INCREASE IN ACOUSTIC DETECTABILITY OF PLODIA INTERPUNCTELLA (LEPIDOPTERA: PYRALIDAE) LARVAE IN STORED PRODUCTS AFTER ELECTRICAL STIMULATION

Abstract

Modern computer and electronic technology has enabled the development of powerful acoustic methods for detection of hidden insect infestations in stored products (e.g., Shade et al. 1990, Shuman et al. 1993, 1997; Pittendrigh et al. 1997), wood (Scheffrahn et al. 1993, 1997), and soil (Mankin et al. 2001). Despite their power, the utility of acoustic tools for detecting insect infestations is limited under conditions of reduced activity such as low temperatures (Maier et al. 1996), molting or pupation (Vick et al. 1988, Pittendrigh et al. 1997), and after disturbance, depending on the species (Mankin et al. 1999, Miyatake 2001). Even under optimal temperatures, stored product insect larvae are quiet in 1030% of 5-minute monitoring periods during growth phases (Vick et al. 1988), and quiescent periods can extend from 3-5 minutes after a disturbance (Miyatake 2001) to >8 h during molting phases (Pittendrigh et al. 1997). The reliability of acoustic surveillance would be improved if effective excitatory stimuli could be applied to inactive insects just before monitoring, particularly if the excitation increased the loudness of the sounds produced. One potential use of acoustic technology that remains largely undeveloped is nondestructive surveying for insect infestations in packaged goods. Plodia interpunctella (Huibner) is a major pest of packaged goods in warehouses and retail stores (Arbogast et al. 2000) and early detection of larvae in packages could reduce high-value product losses and eliminate the buildup of populations. However, the weak sounds produced by 3rd-4th instar (5-15 mg) P interpunctella larvae are difficult to detect through multiple layers of protective packaging. The detectability of these larvae could be improved if artificial stimuli were available to increase the rate and loudness of larval sounds during acoustic monitoring periods. Treatments used previously to stimulate insect activity and increase their visual or acoustic detectability include heat (Shade et al. 1990, Hagstrum and Flinn 1993, Au 1997, Mankin et al. 1999), disturbance, usually of highly mobile species (Minnich 1936, Masters 1979, Roces and Manrique 1996), and electrical stimulation (Vander Meer et al. 1999). Heat has been used in Berlese funnels and related devices (e.g., Edwards 1991) but its activating effects are indirect and may take hours to develop. Electrical stimulation is used to control activity of farm animals but rarely of insects. In a high-school student research project conducted by Brett Miller, 4th-instar P. interpunctella larvae were monitored to determine if commercially available electrical stimulation tools increased larval activity levels and acoustic detectability. Individual larvae were placed on dog food biscuits 1-2 days before testing and kept in separate containers. Stimulation was applied by placing an infested biscuit between the two prongs of an electric prod (Model Sabre Six, HotShot Products, Inc., Savage, MN) (9 kV, 19 mA) and then briefly toggling the actuator. The prongs were 3.5 cm apart and the biscuits were -2.5 cm wide. For control tests, the prod was inactivated by removing its battery pack. Acoustic monitoring was performed by placing each infested biscuit on a 4.5-cm-diameter piezoelectric disk (MuRata Erie model PKM28-2AO, Smyrna, GA) (see Mankin et al. 2000). Sounds generated by moving and feeding larvae were monitored for 180-s periods at 24-26?C in an acoustically shielded chamber (Mankin et al. 1996). On 3 different weeks (blocks), recordings were obtained from larvae in 20 biscuits treated with an active prod and 20 biscuits treated with an inactive prod (120 larvae altogether). Monitoring was done immediately before the biscuit was placed between the electrodes, immediately after the activation treatment, and 10 minutes later to consider the duration of stimulatory effects. The acoustic signal collection and analysis procedures were similar to those described in Mankin et al. (2001). A noise threshold was set at a level that eliminated low-level, extraneous sounds. At this threshold, no sound pulses were registered in control tests with uninfested biscuits, with or without electrical stimulation. A spectral profile of P interpunctella sounds was calculated from averages of 20-50 pulses collected during a noise-free period of recording, and signals above the noise threshold that matched this profile were counted as larval sound pulses using a custom-written computer program, DAVIS (Mankin 1994). Because this study was conducted in an acoustically shielded anechoic chamber, the recordings contained minimal noise and almost all of the sound pulses matched the larval profile. Background noise is prevalent in field studies but insect profiles can be used to screen out all but a small percentage of background noises (Mankin et al. 2000, 2001). A treatment that stimulates increased activity is of most benefit for acoustic detectability when