Project Code STB19-010/PRR19-070
Project Lead
Cezarina Kora and Carl Bélec - Agriculture and Agri-Food Canada
Objective
Conduct on-farm demonstration trials to validate the effectiveness of durable polyethylene insect netting for cabbage maggot management in commercial scale production of brassica vegetables
Introduction
Brassica vegetables are widely produced in Canada, with Ontario and Quebec accounting for 86% of Canadian production in 2023. Sustainable production of these crops is seriously affected by cabbage maggot (Delia radicum) damage. Maggots feed on roots killing or stunting young plants, significantly reducing yields. Management has relied on few available insecticides. However, resistance of this pest to chlorpyrifos, one of the most commonly used insecticides, has been widely reported and, moreover, chlorpyrifos is no longer available for use since 2023.
Previous research by Agriculture and Agri-Food Canada scientists in the Atlantic provinces and British Columbia has shown polyethylene woven netting to be an effective physical barrier against cabbage maggot, a practice widely adopted by European farmers. This project aimed to further validate the efficacy and feasibility of insect netting to mitigate cabbage maggot damage in commercial farm conditions, and transfer this technology to growers in other regions to promote the uptake of netting as an alternative to chlorpyrifos.
Approach
The project work was conducted by a team of the provincial research centre CIEL (French only) in various regions of the Quebec Province between 2020-2022. A mechanical roller (HIWER floating row cover system from Dubois Agrinovation, QC) attached to a tractor was used to place and collect the netting at all sites, except at St-Michel where nets were manually laid. Demonstration trials were set up in 7 commercial brassica crops, including: Chinese cabbage on an experimental farm in the Lavaltrie region (2020); early head cabbage field in Île d’Orléans (2021); two fields of storage cabbage (2 varieties, Promise and Ancoma) in Île d’Orléans (2022); a rutabaga seedling, and a turnip field in St-Anselme (2022); and a field of turnips seedling in St-Michel (2022). In 2020, 13m x 100m sheets of polyethylene insect netting (Wondermesh Limited, UK) were tested with varying mesh sizes (0.6 mm, 0.8 mm, and 1.33 mm) covering 0.4 hectares plots. In 2021 and 2022 trials 0.8 mm netting was placed right after transplanting, with edges tightly secured using steel rebars (~40 per net) and sand bags, and removed right before harvest, except in Île d’Orléans where it was removed earlier for granular fertilizer, drip irrigation installation and for weeding.
Control plots with conventional pesticide treatments were compared to netted plots with bi-weekly scouting for pests, diseases, and weeds. Netting efficacy was assessed by comparing yields and root damage between covered and uncovered plots. Typically, herbicide applications were consistent between the netted plots and those under conventional management. Subsequently, based on various pest pressures (diseases and insects), phytosanitary strategies were determined in consultation with the agronomist advising the producer. In 2021, 0.13 hectares (1 grower) were netted, expanding to 2 hectares in 2022 (3 growers). The number of pesticide sprays and provincial human health and environmental risk indices were calculated and compared between control treatments and plots under netting. A partial cost-benefit analysis was conducted for a 15-year period on 1 hectare unit of cultivated land using 2022 production data from 3 netting demonstration sites (2 storage cabbage and rutabaga). The analysis assumed a 15 year life span for the HIWER system, and that netting and sandbags were replaced every 7 years.
Results
In 2020, there was low root maggot pressure, with minimal egg and larval presence, but, high diamondback and cutworm pressure severely affected yields (data from this trial are not reported).
In 2021, the cabbage field under netting had a notable reduction in pesticide applications and yields were comparable to conventional plots, with average cabbage head weights of 2.74±0.37 kg under netting versus 2.65±0.77 kg in conventional plots. There was one pre-planting herbicide application in netted area, while the conventional plots received 6 insecticide, 3 fungicide, and 1 herbicide applications. Even with the increased relative humidity under netting and no fungicide applications, there was not an observed increase in disease incidence or damage. The grower noted that cabbage heads under netting were ready to harvest 5 to 7 days earlier without affecting yield.
In 2022, the project was conducted on a farm with summer cabbage (Promise variety) planted in mid-May, and another site with winter cabbage (Anacoma) planted in late May. The summer cabbage had high cabbage maggot pressure with 92% of the plants with root damages in the conventional plot compared to 8% in the netted area, at the 12 leaf stage. In the conventional plot, was observed damage covering between 10 to 25% of the root surface, but 24% of the cabbages showed severe damage. In the netted area, the damage covered less than 10% of the root. Winter cabbage had lower cabbage maggot pressure, with only 3 out of 25 observed plants with damage under netting, while in conventional plots 44% of plants showed low severity (<25%) damage. There was no statistical difference between yields in netted and conventional plots (e.g., summer cabbage under netting: 3.26±0.66 kg; conventional: 3.29±0.82 kg). Also, there was a higher damage in conventional cabbage plots by other insect pests (44-90%) compared to netted plots (9-12%), where diamondback moth larvae damage was concentrated at net edges. All conventional cabbage plots had 8 insecticide applications versus 1 or 2 sprays under netting.
Similarly, turnip fields under netting had less damage and were comparable to conventional plots in yields. Average turnip weights for conventional vs netted areas respectively were: 84g vs 102g at St-Anselme, 219g vs 206g in the May seeded field at St-Michel; and 211g vs 211g at the July seeded field at St-Michel. The rutabaga field under netting had lower yields, approximately half of conventional areas, which was attributed to improper fertilization and high flea beetle pressure.
Overall, netting significantly reduced insecticide use with 1-9 less spray applications (33-90% reduction) compared to conventional plots without compromising yield, particularly in cabbage where there were 8-9 fewer sprays. Reducing insecticide sprays led to lower human health and environmental risk indices in netted rutabaga plots (e.g., 359 and 121, respectively) compared to the control (e.g., 2,781 and 2,472, respectively - due largely to 3 chlorpyriphos applications).
The cost-benefit analysis showed that using netting with the mechanized HIWER system is costly relative to insecticide applications. There is an approximate overhead of $20,000 due to the cost of netting material, equipment and labour (for handling the laying, securing in place, and removing). Due to the nature of demonstration trials, it is hard to adjust the benefits of netting in relation to reduced yield/quality of produce without the use of insecticides lost to deregistration.
Conclusion
The insect netting demonstrations have shown that this technology offers several benefits for growers. Adoption of this practice as part of an integrated management system for cabbage maggot in brassica crops could significantly reduce or completely eliminate the need for treatments with chemical insecticides, addressing concerns about insecticide resistance and pesticide load in the environment. The impact for growers is expected to be high, as netting provides an effective alternative solution to the lost insecticide uses for brassica pests. The economic hurdle of adopting a netting system, such as the HIWER machine, may be overcome with cooperative equipment sharing, and subsidy programs.