Genetic diversity and insecticide resistance during the growing season in the green peach aphid (Hemiptera: Aphididae) on primary and secondary hosts: a farm-scale study in Central Chile.

The seasonal dynamics of neutral genetic diversity and the insecticide resistance mechanisms of insect pests at the farm scale are still poorly documented. Here this was addressed in the green peach aphid Myzus persicae (Sulzer) (Hemiptera: Aphididae) in Central Chile. Samples were collected from an insecticide sprayed peach (Prunus persica L.) orchard (primary host), and a sweet-pepper (Capsicum annum var. grossum L.) field (secondary host). In addition, aphids from weeds (secondary hosts) growing among these crops were also sampled. Many unique multilocus genotypes were found on peach trees, while secondary hosts were colonized mostly by the six most common genotypes, which were predominantly sensitive to insecticides. In both fields, a small but significant genetic differentiation was found between aphids on the crops vs. their weeds. Within-season comparisons showed genetic differentiation between early and late season samples from peach, as well as for weeds in the peach orchard. The knock-down resistance (kdr) mutation was detected mostly in the heterozygote state, often associated with modified acetylcholinesterase throughout the season for both crops. This mutation was found in high frequency, mainly in the peach orchard. The super-kdr mutation was found in very low frequencies in both crops. This study provides farm-scale evidence that the aphid M. persicae can be composed of slightly different genetic groups between contiguous populations of primary and secondary hosts exhibiting different dynamics of insecticide resistance through the growing season.

The invasion route for an insect pest species: the tobacco aphid in the New World.

Biological invasions are rapid evolutionary events in which populations are usually subject to a founder event during introduction followed by rapid adaptation to the new environment. Molecular tools and Bayesian approaches have shown their utility in exploring different evolutionary scenarios regarding the invasion routes of introduced species. We examined the situation for the tobacco aphid, Myzus persicae nicotianae, a recently introduced aphid species in Chile. Using seven microsatellite loci and approximate Bayesian computation, we studied populations of the tobacco aphid sampled from several American and European countries, identifying the most likely source populations and tracking the route of introduction to Chile. Our population genetic data are consistent with available historical information, pointing to an introduction route of the tobacco aphid from Europe and/or from other putative populations (e.g. Asia) with subsequent introduction through North America to South America. Evidence of multiple introductions to North America from different genetic pools, with successive loss of genetic diversity from Europe towards North America and a strong bottleneck during the southward introduction to South America, was also found. Additionally, we examined the special case of a widespread multilocus genotype that was found in all American countries examined. This case provides further evidence for the existence of highly successful genotypes or 'superclones' in asexually reproducing organisms.

Worldwide variability of insecticide resistance mechanisms in the codling moth, Cydia pomonella L. (Lepidoptera: Tortricidae).

The activity of detoxifying enzymes (glutathione-S-transferases (GST), mixed-function oxidases (MFO), and esterases (EST)) and the presence of insensitive variants of target proteins (sodium channel and acetylcholinesterase) were examined in individual male and female codling moths. Twenty-nine populations from 11 countries and two laboratory strains were examined. Populations were classified as either unsprayed or sprayed. The ranges of enzyme activities across field populations varied 15-fold, 485-fold and fourfold for GST, MFO and EST, respectively. MFO was the only enzyme whose activity differed in a binomial classification of orchards based on their spray history. Few differences in enzyme activities were found due to sex among populations; and, in these cases, males had higher GST and lower MFO and EST activities than females. Activities of the three enzymatic systems across all populations were positively correlated. Populations from Greece, Argentina and Uruguay had significant percentages of moths with elevated GST and MFO activities. The co-occurrence of moths expressing both elevated MFO and low EST activities was found in conventional orchards from the Czech Republic and France. Chile was the only country where populations from treated orchards did not include a significant proportion of individuals with enhanced enzyme activity. The kdr mutation was found at significant levels in ten populations from five countries, including all French and Argentinean populations. The mutation in AChE was only detected in the Spanish population.