Insecticide resistance traits differ among and within host races in Aphis gossypii

BACKGROUND

The polyphagous cotton-melon aphid Aphis gossypii Glover is structured into geographically widespread host races comprising a few clones specialised on Cucurbitaceae, cotton, eggplant or pepper. To assess insecticide resistance among and within host races, leaf disc bioassays were conducted on aphid clones collected from Cucurbitaceae (genotypes C4 and C9), cotton (genotypes Burk and Ivo), eggplant (genotype Auber) and pepper (genotype PsP4). Molecular diagnostic (PCR-RFLP) and enzyme assays were also performed to detect the basic mechanisms underlying insecticide resistance.

RESULTS

All six clones were susceptible to acetamiprid (neonicotinoid) or carbosulfan (carbamate). Conversely, all clones were resistant to dimethoate (organophosphate) (RF = 4.1-38.1) and carried mutation S431F in the acetylcholinesterase gene. Auber, PsP4 and Burk also carried mutation A302S in this gene, which possibly conferred moderate resistance (RF = 3.7-6.8) to profenofos and monocrotophos (organophosphates). Auber and Burk were highly resistant (RF = 41.2 and 473 respectively) to cypermethrin (pyrethroid). This resistance was likely associated with point mutation super-kdr (M918L) in the voltage-gated sodium channel gene (para gene) or metabolic detoxification mediated by esterase and oxidase enzymes.

CONCLUSION

Multiple resistance to a broad range of insecticides and multiple mechanisms of resistance in some clones could explain to some extent the low genetic diversity observed within A. gossypii host races.

Genetic structure and clonal diversity of an introduced pest in Chile, the cereal aphid Sitobion avenae

In Chile, the aphid Sitobion avenae is of recent introduction, lives on cultivated and wild Poaceae, and is thought to reproduce by permanent parthenogenesis. In order to study the genetic variability and population structure of this species, five microsatellite loci were typed from individual aphids collected from different cultivated and wild host plants, from different geographical zones, and years. Chilean populations showed a high degree of heterozygosity and a low genetic variability across regions and years, with four predominant genotypes representing nearly 90% of the sample. This pattern of low clonal diversity and high heterozygosity was interpreted as the result of recent founder events from a few asexually reproducing genotypes. Most geographical and temporal variation observed in the genetic composition resulted from fluctuations of a few predominant clones. In addition, comparisons of the genotypes found in Chile with those described in earlier surveys of S. avenae populations in Western Europe led us to identify 'superclones' with large geographical distribution and high ecological success, and to make a preliminary exploration of the putative origin(s) of S. avenae individuals introduced to Chile.

Variation in the life cycle and morphology of the tobacco host-race of Myzus persicae (Hemiptera: Aphididae) in relation to its geographical distribution

Morphological variation and life cycle category were examined in 121 clones of Myzus persicae (Sulzer). The clones were collected from tobacco from three localities in Greece (Xanthi, Nea Efessos and Naphplion), one in Germany (Rheinstetten), one in France (Bergerac) and one in Spain (Madrid). Before morphometrics, all aphids were laboratory-reared on potato. The morphological variation was investigated using both canonical variates analysis and a novel non-parametric classification tree method. The life cycle category was examined by rearing the clones for three generations under short day conditions. In Nea Efessos a relative high proportion of clones was found to overwinter as eggs on the primary host. In the other regions all collected clones were non-holocyclic. Intermediate genotypes were found in all regions at percentages ranging from 4.0 to 24.0%. Androcyclic clones were found only in Xanthi, Greece (4.0%) and Rheinstetten, Germany (16.7%). The canonical variates analysis and the tree classification method revealed important intrapopulation polymorphisms in clones from Bergerac, Nea Efessos and Madrid. Both methods separated the populations originating from Greece from those collected elsewhere in western Europe. The observed morphological variation was probably due to genetic differences, since all clones were reared in a common environment. The results are discussed in relation to factors responsible for genetic divergence in M. persicae populations.

Slaves of the environment: the movement of herbivorous insects in relation to their ecology and genotype

The majority of insect species do not show an innate behavioural migration, but rather populations expand into favourable new habitats or contract away from unfavourable ones by random changes of spatial scale. Over the past 50 years, the scientific fascination with dramatic long–distance and directed mass migratory events has overshadowed the more universal mode of population movement, involving much smaller stochastic displacement during the lifetime of the insects concerned. This may be limiting our understanding of insect population dynamics. In the following synthesis, we provide an overview of how herbivorous insect movement is governed by both abiotic and biotic factors, making these animals essentially ‘slaves of their environment’. No displaced insect or insect population can leave a resource patch, migrate and flourish, leaving descendants, unless suitable habitat and/or resources are reached during movement. This must have constrained insects over geological time, bringing about species–specific adaptation in behaviour and movements in relation to their environment at a micro– and macrogeographical scale. With insects that undergo long–range spatial displacements, e.g. aphids and locusts, there is presumably a selection against movement unless overruled by factors, such as density–dependent triggering, which cause certain genotypes within the population to migrate. However, for most insect species, spatial changes of scale and range expansion are much slower and may occur over a much longer time–scale, and are not innate (nor directed). Ecologists may say that all animals and plants are figuratively speaking ‘slaves of their environments’, in the sense that their distribution is defined by their ecology and genotype. But in the case of insects, a vast number must perish daily, either out at sea or over other hostile habitats, having failed to find suitable resources and/or a habitat on which to feed and reproduce. Since many are blown by the vagaries of the wind, their chances of success are serendipitous in the extreme, especially over large distances. Hence, the strategies adopted by mass migratory species (innate pre–programmed flight behaviour, large population sizes and/or fast reproduction), which improve the chances that some of these individuals will succeed. We also emphasize the dearth of knowledge in the various interactions of insect movement and their environment, and describe how molecular markers (protein and DNA) may be used to examine the details of spatial scale over which movement occurs in relation to insect ecology and genotype.

Analysis of clonal diversity of the peach-potato aphid, Myzus persicae (Sulzer), in Scotland, UK and evidence for the existence of a predominant clone

Clones of the peach-potato aphid, Myzus persicae (Sulzer), mostly from Scotland, UK were examined using an rDNA fingerprinting technique. Eighty patterns (genotypes) were found amongst the 276 clones. A large number of clones (30%) from all sample areas in Scotland exhibited the same simple pattern, suggesting the presence of a single M. persicae clone. There was no difference in genotype distributions between M. persicae collected from brassica or potato crops, suggesting that host-adapted genotypes have no advantage in the field. Different fingerprints were randomly distributed in the environment, although clones taken from the same leaf were more often the same fingerprint. Highly distinctive fingerprints, which were more widely distributed, suggest that this technique could be used to follow individual clones. In addition to the common clonal type, multiple fingerprint bands were found over successive years, implying that, in Scotland, local overwintering asexual populations are the most common source of M. persicae in the following year.