A comparison of microwave and infrared radiation to control rice weevils (Coleoptera: Curculionidae) in wheat

Microwave or infrared radiation was used to produce a temperature of 54 ? 1? C in paired samples of rice weevil infested soft winter wheat. Infrared gave greater insect control than did microwave by the following average dif ferences in percentage: 13 for immature stages, 76 for adults, and 56 for Fa progeny of treated adults. A continuation of our efforts to find a promising new method of reducing insect populations in grain has shown that either microwave or infrared radiation can control stored-product insects without leaving undesirable residues. Short exposures of infested commodities to high temperatures can be used to kill insects without adversely affecting the commodity. For example, Tilton and Schroeder (1963) demon strated that infestations of insects within kernels of rice can be ef fectively controlled with infrared radiation. The possibility of using infrared radiation for grain drying has been recognized for many years. Faulkner and Wratten (1969) proposed a large-scale infrared source for drying rice; such a unit could also be used for insect control. Microwave radiation was proposed as an alternative method of obtaining insect control with high temperatures by Hamid et al. (1968). Boulanger et al. (1971) described a large-scale microwave unit that can be used for drying grain or for insect control. As no studies of the comparative efficiency of microwave and infrared energies for insect control had been made, our study was conducted to compare the effectiveness of the same temperature pro duced by microwave radiation and by infrared radiation on all stages of rice weevils, Sitophilus oryzae (L.), infesting soft winter wheat. Methods and Materials Large populations of adult rice weevils were placed in ca. 4,800-g batches of soft winter wheat for 3-day oviposition periods. After oviposition, all adults were removed and the wheat was stored at 27 ? 2? C and 60 ? 3% RH. Insects were placed on the first batch for 3 days, then on the second, third, and so on until adults began 1 Received for publication February 4, 1972. Mention of a proprietary product does not imply endorsement by the USDA. 2 Research Entomologists, Stored-Product Insects Research and Development Laboratory, MQRD, ARS, USDA, Savannah, Georgia 31403. Journal of the Kansas Entomological Society 45:434-438. October 1972. This content downloaded from 157.55. 39.34 on Tue, 17 Jan 2017 17:52:52 UTC All use subject to http://about.jstor. org/terms Vol. 45, No. 4, October, 1972 435 to emerge from the first batch of wheat infested. Seven different batches of wheat containing known ages of rice weevils were used. The 7th batch infested contained only eggs and 1st instar larvae, while the 1st batch contained pupae and adults that had not emerged from the kernels. Different larval instars and pupae were represented in the intermediate batches of infested wheat. In addition, an 8th batch contained adult rice weevils that were placed on the uninfested wheat immediately before treatment and were left on the wheat for a 7-day post-treatment oviposition period. Each of the 8 batches of wheat was divided into twenty-four 200-g samples and placed in 0.47-liter mason jars. These samples were used to provide 5 replications each of microwave and infrared radiation treatments and an untreated check. The infrared radiation source was a Perfection gasfired infrared heater equipped with ceramic panels, and having a rated output of 48,000 BTU/hr and a peak energy emission of 2.5/x. Each sample was spread on a 1225-cm2, 60-mesh brass strainer cloth tray in a layer 1 kernel in thickness and placed beneath the heater for 26 ? 1 sec to attain a temperature of 54? 1? C. The distance between the heat source and the wheat was 65 cm. The microwave radiation source was an Amana Radarange (R) microwave oven which operated on a frequency of 2.45 GHz, by using a 1.6-kw magnetron oscillator. Each sample designated to receive microwave treatment was placed in a 19-cm-diam cardboard pan, placed in the center of the microwave oven, and exposed for 23.5 ? 1 sec giving a temperature of 54 ? 1? C. Immediately after treatment the wheat was returned to the sample jars and the temperature of the wheat was taken by using a banjo thermistor and a Yellow Springs Instrument Model 42 5F digital readout thermometer. This method of temperature determination mea sures only grain surface and interstitial air space temperatures. Due to differences in the acquired temperatures of treated samples, 5 microwave-treated samples were selected in a 53? to 55? C range. Each was paired with an infrared-treated sample of the same tem perature. This procedure was followed for all age groups, Excess samples were discarded. Moisture analyses were made at random from samples before and immediately after the various treatments with an automatic Steinlite(R) moisture meter. The average moisture content of all samples before and immediately after treatment was 11.8 ? 0.5%. After treatment, the samples were maintained at 27 ? 2? C and 60 ? 3% RH. Emerged adults were removed from the wheat 3 times a week, counted, and discarded. Counts were made until emergence was completed or until an overlap of generations was evident. The percentage of mortality, which served as a measure of the effectiveness of the infrared or microwave treatments, was cal culated by dividing the number of insects that emerged from the This content downloaded from 157.55. 39.34 on Tue, 17 Jan 2017 17:52:52 UTC All use subject to http://about.jstor. org/terms 436 Journal of the Kansas Entomological Society Table 1. Average emergence and percentage of reduction in number of adults emerging from wheat containing different ages of immature rice weevils treated at 54 ? 1? C. Reduction in emergence Emergence"1 Infrared Age of microwave insects Untreated Microwave Infrared Microwave Infrared difference (days) (No.) (No.) (No.) (%) (%) (%) 2-5 2,317 352 43 84.9 98.2 13.3 5-8 1,605 163 18 89.9 99.0 9.1 11-14 1,560 278 45 82.2 97.1 14.9 17-20 2,433 328 52 86.6 97.9 11.3 23-26 1,347 285 96 78.9 92.9 14.0 26-29 1,789 555 239 69.0 86.7 17.7 29-32 1,037 232 83 77.7 92.1 14.4 Total 12,088 2,193 576 81.9 95.2 13.3 a Average of 5 replications. treated samples by the number that emerged from the untreated samples, multiplying by 100, and then subtracting from 100%. The mortality of treated adults was determined 7 days after exposure, and all adults were removed and discarded. The progeny of treated adults was determined by counting the emerging Fx adults in the manner described above. Analysis of variance was performed on the results.