Adult Sycanus sp. and M. plana larvae were initially collected from oil palm plantations via direct field sampling, following established procedures (Cuthbert et al. 2021). The founder population of Sycanus sp. was collected from an oil palm plantation in Labuhanbatu Regency, North Sumatra province, Indonesia (2°09′N, 99°43′E). Adult predators were commonly observed on flowering refugia plants, including Helianthus annuus, Cosmos sulphureus, Turnera subulata, and Calopogonium mucunoides. These plants serve as important habitats for natural enemies. To minimize variability typical of field-collected insects, all predators used in the experiment were laboratory-reared F1 Sycanus sp. derived from this parental population and used 15 days after adult emergence. Metisa plana larvae (third-fourth instars) were collected from smallholder oil palm plantations with active infestations. The larvae were carefully removed from the undersides of oil palm leaflets and maintained under laboratory conditions prior to experimentation.

A laboratory experiment was conducted using a completely randomized design (CRD) to evaluate the predatory interaction between Sycanus sp. and M. plana. Oil palm seedlings were used as the host plant and food source for M. plana larvae throughout the experiment. The experiment consisted of five predator-density treatments, each replicated 10 times, for a total of 50 experimental units (each an individual observation arena). All predators were laboratory-reared F1 female Sycanus sp., aged 15 days after adult emergence, standardized to minimize individual variability; only females were used to avoid sex-related behavioral differences and ensure uniform predatory performance. The five treatments comprised 1, 2, 3, 4, and 5 female Sycanus sp. (P1–P5, respectively), each with 10 M. plana larvae. Predators were then introduced into the units containing the assigned number of M. plana larvae, and predation activity and predator–prey interactions were observed.

Behavioral observations were conducted in transparent plastic containers (20 cm × 15 cm × 15 cm) covered with fine mesh to prevent insect escape while allowing ventilation, at 27 ± 2 °C, 70 ± 10% relative humidity, and a 12:12 h (L:D) photoperiod. Before each observation, predators were individually isolated and starved for 12 h to standardize hunger and feeding motivation (Jamian & Adam 2018). Each arena then received M. plana larvae according to the assigned treatment.

Predatory performance of Sycanus sp. against M. plana was evaluated using both qualitative and quantitative observations: (a) prey search time, determined as the interval from predator introduction to the first successful prey encounter; (b) prey handling time, measured as the interval from initial contact until the prey was completely immobilized; (c) prey consumption, the number of larvae consumed during the observation period; (d) larval mortality, assessed at 24-hour intervals after predator introduction using the formula (Ge et al. 2019):

M: mortality of larvae (%); a: number of dead larvae; and b: number of larvae treated.

Data were analyzed in Microsoft Excel 2016 and SPSS version 23. One-way analysis of variance (ANOVA) tested the effects of predator density on prey search duration, prey handling time, larval mortality, and prey consumption. Where treatments differed significantly, means were separated using Duncan’s multiple range test (DMRT) at P = 0.05. Linear regression assessed the relationship between predator density and prey consumption.

The prey-search behavior of Sycanus sp. showed clear temporal patterns throughout the experimental period (Figure 1). The first search was consistently longer than later ones: the initial search for 10 M. plana larvae ranged from 2.11 to 2.25 minutes across all predator densities, with no consistent trend as predator density increased. In contrast, subsequent searches were markedly shorter (1.69 to 2.04 minutes) and decreased slightly at higher predator densities. This shortening after the first capture indicates that the predator shifted to a more efficient hunting state, possibly reflecting a higher encounter probability within the confined arena, although the underlying behavioral mechanisms were not directly quantified.

Total handling time for 10 M. plana varied across predator densities, ranging from 58.1 to 59.1 minutes (Figure 2). These minimal differences, indicate that predator density did not substantially affect the duration of the prey handling process, suggesting that Sycanus sp. requires a relatively constant time to capture, immobilize, and consume prey, regardless of the number of conspecifics present. The mechanism by which Sycanus sp. handles M. plana involves piercing the larval cuticle with its rostrum and extracting internal body fluids by sucking, leading to progressive desiccation and eventual death of the larvae. The death of M. plana was characterized by the larvae exiting their larval sacs, a clear and reliable indicator of successful predation.

Metisa plana mortality was recorded daily for three days predator introduction (days after application, DAA) until few or no larvae remained (Figure 3).

Across all predator-to-prey ratios, mortality was highest on the first day (DAA = 1) and declined thereafter. On DAA = 1, the 5:10, 4:10, 3:10, 2:10, and 1:10 ratios recorded killed 9, 8, 7, 6, and 5 larvae, respectively; mortality then fell sharply to 1–3 larvae on DAA = 2, and by DAA = 3 only 0 to 1 M. plana larvae remained. Most predation therefore occurred soon after Sycanus sp. introduction, with the 5:10 ratio being most effective in achieving early elimination of M. plana.

Predator density had a strong positive effect on prey consumption. The mean number of larvae consumed rose steadily with density from 1.26 ± 0.21 larvae at one predator to 2.05 ± 0.18, 2.74 ± 0.17, and 3.64 ± 0.20 larvae at two, three, and four predators, and 4.43 ± 0.19 larvae at five predators per 10 larvae (Figure 4). Linear regression confirmed a significant positive relationship between predator density and prey consumption, (y = 0.795x + 0.451; R² = 0.9531), indicating that predator density explained about 95% of the variation in prey consumption.

The search behavior of Sycanus sp. during the initial phase – longer durations at first, then progressively shorter intervals – in line with the previous studies on predatory insects, which report that predators often search longer while familiarizing themselves with their environment and the spatial distribution of prey. The rapid decline in search time across subsequent prey encounters suggests that, once the predator located and captured its first prey, it finds additional targets more efficiently in the same arena. This pattern is in line with optimal foraging theory, which holds that predators adjust their search tactics according to previous encounters and environmental cues (Yuliadhi et al. 2021; Srikumar et al. 2014).

The ability of Sycanus sp. to adjust its search strategy upon detecting M. plana could strengthen its role in biological pest control in oil palm plantations. Such flexibility is likely important in agricultural systems, where fluctuating pest populations may demand varied predation strategies (Jamian et al. 2017). Because a predator’s search behavior can shift with environmental complexity and with successive prey encounters, further should examine how prey density and habitat affect search efficiency; more complex environments, in particular, may hinder prey detection and lengthen the search process (Ahmad et al. 2020).

The initial search time is a useful indicator of predator’s potential to control a pest. The shorter search times at higher predator densities may reflect density-dependent interactions among predators, although these were not measured directly. Such effects are likely more pronounced in confined laboratory settings than in the field, where predators and prey are distributed more heterogenously (Stone et al. 2023; Jamian et al. 2017).

The narrow range of handling times across all predator densities (58.1–59.1 minutes for 10 M. plana) indicates that prey handling in Sycanus sp. is governed mainly by physiological and behavioral traits intrinsic to the predator, rather than by external factors such as intraspecific competition (Ullah et al. 2019; Srikumar et al. 2014). The mode of prey immobilization employed by Sycanus sp.—piercing the larval cuticle and extracting body fluids through rostral sucking—is characteristic of the Reduviidae family and matches the mechanisms documented for other harpactorine assassin bugs (Ullah et al. 2019; Hwang & Weirauch 2012). This piercing-sucking action causes progressive desiccation and death, and the emergence of M. plana larvae from their sacs provides a reliable behavioral indicator of successful predation.

The stability of handling time across predator densities contrasts with reports of density-related behavioral effects in some predatory insects, where prey exploitation varies with predator abundance. This constancy suggests that prey capture and consumption are constrained mainly by the physiological processes of immobilizing and feeding on prey rather than by predator density (Ullah et al. 2019). This is also seen in other reduviid predators in which handling time remains relatively constant across different prey densities (Yuliadhi et al. 2021; Ahmad et al. 2020).

Handling time has important implications for biological control. Rapid, predictable handling lets Sycanus sp. process prey quickly and sustain a high predation rate, even in the highly modified environments of oil palm plantations, which supports its integration into an oil palm pest management plan (Wood 2019). Optimizing the density and release timing of the predator could further improve prey suppression. As reported, for other harpactorine reduviids such as Cydnocoris gilvus (Burmeister) preying on Helopeltis antonii Signoret, in which handling time was a key determinant of predation efficiency (Srikumar et al. 2014).

The mortality of M. plana peaked on the first DAA and decline steadily across all predator-to-prey ratios, mirroring the acclimatization of Sycanus sp. after introduction to the experimental arena. The peak was more pronounced at the highest-density, where Sycanus sp. consumed nine M. plana larvae on the first day. This early surge likely results from a combination of hunger (from the 12 h pre-experimental starvation) and the stimulus of newly presented prey in a novel setting, which together trigger exploratory and predatory behavior (Imboma et al. 2020; Ullah et al. 2019).

The subsequent decline in feeding is consistent with reports that predators feed less after initial prey exposure, owing to satiation or reduced prey availability (Yuliadhi et al. 2021; Ullah et al. 2019). This “predation surge” – elevated predation rates immediately after introduction, followed by stabilization into a more sustainable feeding rhythm– has been described for other biocontrol agents (Ullah et al. 2019; Jamian et al. 2017). It indicates Sycanus sp. Exerts its strongest suppression right after release.

Our results indicate predator density had a strong positive effect on the consumption of M. plana larvae by Sycanus sp. As predicted by ecological theory, more predators produced greater total prey mortality, indicating substantial potential for Sycanus sp. to suppress bagworm populations in oil palm systems at adequate densities (Siallagan et al. 2022; Jamian et al. 2017). However, the increase was not proportional; prey consumption rose about 3.5-fold across a fivefold increase in predator density, suggesting density-dependent interactions among predators that may influence prey consumption at higher densities. The regression, which explained over 95% of the variation in prey consumption, indicates a highly predictable predator–prey response under the tested conditions and suggests practical value for anticipating suppression from predator abundance in managed habitats (Adhikary et al. 2025).

Our findings are in line with previous work on reduviid predators, including Sycanus dichotomus Stål and Cosmolestes picticeps (Stål), in suppressing bagworm populations in oil palm ecosystems (Ahmad 2020), and with reports in other predatory Hemiptera where higher predator density enhances encounter rates and prey mortality. Because the per-capita gain diminished at higher densities, the highest prey mortality (at the 5:10 ratio) did not translate into proportionally greater consumption. The intermediate ratio of 2:10 may therefore offer a practical compromise between suppression and the number of predators deployed, although this requires field validation. From a management perspective, maintaining adequate populations of Sycanus sp. could enhance M. plana suppression and support the integration of this predator into sustainable integrated pest management (IPM) programs to reduce reliance on insecticides (Ikhsan 2024; Wood 2019; Priwiratama et al. 2018).