The Lethal Effects of Microwave Radiation on the Adults of Cowpea Seed Beetle, Callosobruchus maculatus Fabricius (Coleoptera: Bruchidae)

The cowpea seed beetle (Callosobruchus maculatus F.) is one of the most injurious insects infesting a wide range of leguminous stored seeds such as Lens culinaris, Vigna radiata, Vicia faba and Vigna unguiculata (1). This insect also causes secondary infestation during pulse storage, and may cause total loss within three months. The bruchids can cause heavy losses in terms of both quantity and quality (4). Heavy infestations of bruchids can cause heating of commodity, which results in quality loss, and mould growth. Currently, the control strategies of stored product insect pests have exclusively relied upon application of fumigants and protectant synthetic insecticides. Chemical control of pests has led to insecticide residues in stored products and insecticide-resistant insect populations (3, 6). Therefore, there is a need for the ecologically benign methods to control cowpea weevil on chickpea. High temperature application has been used to satisfactorily disinfest stored commodities by various technologies. Microwave radiation is one of the most promising biorational pest management tools for farm stored grain and grain processing industries (29). Microwave is a type of electromagnetic energy that provides rapid heating. It appears that high temperatures in a short time period may be lethal to many stored product pests. The present study aimed to assess the lethal effects of microwave radiation on adults of cowpea seed beetle in stored pulses.
The population of C. maculatus was originally collected from Laboratory of Entomology at Ferdowsi University of Mashhad, Iran. The cowpea weevils were reared on chickpea, in a growth chamber at temperature of 28 ± 2 ºC, relative humidity of 60 ± 5 % in the dark. Male and female beetles were treated separately with 2450 MHz at power levels of 90-900 W over a range of irradiation periods from 20 to 960 seconds. Corrected mortality data were subjected to two-way factorial analysis of covariance (ANCOVA) in which the microwave power and insects’ sex were regarded as independent categorial factors and irradiation period as covariate using SPSS Version 16 software. Prior to the ANCOVA test, Levene's test for equality of variances was performed to examine the assumptions for ANCOVA. Moreover, at each radiation power, the values of median lethal time (LT50), i.e., microwave irradiation period needed for the death of 50% insect population, and 90% lethal time (LT90) with 95% confidence limits were estimated separately for male and female beetles by subjecting mortality data to the maximum likelihood program of probit regression analysis using POLO-PC software. This program has a provision for control mortality.
The results indicated that the mortality rates of both male and female adults increased as the exposure period and/or the power of radiation increased. When microwave power levels increased, the time elapsed to achieve maximum lethal effect decreased; this was implied by the results showing maximum mortality achievement in 960 and 80 seconds at 90 W and 900 W, respectively. Male beetles were more susceptible than females, but this difference was not significant based on the LT50 ratio. The LT50 values for the power levels between 90-900 W varied between 674.8 - 41.40 seconds for males and 741.5 - 47.2 seconds for females. The probit analysis showed that the slope values of mortality - exposure time regression lines were in the range of 3.23 - 5.51. According to the likelihood ratio test of parallelism, the slopes of probit mortality regressions differed significantly among various microwave power levels. However, further likelihood ratio test between the paired combinations concerned revealed that the slopes of probit mortality lines differed significantly only between those of 90 and 900W levels. The present results are in agreement with those of Singh et al. (2012) who worked on Callosobruchus chinensis (26). Also, Sadeghi Nasab et al (2004) working on the effect of microwave radiation on three stored product pests, found similar results (25).
The results from our study showed that microwave radiation has a potential to kill insects in stored beans. It was observed that at certain constant power levels, the mortality of beetles increased as the microwave exposure time elongated and vice-versa. Although this strategy can control insect population in stored beans in a short time whilst deserting no chemical residue on food products, the adverse effects of microwave radiation on nutritional quality of food products and seed germination should be investigated. Further research is also recommended to improve methodology for practical disinfection of stored pulses.