Strategus aloeus (L.) (Coleoptera: Scarabaeidae: Dynastinae: Oryctini) is a pest of economic importance in oil palm crops under 5 yrs of age and has caused damage in plantations in Brazil, Colombia, Ecuador, Guyana, Mexico, Peru, Suriname, and Venezuela (Genty et al. 1978; Pardo-Locarno et al. 2005, 2012; Neita-Moreno et al. 2006, 2008; García et al. 2009; Lugo-García et al. 2011; Luna-León et al. 2017). The establishment of new areas, the replanting of oil palm crops, and the accumulated decomposing stipe in the surrounding areas have favored the development of populations of S. aloeus, which find ideal conditions to reproduce, feed and develop within oil palm plantations (Aldana et al. 2010).

In oil palm plantations, biweekly monitoring is conducted to determine the presence of S. aloeus through the galleries they create in the palm. One of the commercially carried-out management practices by palm growers is the periodic application of synthetic insecticides, which leads to insecticide resistance, the destruction of biological controllers like Phileurus didymus (L.) (Coleoptera: Scarabaeidae) and other beneficial insects, and costly production (Aldana 2000). Therefore, this study aimed to identify the beneficial fauna of S. aloeus present in the agroecosystem of oil palm cultivation in plantations of the central zone in Colombia and to determine the potential of P. didymus predation on the larval stage of S. aloeus under laboratory conditions.

Strategus aloeus and P. didymus larvae and adults were collected every 20 d for 3 yrs in 3 commercial oil palm plantations in Puerto Wilches (Santander, Colombia), that have the same environmental conditions, the same planting material and the management of the pest insect is the same (application of chemical insecticides). Plantation 1 (7.2691 °N, 73.8483 °W, 75 m.a.s.l.), Plantation 2 (7.3447 °N, 73.8275 °W, 99 m.a.s.l.), and Plantation 3 (7.2350 °N, 73.8740 °W, 79 m.a.s.l.). Average weather conditions of the study: temperature 27.1 ± 3.4 °C and relative humidity 79.1 ± 27.1%. Insects were sampled between 08:00 AM and 12:00 PM, in replanting lots of the hybrid E. oleifera × E. guineensis, which had decomposing stipes, where the female oviposits, and the larvae and pupae of S. aloeus develop. With the help of a chain saw and a stick, 25 entire stipes were cut down and each stipe was cut into small pieces. During the sampling, all the development stages of S. aloeus and P. didymus were collected to observe the presence of parasitoids and microorganisms that naturally affect the populations of this pest insect. The collected insects were transported to the Cenipalma Entomology Laboratory (Barrancabermeja, Santander, Colombia) in plastic containers with pieces of decomposing stipes for feeding. Larvae and adults were disinfected with 1% sodium hypochlorite for 30 and 60 s, respectively, and then washed 3 times with distilled water. The insects were identified in the laboratory considering the taxonomic traits of external morphology and genitalia of the original specimens and previous studies carried out by Dr. Jhon César Neita Moreno, curator of the Alexander Von Humboldt Institute of Biological Resources Research (Colombia). Voucher specimens were conserved in the entomological collections of Cenipalma (Barrancabermeja, Santander, Colombia).

Disinfected S. aloeus larvae were deposited in 12 L plastic containers containing sterilized decomposing stipe pieces for feeding and covered with a lid that had holes. Disinfected pupae were deposited in 0.7 L plastic containers containing soft sterile decomposing stipe and covered with a piece of black fabric to maintain natural development conditions. The stipe was sterilized for 30 min at 120 °C in an autoclave. The adults of S. aloeus and P. didymus were separated and placed in 40 L plastic containers containing a 13 cm sterile layer of soil mixed with decomposing stipe. Strategus aloeus adults were fed fruits (apple, pear, banana, or plantain), and the adults of P. didymus were fed third-instar larvae of S. aloeus.

The individuals of S. aloeus affected by entomopathogens were collected, placed in a humid chamber to cultivate the microorganism, and sent to the Cenipalma Entomopathogenic Microorganisms Laboratory (Bogotá, Cundinamarca, Colombia) for identification of the pathogen.

Due to the constant presence of the predator P. didymus in the samples, further investigation of its interactions with S. aloeus was conducted. For this, 15 larvae of each instar of S. aloeus were placed individually in 12 L plastic containers that contained sterilized stipe pieces for feeding, then a P. didymus adult collected in the field was added, and observations were made every 2 mins until the predation of the larva was complete. Time of attack, time of death, and time of consumption were recorded.

In a second experiment, a group of S. aloeus larvae and an adult of the predator P. didymus (experimental unit) were placed in 12 L plastic containers containing sterilized stipe pieces. The experiment was developed under a factorial design, in which treatments corresponded to 3 groups of 3rd instar larvae of S. aloeus (5, 7, and 10 larvae per experimental unit, factor 1) in 3 exposure times (6, 12, and 24 h after the experiment was established, factor 2). The bioassay had 5 replicates per treatment. Sampling was performed to quantify the number of predated S. aloeus larvae per experimental unit at different exposure times.

It was observed that the largest number of individuals of S. aloeus and the predator were in the middle and basal part of the stipe. The larval stage of S. aloeus predominated in all the samples. The S. aloeus samples were affected by the predator P. didymus (Figs. 1D and 1E) and the entomopathogenic fungus Metarhizium anisopliae (Metschnikoff) Sorokin (Deuteromycotina: Hyphomycetes) (Fig. 1A). Additionally, this entomopathogenic fungus was found infecting S. aloeus pupae and adults (Figs. 1B and 1C).

Phileurus didymus was present in 64.7% of the samples. In Plantation 1, there was a greater presence of this predator (83.3%), followed by Plantation 2 with 66.7% presence of this predator; and in Plantation 3, the presence of this predator was lowest with 44.4%; likewise, in this plantation, the largest amount of S. aloeus larvae was collected with 1,142 larvae; and in Plantation 1 and 2, 700 larvae were collected per plantation.

It was observed that the females of P. didymus oviposited in the same stipe sites where the larvae of S. aloeus are located. Moreover, Veiga (1985) and Rozas et al. (1991) reported that the Scarabaeoidea superfamily that this predator belongs to has highly varied feeding habits. Some species are saprophagous and phytophagous, although some mycophagous and zoophagous species have been recorded, the latter being the strangest and rarest, limited to the family Scarabaeidae, such as Canthon virens Mannerheim (Coleoptera: Scarabaeidae) and Canthon dives Harold (Coleoptera: Scarabaeidae), predators of the ant Atta spp. (Hymenoptera, Formicidae) (Forti et al. 2012; Araújo et al. 2015; Aquino et al. 2018); Deltochilum valgum Burmeister (Coleoptera: Scarabaeidae) specific predator of millipedes (Larsen et al. 2009); Hybosorus ittigeri Reiche (Coleoptera: Scarabaeoidea) predator of coprophagous insects (Veiga 1985; Rozas et al. 1991); and P. didymus, a predator of Platycoelia valida Burmeister (Neita-Moreno & Morón 2017) and S. aloeus larva, which was evidenced during this study.

Another of the natural enemies of S. aloeus identified was the entomopathogenic fungus M. anisopliae, which was isolated from larvae (Fig. 1A), pupae (Fig. 1B), and adults (Fig. 1C). The entomopathogenic fungus in the development stages of S aloeus, began to be observed in the rainy seasons of the study area as these conditions favored the manifestation of this natural microorganism (personal observation). The incidence in the larval stages was higher in Plantation 1 with an infection percentage of 1.4%. In contrast, in Plantations 2 and 3, lower infection percentages were recorded with 0.1% and 0.9%, respectively. Although the natural incidence of M. anisopliae in the S aloeus populations in the 3 plantations was low, Valencia et al. (2011) and Matabanchoy-Solarte et al. (2015) determined that under laboratory, shade, and field conditions, 2 strains of M. anisopliae caused larval mortality greater than 90%. Indriyanti et al. (2017) also determined that this fungus can kill 100% of Oryctes rhinoceros (Linnaeus) (Coleoptera: Scarabaeidae) larvae after 7 wks of being sprayed on mounds containing the larvae under shaded conditions. Metarhizium sp. can infect all stages of development of O. rhinoceros, causing a greater effect on third-instar larvae (Moslim et al. 2006; 2007).

It was observed that adult P. didymus caused the death of S. aloeus larvae at 4.9 ± 2.5 mins after making contact, and the total time to consume the larva was 5.7 ± 1.3 mins for the first instar and 17.3 ± 3.2 mins for the second instar and 36.1 ± 6.9 mins for the third instar. The adults of the predator P. didymus can kill 1 or 2 larvae of S. aloeus before consuming them. Statistically significant differences were observed in the effect of predation of P. didymus on S. aloeus larvae and the exposure time, given that P. didymus must search for its prey (Fig. 1D) and predate it (Fig. 1E). The percentage predation was 29.3% and 81.3% 6 and 24 h after introducing P. didymus, respectively (Table 1). Significant differences were found in the predation of P. didymus when placed with different size groups of S. aloeus larvae (Table 1). When the group size was larger there was an increased percentage of predation. Fellowes et al. (2007) determined that when the density of beneficial insects in an agroecosystem increase, the natural enemy spends more time finding prey. Acevedo (2020) mentions that beetles' search method to find their prey is with chemical, visual, mechano-sensory, and vibration signals; the chemical signals coming from the prey insects, the main stimuli used in the process of searching by the predator. Due to its presence within the agroecosystem, search capacity, and predation capacity that P. didymus has on the larvae state of S. aloeus, this natural enemy could regulate the populations of this pest insect in oil palm cultivation.

Fig. 1.

Natural enemies of Strategus aloeus. Metarhizium anisopliae infecting S. aloeus in: A. larvae, B. pupae, and C. adult of Phileurus didymus, D. searching for prey and E. preying on 3rd stage larvae of S. aloeus.

Table 1.

Predation of Phileurus didymus on Strategus aloeus larvae (mean ± SD).

The authors thank Ph.D. Jhon César Neita Moreno, curator of the Alexander Von Humboldt Institute of Biological Resources Research (Colombia), for identifying the specimens, the Palmero Development Fund administered by Fedepalma for financial support, and the plantations for logistical support.