1a. Objectives (from AD-416):
The long-term objective of this project is to provide basic and applied information for the development and transfer of sustainable and environmentally acceptable methods and technologies for management and control of potato psyllid and zebra chip disease in the major potato growing regions of North America. The objectives of our project are listed below. Objective 1: Determine differences among diverse haplotypes of potato psyllid found in the Pacific Northwest in vectoring capabilities, fitness traits, and overwintering capabilities. Subobjective 1A: Determine if psyllid haplotypes differ in fecundity, development rates, and coldhardiness capabilities. Subobjective 1B: Determine if psyllid haplotypes are equally rapid at transmitting Liberibacter to potato, and examine whether all haplotypes can transmit the pathogen from mother-to-daughter (transovarial transmission) and from male-to-female during mating. Objective 2: Determine suitability of non-crop plant species for development, reproduction, and overwintering of potato psyllid and as potential reservoirs of the zebra chip pathogen in the Pacific Northwest. Subobjective 2A: Compare suitability of non-crop host plants for development, fecundity, and overwintering success among haplotypes of potato psyllid. Subobjective 2B: Compare host preferences of psyllid haplotypes. Subobjective 2C: Develop molecular methods to determine which host species are sources of psyllids colonizing potatoes. Subobjective 2D: Determine whether non-crop Solanaceae are suitable hosts for the pathogen causing zebra chip.
1b. Approach (from AD-416):
Objective 1: Our hypothesis is that haplotypes differ in biological traits that determine their respective risks to growers as vectors of the zebra chip pathogen. Methods to determine how haplotypes differ in biology will involve: 1). Laboratory-based rearing trials to compare haplotypes in fecundity, egg fertility, and developmental rates; 2). Use of a cold-temperature programmable bath to estimate lower lethal temperatures of each haplotype; 3). Use of electrical penetration graph technology combined with potato grow-outs to estimate how rapidly the zebra chip pathogen can be transmitted by each haplotype; 4). Mating assays between infected and uninfected psyllids to determine if the pathogen is transferred between psyllids during mating; 5). Molecular assays of offspring from infected vs uninfected mothers to determine if all haplotypes transfer the pathogen from mother to offspring. Objective 2: Our hypothesis is that different species of non-crop hosts of potato psyllid will vary in how suitable they are to potato psyllid and to the zebra chip pathogen. Moreover, different haplotypes of the psyllid will vary in what species they prefer for egglaying, and in what species are most suitable for psyllid development and survival. Methods to examine plant suitability to potato psyllid and to the zebra chip pathogen, and to compare suitability of different plant species among the psyllid haplotypes will involve: 1). Standard rearing assays with each haplotype on targeted plant species to determine fecundity and development rates on the different plant species; 2). Choice tests with each haplotype to determine whether haplotypes all prefer the same plant species for egglaying, or whether haplotypes differ in preferences; 3). Development of molecular methods to detect the DNA of specific plant species within the guts of field-collected psyllids, and a comparison of gut contents among field-collected psyllids of the different haplotypes; 4). Inoculation trials to determine whether our targeted plant species are suitable hosts for the zebra chip pathogen, and to determine whether all haplotypes of the psyllid transmit the pathogen to all targeted plant species.
3. Progress Report:
Potato psyllid is the insect vector of zebra chip, an economically important disease of potato in the U.S., including the Pacific Northwest (PNW) where over 50 percent of U.S. potatoes are grown. ARS scientists in Wapato, Washington, in collaboration with scientists at Washington State University, University of Idaho, and the Washington State Potato Commission, monitored psyllid densities on matrimony vine, a common shrub found within the PNW potato growing regions, and evaluated seasonal traits of these shrubs affecting attractiveness to potato psyllid. It was found that matrimony vine in PNW enters a summer dormancy characterized by significant drops in leaf chlorophyll and by heavy leaf drop, which together prompt a burst of dispersal by potato psyllids from these deteriorating shrubs into potato fields. An understanding of seasonal changes in suitability of matrimony vine to potato psyllid allows growers and extension personnel to better anticipate timing of psyllid arrival in fields and better time their psyllid control efforts. ARS scientists in Wapato, Washington, in collaboration with the Washington State Department of Agriculture, finished an extensive screening of annual weedy nightshades for suitability to potato psyllid and to the zebra chip pathogen, showing that a number of sometimes common weed species in the Pacific Northwest are highly suitable hosts for potato psyllid and the zebra chip pathogen. The final stage of the project to be completed in the next several months is development of a “risk index” that ranks each plant species by its potential threat as a source of infective psyllids. That risk index is to be forwarded to growers by posting it on industry web-sites and publishing of the index in the industry newsletters. A molecular tool which can be used to identify (to species) plant DNA in potato psyllid was optimized for use under field conditions and is now being used to determine dietary histories of field-collected psyllids as they begin moving into potato fields. The technology is used to directly determine what species of plants were the sources of psyllids arriving in fields, allowing growers to better predict risk of psyllid arrival in fields based upon composition of vegetation surrounding fields. A new objective was added to this project in April of 2018 to incorporate the technology into ongoing psyllid sampling programs conducted at multiple locations across three states, Oregon, Idaho and Washington. Scientists with Oregon State University, University of Idaho, Oregon State University, and Washington State University will participate in the programs with funding provided by the Washington State Department of Agriculture. The work is expected to substantially improve our understanding of psyllid movements on a landscape scale from non-crop host plants into commercial potato fields in multiple growing regions across the three states. Efforts to better understand the role of non-crop plants as hosts of potato psyllid led to the unexpected discovery that the psyllid is able to reproduce and develop on certain species of morning glories, bindweeds, and bindweed relatives in the Family Convolvulaceae. These results substantially expand the known dietary breadth of potato psyllid to include plants not closely related to potato. The assays additionally led to the discovery of a class of chemicals in non-host Convolvulaceae that prevent psyllid development. These plant-produced chemicals, previously unknown to have anti-psyllid properties, are being examined in bioassays to determine modes of action against potato psyllid. One goal of this work is a search for selective botanical-produced insecticides having effects against potato psyllid and other psyllid species, but with lesser effects against non-target insects.
1. Discovery of new host plants of potato psyllid. Managing potato psyllid and zebra chip disease in potatoes is complicated by the psyllid’s ability to develop on weedy non-crop species which may then act as reservoirs of the psyllid as the insect begins colonizing potato fields in late spring. With cooperators at University of Idaho and Texas A&M University, ARS scientists in Wapato, Washington, screened multiple species of ornamental and weedy plants in the morning glory/bindweed family to evaluate the insect’s ability to develop on these plants. The results unexpectedly showed that dietary breadth of potato psyllid is much broader than previously appreciated, and that several species of morning glories and bindweeds do indeed support egg to adult development of potato psyllid. These plant species should be considered potential field hosts for potato psyllid in potato growing regions where they either occur naturally or where they have escaped cultivation and become naturalized.
2. Molecular gut contents analysis to determine non-crop sources of potato psyllid. Potato psyllid colonizes grower fields from non-crop host plants possibly including annual and perennial weeds or ornamental shrubs, but it is not yet clear which non-crop plant species actually serve as important sources of psyllids. ARS scientists in Wapato, Washington, developed a novel method for gut content analysis of psyllids by direct-sequencing plant DNA from wild-caught psyllids. The method allowed the researchers to identify the dietary history of the psyllids and to track their previous landscape dispersal patterns. This method will be used in future studies to pinpoint the weed sources of potato psyllid in multiple growing regions, by examining psyllids that are captured as they colonize potato fields. This knowledge will enable growers and researchers to better predict which fields are at risk of being colonized by potato psyllid to allow improved planning to reduce the economic impact via reduced yield and lower crop quality.
3. Ranking weedy non-crop hosts of potato psyllid according to potential importance as source of zebra chip disease. Potato psyllid is the vector for the pathogen associated with zebra chip disease of potato, which renders potato tubers unmarketable. The threat of non-crop plant species as potential sources of the zebra chip pathogen is not well understood for growing regions of the Pacific Northwest. ARS scientists in Wapato, Washington through funding provided by Washington State Department of Agriculture finished an extensive screening of weedy and ornamental non-crop plants in the potato family to determine whether these plants are potential field-sources of infective potato psyllids. Several of these plant species, including some that are important weeds in potato growing regions, were found to support psyllid reproduction and development, and to be suitable hosts for the zebra chip pathogen. These findings are being included in a “risk index” that ranks plant species by their respective densities in potato growing regions, seasonal availability, suitability to psyllids, and suitability to the zebra chip pathogen. Growers and extension personnel have a new tool for monitoring the risk of infective psyllids.
Horton, D.R., Miliczky, E., Lewis, T.M., Cooper, W.R., Waters, T., Wohleb, C., Zack, R., Johnson, D., Jensen, A. 2018. New North American records for the Old World psyllid Heterotrioza chenopodii (Reuter) with biological observations (Hemiptera: Psylloidea: Triozidae). Proceedings of the Entomological Society of Washington. 120(1):134-152.