Potential of Solid Gel Formulation of Kaffir Lime Peel Extract Against Postharvest Pest Sitophilus Zeamais on Stored Rice

. Maize weevil, Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae) is a primary pest on maize, rice, wheat, sorghum, and other cereal grains. The purpose of this study was to determine the effectiveness of using kaffir lime peel extract in solid gel formulation as a plant-based insecticide. This study used five treatments, untreated (control), 0.5%, 1%, 1.5%, and 2%. The results of this study are the treatments tested had effects on adult mortality and repellency. The 2% concentration showed the highest adult mortality and repellency . The major volatile organic compounds contained in kaffir lime peel were α -fernesene, α -sinensal, D-citronellol, and citronellal.


Introduction
Rice is the most important staple food for over 3.5 billion people in worldwide, especially in South Asia, East Asia, Southeast Asia, and South America [1].Rice is known as a source of carbohydrates for 90% of Indonesian people.There are several types of rice, such as white rice, brown rice, black glutinous rice, and white glutinous rice [2].Population of Indonesia was reached 270 million people with an annual rice consumption was 81.044 kg/capita in 2022 [3].Rice stocks is not immediately distributed to the public, but needs to be stored to maintain the stock of rice.Rice that is stored in long period can decrease in quality and quantity [4].The shelf-life of rice will affect the quality and quantity of rice because several biological and physical factors.One of the factors is postharvest pest infestation of maize weevil, Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae) [2].
Maize weevil, S. zeamais is one of the primary pests of cereal grains such as wheat, maize, rice, and sorgum [5].Other than grains, S. zeamais can cause damage to processed products such as pasta and biscuits.In addition, it can reduce the quality, such as changes in color, tastes, unpleasant odor, nutritional value, and seed viability.It also can reduce the quantity of ingredients, like grain weight [6].A pair of these insects can reproduce about one million of their species within three months under some favorable conditions.This insect has a very fast life cycle in breeding, causing damage to rice grains to become hollow, easily broken, and crushed into fine flour which can reduce the quality of rice [7].
The disruption caused by S. zeamais to rice is very troubling to the community, which currently ranges from 10-20% damage and can continue to increase without pest control actions [4].Until now, S. zeamais pest control still uses synthetic pesticides in form of fumigating and spraying.Methyl bromide and phosphine fumigants have been used in the control of pests and effective in the food storage and other agricultural needs.These insecticides are rarely or even no longer used due to ozone damage and interfere with insect biological systems so that it can cause pest resistance [8].
Based on the demands of the global market (AFTA, APEC, and WTO), agricultural products must be healthy, environmentally friendly, safe for consumption, and highly nutritious.Therefore, it is necessary to control according with the Integrated Pest Management (IPM) system by utilizing plant-based pesticides [8].This plant-based pesticides made from secondary metabolites of plants, like essential oils can be used as an alternative to pesticides [9].
Essential oils have a distinctive strong aroma and act very well as fumigants, antifeedants, and repellents in insects [10].One of the natural materials that contain secondary metabolites essential oils is kaffir lime.Active compounds in kaffir lime are generally volatile, such as compounds from the terpenoid group [11].Based on previous research, the kaffir lime peel contains a lot of essential oil to be economically used as a household insecticide [12].
Plant-based pesticides that is often done by the community is utilizing plant extracts through spraying.Plant-based pesticides has disadvantages of being perishable and not resistant to sunlight, relatively short shelf-life, and requires repeated spraying which from an economic point of view is not effective and efficient [13].Therefore, efforts are needed to develop more effective plant-based pesticides.
The potential of using kaffir lime peel extract formulated in solid gel is a newest alternative to controlling maize weevil.Gel formulation able to maintain a relatively long fragrance [14].The purpose of this study was to determine the effectiveness of using kaffir lime peel extract as a bioinsecticide in killing pests with solid gel formulations.

Insect culture
Infestation was done by one kg of the cold-sterilized Ciliwung variety of white rice put in glass jar (Ø= 14.5 cm; h= 23.5 cm), then 200 of S. zeamais adults were infested for two weeks [18].The jar was covered with organza and tighted with a rubber band to kept adult inside the jar [15].

Extraction of kaffir lime peel
Kaffir lime with dark green colour, fresh, and no signs of disease or defects were chosen.The sorted kaffir lime washed on tap water, peeled the skin, then dried in oven at 60 °C and pulverized into powder using blender for two minutes.Powder and ethanol 96% were mixed in Erlenmeyer glass (1:5 v/w) then closed tightly using aluminium foil for 24 h.The extract was filtered using filter paper then distilled (65 °C; 120 rpm; 6 h) and stored in sealed brown bottle [16].

Volatile Organic Compounds (VOCs) analysis
VOCs in the extract were identified and quantified using gas chromatography-mass spectrophotometry (GC-MS)-QP2010 Ultra Shimadzu.

Mortality Test
Thirty grams of cold-sterilized white rice (Ciliwung variety) and 15 pairs of S. zeamais adults (1-2 weeks old) from insect culture were introduced to the glass jar (Ø= 7 cm; h= 11.5 cm).An untreated (control) and five concentrations of solid gel formulation were put into the glass jar, separately.The jars were closed tightly and hermetically sealed.Each of treatments were done in five replications.The adult mortality was observed dialy within a week and using the following formula [18]: To obtain the corrected mortality used following formula [20]:

Repellency test
The repellency test was carried out by assembling the two-way transparent cylindrical plastic as shown in previous study [19].Fumigants with various extracts to be tested were placed in one side, with 30 unsexed adults (1-2 weeks old) inserted into the center tube.Number of adults on both sides were observed within 1, 3, 5, and 24 h.Percentage number of adults in both sides were calculated using the following formula [20]: (3)

Data analysis
The experimental units were arranged in a completely randomize design (CRD).The data obtained from this research were statistically analyzed using the analysis of variance (ANOVA 5%) and post-hoc using the least significant difference (LSD) test at the 5% error

Fig. 1.
Volatile organic compounds analysis of kaffir lime peel extract by GC-MS instruments.

Mortality test
Solid gel formulation had a significant effect of different concentration on adult mortality of maize weevil from 1 to 7 days after treatment (DAT) (Fig. 2).Concentration at 2% extract was the highest adult mortality ranged from 4.00-8.67%on daily observation compared to other treatments.While among the lower rates of concentrations (0.5%, 1%, and 1.5%) and untreated were no significant effect on each observation day.However, highest concentration (2%) of kaffir lime peel extract in solid gel formulation was remain considered less effective to cause high adult mortality due to concentration used relatively low.Higher concentration in the future studies needs to be carried out to determine the most effective to control adult of S. zeamais.The solid gel formulation provides slow release of volatile compounds thus it can increase the shelf-life of product [21].Kaffir lime peel extract contains VOCs that causes distinctive odors and have potential insecticidal properties because it contains the main chemical component citronellol (5,38%).The high percentage of citronellol is one of the advantages of kaffir lime peel extract due to decrease the population of maize weevil.Pure citronellol proved to caused adult mortality about 30-64% of rice weevil, S. oryzae (Linnaeus) (Coleoptera: and 28-58% of red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) within 6-72 [22].Solid gel combined with kaffir lime peel extract evaporated and concentrated in the air of the glass jar.This concentrated VOCs inhaled by adults through the respiratory and poisoning their system.

Repellency Test
The percentage of adults number in the untreated area was significantly higher than treated area (Fig. 3).This results were significantly in different concentration and time of exposure.Higher concentration showed lower of number of adults in the treated area and vise versa.The longer of exposure time also showed the lower number of adults in treated area and vise versa.It shows that the potential of the kaffir lime extract in solid gel formulation was considered as good candidate of alternative to repelled adults of maize weevil.This study in line with previous study that pure citronellal showed 38-68% and 28-56% repellency of T. castaneum and S. oryzae respectively [22].As pest repellent in the coleopteran group [23], followed by α-sinensal (8.13%) to produce aroma, and D-citronellol (7.35%) causes interference in the nervous system and metabolism of the insect body and act as a plant-based insecticide [24].

Conclusion
The potential of kaffir lime peel extract in solid gel formulation showed significantly effect on the mortality and repellency of maize weevil.However, 2% of concentration showed 8.67% of adult mortality and this concentration less effective to cause high adult mortality.Meanwhile, the repellency test at 2% of concentration showed the highest repellency against maize weevil.
Number of adults in untreated or treated areaNumber of inserted adults× 100%

Fig. 2 .
Fig. 2. Adult mortality of Sitophilus zeamais in different concentration of kaffir lime peel extract in solid gel formulation.