Spinosad Resistance In Diamondback Moth, Plutella Xylostella(L.)

The diamondback moth (DBM), Plutella xylostella (L.), is one of the most destructiveinsect pests of cruciferous crops and requires globally US $1.0 billion in estimated annualmanagement costs in addition to the crop losses it causes. Because of extensive andfrequent use of conventional insecticides, this pest has developed serious resistance tomany classes of insecticides.Spinosad is one of macrolides with perfect efficacy against DBM resistant toconventional insecticides. In this paper, a resistant strain of DBM to spinosad was selectedand cross resistance pattern, inheritance mode, fitness cost and resistance mechanisms ofspinosad resistance were investigated.1. Selection for resistance to spinosad in DBMThe SZ-Spin strain was selected with spinosad from a field-collected strain (SZ) of thediamondback moth, Plutella xylostella. After 26 generations of selection, resistance tospinosad of the selected SZ-Spin strain increased to 22.4-fold compared with that of the SZstrain, and 60.7-fold resistance compared with that of a susceptible strain (PHI-S). Themean realized heritability of spinosad resistance during selection was 0.09, indicatingresistance development potential of spinosad was low.2. Genetics and the relative fitness of spinosad resistance in DBMThe inheritance mode of resistance to spinosad in diamondback moth was evaluatedfrom log dosage-probit mortality curves constructed from the response of DBM larvae tospinosad treatment. The results of genetic analysis indicated that the resistance of DBM tospinosad was autosomal, incompletely recessive, and probably polygenic. The resistantstrain (SZ-Spin) possessed significantly biological disadvantages, including prolonged larval period, lighter pupa weight, lower mating and oviposition rates compared with theunselected SZ strain. The resistant strain possessed 0.49 of the fitness value relative to theSZ strain.3. Cross-resistance of the spinosad-resistant strain of DBMAfter continuously selected with spinosad for 26 generations, the SZ-Spin strainshowed 22.4-fold resistance compared with the SZ strain. However, the SZ-Spin strain didnot show any cross resistance to abamectin, fipronil, monosultap, chlorfenapyr andindoxacard. It suggests that proper used of mixture or rotation between these insecticidescould slow down resistance development to spinosad in field.4. Biochemical characterization of spinosad resistance in DBMDetoxication enzymes including Glutathione-S-transferases (DCNB, CDNB), generalesterases and monooxygenases (MCOD) were compared among the PHI-S, SZ and SZ-Spinstrains. There was no difference in these metabolic enzyme activities between the SZ andSZ-Spin strains, although GST and MFO activities of the SZ and SZ-Spin were a littlehigher than that of the PHI-S strain. The results suggested that spinosad resistance observedin the SZ-Spin strain was possibly attributed to decreased target site sensitivity, andmetabolic mechanisms may be not important.