Thermal treatments to increase acoustic detectability of Sitophilus oryzae (Coleoptera: Curculionidae) in stored grain.

Hidden infestations of stored–product insect larvae are detected most rapidly by acoustic techniques when the larvae are highly active. Larval activity is periodic, however, and it tends to decrease after the larvae are disturbed or cooled. Because of the practical need for rapid inspection of grain at commercial elevators, several heat treatments were tested as potential methods of increasing larval activity and improving the speed and reliability of acoustic detection under adverse conditions. Samples of grain infested with 4th instars of Sitophilus oryzae (L.) were exposed to different radiant and convective heat treatments after they had been conditioned at 11°C, 17°C, or room temperature for 12–24 h. Relative activity levels were evaluated over periods of 0–12 h based on the mean levels in a 15–min interval, 2 h after the beginning of a trial. In comparisons among treatments with precooled larvae, relative activity levels 5–10 min after brief heat pulses were 2–30 times higher than activity levels in precooled controls exposed only to ambient temperatures (25°C). After 15–25 min, the relative activity levels of these heated larvae remained 2–5 times higher than those of the ambient controls. Brief movement disturbances inhibited activity for ≈20 min at any temperature. These results suggest that, in general, larval detectability is enhanced if cool grain samples are warmed and all samples are left undisturbed for 15–20 min before inspection.