Screening and Applying of Some Plant Essential Oils for Controlling Pulse Beetle, Callosobruchus maculatus (F.) on Mung Bean, Vigna radiata L. (Wilczek) Seeds

Abstract

Callosobruchus maculatus is one of the most serious pests brought into storage containers with harvested mung bean. It cause total loss of the stored crop in few months. In this study, five plant essential oils were compared for their residual contact and fumigant activities against C. maculatus. After getting the most effective essential oil, synergistic effect with sesame oil was investigated. Moreover, this study was created awareness of the value of plant products as the application method for pulse beetle in small holder farmers' storage facilities. The insecticidal activities of plant essential oils extracted from lengkuas (Alpinia galanga) rhizome, citronella (Cymbopogon nardus) leaf, clove (Syzygium aromaticum.) flower bud, cinnamon (Cinnamomum verum) bark and kaffir lime (Citrus hystrix) peel were investigated against the C. maculatus adults under laboratory conditions. Insecticidal activities of plant essential oils varied with different essential oil, exposure period and concentration. In residual contact bioassay, clove oil exhibited the strongest toxicity against C. maculatus adults with LC50 values of 16.05, 12.99 and 7.67 μl/ml at 24, 48 and 72 h, respectively. Moreover, clove oil was the most effective in the fumigation method followed by lengkuas, cinnamon, citronella. Its LC50 values at 24, 48 and 72 h were 297.80, 221.69 and 136.20 ul/1 air, respectively. Synergistic toxicity of sesame oil and clove essential oils were investigated against C. maculatus adults, through residual contact and fumigation tests. The percent mortality of insect were 44.00±2.45 and 46.00±5.10 after exposure to clove oil alone at 24 h by the residual contact and fumigation methods, respectively. These values increased to 48.00±5.83 and 62.00±3.74 after exposure to the plant oil mixture of clove oil and sesame oil ratio (8:2), whereas there was no mortality after treating with sesame oil alone in both application methods. It suggests that sesame oil showed the synergistic effect after mixing with clove oil at the ratio of 8:2 in both application methods. Synergistic toxicity was greater by fumigation than by residual contact application. The most effective ratio of clove oil and sesame oil (8:2) was tested with three application methods of sack coating, seed dressing and fumigation against the C. maculatus. The results showed that inhibition percentage of plant oil mixture depended on concentration and day of exposure after sack coating method. At the 1st month, the movement inhibition of plant oil mixtures was less effective than 63.52 ± 1.47% to inhibit C. maculatus after 7 days of treatment. Plant oil mixtures 3.0% and 5.0% completely inhibited the F1 adult emergence and did not affect the weight loss of mung bean seeds. At the 2nd month, all concentrations of plant oil mixture showed the inhibition percentage less than 50% after 5 days. F1 adult emergence was reduced from 476.75±8.11 to 33.25±4.44 at the concentration of plant oil mixture ranging from 1.0% to 5.0%. The lowest weight loss of 0.27±0.09% was recorded at the concentration of 5.0%. In the seed dressing, the 3.5% concentration of plant oil mixture and chlorpyrifos completely suppressed the adult progeny of C. maculatus, no seed damage and lowest WPI values 0.00±0.00 were observed through the six months storage period. By fumigation with a burner, C. maculatus eggs were tolerant to plant oil mixture with the highest LCso values of 7.81% and the mortality percentage of 58.00+2.58%. On the other hand, C. maculatus adults were susceptible to plant oil mixture with the lowest LC50 value of 3.64% after 72 h in plastic cup. This research provided a scientific basis in applying botanical insecticides against C. maculatus. Further studies should be done for the bioactivity of the plant oil mixture of clove oil and sesame oil (8:2) against other stored-product insect pests. Moreover, there is a need to assess the cost-effectiveness and feasibility of using the plant oil mixture on large scale seed storage.