Fitness Costs As Affected By Rearing Temperature In The Experimental Populations Of Insecticide-resistant And Susceptible Plutella Xylostella (Lepidoptera: Plutellidae)

In this study, the fitness costs in the experimental populations of insecticide-susceptible (S) and-resistant (R) strains of diamondback moth under different rearing temperatures were studied employing bioassay, life table construction and biochemical analyses. Methamidophos-susceptibility in the progenies of S and R strainsof diamondback moth was determined. Furthermore, the effects of heat stress on the expression levels of heat shock protein 70 (Hsp70) in the S and R strains of diamondback moth were also investigated.Four different field populations of diamondback moth were collected from Xiaoyuantian, Jianxin, Shangjie and Ganzhe of Fuzhou, respecitively. It was found that all the collected diamondback moth populations had developed significant resistance to methamidophos and the resistance level varied depending on different field colonies.Significant correlations were found between the methamidophos resistance and bimolecular rate constant (K_i) values of acetylcholinesterase (AChE) in both S strain reared in laboratory and R strain collected from filed. The K_i values to methamidophos in S strain was 17．30 to 39．10 folds of the field population of R strain. The fact indicated that AChE played a criticle role in diamondback moth's methamidophos resistance.The survival of diamondback moth was strongly inhibited by high temperature. Both the R and S strains displayed higher survival rate, emergence rate, copulation rate and hatchability at 25℃than at 33．5℃.High temperature showed significant depression on the fecundity of diamondback moth: 1．Effects on the spawning stage of female diamondback moth. The spawning stage of female moth was 6 to 9 days at 33．5℃, while that could be 14 to 16 days at 25℃. 2．Effects on the fecundity of diamondback moth. The number of eggs and hatchability declined inevitably at higher temperature.It was found that the population growth tendency index (I) in S strain was higher than that in R strain under the same constant temperature, which revealed that the S strain was more adaptive to the changes of environmental temperature. Compared to S strain, R strain showed 0．66-and 0．19-fold I values at 25℃or 33．5℃, respectively.The survival curves of R and S strains at different constant temperature were obviously different. At 25℃, both the survi(?)al curve of R and S strains were Deeveyâ…¡model while that at 33．5℃were Deeveyâ…£model. The average generation period (D) of both strains were shorten with the temperature increase. But at the same constant temperature condition, no significant difference in D value was found between R and S strains. The innate capacity of increase (r_m), finite rate of increse (λ) , in both strains were lower at 33．5℃than at 25℃. Moreover, the values of net reproduction rate (R₀) , r_m,λin S strain were higher than those in R strain at the same constant temperature while the population doubling time (T_d) was opposite between them.At 25℃, there were no significant differences in methamidophos-susceptibility among F₁, F₂ and F₃ of S strain. If the F₁ was reared at 33．5℃, then its offsprings (F₂', F₃') were transferred to be reared at 25℃, the methamidophos-susceptibility of F₂' increased. There were no significant differences in methamidophos susceptibility between F₁ and F₃'.At 25℃, the susceptibility to methamidophos of F₂R strain was higher than F₁, and F₃ was higher than F₂．If the F₁ was reared at 33．5℃, then its offsprings (F₂',F₃') were transferred to be reared at 25℃, the methamidophos-susceptibility of F₂' increased. Though the methamidophos-susceptibility of F₃' declined. It had significant differences with F₃．The susceptibility of R and S strains of diamondback moth to methamidophos in field insectarium under field natural temperature were also determined In June and July, methamidophos susceptibility of S strain mildly increased while no significant differences among that in July, September and October could be found.Pupa of R strain collected from Shangjie in Jun. (F₀) and Oct. (F₀'), 2005, respectively were transferred to a field insecticide-free insectarium when they emerged. The susceptibility to methamidophos of F₁ and F₁' was found to be much higher than that of F₀ and F₀'. But there were no significant differences in the susceptibility to methamidophos among F₁, F₂ and F₄ generation, with LC₅₀ being 114 mg. L^-1 ,103 mg. L^-1 and 112 mg. L^-1 , respectively. Compared to those collected in June, the susceptibility to methamidophos collected on October was lower.The expression of Hsp70 could be induced effectively under high temperature condition. Comparing the lavae and adults of both S and R strains treated at 20℃, the expression of Hsp70 of those under 33．5℃were much higher. The expression of Hsp70 was different with different strains, and that of S strain was much higher than the R strain.In conclusion, it was found in this study that the biological fitness in S strain of diamondback moth was significantly higher than that in R strain at both 25℃and 33．5℃. Low biological fitness might result in the sharp increase of methamidophos of R strain under high temperature conditions (i.e., 33．5℃in Lab or in July in the field insectarium). Higher up-regulated expression of Hsp70 might be involved in higher fitness of S strain.