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Project description

Using population structure to identify effective parasitoid biotypes for biological control of mealy plum aphid, Hyalopterus pruni, in California. (03XA011)
Program Exotic Pests and Diseases Research Program
Principal
investigator
N.J. Mills, Insect Biology, UC Berkeley
Host/habitat Plums; Pluots
Pest Mealy Plum Aphid Hyalopterus pruni
Discipline Entomology
Review
panel
Agricultural Systems
Start year (duration)  2003 (Three Years)
Objectives Characterize the species status, genetic variation, and degree of biotype formation in Hyalopterus aphid populations and associated Aphidius parasitoids in their ancestral Mediterranean home using neutral genetic markers.

Characterize the genetic variation of Hyalopterus populations in California to determine their invasion history and relation to ancestral populations in the Mediterranean.

Test the compatibility of distinct Aphidius biotypes against Californian Hyalopterus to identify the best source of parasitoids from the Mediterranean for introduction to California.

Project
Summary
The mealy plum aphid, Hyalopterus pruni, is an invasive pest and a key barrier to reduced pesticide use in California's $200 million-per-year dried plum and pluot industry. Prospects for biological control of this pest would be greatly improved from a thorough understanding of the genetic structure of mealy aphid and parasitoid populations in their ancestral home in the Mediterranean, and its influence on the virulence of aphid-parasitoid interactions. Microsatellite and mtDNA analysis will be used to characterize the genetic variation in aphid and parasitoid populations, and compatibility tests will be conducted to relate virulence to population structure and identify suitable sources of parasitoids from the Mediterranean for introduction to California.
Final report The mealy plum aphid, Hyalopterus pruni, is an invasive pest in California, where it develops large populations on the underside of prune foliage during early summer, causing reduced growth and fruit yield. Biological control using specialist parasitoids from the mealy aphid's region of origin in the Mediterranean provides an attractive alternative to pesticide usage on prune.

This research project has assisted us in determining the most effective native parasitoids for introduction against mealy aphid, and in the development of a methodology that may be used to conduct future natural enemy explorations in other biocontrol systems.

Multiple genetic markers were examined in mealy aphids collected from California and the Mediterranean. These data indicate the presence of three host adapted species within Hyalopterus that are specific to the host plant from which they were collected. Aphids sampled from plum share one genetic pattern, aphids sampled from almond share a second, and aphids sampled from peach share a third. Interestingly, we have found that apricot trees act as a shared resource for each of the three species, and even permits some hybridization, at least between the plum and almond associated species.

Mealy plum aphids in California are of the same species as the Mediterranean plum group, thus indicating that only H. pruni is an invader. Based on genetic markers, we have identified Spain or Tunisia as the likely source of invasive mealy aphids, and have discovered an entirely separate introduction of aphids in eastern North America.

Genetic analysis of the mealy aphid's native parasitoid, Aphidius transcaspicus, has revealed substantial population structure, but no evidence of host plant associated species like those found in Hyalopterus. Virulence experiments showed no differences in parasitism rates or offspring sex ratio among A. transcaspicus from Spain, Morocco, Israel, and Cyprus, and instead indicated that female size was a better predictor of parasitism success.

Third-year
progress
The mealy plum aphid, Hyalopterus pruni, is an invasive pest in California, where it develops large populations on the underside of prune foliage during early summer, causing reduced growth and fruit yield. Biological control using specialist parasitoids from the mealy aphid's region of origin in the Mediterranean provides an attractive alternative to pesticide usage on prunes. This research project will assist in determining the most effective native parasitoids for introduction against mealy aphid, and in the development of a methodology that may be used to conduct future natural enemy explorations in other biocontrol systems.

DNA sequences from four genes and genotypes for nine microsatellite markers were examined for mealy aphids collected from California and the Mediterranean. These data indicate the presence of three host adapted species within Hyalopterus that are specific to the host plant from which they were collected. Aphids sampled from plum share one genetic pattern, aphids sampled from almond share a second, and aphids sampled from peach share a third. Interestingly, we have found that apricot trees act as shared resources for each of the three species, and even permits some hybridization, at least between the plum and almond associated species.

Mealy plum aphids in California are of the same species as the Mediterranean plum group. Based on microsatellite genotyping and a shared mutation at the mtDNA 12S gene, we have identified the western Mediterranean as a likely source of invasive mealy aphids.

Genetic analysis of the mealy aphid's native parasitoid, Aphidius transcaspicus, has revealed several regional strains, with no evidence of cascading host plant effects on parasitoid population structure. Behavioral assays indicate that parasitoids from Greece seem more aggressive toward invasive H. pruni than Cypriot and Israeli populations. More extensive experiments will help guide the search for the most effective source of parasitoids for biological control introductions.

Second-year
progress
The mealy plum aphid, Hyalopterus pruni, is an invasive pest in California, where it develops large populations on the underside of prune foliage during early summer, causing reduced growth and fruit yield. Biological control using specialist parasitoids from the mealy aphid's region of origin in the Mediterranean provides an attractive alternative to pesticide usage on prunes. This research project will help determine the most effective native parasitoids for introduction against mealy aphid, as well as assist in the development of a methodology that may be used to conduct future natural enemy explorations in other biocontrol systems.

DNA sequences from a single gene for mealy aphids collected from California and the Mediterranean indicate three major groups, or potential species, within Hyalopterus. These groups differ based on the host plant from which the aphids were collected. Aphids sampled from plums share one genetic sequence, aphids sampled from almonds share a second, and aphids sampled from peaches and apricots share a third. Mealy plum aphids in California have the same DNA sequence associated with the Mediterranean plum group. Based on initial microsatellite genotyping, we have tentatively identified Spain as a likely source of invasive mealy aphids, and confirmed that "plum type" genotypes are found on multiple host plants in the Mediterranean.

DNA sequences from the mealy aphid's native parasitoid, Aphidius transcaspicus, show little population structure. Initial microsatellite genotyping from Mediterranean populations, however, has revealed three main regional groups. Parasitoids from Spain and Tunisia are similar, with parasitoids from Greece and Italy forming two distinct groups. Though genetic variability in A. transcaspicus is relatively low compared to H. pruni, parasitoid populations appear to have greater geographic population structure. More extensive genotyping will help guide the search for the most effective source of parasitoids for biological control introductions and elucidate the potential for future host-plant shifts or expansions of the mealy aphids in California.

First-year
progress
The mealy plum aphid (Hyalopterus pruni) is an invasive pest in California, developing large populations on the underside of prune foliage during early summer and causing reduced growth and fruit yield. Biological control using specialist parasitoids from the mealy aphid's region of origin in the Mediterranean provides an attractive alternative to pesticide control, in relation to the negative effects of pesticides on water quality and human health. This research project will determine the most effective native parasitoids for introduction against mealy aphid, as well as assist in the development of a methodology that may be used to conduct future natural enemy explorations in other biocontrol systems.

DNA sequences from a single gene for mealy aphids collected on prune in California's Central Valley, and on plum, almond, peach, and apricot from Spain and Greece indicate the presence of three major groups, or potential species, within the genus Hyalopterus. These groups are based largely on the host plant from which the aphids were collected: aphids sampled from plum share one genetic sequence, aphids sampled from almond share a second, and aphids sampled from peach and apricot share a third. Examples from the aphid group on plum were also found on all other host plants, suggesting that 'plum' aphids have the potential to utilize multiple host plants. Mealy plum aphids in California also share the same DNA sequence associated with the plum group in the Mediterranean.

More detailed experiments are currently being developed to determine the geographic sources of the mealy plum aphids in California and the extent of variation associated with its parasitoid, Aphidius transcaspicus, in the Mediterranean. This will help guide the search for the most effective source of parasitoids for biological control introductions, and elucidate the potential for future host-plant shifts or expansions of the mealy aphids in California.

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