The Coexistence of Oryctes rhinoceros L . and Xylotrupes gideon L . ( Coleoptera : Scarabaeidae ) on Immature Plant in Oil Palm Plantation

The rhinoceros beetle, Oryctes rhinoceros L. (Coleoptera: Scarabaeidae) is a major pest of oil palms. Manjeri et al. (2014) has reviewed the biological and ecological characteristics of O. rhinoceros as an oil palm pest in Malaysia. This noxious pest damages the immature and mature plants there by affecting the oil palm production (Chung et al., 1991; Mariau, 2000; Abidin, et al., 2014; Bedford, 2014). Adult male and female beetles can cause significant damage to the plants but not to the larvae because they feed on the dead plant materials. One attack increases of further attacks and certain trees could be more frequently attacked than would be expected by chance. More than one beetle may infest a tree at the same time, while a neighboring tree may remain un-infested. A study in India found five to six beetles feeding in the same crown (Bedford, 1980). On both oil palms and coconuts, the adult beetles bore through the petiole bases into the central unopened leaves. This causes tissue maceration and the presence of a fibrous frass inside the feeding hole. These pests can reduce 69% of the yield of fresh fruit bunches in the first year; causing the death of 25% of the young plants (Widayanto et al., 2014). On an oil palm estate, decaying palm trunks serve as common breeding grounds, furthermore while empty bunches or other organic waste used as mulch may also become breeding sites (Wan et al., 2009; Corley and Tinker, 2016). Eggs, larvae, pupae as well as adults can be collected by digging into or breaking open its possible breeding sites such as in the heaps of empty fruit bunches and decomposing trunks. Another rhinoceros beetle is Xylotrupes gideon L. (Coleoptera: Scarabaeidae), that is widespread throughout South East Asia. This species is not recorded in the list of oil palm pests of Indonesia (Kalshoven, 1981), however, the adults of X. beckeri Schauf. adults were found to feed on oil palms in Malaysia (Wood, 1968 cit. Bedford, 1975). Furthermore, Kalshoven (1981) explain that the ABSTRACT


INTRODUCTION
The rhinoceros beetle, Oryctes rhinoceros L. (Coleoptera: Scarabaeidae) is a major pest of oil palms. Manjeri et al. (2014) has reviewed the biological and ecological characteristics of O. rhinoceros as an oil palm pest in Malaysia. This noxious pest damages the immature and mature plants there by affecting the oil palm production (Chung et al., 1991;Mariau, 2000;Abidin, et al., 2014;Bedford, 2014). Adult male and female beetles can cause significant damage to the plants but not to the larvae because they feed on the dead plant materials. One attack increases of further attacks and certain trees could be more frequently attacked than would be expected by chance. More than one beetle may infest a tree at the same time, while a neighboring tree may remain un-infested. A study in India found five to six beetles feeding in the same crown (Bedford, 1980). On both oil palms and coconuts, the adult beetles bore through the petiole bases into the central unopened leaves. This causes tissue maceration and the presence of a fibrous frass inside the feeding hole. These pests can reduce 69% of the yield of fresh fruit bunches in the first year; causing the death of 25% of the young plants (Widayanto et al., 2014). On an oil palm estate, decaying palm trunks serve as common breeding grounds, furthermore while empty bunches or other organic waste used as mulch may also become breeding sites (Wan et al., 2009;Corley and Tinker, 2016). Eggs, larvae, pupae as well as adults can be collected by digging into or breaking open its possible breeding sites such as in the heaps of empty fruit bunches and decomposing trunks.
Another rhinoceros beetle is Xylotrupes gideon L.
(Coleoptera: Scarabaeidae), that is widespread throughout South East Asia. This species is not recorded in the list of oil palm pests of Indonesia (Kalshoven, 1981), however, the adults of X. beckeri Schauf. adults were found to feed on oil palms in Malaysia (Wood, 1968cit. Bedford, 1975. Furthermore, Kalshoven (1981) explain that the female beetles bore into the central leaf in the crown of coconut palms in a way similar to O. rhinoceros; consequently part of the frond may break off where the crown leaves unfold. The males are often observed on the newly-opened inflorescences. The larvae live in the soil rich in organic matter, in the heaps of compost and manure, in dead banana stems, and in decaying wood.
The pest, X. gideon, has never been noticed as a significant pest of oil palm in Indonesia. Some adult beetles, mainly the females, are often attracted to light at night. In-depth research on X. gideon as a palm pest is required. A preliminary study on the existence of X. gideon was hence carried out in the oil palm plantation. Breeding sites of X. gideon are similar to O. rhinoceros (Bedford, 1975;Kalshoven, 1981), hence, the study intended to describe the coexistence of O. rhinoceros and X. gideon in the oil palm plantation.
Collection of rhinoceros beetles at their breeding sites by trapping adult beetles is often carried out to monitor and control the pests. A pheromone is used in the traps to effectively capture the beetles. The aggregation pheromone, ethyl-4-methyloctanoate, is produced by the males of O. rhinoceros (Bedford, 2014). The synthetic version of this pheromone has been developed to control 21%-31% male and 67%-79% female beetles in the field (Sudharto et al., 2000). Pheromone traps are useful as monitoring tools, which can be an economical control method particularly in young oil palm replant areas when placed at one trap per 2 ha (Norman & Basri, 2014). Mechanical control by picking up larvae at the breeding sites is an alternative control measure. Labour costs to manually collect the beetles was approximately 71,500 USD annually (Agus, 2018; personal communication). Mechanical control may have significant impact to the population of the adult beetles. Therefore, the second objective of this study was to determine the influence and correlation of adult beetles collected manually from the breeding sites against collected in the pheromone traps.

Study Site
The study site was an immature oil palm plantation, three years after replanting, approximately 10 ha wide and 2 km away from the border, at which the dead old trees were scattered relatively and evenly on plantation. The study was conducted during January to February 2019, in Afdeling II Sei Silau PTPN III, Asahan Regency, North Sumatera Province, Indonesia. An attractant trap containing ethyl-4-methyloctanoate (Bedford, 2014) was erected at the plantation site, approximately 300 cm above the soil surface and as high as the oil palm canopy (Figure 1). The trap was designed as a double vane bucket trap (Lestari, 2018) installed at one trap per 2 ha (Norman and Basri, 2014). Therefore, in this study five traps were installed approximately 400 m apart. The adult beetles at every trap were collected, identified, and recorded daily during 30 consecutive days.

Determining the Breeding Site for Collecting Adult Beetles
The decomposing oil palm trunks are the ideal breeding sites of the rhinoceros beetles (Wan et al., 2009;Corley & Tinker, 2016). At the study site, one trap was set in the middle of the plot where decomposing oil palm trunks (Figure 2) surrounding the young plants that were relatively regular in position; most of them on the ground while some were still upright. Adult beetles were inspected for Pradipta et al.:

Data Analyses
In this study, the term "coexistence" is defined as the state or fact of living or existing at the same time or in the same place between the rhinoceros beetles O. rhinoceros and X. gideon at the oil palm plantation, especially in the breeding sites and traps. The strength of their relationship was determined by interpreting the correlation coefficient (r), resulting from the analysis of simple linear regression and correlation of Y = a + bX, where Y is the population of X. gideon and X is the population of O. rhinoceros, while a is the intercept and b is the coefficient of regression. Analysis tools available in the Microsoft Office Excel program were used for data analysis. Guidelines of Ratner (2009) was used for interpreting the correlation coefficient. Similar analysis was used to determine the influence of beetle collection in the decomposing trunks vs those in the attractant traps.

Pest Status of X. gideon on Oil Palm
This current study is one of the first reports on X. gideon found at an oil palm plantation in the region of North Sumatera Province, Indonesia. Figure 3 shows a pair of the X. gideon found mating on an oil palm frond, likewise, males and females were also found in the decomposing trunks. The male and female adult of X. gideon is not similar to O. rhinoceros (Figure 3 and 4). The X. gideon affects oil palm plants and has the potential to become a pest (Wood, 1968cit. Bedford, 1975, however, there has not been any reports on the economic impact of X. gideon. Therefore, existence of X. gideon in the oil palm plantation is not a threat at the moment and its current status is stated as an incidental or minor pest.

Coexistence of the Rhinoceros Beetles, O. rhinoceros and X. gideon
We found both species of the beetles in the decomposing trunks. Population density of O. rhinoceros adults was approximately 2.7 times greater than that of X. gideon (Table 1). The O. rhinoceros females dominated (p < 0.05) at the breeding sites (148 females vs 73 males) while the males dominated in the pheromone traps (55 females vs 102 males).
The existence of O. rhinoceros did not influence the existence of X. gideon such as indicated by the non-significant coefficient of regression (p > 0.05). Relationship between the two beetle species was weak, meaning that they freely coexisted with other macrofauna.
The aggregation pheromone has been reported to attract more than one species of rhinoceros beetles, however, in this study O. rhinoceros was the only species collected in the attractant traps ( Table 1). Existence of X. gideon was noticed on frond of the oil palm ( Figure 3) and it was also found in the decomposing trunks. Female beetles were also attracted to the light at night (Syaiful, 2019; personal communication).

The Influence of Manual Collection vs Attractant Traps
Destruction of breeding sites and manual collection of adult beetles are the traditional methods used to control rhinoceros beetles (Corley & Tinker, 2016). Trapping using pheromones is becoming a popular method to monitor the beetle populations (Norman et al., 1999). We found that the population densities of O. rhinoceros that were collected by trapping (7.37 ± 5.03) and hand picking (5.23 ± 3.52 beetles/30 days) were relatively equal (p-value = 0.0627). Fluctuation pattern of the population ( Figure 5) shows that the population measurement using trapping follows that of hand picking. This indicates a dependency relationship between hand picking and attractant trapping, and it proved that the breeding sites are an infestation source of the adult beetles.
Statistical analysis showed that the correlation coefficient (r = 0.4328) between the hand picking to trapping indicates a moderately positive linear relationship (Ratner, 2009). The significant (p value = 0.0169) coefficient of regression ( Figure 6) indicates that hand picking significantly influenced the collection of the beetles by attractant trapping. The collection of beetles using attractant traps was approximately 18.73% influenced by hand picking. In Malaysian oil palm plantations, X. gideon beckeri Schauf. adults feed mainly under the surface of rachides, newly emerged fronds, causing them to break. The distal portion of the frond then hangs loose and dies (Wood, 1968cit. Bedford, 1975. The differences in the infestation characteristics of X. gideon vs O. rhinoceros require further investigation. Dominance of females in the beetle population in this current study is in accordance with that of Fauzana et al. (2018) which reported 97 females vs 22 males using a similar pheromone trap. Although aggregate pheromones can invite both male and female beetles, there are actually more female trapped (Witjaksono et al., 2015). The population of X. gideon was also dominated by females (58 Control methods such as hand picking and trapping effectively reduce the number of fertilized females, and hence, it is more effective in reducing the progeny for the following generations. Pheromone trapping is mainly used in monitoring the beetle populations (Norman et al., 1999). During 12 weeks of monitoring using aggregation pheromone traps at a coconut plantation in Jepara Regency, Indonesia, Indriyanti et al. (2018) found that the traps captured a total of 101 beetles that consisted of 90.1% O. rhinoceros and 9.9% other insect species namely Rhynchophorus ferrugineus and X. gideon. The red palm weevil, R. ferrugineus, is presumed to be one of the most aggressive pests that may threaten the oil palm survival in Malaysia, however, currently there is no evidence of the oil palm trees being significantly affected (Azmi et al., 2017).
This current study reveals that the two control measures, hand picking and pheromone trapping, are well integrated in the management and control of the rhinoceros beetles. Next step is to intensify the mechanical control by hand picking in a sustainable manner. Further studies should focus on the economic utilization of the rhinoceros beetles. If the rhinoceros beetle has economic value, the people will be motivated to massively, intensively and continuously collect this pest insect. The pest population will gradually

CONCLUSIONS
Males and females of X. gideon were found at a palm oil plantation at the study location, and coexisted with O. rhinoceros in the decomposing trunks. There was no evidence of X. gideon infestation in oil palm significantly, lead them to be categorized as an incidental pest. The integration of hand picking with pheromone trap can be a good strategy in the management of the rhinoceros beetles.