number entering the next stage, and N denotes the number completing previous stage [5].

(4)

$\overline{F}$ = $\sum (f_i/n_i)$, where F represents fecundity (eggs/female), f represents the total number of eggs laid on that day, and n denotes the total number of adult female mites on that day. Finally, i represents days [5].

(5)

$\overline{L}$ = $\sum (d\times n)$/N, where L denotes lifespan, d is the survival days of female adult mites on that day, n represents the number of female adult mites, and N indicates the total number of female adult mites [5].

(6)

R₀ = N_t/N₀, r_m = lnR₀/t, where R₀ represents the net increase rate, N_t represents population size at time t, and N₀ indicates population size at the initial time, r_m represents the intrinsic rate of increase, t represents the number of days of investigation time [22].

(7)

αij = $\sum p_i{p}_j/\sqrt{\left(\sum p_i^2\right)(\sum p_j^2})$, where αij represents interspecific competition coefficient, αij denotes the competition coefficient of species i and j sharing the same resource, and Pi and Pj represent the proportions of species i and j in each resource sequence [23].

3. Results

3.1. Spider Mite Damage in Rubber Plantations

Our survey of rubber plantations revealed both individual and mixed populations of E. sexmaculatus and O. biharensis. When occurring independently, E. sexmaculatus populations averaged 10.67 individuals, causing a leaf damage rate of 3.25%, while O. biharensis populations averaged 25.74 individuals, with a leaf damage rate of 16.50%. When occurring in mixed populations, the average E. sexmaculatus population reduces to 6.75. In contrast, the population size of O. biharensis increased to 32.62. The leaf damage rate of E. sexmaculatus and O. biharensis increases to 81.25% and 93.75%. This result suggests that O. biharensis inflicts more significant damage to rubber trees than E. sexmaculatus. Moreover, the population of O. biharensis is 4.83 times greater than that of E. sexmaculatus in the mixed populations (Figure 4 and Figure 5).

3.2. Effects of Mixed Populations on Spider Mite Development

Our study of density-dependent responses to intraspecific competition in E. sexmaculatus and O. biharensis revealed no significant difference in generation time across the three treatment densities (

Table 1. Development duration (days ± S.E.) of E. sexmaculatus and O. biharensis on different initial density.

Species	Initial Density	Egg	Larva	Protonymph Instar	Deutonymph Instar	Generation Time
E. sexmaculatus	0 + 5	4.93 ± 0.16 a	2.28 ± 0.23 a	1.73 ± 0.14 cd	1.57 ± 0.19 abc	10.49 ± 0.11 b
	0 + 10	4.00 ± 0.00 b	2.00 ± 0.00 a	2.82 ± 0.09 ab	1.10 ± 0.19 c	9.92 ± 0.18 bc
	0 + 20	4.13 ± 0.08 b	2.37 ± 0.13 a	1.22 ± 0.17 d	2.29 ± 0.33 a	9.99 ± 0.31 bc
	5 + 5	4.36 ± 0.18 b	2.20 ± 0.33 a	1.67 ± 0.26 cd	1.20 ± 0.20 bc	9.50 ± 0.22 c
	5 + 15	4.33 ± 0.21 b	2.53 ± 0.18 a	2.93 ± 0.21 ab	2.21 ± 0.36 ab	12.00 ± 0.45 a
	10 + 10	5.41 ± 0.19 a	2.24 ± 0.15 a	2.39 ± 0.20 bc	2.62 ± 0.37 a	12.67 ± 0.33 a
	15 + 5	4.43 ± 0.24 b	2.48 ± 0.45 a	3.33 ± 0.58 a	2.60 ± 0.56 a	12.84 ± 0.31 a
F, df1, df2, p value		8.308, 6, 35, p < 0.001	0.532, 6, 35, p < 0.001	7.728, 6, 34, p < 0.001	3.414, 6, 34, 0.015	22.269, 6, 34, 0.432
O. biharensis	0 + 5	4.63 ± 0.20 bc	1.97 ± 0.22 ab	1.68 ± 0.23 c	2.99 ± 0.53 a	11.27 ± 0.56 ab
	0 + 10	5.00 ± 0.03 ab	2.11 ± 0.12 ab	2.41 ± 0.37 bc	1.39 ± 0.25 b	10.94 ± 0.06 abc
	0 + 20	4.95 ± 0.02 ab	1.26 ± 0.13 c	2.50 ± 0.32 bc	1.68 ± 0.23 b	10.38 ± 0.20 bc
	5 + 5	4.10 ± 0.28 c	1.60 ± 0.13 bc	2.04 ± 0.38 c	1.83 ± 0.22 b	9.57 ± 0.28 c
	5 + 15	5.38 ± 0.16 a	1.74 ± 0.13 bc	3.64 ± 0.25 a	1.17 ± 0.17 b	11.93 ± 0.05 a
	10 + 10	5.43 ± 0.29 a	1.98 ± 0.26 ab	3.96 ± 0.41 a	1.65 ± 0.37 b	12.08 ± 0.91 a
	15 + 5	4.48 ± 0.23 bc	2.33 ± 0.21 a	3.39 ± 0.49 ab	2.05 ± 0.51 ab	12.21 ± 0.22 a
F, df1, df2, p value		5.689, 6, 35, p < 0.001	3.949, 6, 35, 0.004	6.113, 6, 32, p < 0.001	3.041, 6, 32, 0.018	4.688, 6, 33, 0.001

The data represents mean ± standard error. The means within the same column followed by different letters are significantly different (one-way ANOVA, p < 0.05). initial density = E. sexmaculatus+ O. biharensis.

). The survival rates of both spider mite species declined progressively as intraspecific density increased. Both species showed the highest survival rate at a density of five mites per disc across all developmental stages. Additionally, the survival rate of both species from egg to adult decreased with increased density (Figure 6 and Figure 7). The highest-density group had a survival rate approximately 20% lower than that of the lowest-density group.

In mixed populations, both E. sexmaculatus and O. biharensis showed a significantly shorter development duration when in a 5:5 ratio, while no significant differences were observed in the remaining treatments. The developmental duration is influenced by density. Except for in the 5:5 treatment, the generation time of E. sexmaculatus was longer in mixed populations than in single ones, suggesting that this coexistence prolongs the species’ developmental period (Table 1).

No significant differences were observed in the survival rate of E. sexmaculatus from egg to adult in any of the four density treatments. The survival rate of E. sexmaculatus from egg to adult was reduced when the spider mites coexisted than when in isolation. The survival rate of O. biharensis from egg to adult was highest at an initial density of five mites per disc, whether in singular or mixed populations (Figure 6 and Figure 7).

3.3. Effects of Mixed Populations on Spider Mite Reproduction

The longevity and fecundity of adult female mites in both species were lower across all mixing ratios compared to single-species populations, indicating that coexistence plays a significant role in population potential (Figure 8 and Figure 9). In single populations, the longevity of adult E. sexmaculatus (Figure 8a) and O. biharensis (Figure 8b) females decreased with increasing intraspecific density. The difference in the longevity of adult female O. biharensis mites was significant between the 0:5 and 15:5 groups.

3.4. Interspecific Competition Between Mite Species at Different Temperatures

The interactions between temperature and initial density, temperature and rearing time, and initial density and rearing time significantly affected the population size of E. sexmaculatus. In contrast, O. biharensis was strongly influenced only by the interactions between temperature and rearing time, as well as by initial density and rearing time (p < 0.05) (

Table 2. ANOVA parameters for the primary effects and their interactions.

Spider Mite	Factor	df	F	p
E. sexmaculatus	Temperature	2	5.100	0.007
	Initial density	3	40.163	p < 0.001
	Rearing time	7	3.679	0.001
	Temperature and initial density	6	3.032	0.008
	Temperature and rearing time	9	2.126	0.031
	Initial density and rearing time	18	4.547	p < 0.001
	Temperature and initial density and rearing time	21	0.579	0.927
O. biharensis	Temperature	2	7.308	0.001
	Initial density	3	45.195	p < 0.001
	Rearing time	7	271.637	p < 0.001
	Temperature and initial density	6	0.927	0.477
	Temperature and rearing time	9	3.269	0.001
	Initial density and rearing time	18	7.011	p < 0.001
	Temperature and initial density and rearing time	21	0.721	0.806

).

In single-species treatments, the size of both spider mite populations was heavily dependent on temperature and differed significantly from the 5th to the 40th day (p < 0.05). The population sizes of E. sexmaculatus was largest at 30 °C compared to that at 27 °C and 33 °C at all observation time points. When the population sizes of E. sexmaculatus reached their peaks at 27 °C, 30 °C, and 33 °C, respectively, they increased to 17.52, 34.40, and 2.64 times the initial number, respectively. The population size of O. biharensis showed an overall increasing trend. From the 5th to the 15th day, the O. biharensis population size was greater at 33 °C compared to that at 27 °C and 30 °C. From the 20th to the 40th day, the O. biharensis population size was greater at 27 °C compared to at 30 °C and 33 °C. At the end of the observation, the population sizes of O. biharensis at 27 °C, 30 °C, and 33 °C, increased to 693.32, 417.94, and 357.76 times the initial number, respectively (Figure 10a).

When mixed populations initially comprised 20 E. sexmaculatus and 10 O. biharensis mites, each species’ population size changed drastically from the 5th to the 40th day (p < 0.05). At 27 °C, E. sexmaculatus dominated until the 10th day, when O. biharensis surpassed it in terms of population size. At 27 °C, E. sexmaculatus briefly dominated during the first 10 days when containing initially higher populations. At 27 °C, 30 °C, and 33 °C, when the population sizes of E. sexmaculatus reached their peak, they increased to 7.19, 1.21, and 2.47 times the initial number, respectively. When the population sizes of O. biharensis reached their peak, they increased to 1241.58, 186.37, and 94.73 times their initial number at these three temperatures, respectively (Figure 10b).

Mixed populations beginning with 15 E. sexmaculatus and 15 O. biharensis mites showed significant population fluctuations from the 5th to the 35th day (p < 0.05). The population of O. biharensis dominated throughout the entire observation period. At 27 °C, 30 °C, and 33 °C, when the population sizes of O. biharensis reached their peaks, they increased to 526.22, 139.34, and 77.73 times the initial number, respectively. For E. sexmaculatus, their peak population sizes increased to 7.61, 1.90, and 2.78 times the initial number at three tested temperatures, respectively. The population of O. biharensis generally increased, whereas the population of E. sexmaculatus initially increased and subsequently declined (Figure 10c).

When beginning with 10 E. sexmaculatus and 20 O. biharensis mites, the population sizes of both mites differed significantly from the 5th to the 35th day (p < 0.05). The population of O. biharensis dominated throughout this entire period. At 27 °C, 30 °C, and 33 °C, when the population sizes of O. biharensis reached peaks, they increased to 345.24, 86.34, and 67.88 times the initial number, respectively. This was much larger than the population sizes observed in E. sexmaculatus, their peak population sizes increased to 3.35, 2.65, and 3.42 times the initial number, respectively (Figure 10d).

Across all temperatures, the population size of each species in a combined population is lower than in isolation. When cohabitating, O. biharensis populations grew much larger than E. sexmaculatus, eventually replacing it in all treatments. This phenomenon was intensified by higher temperatures, which shortened the coexistence duration. For example, E. sexmaculatus was replaced by O. biharensis as early as the 20th day at 33 °C (Figure 10).

3.5. Intrinsic Rate of Increase Under Different Temperatures

In single-species populations, the highest r_m was observed at 30 °C for E. sexmaculatus and at 33 °C for O. biharensis. For both species, r_m values peaked on the 5th day at all tested temperatures, except for E. sexmaculatus at 27 °C, which peaked on the 25th day. The r_m was greater in O. biharensis than in E. sexmaculatus, except on the 25th and 30th days under 27 °C and 30 °C (Figure 11).

Mixed populations initiating with 20 E. sexmaculatus and 10 O. biharensis mites contained the largest r_m values at 27 °C in E. sexmaculatus and 33 °C in O. biharensis. For both species, the largest r_m values were recorded on the 5th day, except for O. biharensis at 27 °C which peaked on the 20th day. The r_m values of O. biharensis were greater than those of E. sexmaculatus across all treatments (Figure 11).

When mixed populations started with 15 E. sexmaculatus and 15 O. biharensis, the largest r_m values were documented at 27 °C in E. sexmaculatus and 33 °C in O. biharensis. For both species, the largest r_m values were obtained on the 5th day, except for O. biharensis at 30 °C, which was the highest on the 15th day. Overall, O. biharensis consistently exhibited greater r_m values than E. sexmaculatus, except on the 5th day at 30 °C (Figure 11).

Mixed populations containing 10 E. sexmaculatus and 20 O. biharensis mites reported the largest r_m values at 33 °C in E. sexmaculatus and 30 °C in O. biharensis, both peaking on the 5th day. The r_m of O. biharensis was consistently greater than E. sexmaculatus across all temperatures (Figure 11).

In single-species populations at 27 °C and 30 °C, the r_m of E. sexmaculatus was greater than that of mixed populations, except on the 5th day. At 33 °C, the r_m of E. sexmaculatus in both single and mixed populations exhibited a decreasing trend after the 10th day, suggesting that high temperatures negatively impact population growth and intensify interspecific competition. Except on the 5th day at 30 °C, the r_m values of O. biharensis were greater than E. sexmaculatus across all temperatures and population densities, demonstrating the highly competitive potential of O. biharensis (Figure 11).

3.6. Interspecific Competition Coefficients Under Different Temperatures

When the initial population of E. sexmaculatus was dominant, the interspecific competition coefficient at 30 °C was greater than that at 27 °C and 33 °C, but the differences among the three were not significant. When two species initially coexisted in equal numbers, the interspecific competition coefficient at 27 °C was greater than that at 30 °C and 33 °C, and the difference between 27 °C and 30 °C was significant. When the initial number of O. biharensis dominated, the interspecific competition coefficient at 33 °C was the greatest, and there were significant differences from 33 °C to other two temperatures (

Table 3. Interspecific competition coefficients at different temperatures.

Temperature		Mixed Population
	20 ES vs. 10 OB	15 ES vs. 15 OB	10 ES vs. 20 OB
27 °C	0.4157 ± 0.04 a	0.6466 ± 0.07 a	0.5388 ± 0.42 b
30 °C	0.5152 ± 0.06 a	0.3927 ± 0.04 b	0.5484 ± 0.16 b
33 °C	0.4976 ± 0.05 a	0.5441 ± 0.05 ab	0.6591 ± 0.05 a
F, df1, df2, p	6.493, 2, 6, 0.032	5.975, 2, 6, 0.037	1.017, 2, 6, 0.417

The data represents mean ± standard error. The means within the same column followed by different letters are significantly different (One-way ANOVA, p < 0.05). ES (E. sexmaculatus), OB (O. biharensis).

).

4. Discussion

This study examined the individual and mixed occurrence of two spider mite species under field conditions and assessed their development and reproduction under different densities. We also assessed both intra- and interspecific competition between two rubber tree spider mites, E. sexmaculatus and O. biharensis, which share the same ecological niche. We analyzed the populations of these species in both single and mixed populations under different initial ratios and temperature conditions.

Field investigation demonstrated that two spider mite species damaged rubber plantations alone and in combination. Most of the leaves in rubber plantations were eaten and damaged by both spider mite species, and the damage caused by mixed populations was relatively high. The population of O. biharensis was significantly higher than E. sexmaculatus. In practical production, monitoring both spider mite species and implementing timely prevention and control measures are of critical importance.

Findings showed that the coexistence of two spider mite species prolonged the developmental period of E. sexmaculatus, limited its survival rate and fecundity, and reduced the fecundity of O. biharensis. The survival and reproduction of O. biharensis are greatly affected by its own density. These results support the understanding of competition between the two spider mite species.

Our results demonstrate that the total population size of cohabitating spider mite species was smaller than that of single-species populations at all tested temperatures. There is a known suppressive effect on the population growth of species competing for resources in the same ecological niche [24]. A previous study of Tetranychus urticae, Panonychus ulmi, and Tetranychus viennensis revealed the dominance of Tetranychus urticae, with Tetranychus viennensis prevailing in its absence, providing evidence for interspecific competition between spider mite species [25]. Dominant species can use ecological niche factors to displace weaker ones, as documented in whiteflies, thrips, and spider mites [26].

Our study found that interspecific competition was particularly intense at higher temperatures, with competition coefficients peaking at 33 °C. This is concordant with previous studies that similarly correlate interspecific competition with temperature. One such study on Chilo partellus, Busseola fusca, and Sesamia calamistis determined that the three generally coexist at 15 °C, 20 °C, 25 °C, and 30 °C; however, high temperatures favoured the survival of C. partellus larvae, leading to its dominance in warmer environments [24]. Such discoveries reinforce the trend that competing species with strong temperature tolerances often gain a competitive advantage under high- or low-temperature adversity. Our study showed that at 27 °C, O. biharensis contained a much larger population than E. sexmaculatus in both single and mixed populations, cementing its competitive advantage. We can infer that the interspecific competition between the two spider mite species alters the occurrence pattern of phytophagous mites on rubber trees, a process likely accelerated by high temperatures. Our findings demonstrate that the replacement of E. sexmaculatus by O. biharensis can be hastened by elevated temperatures. This behaviour is mirrored in other insect species. For instance, L. bostrychophila inhibited the population size of L. decolor and L. paeta at 25 °C and 30 °C [12]. At 30 °C, the juvenile survival rate of C. maculatus was higher than that of C. chinensis, while the reproductive capacities of both were greatly diminished [27]. Finally, Sitophilus zeamais and Prostephanus truncatus are known to coexist at 25–35 °C, with S. zeamais dominating at 25–30 °C, while P. runcates prevails at 35 °C [28].

In addition to temperature, the result of interspecific competition is influenced by the initial population density [29]. Species with higher preliminary densities may dominate in the initial competition; however, the species with a stronger competitive ability will gradually claim dominance. In this study, despite starting with a higher initial density, at 27 °C, E. sexmaculatus surrendered its dominance to O. biharensis after the 10th day. A certain population size or density accelerates interspecific competition among insects, potentially inhibiting population growth in one or both species [30]. A previous study revealed that the coexistence between L. bostrychophila and L. decolor led to decreases in population sizes for both, with reductions distinct from single-species populations [12].

The outcome of competition and the growth of phytophagous insect populations are known to be influenced by the intrinsic rate of increase [31]. The population growth potential of E. sexmaculatus and O. biharensis on the rubber variety RRIM600 was assessed, revealing large r_m for both species. Previous studies have reported the strong population growth ability of E. sexmaculatus on tender rubber leaves, with a r_m value of 0.1041 [18]. Our findings show that O. biharensis exhibits an even larger r_m value, reaching 0.457 on RRIM600. Therefore, O. biharensis has a stronger population growth potential than E. sexmaculatus. Interspecific competition affects the intrinsic rate of increase for both competing species, with the degree of influence determined by the strength of competitive ability. In our study, the mean r_m values in O. biharensis were greater than those of E. sexmaculatus from the 5th to the 40th day in both single and mixed populations, indicating a stronger competitive edge. This is supported by a previous study showing that the mean r_m values in Tetranychus urticae were greater than those of Tetranychus truncatus in both individual and mixed populations, indicating the strong competitiveness of T. urticae [30]. Additionally, the highly competitive advantage of Blattella germanica is reflected by its greater reproductive capacity and intrinsic rate of increase compared to Symploce pallens [32].

This study’s results indicate the occurrence and distribution of two spider mite species in the field, as well as inter-species competition, providing a foundation for monitoring the occurrence of two spider mite species. By understanding the occurrence of spider mites, early monitoring and timely prevention and control can be attained to limit the loss of glue production. When the field temperature reaches 30 °C, the monitoring of spider mite species should be strengthened.

5. Conclusions

Taken together, these results indicate interspecific competition between E. sexmaculatus and O. biharensis, two dominant pests of rubber trees. Our results demonstrate that O. biharensis contains a higher competitive potential than E. sexmaculatus at 27–33 °C. However, these studies were conducted in a laboratory setting, which may not be reflective of field conditions due to the influence of natural enemies, hosts, meteorological factors, and pesticides. Future studies should further investigate these interactions in field conditions, with a focus on the influence of natural enemies and the mechanisms underlying competitive displacement.