Molecular Mechanism Of Resistance To Spinosad In Frankliniella Occidentalis

The western flower thrips (WFT), Frankliniella occidentalis (Pergande), is a key pest of vegetable, fruit and ornamental crops. Spinosad is one of the few insecticides that could control F. occidentalis very well. In recent years, the field WFT in Spain has developed high resistance to spinosad. Therefore, the study on spinosad resistance is very important for sustainable control of the WFT. In this paper, the resistance selection, monitoring of the field resistance, fitness analysis and molecular cloning of the target genes of AChR were carried out. The major objectives were to evaluate the risk of the WFT developing resistance to spinosad and to understand the resistance mechanisms underlying it.1. Laboratory selection of spinosad resistance in F. occidentalisResistance selection of F. occidentalis to spinosad was conducted since 2006. The original strain was a laboratory colony which had been reared indoor for more than five years. The results showed that the resistance developed very slowly. Compared to the original strain the resistance level was found to increase by only 30 times. Actually, no resistance occurred in the first two year's selection. With the increase of selection cycles, more and more resistanct individuals were selected. When the susceptility of F. occidentalis to spinosad was decreased, the resistant development could be divided into three phases. The first phase was called ascending phase, during which the resistance ratio was increased continually from 1.08-fold to 14.00-fold. The seconde phase was called evenly ascending phase, the resistance ratio was developed from 15.38-fold to 17.90-fold during three generations. The third phase was another ascending phase, the resistance ratio increased from 21.60-fold to 30.00-fold, which achieved a moderate resistant level.2. Field survey for insecticide resistance in F. occidentalisA field survey of the insectide resistance of F. occidentalis to 13 kinds of insecticides within 6 classes was conducted in Changping and Haidian districts in 2009 by using a modified method of TIBS. Results showed that the sensitivity of the two field populations to every type of insecticide was reduced to some extent. Relatively, the Haidian population was more sensitive than the Changping population. Among the 13 insecticides, spinosad and emamectin were the most effective insecticides to the two populations, and methomyl was the worst one.3. Inheritance mode of resistance to spinosas in F. occidentalisThe inheritance mode of resistance to spinosad in F. occidentalis was evaluated by using the methods of reciprocal crosses and backcross between the susceptible strain and spinosad-resistant strain (80.8-fold).The progeny of reciprocal F1 crosses (resistant male×susceptible male and resistant female×susceptible male) responded similarly in bioassays, with values of 0.51 and 0.42 for D, respectively, which indicated that resistance was autosomal and incompletely dominant. Analysis of dose-response lines indicated that there was no plateaus at the 50% mortality of backcross progeny (F1×Susceptible strain) and at 25% and 75% mortalities of self-cross of backcross progeny. Chi-square analysis from the progeny from backcross (F1×Susceptible strain) and self-cross of backcross suggested that resistance to spinosad was probably controlled by several genes.4. The influence of spinosad resistance on the fitness of F. occidentalisLife tables were constructed to compare the characteristics of development and fecundity in spinosad susceptible and resistant strains with a resistant ratio of 80.8-fold. Compared to the susceptible strain, the mean developmental period of egg, the first instar nymph, the second instar nymph and pupa stages was longer in the resistant strain. Moreover, the adult longevity of the resistant strain was shorter than that of the susceptible strain.The fecundity and the survivorship of the resistant strain were significantly reduced than the susceptible strain.The resistant strain had a fitness value of 0.62 relative to the susceptible strain. The resistant strain possessed developmental and reproductive disadvantages.5. Cloing of a cDNA fragment encodingαsubunit of nAChR from F. occidentalisThe cDNA fragment encodingαsubunit of nicotinic acetylcholine receptor (nAChR) was cloned from F. occidentalis by using of RT-PCR technique.The amplified fragments of Foα1 was 371bp nucleotides in length,from which 123 amino acids were deduced.The sequence analysis demonstrated that the fragment had some features of subunit of nAChR, that was, the loop containing 15 amino acids formed by double sulfur bound between cysteines, some amino acids for nicotine.The cloned fragments had an identity of 82%-95% with otherαsubunit genes of nAChR from other insects.