Background
Quality and quantity of stored grain is constantly changing due to insect and fungal activity. The efficacy of storage method dictates the quality of grain. Traditional chemical pesticides, though effective, were often criticised for issues like increasing insect resistance, chemical residue, environmental contamination and human health risk. The diatomaceous earth based formulations could reduce chemical pesticides usage at some extent. But the slow insect killing and being non-food grade limited wide application. The high recommended dosage (500 to 3500 ppm) results in several adverse effects on grain, including reduce in the flow ability and bulk density, visible residue, extra dust generation during processing. Synthetic amorphous silica (SAS) consists three types: pyrogenic, precipitated, surface-treated SAS. These dusts can be distinguished from natural amorphous silica such as diatomaceous earth by its high chemical purity, the finely particulate nature and characteristics of particles. All types of SAS have been widely used in topical and oral medicines, food and cosmetics for many decades without evidence of adverse human health risks. Based on extensive physico-chemical, ecotoxicology, human health and epidemiology data, SAS as non-chemical method for pest management is revolutionary and advantageous compared to traditional approaches. However, their insecticidal mechanism is poorly understood. The optimal application protocol is not developed. This study described a comprehensive investigation of insecticidal mechanism of SAS particles and their application as an alternative practical stored grain pest control method.
Results
The first study was aimed to investigate the efficacy of different synthetic amorphous silica (SAS) powders against different insect species at multiple developmental stages compared with diatomaceous earth (DE). The stationary stages, egg and pupa, were more tolerant than that of the mobile stages, larva and adult, upon SAS and DE exposure. The insect infestation cannot be completely control by all the SAS and DE. A 100% of hatching rate was observed and more than 32% of pupa emerged in all the dust treated groups. Larva stage was most susceptible to the SAS and DE. Newly emerged adults were more susceptible to SASs and DE than older adults. The outcome for larvae was opposite. Among the three insect species adults, when treated by SAS and DE, T. castaneum was the most tolerant species and C. ferrugineus was the most susceptible. The efficacy of SAS against insects was higher than that of DE. Among of SASs, precipitated SAS performed better than pyrogenic SAS in term of mortality. Hydrophobic SAS powders were more effective against T. castaneum adult, while hydrophilic SAS powders were more effective against T. castaneum larvae, pupae and Sitophilus oryzae adults.
We evaluated the physical property of aforementioned SAS and DE in relation to efficacy. SAS powders have higher specific surface area, total pore volume, oil sorption capacity and smaller particle size than DE. In term of the SAS powders produced by different methods, pyrogenic SAS powders had higher oil sorption capacity but lower total pore volume and specific surface area, and larger particle size than precipitated SAS. Comparing with hydrophilic SAS, the particle size of hydrophobic SAS was smaller while has lower oil sorption capacity. There was a significant relationship between physical property of powder and insecticidal efficacy in SAS without a specific index.
We developed and evaluated a rapid screening protocol to identify electrostatic charge dictates attachment processes during initial contact between SAS and insects. The charge ability of three major stored grain insects, Sitophilus oryzae, Tribolium castaneum and Cryptolestes ferrugineus and four hydrophilic precipitated SAS and one DE was assessed on two insulated surfaces filter paper and glass. After contact with insulation surfaces, synthetic amorphous silica (SAS) and DE carry negative charges due to attaining electrons from insulation surfaces, while stored product insects carry charges of opposite polarity from electron loss. According to Coulomb’s law, the SAS particles would then be passively attracted by insect via the mere effect of electrostatic forces. A linear correlation was observed between electrostatic charge and bioactivity of dust.
After exposure to SAS, the changes in water content and other physiological components of insects led to changes in coloration and gross appearance. The heterogeneous distribution made visual comparisons difficult. Hyperspectral imaging systems with optically tuneable filters can record images at hundreds of contiguous wavelengths (narrow spectral resolution) in the form of a hypercube (three-dimensional hyperspectral data). Hyperspectral imaging coupled with back propagation neural network models was employed to quantify differences in parameters which reflected the response of T. castaneum and S. oryzae to hydrophobic and hydrophilic precipitated SAS. The presence of SAS on ventral and dorsal cuticle of two insect species caused differential values of relative reflectance in visible and short-wave near-infrared ranges. The control samples of all groups were correctly classified by BPNN model and misclassification occurred only with the two SAS treated. These results suggested that the differences in absorption characteristics of cuticular fat and protein contributed to the varied performance. The recognition rate between two SAS treated was within the acceptable identification range. This suggested that both SASs have similar effect on insect with varied degree.
We investigated how these two hydrophobic and hydrophilic precipitated SASs physically influenced insect in intersegmental membrane and their biological effects. Both SASs rapidly reduced insect locomotion to the limiting value within 3.5 hours and 12 hours for S. oryzae and T. castaneum, respectively. In addition, we found that there was significant differential decrease in straightness and upward length which were used as parameters to evaluate insect behaviours. Environmental scanning electron microscope (ESEM) images and data of stride length directly exhibited SAS eroded insect intersegmental membrane and absorbed the vital body fluid, eventually caused irreversible structural damage. The hydrophilic SAS was more effective in changing these parameters in S. oryzae, while hydrophobic SAS was more effective in T. castaneum. Male population was more susceptible than female.
We further evaluated the efficacy of SAS structural treatment combined with a new integrated trap as insect control in the field trial. Insect infestation was monitored by integrated trap utilising insect behaviours. Prior to SAS treatment, five integrated traps captured 1722 g insect inside a warehouse in seven day. Synthetic amorphous silica was aerogelize and dispersed uniformly in different locations of the warehouse. The mortality of five major species of stored grain insect adults reached 100% within three days post exposure.
Conclusion
SAS powders are food-grade, quick, effective, low cost and easy to apply as an insect control method. They don’t have the disadvantages of traditional chemical pesticide regarding to occupational health, environmental and safety concern. Detecting the electrostatic charge is an effective protocol for SAS efficacy evaluation. As an emerging non-destructive and reagent-less analytical technique, hyperspectral imaging proved to be highly efficient in pesticidal effect evaluation. Intersegmental membrane is a promising target site for new inert dust pesticide products
