Project Code: PRR17-020
Project Lead
Suzanne E. Blatt - Agriculture and Agri-Food Canada
Objective
To improve carrot rust fly and carrot weevil control in carrot production in Ontario and the Maritimes by refining emergence prediction models and evaluating efficacy of entomopathogenic nematode products
Background
Carrot rust fly (CRF) (Psila rosae F.) and carrot weevil (CW) (Listronotus oregonensis L.) are two major pests of carrots causing serious economic damage across Eastern Canada. Carrot weevil causes damage mainly in Ontario and Nova Scotia, while CRF causes damage in Ontario and Prince Edward Island. Larvae of both pests create tunnels in the roots of carrots, making them unmarketable. Furthermore, bacteria and fungi may invade the roots through these tunnels and cause important post-harvest damage during storage. Damage is highly variable based on location and year, but carrot losses have been reported to be as high as 70% due to CW and 100% due to CRF. The primary method of control for these pests are applications of organophosphate or pyrethroid based insecticides. However, insecticides often miss reaching their targets, especially when larvae are hidden within the carrot or in soil. In addition, effective insecticide options that are available to growers is decreasing. Some insecticide uses have been phased-out due to regulatory review (e.g. diazinon for CRF) and there is evidence for insecticide resistance developing in pest populations (e.g. CW to phosmet). Timing of insecticide applications is also essential for preventing damage from CW and CRF. Managing applications based on pest presence requires accurate prediction of pest populations, which is typically achieved using Degree Day (DD) emergence models and scouting. Previous DD models were developed for both CRF and CW in Quebec during the 1990s and 2000s. However, these models have not been consistent in predicting emergence of CRF in Ontario and Prince Edward Island and CW in Ontario and Nova Scotia.
The need for more control options to diversify growers’ toolkit and improve management efficiency for these pests was identified as a priority for action under the Agriculture and Agri-Food Canada (AAFC) Pesticide Risk Reduction Strategy for Root Insect Pests of Carrot, Parsnip and Onion. To build upon the results from previous projects funded under this initiative (PRR07-090 and PRR14-020) this two year project aimed to refine and validate existing DD emergence models for these pests in Nova Scotia, Prince Edward Island and Ontario. Alternative management options were also concurrently investigated under the conditions in these regions.
Approaches
Approaches This project was implemented in commercial fields in Nova Scotia, Prince Edward Island and Ontario in 2017 and 2018 and in experimental carrot fields in Nova Scotia in 2018 under the leadership of scientists at AAFC’s Kentville Research and Development Centre in Nova Scotia. Pest population and weather data from in-field or nearby stations were recorded in these provinces during the 2017 and 2018 growing seasons to refine existing DD models. In 2017, commercial carrot crops in Ontario and Prince Edward Island were monitored for adult CRF using yellow sticky traps and, in 2017 and 2018 commercial and experimental fields in Ontario and Nova Scotia were monitored for CW populations using modified Boivin traps. Data was added to the growing database of CW and CRF emergence records and used to update and adapt the respective DD models for these pests in the target regions.
To investigate alternative management options, four commercially available entomopathogenic nematode (EPN) products were evaluated for the control of CRF and CW in each region. Previous laboratory investigations showed promising activity of the tested EPN products against insect pests including weevils and this project aimed to identify if this activity was maintained in field settings. Steiner-system (Steinernema feltiae) and Carpo-system (S. carpocapsae) were obtained from BioBest and applied as foliar sprays at the label rate of 1 million nematodes per metre squared [M/m2] (equivalent to 1000 litres per hectare in a field setting) in Prince Edward Island, Nova Scotia or Ontario in 2017. Two additional EPN products, B-Green system (Heterorhabditis bacteriophora) and Kraussei system (S. kraussei) were applied mid and late season in Nova Scotia and Ontario in 2017 using the same application rate and method as the other products. Based on preliminary results from 2017, a single application of either Steiner-system or B-Green system were applied to each treated plot in Nova Scotia, Prince Edward Island and Ontario in 2018, with varying application timing. To evaluate the efficacy of these products, carrot samples were taken from each treatment plot and were assessed for root damage throughout the season in comparison with the control plots.
The longevity and viability of the entomopathogenic nematodes over the season was also assessed. Soil samples and moisture readings were regularly collected from each plot in Nova Scotia and Prince Edward Island starting at the time of application up to 8 weeks in 2017 and 6 weeks in 2018. Soil samples were placed in a petri dish with 8-10 common waxworms (Galleria spp.) placed on the surface of each dish as a food source for the nematodes. After 1 week the samples were examined to determine the presence of nematodes and mortality of waxworms. A laboratory experiment was also conducted with the nematode products to determine the depth they would travel within substrates to reach a food source (waxworm: Galleria spp.). A single waxworm was flash-frozen (to reduce movement) and then placed at 3, 5, or 7 cm depth within sterilized soil in mailing tubes lined with waxpaper. Each of the four nematode products were applied separately to the surface of soil samples at the label rate of 1 M/m2. After 12, 16, 24, or 48 hours the tubes were disassembled and the waxworm was dissected to evaluated for mortality and nematode presence.
Results
The DD models for CW and CRF that were previously updated for Quebec and Ontario (PRR07-090) were refined using the population and weather data collected in Ontario, Prince Edward Island and Nova Scotia during this project. These results were combined with data that was previously collected in these areas from 2014-2016 (PRR14-020) to improve and validate the models for these regions. The CRF data in Nova Scotia and Ontario were too sporadic and inconsistent during this time and were not sufficient to contribute to refining the DD models for these regions, so only CW results were used for these locations. Using the 7 °C threshold determined previously in Quebec, DD model validation for CW in Nova Scotia showed that initial emergence occurred at 35 DD with peak occurring at 387 DD. This is earlier than observed in Quebec and suggests that growers in Nova Scotia need to begin their monitoring in early May rather than late May or at planting. Oviposition on bait carrots was found to occur as early as 42 DD, which is well ahead of when the carrot crop is sown. Growers using these models can identify their window of opportunity to manage their carrot weevil populations targeting the majority of emerged adults before oviposition begins in the field.
Preliminary results from the 2017 longevity assessment of EPN products showed that two of the nematodes, (S. feltiae and S. carpocapsae) persisted in soils for 4-6 weeks post-application. However, their impacts on CRF and CW damage in field trials were variable by region. No significant differences in damage between plots that received added treatments of mid and late season applications of H. bacteriophora or S. kraussei and control plots were observed in Nova Scotia and Ontario in 2017. Although none of the EPN products showed consistent significant reduction in pest damage during the 2017 trials, S. feltiae showed the greatest reduction overall, primarily in CW populations. Damage was reduced by 8% in Nova Scotia when applied twice but only reduced by 0.9% in Ontario when applied once (late application) compared to the control. The CRF populations on Prince Edward Island showed the most reduction (~7%) when S. feltiae was applied twice compared with a single application during the growing season but there was no significant difference between damage observed in any treated plots and the control. H. bacteriophora and S. feltiae showed promising longevity results in 2017 with infectivity persisting up to 6 weeks post-application. Therefore, these two products were selected for field trials in 2018 and were applied in carrot fields in Nova Scotia, Ontario and Prince Edward Island. As multiple applications of these EPN products would be prohibitively expensive, only a single application of B-Green and Steiner-system to each test plot was evaluated for in all locations in 2018. Timing of applications were selected using current DD models to target CRF pupae and the updated DD model to target CW pupae or adults, which are the most vulnerable life stage of these pests for these treatments. Both EPN products showed significant reductions (~25%) in CW damage when applied early in 2018 in Ontario, but were not effective for CRF in Ontario. Neither EPN product showed significant reductions in damage caused by either pest in Nova Scotia and Prince Edward Island in 2018. Efficacy of these products is affected by longevity and motility of the nematodes, which is highly dependent upon a number of environmental factors including soil moisture, soil type and application timing. In these studies, some EPNs persisted in the field for up to 12 weeks while others were no longer showing efficacy after 3 weeks and this varied across year and site. Laboratory studies with these four EPN species confirmed the choice of species used in the field but also showed the relative efficacy of each species to locate a host across a range of soil moisture levels.. Based upon the overall variable and inconsistent efficacy results from this project, the EPN products that were evaluated are not recommended for commercial use to control CRF and CW in carrot fields in Ontario, Prince Edward Island or Nova Scotia at this time. Further investigations into conditions and strategies for improving their activity is required.