Abstract:
Insect damage in stored grains and grain-based products dates back to early
civilization; Sitophilus oryzae (L.) and Tribolium castaneum (Herbst) are serious
pests. Insecticides are effective management measures but accompany negative
impacts on biotic/abiotic environment, decreased efficacy over time and are
expensive. The combination of treatments, including insecticides, synergizes efficacy
and may overcome the above limitations. The exposure to bacterial formulation
spinetoram, pyrethroid deltamethrin and high temperature are effective treatments but
their effects on certain species when used alone or in combination are poorly
understood. This study evaluated the single and combined effects of spinetoram,
deltamethrin and heat on the mortality of S. oryzae and T. castaneum adults following
a completely randomized design. The insect adults were pre-exposed to different
concentrations of deltamethrin or deltamethrin+spinetoram combinations.
Subsequently, S. oryzae and T. castaneum were held at 42°C and 46°C, respectively.
Insect mortality at different durations was determined. The lethal time to kill 50% of
the population (LT50) determined heat tolerance. Without exposure to heat,
deltamethrin+spinetoram combination caused higher mortality than deltamethrin
alone in S. oryzae but not in T. castaneum. Increased duration at high temperature
increased the mortality in both species. The heat tolerance (LT50) was reduced in S.
oryzae when pre-exposed either to deltamethrin alone or deltamethrin+spinetoram
combination, at all the concentrations tested. However, T. castaneum adults reduced
heat tolerance (LT50) at 15 ppm either with deltamethrin alone or
deltamethrin+spinetoram combination. In general, pre-exposure to deltamethrin and
spinetoram combination synergizes mortality and reduces the survival (LT50) at high
temperature (LT50) compared to deltamethrin alone in S. oryzae but not in T.
castaneum. Pre-exposure to deltamethrin and spinetoram reduces heat tolerance,
enabling pest management at shorter exposure durations and lowering heat energy
costs. Future experiments need to address the limitations identified in this study for
improvements.
