An experiment conducted at the seed center laboratory at Tamil Nadu Agricultural University during June 2022, compared the efficacy of diatomaceous earth and kaolinite clay against rice weevil (Sitophilus oryzae) infestations in stored wheat. Adult weevils exposed to treated wheat grains exhibited dose-dependent mortality. Diatomaceous earth demonstrated rapid insecticidal activity, achieving 100% mortality at the highest dosage (1g/100 g wheat) within seven days while causing minimal grain damage. In contrast, kaolinite clay exhibited slower action, achieving 90% mortality at 30 days, with moderate grain protection. Additionally, diatomaceous earth significantly reduced progeny emergence. These findings highlight diatomaceous earth as a more effective and faster-acting solution for controlling rice weevils in stored wheat compared to kaolinite clay, emphasizing its potential for integrated pest management strategies in grain storage facilities.

Diatomaceous earth, Kaolinite clay, Rice weevil, Inert dusts.

Grains are an essential food source globally, rich in important nutrients that support overall health. Whole grains, in particular, help to manage cholesterol, control weight, and regulate blood pressure. Studies indicate regular consumption of whole grains can significantly reduce the risk of chronic diseases like diabetes and heart disease (Mattia et al., 2022). However, a major challenge is the loss of grains after harvest, especially during storage, which threatens global food security. Reducing these losses is vital to ensure the availability of this essential nutrient source for future generations (Hodges et al., 2011; Stathers et al., 2013).

A major cause of grain losses after harvest is insect pests such as the rice weevil. Traditional pest control methods usually involve chemical insecticides and fumigants. While these chemicals can be effective, they come with serious problems such as pests developing resistance (Benhalima et al., 2004; Collins et al., 2005), environmental pollution (Rajashekhar et al., 2012), and potential health risks to humans (Aulicky et al., 2019). As a result, there is an increasing demand for sustainable pest management alternatives. One such alternative is using inert dust and ash, which control grain pests by physically disrupting them. The effectiveness of these materials depends on factors like the type of material, the pest species' susceptibility, the concentration used, the duration of exposure, and the adherence of the dust to the pests (Christos et al., 2016; Kanteh et al., 2016; Ofuya and Akhidue 2005; Shah et al., 2006; Shams et al., 2011).

The rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), is a significant and destructive pest of wheat and various other crops (Hatami et al., 2011). This pest is responsible for considerable reductions in both the quantity and quality of stored grains globally (Arannilewa et al., 2002). The adult weevils bore circular holes into the grains, consuming mainly the endosperm and thereby diminishing carbohydrate content. The larvae preferentially feed on the grain germ, leading to substantial losses in protein and vitamins. Traditionally, synthetic pesticides and phosphine gas have been widely used to manage stored grain insect pests, including rice weevils, across the world (Anwar et al., 2003). However, due to the rise in phosphine resistance, particularly in countries like Australia and India, control measures are becoming increasingly ineffective (Mau et al., 2012; Ali et al., 2013). While chemical insecticides such as deltamethrin (Decis 2.5 WP; 40 mg/kg) (Mishra and Panday 2014), malathion, and fenvalerate (Singh et al., 1998) have shown effectiveness against rice weevils, their usage comes with significant drawbacks, including residual toxicity, pollution, and negative impacts on both food safety and human health, along with rising costs (Lu and Wu 2010). Therefore, considering the environmentally friendly nature, consumer safety, and reduced likelihood of inducing pest resistance associated with inert dusts, the present study investigated the effectiveness of diatomaceous earth and kaolinite clay as potential solutions for managing rice weevil populations.

Adult rice weevils (Sitophilus oryzae) used in this study were obtained from the seed center laboratory at TNAU AC and RI, Coimbatore. These weevils were cultured on a standard wheat variety. Standardized containers, each filled with 100 grams of wheat, were treated with selected inert dusts of diatomaceous earth and kaolinite clay at increasing dosages of 0.25, 0.5, and 1.0 grams per 100 grams of wheat. A control group with untreated wheat was also included. Each treatment, along with the control group, was replicated three times.

Table 1: Treatment details for the experiment diatomaceous earth vs kaolinite clay against rice weevil.

Twenty adult weevils were introduced into each container. To monitor mortality, observations were made at regular intervals: 5, 10, and 15 days after the weevils were introduced. During each observation, dead insects were counted and removed, allowing the remaining live weevils to continue reproducing. The containers were revisited at later stages (25, 35, 40, and 50 days) to assess the impact on the next generation (F1 progeny) by counting the emerged adult offspring. Upon observing any F1 progeny, treatments in those containers were continued until complete mortality was achieved. Ultimately, the mortality rate and grain damage were determined at the end of the experiment using the following formula,

Observations made on the mortality of rice weevils in the treatments at different dosage

C

Diatomaceous earth 

Short-Term Effects (3 Days After Treatment). Three days after treatment (3 DAT), diatomaceous earth demonstrated significant effectiveness in eliminating adult insects. The study identified a dose-dependent relationship, with higher dosages resulting in higher mortality rates (Fig. 1). At the lowest dosage (0.25g), the average mortality rate was 27.6%, while the highest dosage (1g) achieved a more substantial mortality rate of 77% (Table 2).

Long-Term Effects (7 Days After Treatment). Seven days after treatment (7 DAT), diatomaceous earth exhibited remarkable effectiveness. The highest dosage (1g) resulted in the complete elimination of all adult insects, highlighting its potency. Even at lower dosages, significant mortality rates were observed: 66% at 0.25g and 83% at 0.5g (Table 3). Importantly, grain damage was minimal across all dosages, with the highest being 8% at the 0.25g dose and the lowest being 3% at the 1g dose. This indicates that diatomaceous earth not only effectively eliminates adult insect populations but also protects grains from pest damage over a longer period.

Fig. 1. Effects of diatomaceous Earth on Rice Weevil

Kaolinite clay 

Short-Term Effects (15 Days After Treatment). The findings indicated that kaolinite clay effectively controls adult insect populations over an extended period. Within the initial 15 days of treatment, a dose-dependent trend in mortality rates was observed. The highest dosage (1g) resulted in an average mortality rate of 44%, while the 0.5g dosage yielded 22% mortality, and the 0.25g dosage showed no mortality (Table 4).

Table 2: Effect of Diatomaceous Earth on Rice weevil @ 3 DAT.

Table 3: Effect of Diatomaceous Earth on Rice weevil @ 7 DAT.

Table 4: Effect of Kaolinite clay on Rice weevil @ 15 DAT.

Table 5: Effect of Kaolinite Clay on Rice weevil @ 30 DAT.

Long-Term Effects (30 Days After Treatment). After 30 days, the impact of kaolinite clay became more pronounced and displayed a significant dose-dependent relationship (Fig. 2). The highest dosage (1g) achieved an impressive 90% reduction in insect populations. Even the lower doses were effective, with the 0.5g dosage resulted in 77% mortality and the 0.25g dosage achieved 83% mortality (Table 5). 

Additionally, kaolinite clay proved to be a valuable protector of grains, minimizing damage across all dosages. Grain damage was lowest at 2% with the highest dosage and up to 9% with the lowest dosage, demonstrating that kaolinite clay is a promising and sustainable alternative to traditional pesticides.

Fig. 2. Effects of kaolinite Clay on Rice weevil.

Our results were on par with the previous studies where, the inert dusts, particularly kaolinite clay and diatomaceous earth, reported to effectively prevent the adult weevil emergence, achieving a (0% emergence rate) and protecting seeds from damage and weight loss at all tested concentrations (1g, 5g, and 10g per 100g of seeds) (Akbar et al., 2021).  Another study revealed that, among the treatments, diatomaceous earth was the most effective against all three insect species tested, followed by kaolinite clay and katelsous. However, the effectiveness of those treatments decreased over time (El-Sayed et al., 2010).

Arthur and Throne (2003) found that rice weevils are particularly susceptible to diatomaceous earth compared to other insects. They also noted that factors such as grain temperature and moisture content can impact the effectiveness of this treatment. Our study's findings are also consistent with these observations.

Overall, this study reinforces the potential of diatomaceous earth and kaolinite clay as sustainable alternatives to chemical insecticides for protecting stored grains. Although the efficacy of these treatments may decrease over extended storage periods, their initial effectiveness in controlling pest populations and ensuring consumer safety makes them valuable components of integrated pest management strategies.

Future research should focus on optimizing the use of these inert dusts as an IPM component and exploring additional factors that could affect their performance against different storage pests under various storage conditions.

Sake Manideep, Talapala Sai Kumar, S.V. Sangeetha, S.N. Salma Banu, S. Padmashree, K. Kruthi Reddy and K.S. Kedswin  (2024). Diatomaceous Earth vs. Kaolinite Clay for Rice Weevil Control in Wheat. Biological Forum – An International Journal, 16(7): 94-97.