The insecticidal activity of two types of silica (normal or coarse [CS] [150mm] and nano-silica particles [NSPs] [35 nm]) was determined using Callosobruchus chinensis (L) as the test insect. When the unsexed adults were exposed to broad bean seeds admixed with different concentrations of both tested silica (0.5, 1.0 and 2.0 g/100 g seeds) for different periods (48, 96, 144 and 192 hrs), it was found that NPS had more insecticidal activity against the bruchid beetle C. chinensis. Moreover, it was found that as the exposure time and concentration increased, the mortality of the adults increased. For each concentration of the tested silica with different exposure time, the LT50 of the bioassay tests showed that the calculated LT50 values of NSPs (83.44, 72.23 and 67.52 hrs) were less than those of CS (140.00, 131.44 and 120.15 hrs at the three tested concentrations of 0.5, 1.0 and 2.0 g/100g seeds, respectively). Therefore NSPs are more biologically active than CS against C. Chinensis. The adults survived treatment were left in the same bioassay jars to reproduce the first generation (F1) to determine the delayed effect of such tested concentration of both silica types. The results showed that there were no emerged adult insects at the concentrations of 1.0 and 2.0 g NSPs /100 g broad bean seeds. The numbers of the emerged insects after treatment with coarse silica was decreased as the concentration increased giving a range of emerged adults of 2.3-10.0 as compared with untreated check (control), which produced 24.7 adults. The pattern of picking-up silica particles of both tested silica types was also illustrated showing more pick-up of particles in case of using NSPs concentrations. Nano-silica particles (NSPs) were found to be effective against the bruchid beetle C. chinensis and could be effectively used in the Integrated Pest Management (IPM) program for C. chinesis and for other stored grain insect-pests.
Keyword: Callosobruchus Chinensis, nano-silica particles, LT50, Pick-up