This project will examine the responses of the principal aphid vectors of two important disease-causing plant viruses to changes in the volatile organic compounds (VOC) produced by plants infected by these viruses. Potato leafroll virus (PLRV) is vectored primarily by the green peach aphid (GPA). Barley yellow dwarf virus (BYDV-PAV) is vectored primarily by the bird-cherry oat aphid (BCOA). We have discovered that wingless forms of these two aphid species are attracted to or arrested by VOC produced by their respective virus-infected host plants. These findings represent the first evidence of which we are aware showing that VOC from virus-infected plants influence vector behavior. The long-term goal addressed by this project is to improve understanding of the effects of virus-induced VOC on aphid vectors as a basis for innovations for monitoring vector populations and influencing their movements. Three objectives are proposed in each system: 1) Determine the responsiveness of winged forms to infection-induced changes in VOC from their host plant, 2) Determine the changes in aphid responses to VOC emitted during the progression of the disease, 3) Identify the specific infection-induced VOC causing aphid arrestment or attraction to infected plants. Results of each of these objectives in the two virus systems will be compared to discover commonalities in the mechanisms involved.
1) Determined the responsiveness of alatae of each species to infection-induced changes in VOC from their host plant; 2) Determine the changes in aphid responses to VOC emitted during the progression of the disease in each system; 3) Identify the specific infection-induced VOC causing GPA arrestment on PLRV-infected potato and BCOLA arrestment on BYDV-infected wheat
To meet Objective 1, bioassays will be conducted to determine how the winged forms of the two aphid species respond to virus-infected and noninfected plants of each plant species. In a single replicate of these experiments, infected and noninfected potato plants or wheat plants will be placed randomly within a vented plexiglass cage. A sample of winged aphids will be released from a platform in the middle of the cage and their locations will be determined after 24 h. Refinements on this bioassay will assess immigration and emigration rates of the aphids.
To meet Objective 2, wingless aphids will be used, following a protocol we have already shown to be effective in this context. Groups of aphids will be placed on screening above control leaves or above virus-infected leaves and their rate of emigration determined. This test will be performed using leaves of two different age classes within the plant and at weekly intervals beginning immediately after infection until disease symptoms have progressed to obviously damaging levels. Aphid responses will be compared to detect a change in response as the disease progresses. Volatile compounds in the headspace of the plants will also be analyzed at each of these time intervals to detect changes in the signal to which the aphids are responding.
To meet Objective 3, the most attractive or arrestant blend of volatiles discovered while meeting Objective 2 will be collected in sufficiently large quantity for replicated bioassays. The extracted volatile blend will be deposited at physiological concentrations onto filter paper model leaves and aphid responses will be measured with the emigration bioassay described above. Electroantennogram recordings will also be made from the aphids to help determine which parts of the blend appear to be most active physiologically. Fractions of the volatile blend will be created using preparative gas chromatography and these fractions will be recombined and tested. If active, subsequent bioassays will be conducted with selected fractions removed to determine which fractions are most important alone or in combination for triggering the aphid response. Should an individual fraction be found attractive or arrestant, it will be further fractionated to isolate the activity. Individual components whose activity is suggested by this process will also be tested. Depending on aphid responses, the bioassay will be modified to determine if some of the components cause attraction vs arrestment (by changing their location at the beginning of the experiment). The information generated in this objective will be compared with the volatiles collected from plants in Objective 2 to determine whether changes in these compound concentrations help explain any differences in aphid response to the blends.