Resistance development and recovery of susceptibility in relation to egg size change in the diamondback moth, Plutella xylostella, treated with fenvalerate

摘要

The diamondback moth ( DBM) treated with sublethal doses of fenvalerate lays more but smaller eggs (insecticide hormoligosis). The smaller eggs and newly hatched larvae exhibit lower viability ( Fujiwara et al. 2002). Thus, resistant strains of DBM that are selected by sublethal doses of fenvalerate may lay smaller eggs and have a reproductive disadvantage. Furthermore, such selective disadvantages may allow recovery of susceptibility in the absence of insecticide sprays. Firstly, we established DBMstrains resistant to fenvalerate and examined the effects of sublethal doses on egg size and fecundity. The eggs of the selected strains became significantly smaller than those of the non-selected strains within 2 or 3 generations. The survival rate of the selected strains was significantly lower, and the fecundity of females of the selected strains tended to be higher. Secondly, we carried out reciprocal cross experiments between the resistant ( RR) and susceptible (SS) strains to determine inheritancemode of resistance development and reduction of egg size. The mode of inheritance of fenvalerate resistance was incompletely recessive with no sex-link. The RS and SR strains have an egg size close to that of their own matrilineage. The inheritable modes of resistance and egg size were different from each other. The survival rate of the RS strains did not differ from that of the SR strains. Fitness disadvantage of the resistant strains in terms of diminished egg size influenced the recovery rate of susceptibility in the absence of insecticide selection. In particular, the recovery rate was higher under harsh environment conditions, e. g. low humidity and high temperature, than under moderate conditions.