| Cotton bollworm, Helicoverpa armigera (Hübner) is a pest belonging to lepidoptera, noctuidae, which is a worldwide important agricultural pest. Once the environmental conditions is fitting to grow, Helicoverpa armigera with extensive host will outbreak largely and threat the production of crop. Nowadays, cotton bollworm has developed different degree resistance to insecticides of organochlorine, organophosphate, carbamate, pyrethroids after applied in the field for controlling the pest for long time. Methoxyfenozide is one of the insect growth regulators with excellent effect in cotton field to control Helicoverpa armigera at present. In this paper, we studied cross-resistance, realized heritability and biochemical mechanisms of resistance to methoxyfenozide in Helicoverpa armigera.1.Selection of resistance to methoxyfenozide and cross resistance to other insecticides in Helicoverpa armigeraThe resistant population(R2) with 30.57-fold resistant ratio has been treated 31 times during the whole 35 genenrations by methoxyfenozide in the laboratory. The value of methoxyfenozide LC50 rangs from 13.61μg/mL to 416.07μg/mL. After 4 times of anti-selection with methoxyfenozide, the value of LC50 has dereased from 13.61μg/mL to 5.73μg/mL in the susceptible population(S). The resistance population(R2) and the susceptible population(S) of Helicoverpa armigera with resistance ratio of 72.61 are used for evaluating cross resistance to 12 common insecticides with leaf-dipping method in the laboratory.The results showed that the resistant population has medium cross-resistance to tebufenozide with the cross-risistance ratio of 13.57 fold. there are a little cross-resistance to indoxacarb with ratio of 3.05, and no cross-resistance to phoxim, chlorpyrifos, methomyl, beta-cypermethrin, deltamethrin, chlorfenapyr, lufenuron, chlorfluazuron, hexaflumuron and emamectin in the resistant population. The results suggested that it could be helpful to delay resistance development to methoxyfenozide in Helicoverpa armigera when applied the insecticide in alternation with the insecticides with no cross-resistance in the field.2. Realized heritability of resistance to methoxyfenozide in Helicoverpa armigeraIn order to evaluate the resistance risk of methoxyfenozide, Realized heritability (h2) of resistance of the resisitant populantion(R1,32.99-fold) in different selection stages was evaluated based on the Tabashnik's methods. The results suggested that h2 for the entire selection experiment was 0.0830. h2 to methoxyfenozide in the early, middle and latter selection stages were 0.0715, 0.0768 and 0.0947 respectively. Assuming that slope=2.0 (δp=0.5), the h2 was half of indoor estimation value, it required 34.48 and 27.40 generations of Helicoverpa armigera to obtain 10-fold increase of methoxyfenozide in LC50 under selection pressure at 80% and 90% mortality for each generation of selection. The number of generations required for 10-fold increase would be even more in field due to the changes in allele frequency and environmental variation or both. Therefore, methoxyfenozide could be regarded as an effective insectcide to controling Helicoverpa armigera for a long time at a rational application frequency in the field. 3. Biochemical mechanisms of resistance to methoxyfenozide in Helicoverpa armigeraThree synergists of piperonyl butoxide, phosphate defoliant and diethyl maleate have significant synergism to methoxyfenozide with the synergism ratio of 2.76, 3.15 and 6.14 fold to methoxyfenozide respectively in the resistant population(R2), but no synergism in the susceptible population(S). Activities of four enzymes were compared between the resistant polulation and the homologous susceptible population. The results revealed that enhancement of activity for mixed function oxidase, general esterases and glutathione-s-transferase, but not for acetylcholinesterase, should contribute to the methoxyfenozide resistance in the resistant population of the pest. |
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