Field Population Dynamics Monitoring Of Bactrocera Cucurbitae And Its Resistance Risk Assessment To Two Insecticides

The melon fly, Bactrocera cucurbitae (Coquillett), is a destructive insect of cururbit and other fruits, and has seriously damaged the cucurbitaceous vegetables in recent years in Guangxi. Nowadays, Chemical control is one of the most effective control measures for the melon fly. In this paper, the seasonal population dynamics of the melon fly in Nanning and Beihai, Guangxi province, were investigated by using the traps baited with parapheromone, the resistance levels of the field population of the melon fly in Nanning, Beihai and Qinzhou were detected with residual film method, and the resistance risk of the melon fly to Emamectin Benzoate and Cyhalothrin were evaluated.The main results were summarized as follows:1. Annual monitoring of the population dynamics of B. cucurbitae was conducted by ising the Steiner traps baited with Cue-lure throughout the year during2010and O11Nanning nd Beihai, Guangxi Province. The results showed that the field population of the fly in Nalapo Village, Nanning City, remaintained at a low level from early September to next early une, with the means of number4.6males/trap during15days of the captured fly. And it ncreased steadily from mid June and reached the maximun in early August, with23.6male/rap. In Fucheng Village, Beihai City, B. cucurbitae adults appeared throughout the year in xperimental field. There were two peaks for the field population, one was from June to Lugust, and the other one was from November to December, the average numbers for eachpper were108.07and281.33, respectively. The trapped numbers of melon flies in orchard, rinjaul and melonry were8304,6276and2341, respectively; suggestting the population ensity of the melon flies in orchard was highest. 2. The Standard bioassay was set up for melon fly. Assay of the toxicities of Emamectin Benzoate to B.cucurbitae by three bioassays including topical application, residual film and feeding method were compared under the same conditions.The result showed that coefficient of variation of the topical application and film bioassay were3.84%, and9.94%, and the topical application was the best method due to its high reproducibility and stability, however, higher technical and more time-cost running were required. Thus, the film bioassay was chosen for resistance monitoring and risk assessment of B.cucurbitae.3. Susceptible toxicity baselines of melon fly were established. The toxicity of twelve insecticides, such as Emamectin Benzoate, Chlorpyrifos, Cyhalothrin, Avermectin, Imidacloprid, Bifenthrin, Profenofos Emulsifiable Concentrate, Imidacloprid, Phoxim, Acetamiprid, Dipterex and Cartap on the sensitive population of B.cucurbitae were measured with Film bioassay. The results showed that the toxicities were in the order:Emamectin Benzoate> Chlorpyrifos> Cyhalothrin> Avermectin> Imidacloprid> Bifenthrin>Profenofos Emulsifiable Concentrate>Imidacloprid>Phoxim>Acetamiprid> Dipterex>Cartap. The LC50of Emamectin Benzoate and Cartap were0.2812mg/L and25.0394mg/L, respectively.4. The resistant levels of the field populations B.cucurbitae from Nanning, Qinzhou and Beihai in Guangxi, to four insecticides were determined. The results showed that the resistant ratios of population tested to Emamectin Benzoate, Cyhalothrin, Profenofos Emulsifiable Concentrate and Dipterex were from1.40to3.63,1.33to1.80,1.16to3.32and1.68to2.38, respectively, suggesting that the field populations tested with four insecticides were still in sensitive stages.5. The susceptible strain of B.cucurbitae was continuously selected for five generations with Cyhalothrin under selective pressure of30%to70%mortality for each selective generation in laboratory, the LC50value was up to0.6852mg/L from0.3506mg/L, the resistant ratio to Cyhalothrin of the resistant strain had increased by1.95-fold compared with the susceptible strain. The susceptible strain of B.cucurbitae was continuously selected for seven generations with Emamectin Benzoate,the LC50value increased to1.0771mg/L from0.4018mg/L,the relative resistance index was2.68-fold.The resistance realized heritability (h2) and resistance risk of B.cucurbitae to the two insecticides were estimated with threshold trait analysis method.The realized heritability of resistance were0.1408and0.1316. To obtain a10-fold increase in resistance to the two insecticides, the pest required9.83-4.49and16.72-7.57generations under the selective pressure or death rate of50%to90%for each generation.6. The biological characteristics of melon fly in different strains were observed. The results showed that the fly population selected by Cyhalothrin within5generations was significantly different from the control (sensitive population without insectide selection) in several biological indices, such as duration of larva and pupa,the emergence of adults, pre-ovipositon of female adults. All the above indices in the selected population were smaller than that in the control. There were no significant differences between the sensitive and resistant strain of the egg duration and the weight of mature larvae.In the fly population selected by Emamectin Benzoate within7generations, the biological indices such as duration of larva, pre-ovipositon and the first seven days of oviposition were significantly different from the control; There were no significant differences between the control and resistant of the egg duration, the pupal duration and the emergence of adults.