Effect of grain type on the insecticidal efficacy of SilicoSec against Sitophilus oryzae (L.) (Coleoptera: Curculionidae)
Introduction
During recent years, inert dusts have received increased attention as grain protectants, and are considered among the most promising alternatives to residual insecticides in stored-grain protection. These dusts can be classified into four broad categories (Subramanyam and Roesli, 2000): (a) those based on sand, kaolin, ash and clays, (b) several minerals, such as katelsous, dolomite, magnesite, lime and salt, (c) synthetic silicates, which are purified compounds of silicon dioxides and (d) diatomaceous earths (DE), which are the fossilized remains of diatoms (diatoms are unicellular algae which occurred during the Eocene and Miocene periods). This last category is the most appealing to many researchers since DEs combine effectiveness, low mammalian toxicity and natural origins. Although several modes of action of DE have been proposed, it is generally accepted that, as the dust particles are trapped by the bodies of the insects, damage occurs to the insects’ cuticle, because of absorption of cuticular wax that leads to water loss and desiccation (Korunic, 1998; Mewis and Ulrichs, 2001). Since the mode of action is a physical method, it is postulated that physiological resistance will not occur (Golob, 1997; Korunic, 1998).
The rice weevil, Sitophilus oryzae (L.), is a cosmopolitan pest which is considered to be one of the most destructive species in stored grain. It is classified as a primary pest, which can easily infest sound seeds, enabling other species (the secondary pests), which are not capable of breeding on intact grains, to cause additional damage (Hill, 1990; Rees, 1995). Since its larvae develop in the internal part of the kernels, the infestation is not visible at the first stages. Moreover, S. oryzae has developed a considerable level of resistance, to many traditional residual protectants (Arthur, 1996). For instance, the rice weevil is considered to be one of the most resistant stored-product insect species to pyrethroids, and usually cannot be controlled by application rates that are effective against most other stored-grain beetle species (Samson and Parker, 1989; Arthur (1992), Arthur (1994), Arthur (1999)). These issues highlight the need for assessing other substances that can be effective against this pest.
Newer DE formulations have proved to be very effective protectants against several stored-product beetle species, including S. oryzae (Korunic, 1998; Fields and Korunic, 2000; Arthur, 2002). However, there is still inadequate information about the factors that affect the insecticidal action of DE. In most of the studies evaluating DE efficacy the grain used was wheat, while there are few works using maize, rice and, especially, barley. One factor that seems to be crucial in determining the residual efficacy of DE is the type of the grain the dust is applied to (Subramanyam and Roesli, 2000). The aims of our study were: (a) to obtain information about the effectiveness of the DE formulation SilicoSec® against the rice weevil on four different types of grain, (b) to evaluate the influence of the dose rate of the DE and (c) to examine the ability for progeny production on treated grain.
Section snippets
Materials and methods
A sample of SilicoSec® was obtained from Agrinova GmbH (Germany). SilicoSec is a relatively new DE formulation of freshwater origin, and contains approx. 92% SiO2, 3% Al2O3, 1% Fe2O3, and 1% Na2O. Untreated, clean, with very little dockage and infestation-free peeled rice, paddy rice, maize and barley were used for experimentation. Exposure trials were carried out on each grain type treated with SilicoSec at the rates of 0.125, 0.25, 0.5, 1 and 1.5 g/kg of grain. For each trial (grain-dose
Results
The main effects, dose rate (F4,436=140.41), exposure interval (F3,436=656.47) and grain type (F3,436=434.04) were all significant (P<0.001). In addition, all associated interactions, dose rate×exposure interval (F12,436=11.54), dose rate×grain type (F12,436=17.60) and exposure interval×grain type (F9,436=35.53) were also significant (P<0.001).
Regardless of the dose rate used, the mortality of S. oryzae adults after 24 h of exposure to DE was low (Fig. 1). In general, more weevils were dead at
Discussion
Our study indicates that adult mortality of the rice weevil on grain treated with SilicoSec increases with exposure time. This stands in accordance with previous studies assessing other DE formulations against S. oryzae (Subramanyam and Roesli, 2000; Arthur, 2002). Apparently, this is also the case with most residual protectants (Evans, 1985; Arthur (1994), Arthur (1999)). According to Subramanyam and Roesli (2000) the “speed of kill” is perhaps more important than 100% mortality, because
Acknowledgements
We thank M. Erb-Brinkmann and B. Straube (Agrinova GbmH, Germany) for providing the SilicoSec quantity for experimentation.
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