Evaluating row covers for early season management of cabbage maggot in brassica vegetable crops

Project Code: PRR14-010

Evaluating row covers for early season management of cabbage maggot (Delia radicum) and other pests of leafy brassica vegetable crops

Project Leads

Peggy Dixon and Scott Anderson - Agriculture and Agri-Food Canada

Objective

Demonstrate the efficacy and benefits of polyethylene mesh row covers in managing key insect pests of brassica vegetables in Atlantic Canada and British Columbia

Background

Cabbage maggot (Delia radicum L.) causes significant economic damage to brassica crops grown across Canada. Sustainable management of this pest issue was identified as a priority through consultation with the expert working group leading the Pesticide Risk Reduction (PRR) Strategy for Cabbage Maggot Management in Brassica Crops. Growers require reduced risk alternative options to minimize reliance on insecticide sprays and allow integrated management of this pest. A previous project conducted under this strategy in 2010–2012, showed that polyethylene mesh row covers (commonly known as 'insect netting') successfully provided season long protection of rutabaga from cabbage maggot damage. However, unlike rutabaga, leafy brassica vegetables are only sensitive to the first generation of cabbage maggot, so these crops do not need season-long protection. Removing netting sooner, could prevent weed control challenges associated with this technology.

This project aimed to evaluate the effectiveness and demonstrate the benefits of different gauges of insect netting to protect leafy brassica vegetables from insect pests early in the season, when they are most vulnerable to damage. It also aimed to transfer the knowledge on adopting the netting technology to growers.

Approaches

Field trials were conducted in 2014 and 2015 with broccoli and radish in Agassiz, BC; with broccoli and kale in Kentville, NS; and with broccoli in St. John's, NL. Each site had 20 plots testing five different treatments, including 3 mesh sizes: large (1.33 mm, Crop Solutions Ltd, UK), medium (0.80 mm, Wondermesh Ltd., UK), and small (0.60 mm, Crop Solutions Ltd, UK) gauge netting, along with uncovered control with no insecticide applications, and uncovered control with chlorpyrifos drench applications (industry standard). Netting performance was assessed for key pests, such as cabbage maggot, flea beetle (Phyllotreta cruciferae), imported cabbageworm (Pieris rapae) and diamondback moth (Plutella xylostella). Plants were grown from seed in the greenhouse and seedlings were transplanted in the field at 5-6 weeks old. The netting plots were covered immediately after transplanting using the Hiwer machine and securing the net along all edges with soil or sandbags and removed a few weeks later. A pre-transplant herbicide was applied to all plots.

Broccoli, kale and radish plants were assessed for root damage caused by cabbage maggot feeding using the canola rating scale [Dosdall, L. M., et al. 1994. Susceptibilities of species and cultivars of canola and mustard to infestation by root maggots (Delia spp.). Can. Entomol. 126: 251-260] in all sites in 2014, and in BC only in 2015. Root damage was used to calculate the cabbage maggot infestation index [King, K.M., and A.R. Forbes, 1954. Control of root maggots in rutabagas. J. Econ. Ent. 47:607-615]. Plant biomass, marketable yield, and maturity were measured to monitor effects of netting. The other insects were assessed at mid-season and at harvest, including feeding damage and egg counts for flea beetle, as well as larvae and pupae counts of the cabbageworm and diamondback moth.

The project established excellent collaborations among federal scientists, technology transfer experts, provincial specialists and local crop consultants coordinating numerous outreach activities conducted in all three regions to increase awareness and promote adoption of insect netting technology.

Results

All tested gauges of insect netting were effective in early season management of cabbage maggot, cabbageworm and diamondback moth in broccoli plots. Netting protected broccoli from cabbage maggot damage at equivalent or to a greater extent than conventional insecticide applications. Also, netting significantly reduced cabbage maggot infestation index (ranging from 0.4 to 9 under cover) compared to uncovered, untreated (44 and 100) and chlorpyrifos treated (30 and 84) plots in 2014 and 2015, respectively. Broccoli grown under netting had significantly higher marketable yields (4.3-5.7 t/ha) compared to uncovered, untreated and chlorpyrifos (2.9 t/ha each) treated plots in 2015, with less pronounced advantage in 2014 (for example, 8.2 t/ha vs 6 t/ha, respectively). There were significantly less larvae of imported cabbageworm and diamondback moth on covered broccoli compared to uncovered plots. Flea beetle populations were insufficient for analysis.

In general, broccoli matured more quickly when grown under netting compared with uncovered plots. In 2015, approximately 60% of covered broccoli had matured and was harvested at the first harvest date, compared to ~40% of broccoli from uncovered plots.

In kale, there were no significant differences in infestation indices, harvested biomass, or percent marketable weight in 2014. In 2015, significantly higher marketable yield was observed in kale under netting compared to uncovered controls, perhaps due to greater numbers of cabbageworm and diamondback moth present in control plots. While netting was not effective in reducing cabbage maggot damage, it may be a useful tool to prevent damage from other leaf-feeding insects in kale.

Radish plots, included in this project in 2015, also showed a significant reduction of cabbage maggot infestation and root damage when grown under all netting gauges compared to uncovered plots. There was also higher root yield and plant biomass in covered radish.

Conclusion

Generally, netting showed promise as an alternative practice in protecting leafy brassicas from cabbage maggot and other insect pests. Some leafy brassicas are higher value than rutabaga, making netting technology more cost effective. Significant knowledge and technology transfer efforts were made to inform growers about the efficacy and benefits of using insect netting resulting from this project. Outreach activities included on-farm demonstrations, grower tours of experimental sites, presentations, and a video showing the use of specialized machinery to install and remove the netting (Insect Netting Technology in Atlantic Canada). This technology is considered an excellent option that can be adopted in small farms, organic production, and also on a much larger scale conventional farms, to effectively control key insect pests in brassica crops without the use of insecticides.