Continuing to watch the winds: the origin and arrival of migrant aster leafhoppers and diamondback moths
Date: August 15, 2024
Term: 4 years, ending August 2024
Status: Completed
Researcher(s): Tyler Wist, Tim Dumonceaux, Chrystel Olivier, AAFC; Sean Prager, Jack Gray, University of Saskatchewan; Keith Hobson, University of Western Ontario; Jose Andres, Cornell University.
SaskCanola Investment: $138,325
Total Project Cost: $276,650
Funding Partners: WGRF
Grower Benefits
Study results from wind trajectory and stable isotopes indicated that aster leafhoppers originated from a Great Plains migration event, while wind trajectory results indicated diamondback moth originated from the Pacific Northwest, most likely the states of Oregon and Washington. The migrations of these two insect pests do not seem to be closely linked.
Several perennial and biennial plants, including alfalfa, were identified as positive with aster yellows prior to any aster leafhopper activity in Saskatchewan indicating that these had been infected with Aster Yellows phytoplasma the previous year and carried over the infection from one growing season to the next as a potential “green bridge” between seasons.
Using the aster yellows index together with dates of leafhopper arrival leads to better prediction and improved risk determination for aster yellows outbreaks, and faster communication to growers to help with pest management and decisions of whether to spray or not.
A better understanding of the common source for migrating diamondback moths helps indicate potential insecticide resistance issues from different populations and ensures efficacy of products used by growers when the pest occurs at outbreak levels.
Project Description
Aster leafhoppers and diamondback moths migrate to the Canadian Prairies in spring and early summer on wind currents originating in the USA. Infestations of these insects are tied to migration on the winds since neither of these insects overwinters well in cold Canadian winters. Early detection and monitoring of both migratory insects is essential for predicting outbreaks and informing management and control decisions in canola crops.
Building on a previous project, this study continued aster yellows monitoring and aster yellows risk estimation activities. The study collected data from the provincial network of diamondback moth pheromone traps in canola supplemented by yellow sticky cards for leafhopper catching as well as project specific traps established at sentinel sites. The study also included the use of wind trajectories and stable isotopes to continue gathering data on diamondback moth and aster leafhopper migration flights and origin in each season. Using maternally-inherited mitochondrial COI gene analysis and SNP markers, the study also differentiated diamondback moth populations among pheromone trap catches arriving in spring and early summer in western Canada. The hypothesis of an alfalfa green bridge was also evaluated. The project included both laboratory and field experiments.
The results from wind trajectory and stable isotope data collection indicated that aster leafhoppers were the result of a Great Plains migration event and aster yellows outbreaks in Canada are likely linked to drought in winter wheat in the leafhopper source areas of Kansas and Nebraska. Novel molecular tools developed for aster leafhoppers showed that source populations of migratory leafhoppers could not be determined based on molecular markers alone, likely due to their migratory nature and gene mixing. For diamondback moth, the wind trajectory results showed they originated from the Pacific Northwest, and likely the states of Oregon and Washington. The study also showed that the migrations of these two insect pests do not seem to be closely linked, except that they are both arriving from more southerly latitudes.
Detecting aster leafhopper populations early is crucial for aster yellows management, because aster leafhoppers vector the pathogen Aster Yellows phytoplasma (AYp), which causes Aster Yellows (AY) disease. This disease affects crops across the Prairies including oilseeds such as canola and flax, which are the most susceptible, and cereals, which are less susceptible. The project demonstrated that the aster leafhopper migration into Western Canada usually occurs around the May long weekend either in the third or fourth week of May. Alfalfa fields and roadside ditches provide habitat for migrant leafhoppers where they are concentrated and can be trapped and tested for AYp before the crop has emerged. Researchers tested the migrant leafhoppers with new molecular tools that are more sensitive and much faster, to determine their risk to that year’s crops.
The project showed that migrant leafhoppers collected early in the season had a low infection rate with AYp. This suggests that in most years, aster yellows are not brought into Canada in any significant way in the migrant leafhoppers, with the exception of drought years in the leafhopper source regions. The AYp test results of the offspring of the migrant or F1 generation typically showed a higher percentage of infected leafhoppers than the initial migrants. This supports the findings that the AYp infection is carried over from one growing season to the next by a weedy reservoir and leafhoppers pick up the infection as they reproduce in Saskatchewan. Several perennial and biennial plants, including alfalfa, were identified as positive with aster yellows prior to any aster leafhopper activity in Saskatchewan indicating that these had been infected with AYp the previous year and carried over the infection from one growing season to the next as a “green bridge” between seasons.
Lab experiments revealed that in a ditch setting with two dominant plants, aster leafhoppers chose brome grass almost exclusively over alfalfa for feeding and reproduction. When seedling-stage canola plants are present, many of the leafhoppers will move into the canola field and begin to feed on canola, even without any mechanical disturbance in the ditch area. As the percentage of infected leafhoppers went up, so did the number of aster yellows symptomatic plants.
The project also used molecular genetic techniques to assess genetic variations among the diamondback moths, which can have varying levels of resistance to insecticides and other biological characteristics. The results showed that diamondback moth haplotyping did not reveal any insights into their source locations, but did reveal a dominant haplotype that the majority of the tested moths could be classified into. This dominant haplotype likely indicates that each year there is a common source for migrating diamondback moths into Saskatchewan.
Overall, the project results provide for better prediction and improved risk determination for aster yellows outbreaks. Improved and faster communication of the Aster yellows indexes to growers helps with pest management and decisions of whether to spray or not. As well, a better understanding of the common source for migrating diamondback moths helps indicate potential insecticide resistance issues, ensuring efficacy of products used by growers when the pest occurs at outbreak levels.