Over the past 5 years, sterile onion maggot flies have been released at several onion fields in Ontario to see if onion maggots could be managed without a Supresto 75 WS seed treatment, or chlorpyrifos drench.
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. Its first generation can kill seedlings or newly-transplanted onions (Figure 1A), its second generation targets the plant before bulbing (Figure 1B) and the third generation creates wounds that compromise storage quality.
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.
With the phase-out of chlorpyrifos, onion maggot management in direct seeded onions now relies heavily on neonicotinoid seed treatments, such as Supresto 75 WS. For transplanted onions started in the greenhouse, there are no insecticide drenches currently registered for control. Without these insecticides, onion maggot populations may increase dramatically.
A lot of effort has gone into looking at alternative management strategies for onion maggot control. Discing in crop residue immediately after harvest will help to speed up decomposition and make it more difficult for the Delia spp. flies to complete their lifecycle and reduce the maggot pressure the following year.
Other management strategies exist, however, they require more planning and/or labour. Floating row covers are not an option for onion production, and hooped exclusion nets are costly, time consuming and ultimately not an economical option for large scale production. An effective management strategy that is often not practical for most growers, is a long-term crop rotation with fields spaced many kilometers apart. Onion maggots can travel several kilometers in search of a favourable crop, but theoretically the greater the distance from last years onion field should reduce the overall maggot pressure.
As far as biological controls are to be considered, products like Nemasys containing the beneficial nematode Steinernema feltiae, have been tested for onion maggot control at the University of Guelph – Ontario Crops Research Centre – Bradford (Muck Crops Research Station). These beneficial nematodes, applied as a drench, have shown no significant differences in a reduction of maggot damage or improvement of yield, however there was low onion maggot pressure for the trial overall.
In Quebec, Sterile Insect Technology (SIT) proved to eliminate the application of soil and foliar chlorpyrifos insecticides (before they were phased-out) while maintaining onion yields comparable to pesticide-based programs. On a weekly basis, sterile male onion maggot flies are released into the field and as a result, the eggs that these pre-existing, wild female flies lay end up not being fertile. Over time, the wild onion maggot fly population decreases to the point where the release rates of sterile flies can be decreased by up to 90%.
In Ontario, Sterile onion maggot flies have been released at two field sites in Scotland (Brant county) and Exeter (Huron county) over the past 4 and 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. At both locations, no chlorpyrifos was applied on fields where sterile flies were released. Sticky cards were monitored weekly and sterile flies were dyed pink before they were released so that they could be differentiated on the sticky cards (Figure 1C).
At the Scotland location, the fields in 2019 and 2020 had sterile flies released but were in close proximity to untreated onion fields both years. These other fields may have acted as a refuge for wild flies and may have contributed to the variance in pest pressure and the spike in flies that is seen in mid-June of 2020 (Figure 2). In 2021 and 2022, all onion fields in the area had sterile flies released and the overall pressure of wild onion maggot flies has since dropped dramatically.
At the Exeter location, onion fields were planted adjacent to one another (500 m or less) year after year from 2018 to 2021 making it easier for flies to find a suitable crop to lay eggs next to the following spring. The 2022 onion field was planted was 1.7 km south of where the 2018-2021 fields were, which may have also accounted for the large decrease in onion maggot fly pressure (Figure 3).
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. These results seem to indicate that even with continuous cropping of onion sets in the same area for five years, wild onion maggot flies were effectively controlled by releasing sterile flies on a weekly basis without a clothianidin / imidacloprid seed treatment or a chlorpyrifos drench.
Onion maggot flies and their larvae are considered community pests, meaning that even if you manage the insect well in your 30-acre onion field, but your neighbour chooses not to, their flies 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 results from Scotland demonstrate the importance of widespread adoption in dense onion growing areas.
A full report of this trial along with reports from 2018-2021 are posted below. If you are interested in applying this management strategy on your farm, please contact myself at firstname.lastname@example.org, or Anne-Marie Fortier at email@example.com.