Protecting Rice Grains from Sitophilus oryzae L. Infestation using Plant Extracts under Controlled Conditions

Introduction

Rice (Oryza sativa Linn), a staple food for over half the world’s population, faces a significant threat during storage, insect infestation. This not only reduces rice quality and commercial value but also impacts global food security. According to the Food and Agriculture Organization of the United Nations [1], approximately one-third of the world’s post-harvest crop is lost to waste.

Roughly 75% of warehouse insects belong to the order Coleoptera, with the rice weevil (Sitophilus oryzae L, Coleoptera: Curculionidae) being the most significant and damaging species. This primary pest can cause substantial losses to stored grains, ranging from 18 to 30% in warm regions. Both adult and larval stages of the rice weevil can attack and damage stored grains [2].

While synthetic pesticides offer control, they rise environmental and health concerns due to potential harm to non-target organisms and the development of resistant pests [3].

As a result, there is a growing focus on replacing conventional organic chemical insecticides with safer materials and methods to preserve the environment and minimize the risk of residual effects. Promising alternatives include the use of powders, plant extracts, volatile and non- volatile oils, and inert dusts, etc. [4, 5, 6].

Utilizing plants that contain biologically active compounds offers a promising alternative to harmful chemical pesticides, some plant-derived compounds act as repellents or anti-feedants, deterring insects from consuming the treated material [7].

This research explores plant-based insecticides as a safer alternative to conventional methods. It complements a previous study that investigated the toxicity of similar extracts to adult rice weevils. Here, we focus on the efficacy of extracts from three specific plants: neem (Melia azedarach L.), pepper tree (Schinus molle L.), and common thyme (Thymus vulgaris L.). These extracts are assessed for their ability to control adult rice weevils (Sitophilus oryzae L.), a major storage pest. The study investigates how these weight extracts impact Cumulative mortality rate of adult weevils, adult emergence rates, grain damage rate and grain loss. The goal is to identify plant-based alternatives with potential for controlling rice weevils during storage. Effective plant extracts could offer a safer and more sustainable approach to protecting this vital food source.

Materials and Methods

The study was conducted in an incubator maintained at a constant temperature of 26±2°C and humidity of 65±5% at the insect laboratory.

Adult rice weevils (Sitophilus oryzae) were collected from infested wheat grains and identified based on their morphological characteristics [2, 8]. These adults were used to establish a stable laboratory population of the insect, maintained in an incubator under the same temperature and humidity conditions used for the experiment.

The wheat grains (one potential host for the rice weevil) were thoroughly cleaned to remove impurities and any foreign matter. To eliminate any potential insect infestation, the grains were placed in a refrigerator for hours. To prevent mold growth, the grains were subsequently exposed to dry air and stored in the refrigerator at 4°C until use in the experiments.

Ripe fruits of S. molle and M. azedarach were harvested in late autumn from plants growing in the University Garden. The fruits were thoroughly washed to remove dust and any adhering materials and then dried under controlled laboratory conditions. Dried green thyme leaves (T. vulgaris) were purchased from local markets. The dried plant parts were ground using an electric grinder.

Two hundred grams of each of the aforementioned plant powders were placed in separate glass beakers. Each powder was mixed with twice the amount of acetone, stirred thoroughly, and covered with aluminum foil. The mixtures were then kept in the dark for 48 hours. After the extraction period, conical funnels and filter paper were used to filter the extracts. A Rotary Evaporator with 120 cycles and a solvent volatilization temperature of 56°C was employed to recover the acetone, resulting in 100% concentrated extracts. From each extract, two additional concentrations, 50% and 25%, were prepared.

The resulting extracts were stored in airtight, labeled graduated containers and kept in the refrigerator until use.

The experiment was conducted to evaluate the efficacy of the three plant extracts (T. vulgaris, S. molle, and M. azedarach) at three different concentrations (25%, 50%, and 100%) against adult rice weevils. Three hundred newly emerged rice weevils were randomly divided into three groups: an untreated control group and three treatment groups, each consisting of 90 weevils. The weevils in the treatment groups were further divided into three subgroups corresponding to the three extract concentrations. Ten grams of whole wheat grains were placed in each plastic cup, and 0.25 ml of the respective extract solution was applied to the grains. After allowing any potential solvent residues to volatilize slightly in the air, 30 adult rice weevils were introduced into each cup. The cups were covered with muslin and incubated at the specified temperature and humidity. The mortality rate of adult rice weevils was monitored at regular intervals (days 1, 2, 3, 4, 9, 15, and 20) following the treatment. The experiments were continued for 50 days, and the following observations were recorded:

Results

Effectiveness of Extracts of S. molle, M. azedarach and T. vulgaris on Adult Mortality Rate of the Rice Weevil, Sitophilus oryzae

The results presented in Table 1 demonstrate the efficacy of S. molle, M. azedarach, and T. vulgaris extracts in enhancing the average cumulative mortality rate of S. oryzae adults following treatment with three concentrations (25%, 50%, and 100%) at different time points (1,2, 3, 4,9,15, and 20 days post-treatment).

In general, all tested extracts at the three investigated concentrations exhibited efficacy in increasing the average cumulative mortality ratio, with values ranging from 66.67 to 100%. However, an exception was observed for the treatment with S. molle extract at a concentration of 25%, where the average efficacy values initially ranged between 0 and 72.00% during the first three days of treatment but subsequently declined, reaching 18.52% after the fifteenth day of treatment.

Significant differences were observed between the efficacy of S. molle extract and those of M. azedarach and T. vulgaris extracts at a concentration of 25% across all time points examined. On the fourth day of treatment, the respective efficacy values were 38.89%, 71.90%, and 66.67%, indicating the superiority of the latter two extracts. Conversely, no significant differences were detected at concentrations of 50 and 100%.

Both M. azedarach and T. vulgaris extracts exhibited the highest efficacy at all tested concentrations and across the examined treatment periods. The efficacy values were comparable at a concentration of 100% after treatment times of 1, 3, 4, 9, 15, and 20 days with no significant differences between the two extracts. However, at a concentration of 25%, a significant difference was observed on the ninth day of treatment, with efficacy values of 78.06% and 69.44% for M. azedarach and T. vulgaris extracts, respectively.

The effectiveness of the extracts in increasing the average cumulative mortality ratio increased with increasing extract concentration at each time point. Significant or apparent differences were observed in the effectiveness values among the different concentrations. After 20 days of treatment, the average cumulative mortality ratios for the three extracts of S. molle, M. azedarach, and T. vulgaris at the tested concentrations of 25%, 50%, and 100% were 18.52%, 81.74%, and 84.76%, respectively, for S. molle, 70.74%, 83.16%, and 86.67%, respectively, for M. azedarach, and 67.72%, 82.54%, and 86.67%, respectively, for T. vulgaris.

Despite the overall decline in efficacy values with increasing treatment time due to the high mortality rates observed in the untreated control, the effectiveness of the extracts in increasing the average cumulative mortality ratio of rice weevil adults was dependent on the complex interplay between extract type, concentration, and treatment time. The highest efficacy values, reaching 100%, were observed for treatments with M. azedarach and T. vulgaris extracts at concentrations of 25%, 50%, and 100%, as well as for S. molle extract at concentrations of 50% and 100% after one day of treatment. Conversely, the lowest efficacy values, 0%, were recorded for treatments with S. molle extract at a concentration of 25%.

Discussion

It is evident from the results of the previous tables that the three plant extracts, T. vulgaris, M. azedarach, and S. molle, effectively increased the adult rice weevil mortality when applied at the three tested concentrations (25%, 50%, and 100%). These findings align with previous studies that have demonstrated the insecticidal properties of these extracts. The effectiveness of these extracts may be attributed to the presence of compounds with either contact toxicity or anti-nutritional effects, leading to high mortality rates among adult rice weevils. The observed differences in efficacy among the extracts can be attributed to variations in their chemical composition. For instance, the high efficacy of T. vulgaris extract is often associated with the presence of thymol, aromatic oil with known insecticidal properties. Studies have shown that thymol can affect the longevity of insects and induce mortality [12, 13].

Several studies have also demonstrated that the effectiveness of M. azedarach stems from the presence of bioactive compounds with anti-nutritional and insecticidal properties. These compounds include triterpenoids and limonoids found in the leaves and fruits of M. azedarach, as well as azdrechtin [14, 15, 16, 17, 18].

Additionally, the fruits and leaves of M. azedarach contain flavonoids, catechin, and two kaempherol compounds, all of which exhibit insecticidal activity [19].

Saljoqi, et al. [20] demonstrated that the extract of M. azedarach fruit exhibited higher efficacy against rice weevil adults, with a mortality rate of 62%, compared to the leaf extract, which had a mortality rate of 34%. In the untreated control, no mortality was observed after four days of treatment [20]. Benzi, et al. [21] suggested that the effectiveness of S. molle fruits is likely attributed to the presence of limonene and caryophylline, which inhibit insect nutrition and contribute to high mortality rates among adult rice weevils [21].

The efficacy in increasing the average cumulative mortality rates of adult rice weevils increased with increasing extract concentration due to the rising proportions of toxic or antinutritional components, leading to higher mortality rates among rice weevil adults. This finding aligns with observations from several studies [14, 22].

The plant extracts of T. vulgaris, M. azedarach, and S. molle provided significant protection to wheat grains, particularly at concentrations of 50% and 100%, effectively reducing the number of adult rice weevils emerging from treated grains. This reduction is attributed to high mortality rates among adult rice weevils due to various mechanisms, including poisoning, anti-nutritional effects, and disruption of egg-laying. Furthermore, the extracts demonstrated efficacy in reducing grain weight loss and the percentage of grain damage by inhibiting insect activity and feeding. Govinden K, et al. [17] reported that grain weight loss in S. oryzae-infested grains treated with A. indica extract was minimal, at 13.01%, compared to the untreated control, which exhibited a significant loss of 49.22%, after 60 days of treatment [17]. These findings align with the growing interest in exploring environmentally friendly alternatives to synthetic insecticides for controlling rice weevil infestations.

Both M. azedarach and Ageratum conyzoides L. powders demonstrated significant efficacy against rice weevils compared to the untreated control. This efficacy was evident in higher cumulative mortality rates, lower monthly increases in insect populations, and reduced grain damage and weight loss during a six-month storage period [23]. The use of plant materials in general has been shown to have an effect on stored grain pests. Recent studies demonstrate the efficacy of plant powders from Pistacia lentiscus, Elettaria cardamomum, and Boswellia carterii against S. granarius L. (Sg) and T. confusum Jac. (Tc). These powders completely suppress adult emergence and possess strong repellent properties [24].

In general, the effectiveness of plant extracts stems from the presence of anti-nutritional compounds that deter egg- laying or prevent hatching, thereby reducing adult emergence or affecting insect development and longevity. These findings align with the observations of several researchers [13, 15, 16, 17, 18].

Our results also demonstrated a significant increase in efficacy with increasing extract concentration, attributed to a rise in the proportion of active substances and components that exert diverse effects on insects. These findings align with observations from several studies [14]. Biodegradable plant materials like S. molle and M. azedarach provide a sustainable way to control insect pests without harming grains or the ecosystem. These plants can be used alone or integrated into IPM strategies [13].

Conclusion

The present study demonstrates the promising potential of plant extracts derived from T. vulgaris, M. azedarach, and S. molle as eco-friendly alternatives for controlling rice weevils.

These extracts exhibited significant insecticidal activity, effectively reducing adult mortality, suppressing adult emergence, and minimizing grain damage. The observed effectiveness demonstrates a potent and sustainable approach to managing rice weevil infestations. This study contributes to the growing body of research supporting the use of plant- based solutions for pest control, paving the way for future advancements in environmentally friendly pest management strategies. Before widespread application in grain storages, further research is needed to determine the potential health effects of the studied extracts, any residual solvent traces left after volatilization, the economic feasibility of large-scale use, compatibility with existing storage practices, and the potential for resistance development in rice weevils.

Conflicts of Interest

The authors declare no conflict of interest