Ecological specialization of the aphid Aphis gossypii Glover on cultivated host plants.

Many plant-feeding insect species considered to be polyphagous are in fact composed of genetically differentiated sympatric populations that use different hosts and between which gene flow still exists. We studied the population genetic structure of the cotton-melon aphid Aphis gossypii that is considered as one of the most polyphagous aphid species. We used eight microsatellites to analyse the genetic diversity of numerous samples of A. gossypii collected over several years at a large geographical scale on annual crops from different plant families. The number of multilocus genotypes detected was extremely low and the genotypes were found to be associated with host plants. Five host races were unambiguously identified (Cucurbitaceae, cotton, eggplant, potato and chili- or sweet pepper). These host races were dominated by asexual clones. Plant transfer experiments using several specialized clones further confirmed the existence of host-associated trade-offs. Finally, both genetic and experimental data suggested that plants of the genus Hibiscus may be used as refuge for the specialized clones. Resource abundance is discussed as a key factor involved in the process of ecological specialization in A. gossypii.

Variation among strains of Aedes aegypti in susceptibility to oral infection with dengue virus type 2.

We compared 18 Aedes aegypti strains for oral susceptibility to dengue virus type 2 (DEN-2) using a feeding protocol in which all parameters remained constant, including the titer of the infectious bloodmeal. For most strains, no significant variation between replicates was observed. Comparisons between pairs of strains showed variation of different degrees, and allowed us to characterize the strains with respect to their oral susceptibility to DEN-2.

Analysis of inheritance of oral susceptibility of Aedes aegypti (Diptera: Culicidae) to dengue-2 virus using isofemale lines.

Isofemale lines were compared to determine possible genetic variation in oral susceptibility within the Fare strain of Aedes aegypti (L.) to dengue-2 (DEN-2) virus. Three groups of 12 isofemale lines each were tested statistically using the SAS CATMOD procedure of analysis of variance. The "isofemale line" effect was highly significant, demonstrating genetic variability in oral susceptibility among females. The length of time eggs were stored before hatching influenced the oral susceptibility of the adult mosquitoes for DEN-2 review.

Projection pursuit nonparametric regression applied to field counts of the aphid Macrosiphum euphorbiae (Homoptera: Aphididae) on tomato crops in greenhouses.

The number of plants that sampling requires in aphid population studies often exceeds one hundred. Thus, only quick and nondestructive methods can be used to sample this pest in a reasonable time interval. We propose a visual method for estimating the density of the aphid Macrosiphum euphorbiae (Thomas) on tomato plants reared in greenhouses. After approximately 1min of visual observation plants can be assigned to abundance classes, the boundaries of which are roughly the powers of square root 10. Precise counts were collected simultaneously on sets of reference plants from the same greenhouse. Projection pursuit nonparametric regression was then used to provide unbiased estimates of aphid densities from the abundance classes and several easily gathered explanatory variables. The robustness of the method was evaluated by testing the models on the complementary data sets from plants in which the aphid densities were precisely counted. In both single and twin-row cultural conditions, for the reference and complementary data sets, the order of magnitude of the error was less than one class rank per plant. The investigation time was reduced by approximately 10-fold compared with the exact counting method. This easy-to-teach field method could be useful in large-scale population surveys and for optimizing integrated pest management strategies.

Does insecticide resistance alone account for the low genetic variability of asexually reproducing populations of the peach-potato aphid Myzus persicae?

The typical life cycle of aphids includes several parthenogenetic generations and a single sexual generation (cyclical parthenogenesis), but some species or populations are totally asexual (obligate parthenogenesis). Genetic variability is generally low in these asexually reproducing populations, that is, few genotypes are spread over large geographic areas. Both genetic drift and natural selection are often invoked to account for this low genetic variability. The peach-potato aphid, Myzus persicae, which encompasses both cyclical and obligate parthenogens, has developed several insecticide resistance mechanisms as a consequence of intense insecticide use since the 1950s. We collected asexually reproducing M. persicae from oilseed rape and examined genetic variability at eight microsatellite loci and three insecticide resistance genes to determine whether their genetic structure was driven by drift and/or selection. We identified only 16 multilocus microsatellite genotypes among 255 individuals. One clone, which combined two insecticide resistance mechanisms, was frequently detected in all populations whatever their location over a large geographical area (the northern half of France). These unexpected findings suggest that drift is not the unique cause of this low variability. Instead, the intensification of both insecticide treatments and oilseed rape cultivation may have favored a few genotypes. Thus, we propose that selective pressures resulting from human activities have considerably modified the genetic structure of M. persicae populations in northern France in a relatively short period of time.

Oral susceptibility of Aedes albopictus to dengue type 2 virus: a study of infection kinetics, using the polymerase chain reaction for viral detection.

Female Aedes albopictus mosquitoes, aged 1 week, were infected with DEN-2 dengue virus. The kinetics of infection in mosquito brain and mesenteron were monitored using DNA probes with polymerase chain reaction (PCR) amplification of target DNA sequences coding for DEN-2 virus envelope protein, compared with the standard immunofluorescence assay technique (IFA). Rates of virus detection in the mesenteron of orally infected mosquitoes rose to 38% by day 4 post-inoculation, then declined until day 8, followed by irregular peaks around days 11-14 and subsequently. In mosquito head squashes, virus was detected from day 4 onwards, reaching 38% positive by day 18. Salivary glands of all the same females were found to be positive for virus by day 8 onwards. Parenterally infected Ae.albopictus females were all positive for DEN-2 in the brain and salivary glands 8 days post-inoculation. In every case, results obtained with the PCR matched those from the IFA. Our DNA probe with PCR procedure can therefore be utilized as a sensitive and reliable method for studies of DEN-2 vectors.

Variation in clonal diversity in glasshouse infestations of the aphid, Aphis gossypii Glover in southern France.

Aphis gossypii is an aphid species that is found throughout the world and is extremely polyphagous. It is considered a major pest of cotton and cucurbit species. In Europe, A. gossypii is assumed to reproduce exclusively by apomictic parthenogenesis. The present study investigates the genetic diversity of A. gossypii in a microgeographic, fragmented habitat consisting of eight glasshouses of cucurbit crops. This analysis, which was based on the results from seven microsatellite loci, has confirmed that A. gossypii populations in southern France are primarily asexual, as only 12 nonrecombinant genotypic classes (clones) were identified from 694 aphids. Moreover, a high proportion of the aphids (87%) had one of three common genotypes. No significant correlation was found between genotypic class and host plant species. Within a glasshouse population of A. gossypii, a significant reduction in clonal diversity was observed as the spring/summer season progressed. The final predominance of a clone could result from interclonal competition. At the microgeographic level (i.e. glasshouses within a 500-m radius), significant genetic subdivision was detected and could be attributed to founder effects and the limitation of gene flow imposed by the enclosed nature of the glasshouse structure. Finally, the three common clones of A. gossypii detected in 1996 reappeared in spring 1997 following the winter extinction, together with rare clones that had not previously been seen. The probability that A. gossypii overwinters within refuges at a microgeographic scale from which populations are renewed each spring is discussed.

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera).

In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (parthenogenetical) reproducing individuals occur sympatrically. We found in the laboratory that thelytokous wasps are able to mate, receive and use sperm of arrhenotokous males. Using nuclear (amplified fragment length polymorphism, virus-like protein) and mitochondrial (restriction fragment length polymorphism) markers, we show the occurrence of gene flow from the arrhenotokous to the thelytokous mode in the field. Our results reinforce the paradox of sex in this species.

Incidence of insecticide resistance alleles in sexually-reproducing populations of the peach-potato aphid Myzus persicae (Hemiptera: Aphididae) from southern France.

Intensive chemical treatments have led to the development of a number of insecticide resistance mechanisms in the peach-potato aphid Myzus persicae (Sulzer). Some of these mechanisms are known to be associated with negative pleiotropic effects (resistance costs). Molecular and biochemical methods were used to determine the genotypes or phenotypes associated with four insecticide resistance mechanisms in single aphids from sexually-reproducing populations in southern France. The mechanisms considered were E4 and FE4 carboxylesterase overproduction, modified acetycholinesterase, and kdr and rdl resistance-associated mutations. A new method for determining individual kdr genotypes is presented. Almost all resistant individuals overproduced FE4 carboxylesterase, whereas modified acetylcholinesterase was rare. Both the kdr and rdl resistance mutations were present at high frequencies in French sexually-reproducing populations. The frequencies of insecticide resistance genes were compared before and after sexual reproduction in one peach orchard at Avignon to evaluate the potential impact of selection on the persistence of resistance alleles in the over-wintering phase. The frequencies of the kdr and rdl mutations varied significantly between autumn and spring sampling periods. The frequency of the kdr mutation increased, probably due to pyrethroid treatments at the end of the winter. Conversely, the frequency of the rdl mutation decreased significantly during winter, probably because of a fitness cost associated with this mutation.