A Population Study of reduviidae insect in oil palm plantation with different historical Darna trima caterpillars’ infestations stages

Enriching biodiversity using beneficial refuge plants can expedite the recolonization process of predatory insects for sustainable control of Darna trima nettle caterpillar infestation in oil palm plantations. The recolonization process of predatory insects, including Reduviidae, will affect the differences in environmental factors, feeding availability, and landscape type. This research objective is to compare the population of Reduviidae insects (predator) in different historical D. trima infestation stages in oil palm blocks with various refuge plants, i.e., Antigonon leptopus, Cassia cobanensis, Turnera subulata, and Nephrolepis biserrata. The research was conducted in an existing oil palm plantation in Central Kalimantan from July to December 2022. An experimental 2-factor nested design involving the history of D. trima attacks and the type of refuge plants was implemented in 8 blocks (ca 40 ha each) to enumerate of predatory insect population using visual observation, sweep net, and yellow trap methods. The study revealed that the dominant Reduviidae species were Cosmolestes picticeps and Sycanus dichotomus. The population of C. picticeps was 35% higher in blocks with no historical D. trima outbreaks are significantly different from blocks with historical outbreaks. In both locations, the highest populations of Reduviidae insects were found in C. cobanensis and N. biserrata plots at 61-72 individuals and 58-67 individuals, respectively. Combining both refuge plants in oil palm blocks exhibited the most significant potential as a microhabitat for Reduviidae insects and for enhancing recolonization. To further enhance the role of S. dichotomus as an environmental service in the agricultural ecosystem, it is necessary to study proper propagation and augmentation techniques in oil palm plantations.


Introduction
Harmonizing an agricultural ecosystem is the main focus in oil palm plantations, including minimizing the leaf-eating pest outbreak potential that may affect oil palm yield production.Pest infestations in oil palm plantations occur due to an ecological imbalance between pests and their natural predators [1].Environmental incompatibility, feed source availability, and landscape type can disrupt the increment of leaf-eating pest infestation and decline its predatory insect population.
Darna trima is one of the significant palm oil leaf-eating caterpillars that can cause yield losses of 70% to 90% in Fresh Fruit Bunches (FFB) during the first and second year, respectively [2].Each D. trima can defoliate up to 16 mm in one short lifecycle of 35-40 days [3].In a specific oil palm plantation in Central Kalimantan, D. trima experienced an outbreak from 2018 to 2022.The population increased from 7.69 to 25.70 larvae per frond, defoliating up to 12.3 to 41.1cm per frond.The leaf damage by D. trima can interfere with the photosynthesis process and the supply of carbohydrates the oil palm plant needs for its bunches formation.The insecticides applications have been widely used in oil palm plantations and potentially harm the environment, leaf-eating pests, natural enemies (parasitoids and predatory insects), as well as human health [4][5][6].Controlling D. trima using the insecticide with active ingredients Cypermethrin, Deltamethrin, and Triclofor also potentially kills some leaf-eating pest predatory insects [7].Thus, the management of the oil palm plantation in the research site has decided to use insecticides only if the leaf-eating pest population exceeds the economic threshold level.The Early Warning System (EWS) activity, beneficial refuge plant, and leaf-eating pest predatory insect propagation as an integrated leaf-eating pest management method also have been implemented as ecosystem services in the oil palm plantation site.
Enriching floral biodiversity under the oil palm canopy (e.g., refuge plants Antigonon leptopus, Turnera subulata, Cassia cobanensis, and Nephrolepis biserrata) has a positive correlation with the predatory insect population as natural enemies of D. trima [8].The composition of volatile compounds produced by these refuge plants can attract the predatory insect population [9].The refuge plants provide a food source and shelter for the predatory insect from its competitors to improve their life survival rate.
The Reduviidae, specifically hemipteran insects, have a wide distribution and are agricultural ecosystem stabilizers [10].The Reduviidae insects, i.e., Eucanthecona furcelata, Zelus renardii, Cosmolestes picticeps, Sycanus dichotomus, Arilus cristatus, Apiomerus crassipes have been reported as natural enemies of oil palm leaf-eating pests [11][12][13].Improving landscape conservation in oil palm plantations is vital to enhance the quality of predator insect habitats and support their population increment.This research objective is to analyze the effect of enriching refuge plants (A.leptopus, C. cobanensis, T. subulata, and N. biserrata) and the history D.trima infestation impact in the oil palm plantation for the population of Reduviidae predatory insects in an oil palm plantation in Central Kalimantan.

Research location
The research was conducted at eight (8) blocks, a total of 348 hectares, in a palm oil plantation in Central Kalimantan.The selection of research locations was based on historical census data from the years 2018 to 2022, comprising four blocks (172 ha) with a high of D. trima infestations (above ten larvae per frond) and another four blocks (176 ha) with a low infestation (below the threshold level 5 larvae per frond).The refuge plants were planted alongside the Collection Road, except for Nephrolepis biserrata, planted within the palm inter-row.For A. leptopus, T. subulata, and C. cobanensis, the distance between refugee plant plots was 24 meters.The planting arrangement for A. leptopus and C. cobanensis involved spacing each plant at 0.6 x 0.6 m², with 21 plants per plot.T. subulata plant plot was 3.6 x 1 m 2 with a planting distance of 1 x 1 m 2 and contained ten plants/plot.Each observation block had different refuge plants based on historical high and low D. trima infestation levels.Visual.Two observers visually monitored The Reduviidae insects for 15 minutes in each plot.The observers positioned themselves at a corner opposite the observation plot and moved clockwise every 3 minutes.All observed insects were counted and documented for population and identification purposes.
Sweep-net.Monitoring Reduviidae insects using the sweep-net trapping method involved 50 diagonal swing sampling patterns with a zigzag design [14].The insect collection technique used an American sweep net type with a constant speed (<8 km/hour) in each transect by swinging the net onto 25% vegetation and rotating the net 180 degrees at the end of collection on the 4 th step [11,15].The trapped insects were placed in plastic bags filled with alcohol (70%) for population counting and identification in the laboratory.
Yellow Trap.Reduviidae insects were monitored using yellow traps in each plot.The traps were placed on top of the 10 m 2 refuge plant plot, which had a 2.5 m 2 square hole in the middle (Figure 2).The yellow traps were placed in the evening (3.30 pm to 5.30 pm) and observed for three days.The trapped insects were counted and identified in the laboratory.

Insect identification
The reduviidae insects obtained from the sampling activities were documented using the Olympus SZ61 stereo microscope and a digital camera.

Research Design and Data Analysis
The research was carried out at a block scale using a two-factor nested design, with the first factor being the historical infestation level category and the second factor being the type of refuge plant.Multivariate tests were performed for data analysis.The collected data were analyzed using SPSS ver.20.

Population of Reduviidae
The total Reduviidae insect population from three sampling methods in the historical low D. trima infestation was significantly higher compared to the historically high infestation area (Table 2).The Reduviidae insect population was not significantly different in both historical D. trima infestation levels.However, the number of insects observed using the sweep-net method was significantly higher in the historical low D. trima infestation.On the other hand, the observed Reduviidae insect population using the yellow trap method was inversely proportional to the visual or sweep-net methods.The Reduviidae insect commonly has three pairs of legs on the first segment near the antenna, tibia, and femora, with sticky glands for manipulating and capturing prey [16] (Figure 4b.d).The Reduviidae insect legs (i.e., Cosmolestes sp. and Sycanus sp.) are equipped with teeth structures on the ventral side (base of the claws) [17,18], and they have wings, which may aid them in avoiding sticky traps in the yellow traps.

Figure 3. Comparison of Reduviidae populations in each observation method
The Reduviidae insect in the research site consisted of two species, i.e., C. picticeps and S. dichotomus.(Figure 3).The population of C. picticeps was more dominant at 98.6% than S. dichotomus.Therefore, it was suspected that the role of C. picticeps as a predator insect was superior to S. dichotomus.The abundance of C. picticeps was higher than S. dichotomus in the oil palm plantation ecosystem.However, it is not affected by the leaf-eating pest population level [8,19].Reduviidae is a family group of insects that can feed on more than one prey species (polyphagous) [10].Hence, their population will remain high if another prey or food source exists in the historically low D. trima infestation area.
The highest population of C. picticeps was found in four blocks with similar refuge plant species, i.e., C. cobanensis (P2, P6) and N. biserrata (P4, P8).Blocks with both refuge plants had a higher C. picticeps population than blocks with. A. leptopus and T. subulata refuge plants.It was noted that the blocks with A. leptopus refuge plant were the lowest with visual observation and Sweep-net methods.The leaf-eating pest predator population is higher in the blocks with fern plants than in the nectariferous [20].N. biserrata ferns are benefited as shelter and habitats for predatory insects.Therefore, these ferns can support the biodiversity of insects in oil palm plantations [21].The findings from the study in both historical D. trima infestation levels showed that C. cobanensis was highly Attractive for Reduviidae predatory insects.The extrafloral nectar of C. cobanensis, which was produced on its frond axils, contained 208 mg/ml of sucrose, 82 mg/ml of fructose, and 38 mg/ml of glucose as a nectar source for predators and parasitoids, which could increase the populations of predatory and parasitoid insects in the oil palm ecosystem [22][23][24].The landscape in the study site, such as soil cover vegetation, the height of palm oil trees, and the beneficial plant species numbers, had a strong influence on maintaining the abundance and stability of predator insects in the oil palm ecosystem [19], as well as to support the habitat diversity and food sources for the predatory insects.

3.2
The role of C. picticeps as an ecosystem service C. picticeps is an ecosystem service.The population of C. picticeps, which was observed using three research methods, showed higher values in the blocks with historically low D. trima infestation and were significantly different from those with historically high D. trima infestation.Therefore, the role of C. picticeps has a higher potential to be a natural enemy for D. trima in the oil palm plantation in Central Kalimantan [19].The previous study also noted that the C. picticeps shows a dominant role as a natural enemy for bagworm larvae [25]  Regarding C. picticeps monitoring methods in the field, the total population of C. picticeps from two observation methods (i.e., visual and sweep net) had a similar pattern.Both observation methods were more suitable for the Reduviidae insects monitoring in the field with the Yellow Trap method.Thus, a reassessment of using large-scale yellow traps for Reduviidae insect monitoring in the oil palm ecosystem is necessary.However, it is worth noting that the yellow color attracts specific parasitoids and predators [26].4. The population of S. dichotomus as an ecosystem service for leaf-eating predatory pest insects still needs improvement in the oil plantation in central Kalimantan through S. dichotomus augmentation method and the refuge plant enhancement in the oil palm ecosystem.

Conclusion
Two species of Reduviidae Hemipteran, namely Cosmolestes picticeps and Sycanus dichotomus, were identified at the research site in the oil plantation in Central Kalimantan, with a total population of 98.6% and 1.4%, respectively.The population of Reduviidae predatory insects was relatively high at 230 individuals in the block with a historically low D. trima infestation and significantly different from the block with a historically high D. trima infestation at 170.5 individuals.The population of predatory insects (Reduviidae) was high in the oil palm blocks with refuge plants Nephrolepis biserrata and Cassia cobanensis.Combining both refuge plants may improve the role of Reduviidae predatory insect as an ecosystem service in oil palm plantations.The role of S. dichotomus in the oil palm ecosystem requires improvement through appropriate ecosystem management.Further research is needed to investigate the suitable propagation method and augmentation activities for S. dichotomus to support sustainable integrated leaf-eating pest management in oil palm plantations.

Figure 1 .
Figure 1.Experimental plot design for beneficial plant 2.2.Sampling Method The Reduviidae predatory insect population was sampled in eight (8) research blocks using three trapping techniques: visual, sweep-net, and yellow trap.The visual observation and sweep-net methods were repeated thrice each from 7 am to 10 am.Visual.Two observers visually monitored The Reduviidae insects for 15 minutes in each plot.The observers positioned themselves at a corner opposite the observation plot and moved clockwise every 3 minutes.All observed insects were counted and documented for population and identification purposes.Sweep-net.Monitoring Reduviidae insects using the sweep-net trapping method involved 50 diagonal swing sampling patterns with a zigzag design[14].The insect collection technique used an American sweep net type with a constant speed (<8 km/hour) in each transect by swinging the net onto 25% vegetation and rotating the net 180 degrees at the end of collection on the 4 th step[11,15].The trapped insects were placed in plastic bags filled with alcohol (70%) for population counting and identification in the laboratory.Yellow Trap.Reduviidae insects were monitored using yellow traps in each plot.The traps were placed on top of the 10 m 2 refuge plant plot, which had a 2.5 m 2 square hole in the middle (Figure2).The yellow traps were placed in the evening (3.30 pm to 5.30 pm) and observed for three days.The trapped insects were counted and identified in the laboratory.

Figure 2 .
Figure 2. Design of yellow trap placement

Table 1 .
Detailed data on research location (10)h block has ten(10)experimental plots

Table 3 .
The population of C. picticeps in the oil palm plantation in Central Kalimantan

Table 4 .
Population of S. dichotomus in the oil palm plantation in Central Kalimantan