Over the past 7 years, two growers opted to release sterile flies as their onion maggot management strategy going forward and stopped using Lorsban or Sepresto 75 WS for onion maggot management.
Biology and Identification
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 and can cause distorted growth (Figure 1B), while the third generation creates wounds that compromise storage quality making it easier for fungal pathogens (like Fusarium) to gain entry (Figure 1C). With the phase-out of chlorpyrifos in December 2023, onion maggot management in direct seeded onions now relies heavily on a neonicotinoid seed treatment, Sepresto 75 WS. The seed treatment is working well protecting direct seeded onions from onion maggot larvae, but for onions started in the greenhouse for transplanting, the seed treatment may be worn off before or soon after they are transplanted in the field, leaving these newly transplanted onions at risk for large amounts of onion maggot damage.
Managing Onion Maggot without Insecticides
Options for managing onion maggot are limited. Discing in crop residue immediately after harvest will help to speed up decomposition and make it more difficult for the Delia flies, like onion maggot, to complete their lifecycle. This will likely reduce the maggot pressure the following year, but fly levels will still be above the economic tolerance the following field season. Long-term crop rotations involving fields that are separated by several kilometers may also help, but onion maggots can travel long distances in search of a favourable crop, so this strategy is not practical for most. Floating row covers are not an option for onion production, and hooped exclusion nets are costly, time consuming, and not an economical option for large scale onion production.
How are onion maggot flies sterilized and released?
In Quebec, Sterile Insect Technology (SIT) proved to eliminate the application of chlorpyrifos insecticides (before they were phased-out) while maintaining onion yields comparable to pesticide-based programs. Onion maggot pupae are reared in the Phytodata fly rearing facility (Figures 2 & 3). Pink dye is added to help differentiate the difference between wild flies and sterile flies that are captured on sticky cards in the field later. The pupae are irradiated prior to being shipped to the farm at a specific dose that makes them sterile (Figure 4). Once the pupae arrive at the farm, adult onion maggot flies will emerge in emergence boxes over the next 3-4 days (Figure 5). Once the majority of the pupae have emerged, they are released into the onion field. These sterile files mate with wild females, and as a result, the eggs that these pre-existing, wild female flies lay end up not being fertile.
Monitoring after Releasing Sterile Flies
In Ontario, sterile onion maggot flies have been released and monitored at two field sites in Exeter (Huron county) and Scotland (Brant county) over the past eight and seven years respectively. Both of these locations had very different onion production systems. At the Scotland location, onions were transplanted at a rate of ~345,000 plants per hectare (~140,000 plants per acre) while at the Exeter location, seeds were broadcast at a rate of ~20 million seeds per hectare (~8 million seeds per acre) and harvested as onion sets. Sticky cards were counted weekly and since sterile flies were dyed pink they could be differentiated on the sticky cards from their wild counterparts. The average number of wild flies per trap per week at each field site has decreased and have remained low once all the fields in the area had sterile flies released on a regular basis despite no seed treatment or insecticides aimed at maggot flies (Figure 6 & 7). At harvest, fewer bulbs have been found to be distorted or have maggot damage, and as a result, there are fewer culls that are linked to onion maggot feeding.
The overall number of flies released per season depends on the current level of onion maggot pressure. Over repeated field seasons, the wild onion maggot fly population decreases to the point where the release rates of sterile flies can be decreased by up to 90%. Releases generally start at around 100,000 flies/ha/season and are reduced once wild fly levels fall to an acceptable rate. In Quebec, growers have been releasing an average of 16,000 flies/ha/season in dry onions and 25,000 flies/ha/season in green onions over the past several years. In our two long term areas of study, sterile fly releases have completely replaced insecticides aimed at onion maggots at both locations, and the amount of onion maggot damage has dropped below levels that were observed in the last decade when chlorpyrifos was used.
This strategy is only effective on onion maggot, so seedcorn maggotfly populations will not be managed with this technique. Phytodata is currently working to commercialize rearing and sterilization of seedcorn maggot and cabbage maggot flies. Onion maggot is a community pest, 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. Currently four farms in Ontario are releasing sterile onion maggot flies throughout the growing season, and they are all in locations that are 10 or more kilometers from any other commercial onion fields.
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. While sterile fly releases generally start the first week of May, Phytodata does require time to rear and sterilize additional flies.
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