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By Brian Nault, Department of Entomology, Cornell AgriTech, Cornell University
Christy Hoepting, CCE Cornell Vegetable Program
There has been a great deal of interest across the U.S. about how best to protect next year’s onion crop from maggots given the limited supply of spinosad seed treatment (Regard SC). This article focuses on what insecticide seed treatments should be used for managing maggots (Fig. 1) as well as what fungicide seed treatments should be used to manage the disease complex (damping off, stunting and onion smut) in major production regions across the U.S. In all cases, there are no rescue treatments once onions are either infested with maggots or infected with these pathogens. Therefore, selecting effective seed treatments is especially critical for protecting the onion crop.
Maggot Pests, Early-Season Diseases
Onion maggot (Delia antiqua) is the dominant maggot species that infests onion in the Great Lakes region, while seedcorn maggot (Delia platura) is the principal species in all other production regions. In the Great Lakes, both maggot species routinely damage onions, and this requires a strategy that will control both.
Onion smut (Urocystis cepulae) is a fungal disease that infects onion seedlings and can either kill seedlings or render bulbs unmarketable (Fig. 2). Onion smut is a major disease of onion grown in the Great Lakes region and can be a problem in certain locations in the western U.S., but it is typically not a concern in other production areas. In contrast, Rhizoctonia spp., which are one of the causes of stunting, are not a major issue for onions in the Great Lakes, but can be a problem elsewhere.
Differences in pest and disease status between onion production regions should be considered when selecting early-season management approaches, especially seed treatments. In Table 1, active ingredients are listed as well as their relative efficacy on onion maggot, seedcorn maggot, onion smut and damping off diseases.
Insecticides
Regard
Regard SC is a formulation of the active ingredient spinosad, which has excellent activity on both onion maggot and seedcorn maggot. This active ingredient is owned by Corteva, and Syngenta has been contracting with Corteva to offer it as part of its FarMore FI500 seed treatment package. In addition to Regard, the FarMore FI500 package includes the insecticide Cruiser 70WS (thiamethoxam) and the three fungicides in FarMore F300 (Apron XL, Maxim and Dynasty) (Table 1).
Earlier this year, Syngenta ended its contract with Corteva. There is a limited supply of Regard remaining and it will likely be gone by the end of the year. There are no plans for Regard to be available after supplies are gone. There is uncertainty as to whether spinosad will be available as an onion seed treatment in the future.
Trigard
Trigard OMC will be available for the 2023 season. Syngenta will continue to offer “FarMore FI500 with Trigard” as a seed treatment package. This FarMore package will include both insecticides Trigard and Cruiser, as well as the FarMore F300 fungicide package (Table 1). Trigard also can be applied without Crusier. Trigard currently works very well on onion maggot, but it is not very effective against seedcorn maggot.
Cruiser
Cruiser is poor on onion maggot control, but may provide limited control of seedcorn maggot (Table 1). The combination of Trigard and Cruiser should protect the onion crop from maggot damage in the Great Lakes region, but it is not a viable option for seedcorn maggot control in other production regions.
Sepresto
Sepresto contains two insecticide active ingredients, clothianidin and imidacloprid. Sepresto has not performed well against onion maggot in New York, but has performed fair to good against seedcorn maggot in the western U.S. (Table 1). In the absence of Regard SC, Sepresto would be the best seed treatment option for seedcorn maggot control.
Non-Seed Treatment Options
There are not effective insecticide options other than seed treatments for maggot control in the Great Lakes region. Past research in New York in muck-grown onion examined the following products and none controlled maggots: 1) in-furrow drench treatments of Entrust SC (spinosad), Admire Pro (imidacloprid), Verimark (cyantraniliprole), Coragen (chlorantraniliprole) and Capture LFR (bifenthrin); 2) pre-plant incorporated treatment of Diazinon AG500 (diazinon); 3) post-emergent banded applications (two-leaf stage when flies were active) of either Radiant (spinetoram), Exirel (cyantraniliprole) or Mustang Maxx (zeta-cypermethrin).
In Washington state, Diazinon AG500 (diazinon) is effective when applied pre-plant and mechanically incorporated.
Insecticide Resistance Concerns
Resistance to insecticides is a concern for onion maggot, but not as much for seedcorn maggot (based on their different life histories). Onion maggot has developed resistance to insecticides like chlorpyrifos (e.g., Lorsban). The threat of onion maggot developing resistance to spinosad (Regard) and to cyromazine (Trigard) is relatively high because both active ingredients have been used as seed treatments on onion for many years. Regard and Trigard belong to different modes of action: IRAC Groups 5 and 17, respectively (Table 1). Therefore, the best approach for avoiding resistance is to rotate Regard and Trigard on an annual basis for onion maggot control. Given the limited availability of Regard for 2023 and uncertainty about spinosad’s future availability as a seed treatment, rotating active ingredients will be difficult to impossible until a new chemistry becomes available.
2023 Insecticide Seed Treatment Recommendations
FarMore F1500 with Regard
FarMore FI500 with Regard provides excellent protection against onion maggot and seedcorn maggot in all production regions (Table 1). In the Great Lakes, this option would be especially good if FarMore FI500 with Trigard was used in 2022 because rotating to another chemistry class will mitigate resistance development to spinosad and cyromazine. Moreover, if spinosad is not available as a seed treatment in the future, FarMore FI500 with Trigard can be used in future years until a new product becomes available.
FarMore F1500 with Trigard
FarMore FI500 with Trigard should provide excellent protection against onion maggot and adequate protection against seedcorn maggot in the Great Lakes (Table 1). This option would be especially good if FarMore FI500 with Regard was used in 2022 because rotating to another chemistry class will mitigate resistance development to spinosad and cyromazine. Note that if Trigard is applied in the absence of Cruiser (e.g., not part of FarMore FI500 package), seedcorn maggot will not be adequately controlled. For onion maggot, Trigard/Regard + Cruiser improved control only slightly (by 5-10 percent), if at all, compared with Trigard/Regard alone. This is not a good option for regions where seedcorn maggot is the dominant species because Trigard will not be effective.
Sepresto
Sepresto may provide adequate maggot control. In the Great Lakes, Sepresto could be an option in onion fields where onion maggot damage is expected to be low. If seedcorn maggot pressure is high, Sepresto could be added to Trigard to manage the maggot complex. However, the combination of Sepresto + Trigard has not been evaluated for efficacy or phytoxicity. Research trials in 2023 will examine this option. In the western U.S., Sepresto has consistently worked well for seedcorn maggot control in Tulelake, California, but it provided inconsistent results in Washington trials.
Fungicides
Onion Smut Control
None of the three fungicides included in the FarMore F300 package have activity on onion smut, but they do have some activity on damping off pathogens (Table 1). Growers in the Great Lakes region and some locations in the western U.S. will have to order EverGol Prime for control of onion smut. Since it first became available in 2019, EverGol Prime has become the premier option for onion smut control as its performance is far superior to Pro Gro.
When EverGol Prime is used with FarMore F300, it is not necessary to order additional seed treatments for control of damping off. Mancozeb and/or mefenoxam (Ridomil) could be applied as an in-furrow drench for additional protection against damping off. If EverGol Prime is used without a FarMore package, then another fungicide with activity on damping off should be included in the seed treatment (Table 1). Generally, a mancozeb in-furrow drench is not needed for control of onion smut with EverGol Prime seed treatment. However, some red onion varieties that appear to be more susceptible to onion smut have benefited from inclusion of a mancozeb in-furrow drench in addition to EverGol Prime seed treatment for improved control of onion smut, according to on-farm demonstrations in New York where the mancozeb drench was turned off compared to where it was used in the rest of the field.
Authors’ note: This article includes contributions from Rob Wilson, University of California; Stuart Reitz, Oregon State University; and Tim Waters, Washington State University.
