Improved decision-making for monitoring and management of alfalfa weevil in alfalfa crops on the prairies
Project Code PRR13-010
Julie Soroka Agriculture and Agri-Food Canada
To make available alfalfa weevil forecasting tool and information on naturally occurring parasitism to alfalfa growers in prairie regions
Summary of Results
Alfalfa weevil is an important pest in hay and seed alfalfa production on the prairies. Insecticide sprays and early cutting are the primary control methods for this pest. However, these control measures must be timed at certain stages of weevil growth for maximum benefit of the application. Accumulative growing degree day (GDD) models have been previously established for prediction of alfalfa weevil development in Ontario and the United States (US) and used effectively for years to successfully manage the pest in these areas. However, a validated model that accurately forecasts alfalfa weevil in the Prairie regions was not available.
This project aimed to assess the performance and validate these existing Ontario and US models under farming conditions in the main alfalfa production regions of the Canadian Prairies. The objective was to identify and recommend the best fitting model for use by prairie alfalfa growers. The project also aimed at investigating the biological control profile of alfalfa weevil in these regions. The parasitoid wasp Bathyplectes curculionis, which arrived in Alberta many years ago from the western US, and is established in eastern North America following deliberate introductions, has been a successful biological control agent for managing alfalfa weevil. However, the presence or prevalence of this species across the Prairies was not well known.
The end goal was to provide the information generated through this project in a suitable format to help prairie alfalfa growers make informed decisions about the need or timing of control measures to ensure protection of pollinators, parasitoids and other beneficial organisms present in their crops.
Thirteen alfalfa fields in each of two years (2013 and 2014) were monitored for the presence of alfalfa weevils across the three Provinces: Alberta (5 fields), Saskatchewan (5 fields), and Manitoba (3 fields). Monitoring was conducted by stem and sweep sampling at approximately weekly intervals from the time of plant regrowth in spring until mid- or late summer. The weevil numbers and growth stage, as well as alfalfa height and growth stage per date and location were recorded for each sampling.
Three weevil development models (Guppy-Mukerji, Harcourt and North Dakota) were tested, along with weather data from three different sources in an effort to determine the combination most closely aligned with the observed field data related to the emergence of weevil larvae. The target was to accurately predict timing of peak occurrence of second instar larvae, because it is the optimum time for weevil management.
Contents of sweep captures were also examined for the presence of adult B. curculionis, the most common parasitoid of alfalfa weevil in North America, and other natural enemies.
Prediction: Both the Harcourt and North Dakota models yielded better prediction of weevil population peaks than the Gupta-Mukerji model. Temperature data source affected prediction model accuracy, with the best predictability achieved utilizing the within-field and the Modified Agriculture and Agri-Food Canada (AAFC) model generated temperature data. Weevil populations usually peaked when alfalfa was approximately 53 centimeters tall.
This project determined that a threshold of about 230 GDD with the Modified AAFC temperature accumulations (base 9 degrees Celsius) will provide the best prediction for the peak of second instar larvae populations which is also the stage at which an economic threshold can be applied. Based on this information, a new warning system was made available to prairie alfalfa growers in the 2015 growing season delivered through the Prairie Pest Monitoring Network in the form of weekly updated degree day maps. The maps provide growers with a record of accumulated growing degree days, so that they can estimate the stage of development of the weevil in their area and determine when control should be considered if weevil numbers warrant action.
Biocontrol: Adults of the parasitoid wasp B. curculionis were captured at 10 of the sites sampled, including three Manitoba locations. Once found only in southeastern Alberta and southwestern Saskatchewan, the parasitoid has recently become well distributed across Saskatchewan, and now, through this project, it has been identified also in Manitoba. Large numbers of B. curculionis were collected occasionally in some fields (for example 74 wasps in 100 sweeps near Swift Current, Saskatchewan, on June 11, 2014). The greatest numbers of the wasp were collected in mid-June, about 2-3 weeks earlier than the peak number of alfalfa weevil larvae. This suggests that the cut or spray would be best timed when second instar larvae are at peak and not before, to maximize wasp egg-laying.
The presence of the parasitoid B. curculionis and other biological control agents encountered in the project is promising for the potential for natural suppression of alfalfa weevil populations. Optimizing management of alfalfa weevils while protecting beneficial insects will contribute to sustainable production of alfalfa.
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