| Bradysia odoriphaga Yang and Zhang (Diptera:Sciaridae) is a serious pest of Chinese chive, Allium tuberosum Rottle ex Spreng, which commonly known as chive maggot. The larvae caused damage mainly by eating the roots and stems of the Liliaceous vegetables, aggregating together and injuring the plant leading to large reductions in production and irreparable losses. With the large scale expansion of the greenhouse leek planting, its occurrence trend became increasing year after year. Successive planting plot happen can be particularly serious. In general, farmers controlled the fungus gnat using organic phosphorus pesticide such as chlorpyrifos and phoxim, but with the extending of the using time, the control effect getting lower and lower in field condition. Insect growth regulators (IGRs) were a series compound with the activities of destructing normal physiological activities of insects, which has the characters of higher animal safety and environmental friendly. Different types of IGRs have been developed since 20th Century 90 years and began to be used instead of conventional pesticides such as organophosphates and carbamates, which has important application value in Integrated Pest Management (IPM) control strategies. To select the efficient IGRs pesticides against B. odoriphaga, comparison toxicity effect of hexaflumuron, chlorfluazuron, chlorbenazuron, cyromazine, pyridalyl and pyriproxyfen on 2nd instar and 4th instar larvae of B. odoriphaga were tested using stomach-contact combination toxicity method. On this basis, the toxic characters and toxicity of hexaflumuron against different developing states of B. odoriphaga were studied, as well as the combined effects of pyriproxyfen and hexaflumuron on 4th instar larvae, and different developmental days age of eggs on their mortality of newly hatched larvae of B. odoriphaga, besides, the greenhouse pot and field trials tests were conducted to evaluate the efficacy of the pesticide mixtures in field conditions. The main results of all experiments are as follow:1. In the bioassay with chitin synthesis inhibitors of hexaflumuron, chlorfluazuron, diflubenzuron and cyromazine, juvenile hormone analogues of pyriproxyfen and pyridalyl, ecdysone analogs oftebufenozideto larvae of B. odoriphaga, we found all the IGRs got the slower lethal activity than conventional insecticides. When treated the larvae, their biggest mortality emerged at the period from late instar to prepupae and even when the adult emergence. Chitin synthesis inhibitors have higher lethal bioactivity than ecdyson analogue and juvenile hormone analogue against larvae of B. odoriphaga. Observed 96 h after treated, the toxicity of hexaflumuron against 2nd instar larvae is the largest, as the same result of 4th instar larvae treated by hexaflumuron observed until adult emergence, the toxicities were 0.50 mg/L and 0.12 mg/L, respectively.2. Stomach-contact combination toxicity methods, the dripping ration liquid methods, and the dry film residue method under laboratory conditions were used to determine the toxicity of eggs, larvae, pupa and adult of B. odoriphaga. Result shows that observed until 24 h after eggs hatched, the LC50 values of 0.5-1,1.5-2 and 2.5-3 day-old eggs were 2.27,1.60 and 0.64 mg/L in orders. It was more toxic to the developed eggs (1.5-3 day-old) than the new laid eggs (0.5-1 day-old). Observed the different developmental stages until adult emergence, the LC50 values of 2 day-old eggs,2nd instar larvae,4th instar larvae,1 day-old pupae and 3 day-old pupae of B. odoriphaga were 0.06,0.30,0.34,24.05 and 27.42 mg/L when treated by hexaflumuron separately. The toxic level decreased progressively with the developmental process of B. odoriphaga. The toxicity of pupae and adults were lower than larvae. At the dose of 100 mg/L, the emergence rates of 1 and 3 day-old pupae were above 10% and 20% respectively. Besides, the corrected mortality rate of adults treated by dry film residue method was less than 20% after 24 h, at dose of 100 and 1000 mg/L. Hexaflumuron has negative impact against each developmental stages of B. odoriphaga, which can inhibit the hatching rate of eggs and due to decrease survival rate of newly hatched larvae. The toxic speed of hexaflumuron against larvae is slow, and the significant toxic effect is showed until pupae even adult emergence period. Lower toxicity was showed on pupae and adult periods, but hexaflumuron does have side effect on eggs hatching rates of next generation.3. Joint toxicity of pyriproxyfen and hexaflumuron at the ratio of 1:40,1:20,1:10,1:3,1:1, 3:1,10:1,20:1,40:1 against 4th instar larvae of B. odoriphaga was determined. Experimental results showed that the whole ratios of pyriproxyfen and hexaflumuron with different mechanisms of action showed synergistic effect against the 4th instar larvae observed until adult emergence, the co-toxicity coefficient of 1:3,1:1 and 3:1 is higher than other ratios, and the ratio of 1:3 shows the highest synergies.4. The single and mixed used of pyriproxyfen and hexaflumuron against different developmental eggs of B. odoriphaga in laboratory bioassay. Results indicated the younger eggs were susceptible than the older eggs when treated with pyriproxyfen. The mortality of younger eggs reached 92.5% at 5μg (a.i.) mL-1 and 100% mortality at 10μg (a.i.) mL-1 of pyriproxyfen. However, hexaflumuron had strong effects against the developed (1.5-3 days) eggs and the survival rate of the young larvae. When combined, the ovicidal and larvicidal ictivity was markedly increased on all stages of the eggs. The co-toxicity coefficients for these three stages of the eggs was 324.48 for eggs 0.5-1day-old,149.12 for eggs 1.5-2day-old and 190.92 for eggs 2.5-3day-old, respectively. All results showed synergistic effects to a varying degree.5. The pot tests indicated that pyriproxyfen, hexaflumuron at the ratio of 1:1 has good control effectiveness of 4th instar larvae of B. odoriphaga. The control effect of mixture was 81.48% at the rate of 20 mg/L; adult emergence inhibiting effect was significantly higher than that of the same concentration of each single agent in the treatment, which can effectively reduce the number of the generations. hexaflumuron at 4995 g/ha2 of effective components after 130 d, the amount of adult was 63.91%-73.81% lower than the control, and the effect of IGRs treatment is equivalent to the neonicotinoids treatment, which has the same results with the pot tests. |
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