Resistance of oil palm seedlings against red spider mite attack through boron fertilizer application

Red spider mites are one of the pests that attack oil palm seedlings, especially in the dry season. The damage caused by red spider mites in oil palm nurseries turns the leaves brown and necrotic and the growth of seedlings becomes weak and prone to infection with various types of disease. The objective of this study was to assess the effect of boron fertilizer application on red mite attacks in oil palm nurseries. There are four boron concentrations tested in this experiment (0, 0.5, 1, and 1.5 ppm). Each treatment had one seedling, laid out as a randomized block design, and each treatment was replicated ten times in the nursery area. In this study, there was no artificial infestation of red spider mites, but red spider mites were allowed to attack naturally. The population of red spider mites declined in the application of boron fertilizer solution with a concentration of 1 ppm, which reduced by an average of 44.9a individuals/leaf (decreasing population to 69%). Meanwhile, in oil palm seedlings with no application of boron fertilizer (0 ppm), the population of red spider mites increased with an average of 40.8b individuals/leaf. An increase in boron content in leaves correlated with a decrease in red spider mite populations that attack oil palm seedlings. In this experiment, the resistance of oil palm seedlings to red spider mites attack is influenced by the boron nutrient content in the leaves.


1.
Introduction Oil palm is one of the plantation commodities that has an essential role in economic activities in Indonesia because of its ability to produce much-needed vegetable oil for the industry sector.In 2019, there was an increase in oil palm plantation area and CPO production compared to the previous year to 14,46 million hectares with a production of 47,12 million tons.In 2020 there was an almost stagnant increase of 0,90 percent to 14,59 million hectares.Palm oil export volume tends to increase from 2016 to 2019.However, in 2020, the volume of oil exports palm oil has decreased.In 2017, palm oil exports experienced the most significant surge, reaching a volume of 29,07 million tons, showing a growth of 19,45 percent compared to the previous year [1].
Efforts to increase the production and quality of oil palm continue to be conducted as best as possible to meet market demand.Various aspects need to be considered to enhance oil palm production, including post-harvest handling, precision fertilization, best agronomic practices and using superior planting materials in the nursery.Nurseries play a vital role in deciding the success of oil palm cultivation and can be considered the first step in the entire cultivation process.It can be said that the nursery is the first step of the whole series of oil palm cultivation activities.Effective nursery management significantly influences both the quality and quantity of plantation yields [2].The oil palm nursery typically involves two stages: the pre-nursery stage and the main nursery stage.The pre-nursery phase lasts for three months and needs shade.The purpose of this stage is to ensure consistent plant growth before transplantation to the main nursery [3].Main nursery start with planting the three months old from pre-IOP Publishing doi:10.1088/1755-1315/1308/1/012011 2 nursery in larger polybags until the seedlings are ready to be planted in the field, mainly after nine months or 12 old months seedling.One of the factors that determine good management of oil palm nurseries is controlling the attack of pests and diseases.
During the nursery phases, various diseases have been documented, including Curvularia seedling blight, Helminthosporium leaf spot, Anthracnose, Chlorotic streak, and nursery spear rot [4].While the pests that attack are Aphids, Mealybugs, Grasshoppers, Spodoptera spp., Eldana saccharina, Dynastes centaurus, Cockchafer larvae, Cockchafers, Cutworms, and Red Spider Mite [5].The plant mite sucks the liquid on the underside of the young leaves.The damage caused by red spider mites in oil palm nurseries is that the leaves turn brown and necrotic.The growth of seedlings becomes weak and stunted, so the seedlings are susceptible to infection with several types of disease.At a later stage, the seedlings will easily die.Red spider mites maybe have a severe effect, especially in the dry season and poor irrigation.Explosions of red spider mite attacks can occur due to the selection and use of inappropriate pesticides, which results in the death of the natural enemies.Using conventional pesticides for plant pest and disease control raises significant concerns about food safety, environmental impact, and the development of pesticide resistance.These concerns have necessitated the exploration of alternative pest management methods [6].Therefore, more environmentally friendly control techniques are needed.
Mineral nutrients play a crucial role in the growth and development of plants and microorganisms.Moreover, they also significantly influence plant-pest and disease interactions.Additionally, mineral nutrients are integral components of plants and regulate metabolic activities which are associated with plant resistance and pathogen virulence [7].The nutritional status of plants serves as the first line of defense and can determine their susceptibility to pests and pathogens [8].Visual indicators such as leaf color are important in assessing pest vulnerability.Insufficient nutrient content in leaves can damage or hinder the formation of chlorophyll, resulting in changes in leaf color and increased intensity of pest attacks.Symptoms of nitrogen, potassium, and magnesium deficiencies were observed in a sand culture experiment involving young oil palms, showing uniform yellowing, marginal yellowing, and orange chlorosis, respectively [9].Nutritional deficiencies can discolor leaf surfaces and make plants more susceptible to pests.For example, the Asian citrus psyllid, Diaphorina citri, tends to settle on yellow reflective surfaces [10].The application of potassium has been shown to decrease the occurrence of foliar and soil-borne diseases [11], while boron (B) applications have been found to reduce spider mite damage [12].The function of boron in the cell wall structure, plant membranes, and plant metabolism contributes to the reduction in disease severity [6].Specifically, plant nutrients can influence the resistance of plants to pest and disease attacks.This study aimed to examine the impact of boron fertilizer application on red mite infestation in oil palm nurseries.

Location and Time
This research was conducted on Sungai Bahaur Estate (SBHE) oil palm nurseries and Analytical Laboratory of the Research Department of Bumitama Agri Ltd, Pundu Village, East Kotawaringin Regency, Central Kalimantan, Indonesia.The research was conducted from March -June 2022.Rainfall measurements during observations were carried out through an ombrometer and recorded every day.

Population Dynamics of Red Spider Mite
There are four boron concentrations evaluated in this experiment namely 0, 0.5, 1, and 1.5 ppm.Each treatment had 1 seedling, laid out as a randomized block design, and each treatment was replicated ten times in the nursery area.The distance between test oil palm seedlings is as far as 90 cm square.The oil palm seeds used are 5 months old.In this study, there was no artificial infestation of red spider mites, but red spider mites were allowed to attack naturally.The oil palm seeds used in this experiment are naturally attacked by red spider mites.Affected oil palm seedlings have symptoms of leaf color changing to a shiny brownish color.Red mites are on the underside leaves of the attacked seedlings.Before the application of boron fertilizer was carried out, the number of red spider mites on one affected leaf in each test plant was calculated first and leaf samples were taken to see the boron nutrient content in each test oil palm seedlings.The boron content was observed from the composite sample of leaves using a spectrophotometer [13].
Boron fertilizer is applied by spraying with a hand sprayer on all leaves evenly.Three weeks after treatment (WAT), the population of red mites was counted on the same leaves as before spraying boron fertilizer and leaf samples were taken again to analyze the boron nutrient content in the leaves.Preliminary studies revealed that an accurate evaluation of the damage could be documented after three weeks following the introduction of mites [12].Soil samples as planting media for oil palm seedlings were also taken to analyze the nutrient content in the soil.During the observation, no pesticides, and other fertilizers were applied.

Data Analysis
Data analysis of the population's changes of red spider mites was carried out using Analysis of Variance (ANOVA) and if it had significant differences, it was further tested by Duncan Multiple Range Test (DMRT) with α = 5 %.

3.
Results and discussions

Red Spider Mite Population
The red spider mites on oil palm seedlings treated with boron fertilizer solution showed a decrease in population.On the other hand, oil palm seedlings without the application of boron fertilizer solution showed an increase in the population of red spider mites (Table 1).The greatest population decline occurred in applying boron fertilizer solution with a concentration of 1 ppm, which was reduced by an average of 44.9 individuals/leaf.Meanwhile, in oil palm seedlings that were not applied with a solution of boron fertilizer (0 ppm), there was an increased population of red spider mites in an average of 40.8 individuals/leaf.Development of egg to nymph, nymph to adult stage takes 4.66 ± 0.19; 1.75 ± 0.14; 1.92 ± 0.12; and 1.72 ± 0.08 days.Development time from egg to adult stage is 10.15 ± 0.16 days.For 16 days, each female can produce eggs as much as 108.3 ± 3.23 [14].Natural resistance to pests and diseases attack is a property of resistance carried out by plants and is influenced by genetic and environmental factors.The natural resistance of plants can be mechanical resistance with changes in the structure of organs or tissues with the accumulation of lignin or cellulose or biochemical resistance with the synthesis of resistance elements such as polyphenol compounds or tannins [15].The primary function of plant nutrients is as a constituent of plant cells or tissues.Boron is reported to work by metabolizing nucleic acids, carbohydrates, proteins, phenols, and auxins [16].Boron absorbed plants in the form BO3 -and it works to facilitate glucose polymerization to cellulose to thicken cell walls so that it is more resistant to attack pests and diseases [15].The red spider mites attack host plants by sucking the contents of plant cells through piercing and sucking mouthparts.The application of boron fertilizer plays a role in thickening plant cell walls so that plants have better resistance to red spider mites' attacks.An increase in boron content in leaves correlated with a decrease in red spider mite populations that attack oil palm seedlings (Figure 1).

Figure 1. Correlation between boron nutrient content in leaves and population of red spider mite
Rainfall is an environmental condition that is not suitable for the development of red spider mites.The red spider mites prefer hot, dusty, and general conditions on the plants near the road dusty or at the edge of the park [17].Plants that are affected by drought are more susceptible to red spider mites.The population of the red spider mite will decrease rapidly in late summer when the population has high predators.The condition of the host plant becomes not favorable, and the weather turns cold and rainy [18].In addition to the rainfall factor, routine watering in the morning and evening on oil palm seedlings also suppresses the population of red spider mites.Regular watering is carried out to meet the water needs of oil palm seedlings of 2 L per day.The addition of boron fertilizer solution increased the boron nutrient content in the leaves (Figure 3).In the control treatment, there was also an increase in boron content, but not as much as in the treated plants.The increase in boron content in oil palm seedling leaves is influenced by foliar spraying.Application through the leaves causes the absorption of nutrients to be faster than the sowing method.In many species of plants, absorption of B through the leaves is more effective than through roots [19].However, avoid applying too much boron fertilizer to plants.The application of excess boron fertilizer is toxic to plants (Figure 4).Accumulation of excess boron is toxic to plants, with symptoms of interveinal necrosis and some species avoid it through the guttation mechanism [20].The principle of implementing integrated pest management (IPM) is keeping plants in a healthy condition and fulfilling their nutrient needs.In healthy conditions, plants become more resistant to pests and diseases attacked.Fertilization (inorganic & organic) is one way of inducing natural resistance to plants and increases the ability of the plant to defend itself against pressure from environmental factors, including pests and disease.The genetic control of pest and disease resistance is facilitated through physiological and biochemical processes interconnected with the nutritional condition of the plant or pathogen [7].However, providing excessive nutrition to plants can also make plants susceptible to attack by pests and diseases.Excessive nitrogen fertilization can increase the susceptibility of apple and pear plants to fire blight disease (Erwinia amylovora) [21].High K levels increase the disease severity of rice white tip (Aphelenchoides oryzae) [22].The overuse of phosphorus fertilizers enhances susceptibility to infection by the fungal pathogen Magnaporthe oryzae in rice [23].Therefore, the fulfillment of plant nutritional needs must be under the fertilizer recommendations as plant needs.The potential for pest and disease explosion can be prevented through proper nutrition management.

Conclusions
The resistance of oil palm seedlings to red spider mites attack is influenced by the boron nutrient content in the leaves.Keeping plants healthy by meeting their nutrient needs is an essential element in implementing integrated pest management.

Figure 3 .
Figure 3. Boron nutrient content in tested oil palm seedling

Figure 4 .
Figure 4. Symptoms of leaf turn brown and necrotic caused by excess boron application

Table 1 .
The effect of boron solution application on the red spider mite population * The + orindicates an increase (+) or a decrease (-) in the red spider mites population ***WAT (Weak after treatment) * Number in the same column followed by different letters indicate significant differences in the Duncan Multiple Range Test with α = 5 % *