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Introduction
Elaeis guineensis Jacq. is an oil palm that can live for more than 100 years (Hang & Sharma, 2000). Under conventional agricultural conditions and from a practical standpoint, the useful life of the palm can be around 25 years (Hang & Sharma, 2000) or when it reaches a height of 12 m; when this height is exceeded, harvesting the fruit becomes uneconomical (Loh & Mukesh, 1999).
Globally, the oil palm represents an excellent alternative for commercial development and expansion due to its high productivity, as its oil yield is higher than that of most other oil crops (Hinrichsen, 2016). In recent decades important agronomic advances have been achieved in oil palm: (1) plantation renewal without eradicating all plants, (2) precision fertilization in relation to soil type, (3) recycling of industrial waste from the palm and (4) organization of small and medium producers to industrialize their products and market directly in internal and external markets (Clare, 2004).
Adult oil palm plantations are characterized by considerable height and low productivity (Aznab & Mohd, 2002; Idris, Azman, & Chang, 2001; Loh & Mukesh, 1999), which constitutes a risk for those harvesting the fruit. Bud rot is another factor that produces losses to the farmer, although in lower proportion and intensity (Rivas-Figueroa, Moreno, Rivera-Casignia, Herrera-Isla, & Leiva-Mora, 2017).
Biomass generated in the oil palm is wasted, as farmers fell and burn the trunks, as well as apply systemic insecticides to them (Garbanzo, 2016). These practices are dangerous for the environment and cultivation, creating soil fertility problems, low productivity, microfauna and vegetation mortality, and high pollution levels (Arboleda, 2008). An alternative to avoid burning is the use of the felled trunks, at the end of the renewal phase of the oil palm, to rear larvae of Rhynchophorus palmarum (Linnaeus, 1758) Csiki E., 1936 (Coleoptera: Curculionidae) (Ávila, Bayona, Rincón, & Romero, 2014; Delgado, Couturier, Mathews, & Mejia, 2008). Due to its nutritional properties, the larva of the R. palmarum insect has become an important alternative food for the inhabitants of the Latin American Amazon (Cerda et al., 2001; Choo, Zent, & Simpson, 2009; Delgado et al., 2008; Sancho, Álvarez, & Fernández, 2015; Vargas, Espinoza, Ruiz, & Rojas, 2013). This polyphagous insect reproduces in the internal meristem of the tissue of the oil palm E. guineensis and other Amazonian palms (Hodel, Marika, & Ohara, 2016; Rodríguez-Currea, Marulanda-López, & Amaya, 2017).
The objective of this study was to establish the relationship between the characteristics of felled E. guineensis trunks and rearing of R. palmarum larvae in the Peruvian Amazon.
Materials and methods
Study area
The study area comprised three districts: (1) San Pedro-Campo Verde, with elevation of 200 m, temperature of 22.3 °C, annual rainfall of 1 726 mm and annual relative humidity of 80 %; (2) Tahuayo-Neshuya, with an elevation of 206 m, temperature of 25 °C, annual rainfall of 4 954.3 mm and annual relative humidity of 87 %; and (3) Maronal-Curimaná, with an elevation of 172 m, temperature of 25.5 °C, annual rainfall of 4 583.2 mm and annual relative humidity of 87 %.
Data collection procedure
The study was conducted during the months of January to July 2016 in nine oil palm plots, located in the hamlets of the districts of San Pedro-Campo Verde, Tahuayo-Neshuya and Maronal-Curimaná. Non-probability sampling was carried out. The length (m) and diameter (cm) of the felled trunks were measured with a 50-m measuring tape and a Vernier caliper (precision: 0.01 mm), respectively. Three trunk sections were evaluated in 12 replicates: basal part (which contains the roots), middle section (between the base and apex of the trunk) and the apex. In each section the thickness of three tissues or structural layers was measured: bark, sclerenchyma and central cylinder. Subsequently, 75 soil samples were taken to determine the texture class in the laboratory, using the Bouyoucos method (Beretta et al., 2014). The samples were obtained at a depth of 50 cm, in 31 replicates for San Pedro-Campo Verde, 15 for Tahuayo-Neshuya and 29 for Maronal-Curimaná.
After measuring the length and diameter of all trunk sections, an attractant made from a ferment ("masato") of cassava (Manihot esculenta Crantz) was applied. To do this, three V-shaped cuts were made in each trunk section, as well as three cross cuts on the sides of the trunk; 100 mL of the attractant were introduced with a brush in each cut. At 58 days the R. palmarum larvae were collected from the trunk sections where the attractant was applied. This ferment is used by indigenous communities to capture larger numbers of adult beetles and increase the production of larvae for local consumption (Delgado et al., 2008).
Data analysis
The data on the length and diameter of the three layers (bark, sclerenchyma and central cylinder) in the three oil palm trunk sections (basal, middle and apical) were tabulated and ordered according to the number of R. palmarum larvae in the three sites studied. Two-way factorial fixed-effect (Type 1) analysis of variance (ANOVA) and comparison of means with the Tukey test (P < 0.05) were performed; the length and diameter of the oil palm trunk were the independent variables, and the abundance of larvae was the response variable. The Spearman coefficient (rs) was used to relate the number of R. palmarum larvae to the length and diameter of the trunk, the diameter of the basal, middle and apical central cylinder of the oil palm, and the soil texture class. All analyses were performed with InfoStat™ (Di Rienzo et al., 2014).
Results and discussion
Table 1 shows that the length (F = 9.62; P = 0.01) and diameter (F = 5.72; P = 0.04) of the adult oil palm trunk were statistically different among sites. The Tahuayo-Neshuya district had the lowest numerical values of these variables.
Table 1 Length and diameter of felled adult oil palm (Elaeis guineensis) trunks in three districts of Pucallpa, Ucayali, Peru.
| District | Longitude (m) | Diameter (cm) |
|---|---|---|
| San Pedro-Campo Verde | 7.82 ± 0.25 ab | 69.95 ± 3.12 b |
| Tahuayo-Neshuya | 6.90 ± 0.44 a | 52.53 ± 12.29 a |
| Maronal-Curimaná | 8.53 ± 0.60 b | 71.70 ± 4.06 b |
Means with a different letter in the same column are statistically different according to Tukey’s test (P < 0.05). ± standard error of the mean.
The length and diameter of the adult oil palm trunk vary during development until the trees reach maturity or the end of their useful life (Cayón-Salinas, 1999). Differences in trunk length and diameter among districts were probably due to the type of crop management that consisted of fertilization, pruning and weeding, as well as the age of the oil palms (Cayón-Salinas, 1999; Paramananthan, 2013). The diameter differences could be attributable to a coating that the petiolar base formed on the trunk in the district of Tahuayo-Neshuya, but not in the districts of San Pedro-Campo Verde and Maronal-Curimaná. Oil palm planting density was 143 plants·ha-1 in the three districts.
The oil palm stem has a growth rate of 25 to 30 cm·year-1 (Bonneau, Vandessel, Buabeng, & Erhahuyi, 2014) and can reach a maximum of 15 to 20 m in height (Mosquera et al., 2016). Crop management consists of fertilization, pruning, weeding and other agroecological activities that play an important role in trunk growth (Bonneau, Impens, & Buabeng, 2018; Woittiez, van Wijk, Slingerland, van Noordwijk, & Giller, 2017). When this research was carried out, the oil palms were between 16 and 24 years old, which means that they were near the end of the productive phase of a commercial crop that is from 6 to 25 years (Mosquera et al., 2016). The recorded heights did not reach 15 to 20 m, which are corresponding measurements for ages over 30 years, when production is almost nil (Mosquera et al., 2016).
The length of the oil palm trunk sections did not correlate with the number of R. Palmarum larvae (rs = 0.47, P = 0.19). Similarly, the diameter of the trunk sections and the number of R. palmarum larvae did not correlate (rs = -0.11, P = 0.76). This insect oviposits in the exposed internal tissues of the oil palm to ensure that the larva has access to food. The success of oviposition depends on certain specific conditions. Firstly, it has been determined that insects are attracted by the odor of the substratum from which they feed and oviposit, and the attractants that may be added; secondly, the space and soft tissues are important factors that allow larvae to feed and protect themselves from their natural enemies (Aldana de la Torre, Aldana de la Torre, & Moya, 2010; Choo et al., 2009; Monzenga, Le Goff, Kayisu, & Hance, 2017; Pérez & Iannacone, 2006; Rodríguez-Currea et al., 2017). Sánchez, Jaffé, Hernández, and Cerda (1993) and Choo et al. (2009) indicate that when the larvae reach their last stage, it is common to find competition among individuals of the same species that leads to predation by cannibalism. Therefore, stem (trunk) height may not be associated with the rearing of R. palmarum larvae.
Table 2 shows the thickness values of the oil palm trunk layers. According to the results, the central cylinder is thicker than the bark and the sclerenchyma. The bark and the central cylinder had the largest thickness in the basal third, while the sclerenchyma was similar in all three sections. Regarding the study sites, the bark, sclerenchyma and central cylinder were smaller in the Maronal-Curimaná district. In all three districts, the central cylinder had higher values than the bark and sclerenchyma,
Table 2 Measurements of the three thickness layers of the oil palm (Elaeis guineensis) trunk in Pucallpa, Ucayali, Peru.
| Variables | Thickness of the trunk layers (cm) | ||
|---|---|---|---|
| Bark | Sclerenchyma | Central cylinder | |
| Trunk third | |||
| Apical | 1.07 ± 0.60 a | 0.65 ± 0.23 a | 31.72 ± 3.14 a |
| Middle | 1.43 ± 0.66 a | 0.74 ± 0.23 a | 34.84 ± 4.92 a |
| Basal | 1.97 ± 0.66 b | 0.80 ± 0.29 a | 52.22 ± 3.08 b |
| District-site | |||
| San Pedro-Campo Verde | 1.88 ± 0.72 c | 0.75 ± 0.29 ab | 39.27 ± 12.20 ab |
| Tahuayo-Neshuya | 1.47 ± 0.66 b | 0.78 ± 0.22 b | 43.38 ± 14.65 b |
| Maronal-Curimaná | 1.09 ± 0.60 a | 0.64 ± 0.22 a | 35.93 ± 7.75 a |
Means with the same letter in a column are not statistically different according to Tukey’s test (P > 0.05). ± standard error of the mean. Trunk third: F bark = 17.39, P < 0.0001; Fsclerenchyma = 2.85, P = 0.06; F central cylinder = 61.33, P < 0.0001. District-site: F bark = 12.72, P < 0.0001. F sclerenchyma = 3.21, P = 0.045; F central cylinder = 3.47, P = 0.03.
Rhynchophorus palmarum developed, until its last larval stage, in the central cylinder of the three cut trunk sections (basal, middle and apical); this was evidenced by the state of advanced decomposition of the central cylinder at the time of evaluation. Therefore, it follows that the preferred site of adults for oviposition is the central cylinder; however, there was no relationship between the number of R. palmarum larvae and the diameter of the apical-central cylinder (rs = 0.44; P = 0.23), middle-central cylinder (rs= 0.56; P = 0.11) and basal-central cylinder of the trunk (rs = -0.32; P = 0.39).
In the central cylinder of the apical third (the furthest from the ground) of the felled oil palm trunk, there is great meristematic activity, foliar growth and sexual differentiation of the inflorescence; according to Ávila et al. (2014) and Delgado et al. (2008), the tissue of the central cylinder’s apical third is the most appetizing and soft for oviposition and for the larval development of R. palmarum in its initial stages. The apical meristem decomposes easily under adequate moisture and shade conditions, so it would be expected that the number of larvae is related to the characteristics of the tissues and the diameter of the central cylinder (Abe, Hata, & Sone, 2009; Ávila et al., 2014; Cayón-Salinas, 1999; Monzenga et al., 2017; Van Itterbeeck & van Huis, 2012). In future studies, the degree of decomposition should be correlated with the abundance of R. palmarum larvae.
Delgado et al. (2008) indicate that the trunk’s apical part has a greater number of R. palmarum larvae compared to its basal part. In the present study there was no significant relationship between the diameter of the central cylinder’s apical third and the number of larvae found in R. palmarum. The basal third, due to its support function for oil palm, contains lignified fibers that are harder and perhaps not suitable for the feeding, growth and development of the larvae; however, when there is moisture and shade, the development of larvae in the lignified base of the felled trunk is observed (Ávila et al., 2014; Choo et al., 2009; Delgado et al., 2008).
Table 3 shows the soil texture classes in each district. The results indicate that there is a higher percentage of clay loam soil in Tahuayo-Neshuya, sandy loam in San Pedro-Campo Verde and clay in Maronal-Curimaná. The percentage of clay loam soil (dominant texture) and the number of R. palmarum larvae found per district showed a positive relationship between the two variables (rs = 1.00, P < 0.01). The number of R. palmarum larvae was statistically different in the three sites (F = 17.40; P < 0.001; Levene = 1.28).
Table 3 Soil texture class of three districts with oil palm (Elaeis guineensis) production in Pucallpa, Ucayali, Peru, and number of Rhynchophorus palmarum larvae.
| Soil texture | San Pedro-Campo Verde (%) | Tahuayo-Neshuya (%) | Maronal- Curimaná (%) |
|---|---|---|---|
| Sandy loam | 35 | 0 | 10 |
| Loam | 16 | 0 | 17 |
| Clay loam | 29 | 53 | 17 |
| Clay | 13 | 13 | 21 |
| Clay sandy loam | 7 | 7 | 21 |
| Silty clay | 0 | 27 | 14 |
| Total | 100 | 100 | 100 |
| Number of larvae | 56.67 ± 39.23 a | 145.67 ± 39.67 b | 50.00 ± 28.14 a |
The number of larvae was different among districts according to Tukey’s test (P < 0.001). ± standard error of the mean.
Texture is a relatively stable property that influences management, movement of water and air, determination of the genesis, infiltration and soil moisture (Paramananthan, 2013). In this context, the trunks found in these soil texture classes are considered good food substrates because they have less fiber, are rich in carbohydrates and proteins and could support rapid larval growth (Paramananthan, 2013; Woittiez et al., 2017). Clay soils are preferable because they favor the root growth of the oil palm and retain adequate amounts of moisture; this is why the trunks are better developed in soils with clay loam texture (Ortiz & Fernández, 1994).
The results obtained from the reproduction of R. palmarum show the importance of the soil texture class in trunk growth. This is reflected in a greater number of R. palmarum larvae in the hamlet of Tahuayo-Neshuya with the increase in the clay loam texture and a lower number of larvae in San Pedro-Campo Verde and in Maronal-Curimaná with a texture different from the clay loam texture (Table 3).
Conclusions
The diameters of the oil palm Elaeis guineensis trunk sections were different among sites, with the central cylinder representing the greatest proportion of the trunk being the largest. There is no relationship between the number of Rhynchophorus palmarum larvae and the length and diameter of the trunk. Nor was there a relationship between the diameter of the central cylinder of the three-thirds (apical, middle and basal) of the trunk and the number of larvae. Therefore, trunk characteristics are not associated with the development of R. palmarum. However, the clay loam soil texture did have an important role since it showed a positive relationship with the number of larvae.
