In 2018 and 2019, before the phase-out of chlorpyrifos, two growers opted to release sterile flies as their onion maggot management strategy going forward. Over the next few years, populations of onions maggots have decreased and so has the damage observed in the field.
The onion maggot (Delia antiqua) is the larval stage of the onion maggot fly which can cause severe damage to every stage of onion development. In onions, one maggot can kill up to 20 plants early in the season and without a management strategy, onion maggot can reduce plant stands by over 50%. Later generations of onion maggot can cause distorted growth and open wounds making it easier for pathogens to gain entry and reduce the storage life of the bulb (Figure 1).
Onion maggot management has relied heavily on chlorpyrifos (Lorsban and similar products) and now with the use pattern being phased out in Canada, neonicotinoid seed treatments, such as Sepresto 75 WS are the primary management strategy for direct seeded onions. For transplanted onions started in the greenhouse, there are no insecticide drenches currently registered for control once they are transplanted in the field.
In Quebec, Sterile Insect Technology (SIT) has been used for over a decade to eliminate the application of chlorpyrifos, and the program is now used for onion maggot management on roughly 2500 acres of onions. Flies are reared in a facility at the Phytodata Research Company in Sherrington, Quebec. Pupae are irradiated, dyed pink (to allow them to be distinguished from the wild population on sticky cards) and are shipped to release points where growers allow flies to emerge in emergence boxes and then release them on the edge of onion fields on a weekly basis (Figure 2).
In Ontario, sterile onion maggot flies have been released at two field sites in Scotland (Brant county) and Exeter (Huron county) over 5 years. Both of these locations had very different onion production systems. At the Scotland location, onions were transplanted and planted at a rate of 140,000 plants per acre while at the Exeter location, seeds were broadcast at a rate of ~8 million seeds per acre and harvested as onion sets (Figure 3). At both locations, no chlorpyrifos has been applied for the past 5 years. Sticky cards were monitored weekly and sterile flies could be differentiated from wild flies by the pink dye described above.
At both Ontario locations, onion fields were transplanted or seeded yearly within ~3 km of the previous year’s field. Many years the fields were directly adjacent to the previous year’s field. Populations of wild flies in 2022 and 2023 were lower than all previous years and the number of plants harvested with onion maggot damage has decreased dramatically. At harvest, fewer bulbs are found to be distorted and as a result, there are fewer culls.
In Exeter, fields of onion sets were planted adjacent to one another (500 m or less) year after year from 2018 to 2021 and again in 2022 to 2023 making it easier for flies to find a suitable crop to lay eggs next to the following spring. The 2022 and 2023 fields were planted ~2 km south of where the 2018-2021 fields were. Despite growing onions in fields adjacent to each other or following a short crop rotation, populations of wild flies did not increase to levels high enough to cause observable damage at the Exeter field site. In 2023, the concentration of sterile flies was halved from an average of ~100 000 flies per hectare per week to ~50 000 flies, yet populations of wild flies remained low.
Sterile flies have replaced Lorsban at both of these locations, and the amount of onion maggot damage has dropped below levels that were observed in the last decade when chlorpyrifos was used.
There are limitations to using sterile flies as the primary onion maggot management strategy. The first limitation is that both locations are uniquely isolated from other onion fields. Onion maggot flies are community pests, meaning that even if you manage the insect well in your 30-acre onion field, but your neighbour chooses not to, the wild flies from their field may easily find your field. This is a concern for onions since the majority of cooking onion production in Ontario is concentrated in muck soils with very small distances between fields.
The second limitation is that only sterile onion maggot flies are being released. Seedcorn maggot (Delia platura) is sometimes responsible for damage in onion fields, and Phytodata is still working on commercializing SIT for seedcorn maggot.
Previous year’s results can be downloaded using the links below. If you are interested in applying this management strategy on your farm, please contact myself at travis.cranmer@ontario.ca, or Anne-Marie Fortier at afortier@phytodata.ca.