Integrated pest management: sterilized pink onion flies in onion crops

Ten years after they were first introduced on a large scale in onion fields in Montérégie, sterilized pink onion flies have successfully replaced a controversial insecticide, while generating a dramatic reduction in the natural populations of onion maggots (Delia antiqua), the main onion pest in Canada.

The sterile insect technique (SIT) works in a surprisingly simple and ingenious way. It essentially uses one pest to combat another. To do so, onion flies that are factory reared and sterilized through irradiation ahead of time are released into fields. When the sterilized males mate with the females in the crops, the females lay empty eggs, preventing the pests from multiplying.

The associates of Delfland, major vegetable growers located in Napierville in Montérégie didn’t think twice about making the shift to pink flies in 2011. “The most important thing for us was to be able to stop using Lorsban (chlorpyrifos) at seeding,” says Yvon Van Winden. “All the workers who had to handle the insecticide were happy to no longer be working with this hazardous product.”

In 2018, Québec’s Ministère de l’Environnement included chlorpyrifos on its list of the five most high-risk pesticides. Its use will be banned after 2023, and its approval has been withdrawn for almost all current agricultural usage.

Jean-Claude Guérin, of Maraîchers J.P.L. Guérin & Fils (Sherrington, Québec), also opted for sterilized pink onion flies in his onion fields. He, too, did so for health reasons, but mainly for the environment. “My goal was to use a minimum of pesticides,” says the grower. “I’m constantly trying to find alternative pest control methods, while ensuring that my business remains economically viable.”

In fact, the environmental benefits were clearly felt in Québec’s RCM of Jardins-de-Napierville, where several onion growers replaced chlorpyrifos with SIT. The concentration of chlorpyrifos in the Gibeault-Delisle Brook decreased to such an extent that it became undetectable in 2019, while it was present in 100% of the water samples taken in 2005-2007, 77% in 2013 and 40% in 2014.

Imported and adapted to Québec

For several decades, the Netherlands has been using the mass release of sterilized individuals to control onion maggots. In Québec, Phytodata Research Company Inc., a member of PRISME Consortium, developed and refined this technique, adapting it to local conditions.

In the 2021 season, roughly 30 Québec growers, producing nearly 1,000 ha of onions, green onions and shallots, used SIT. This represents 40% of the area designated for onion production in the province.

The sterile flies are distributed manually throughout the onion growing season, namely from May to September–October. They are released once a week on the edges of fields, near ditches and in nearby vegetation, all preferred breeding sites for onion flies.

The technique’s success is based on the careful screening of pests to monitor changes in populations, identify species and quantify their presence. “To monitor adult populations, we use insect sticky traps,” explains entomologist Anne-Marie Fortier, Assistant Scientific Director at Phytodata. “We count the natural populations on each farm to adapt the release rates over the course of the season and subsequent years.”

A dramatic reduction in onion flies

The beauty of the pink flies is that the more we use them the less we need them. “The average phase-in rate (number of flies/ha) decreased by nearly 90% in the first five years of use,” says Fortier. “In 2011, it was around 160,000 flies/ha for onions, while today, it is around 20,000.”

This trend can be explained through the gradual reduction in natural populations as growers implement the technique. Since Québec onion production is concentrated in Montérégie (80% to 90% of total volume), the effectiveness of the pink flies was quickly demonstrated. Jean-Claude Guérin noticed it in his area: “The more sterilized flies are used in a given region by several growers, the more we can decrease application rates.”

The reduction in phase-in rates of flies helps make the technique economically viable for growers. The cost per hectare for an average rate of 20,000 flies is around $280. According to growers, that amount is comparable to the cost of purchasing pesticides.

In addition, subsidies are available to encourage the adoption of SIT. The purchase of pink flies is eligible under component 1 of the Prime-Vert program. The MAPAQ will reimburse 70% to 90% of costs, up to $12,000 annually and $60,000 over five years.

The companies that produce large onion crops, such as Delfland (160 ha) and Maraîchers J.P.L. Guérin (70 ha), easily reach this ceiling. Although they don’t benefit from the subsidy for subsequent applications, going back to chemical insecticides is out of the question for these growers.

Coming soon: sterilized cabbage flies

The success of pink flies on onion production has inspired another initiative, this one targeting a cruciferous pest: the cabbage maggot. Two research projects are currently assessing the effectiveness of SIT on radishes, daikon radishes, broccoli, cauliflower, turnips and Chinese cabbage.

Delfland has been working closely with Phytodata to develop sterilized cabbage flies. For this vegetable grower, a major producer of daikon radishes (40 to 45 ha/year), there was a great deal at stake. “We considered completely abandoning this crop because of damage caused by cabbage maggots,” says Guillaume Cloutier, an agronomist and Delfland associate. The damage remained considerable, despite numerous applications of insecticide. Now, with sterilized insects, our problems are solved. We’ve gone from seven applications of pesticide to none for this pest.”

Although the technique is clearly effective in the field, mass rearing of the flies has proven to be an issue. For onion flies, the artificial diet for rearing purposes and the method of induction during diapause (period of dormancy) are well known. It is therefore possible to produce a large quantity of individuals ahead of time and refrigerate them while waiting for use of the field.

Fortier explains that cabbage fly rearing techniques need to be further developed. “We’re not aware of the right conditions for rearing diapausing cabbage flies, and we don’t have an adapted artificial diet in place. For now, we’re rearing using fresh rutabaga as needed, which prevents large-scale rearing.”

Research is nevertheless underway to overcome these production obstacles, and the results are promising. Current progress suggests that, in 2023, sterilized cabbage flies will be commercially available to more cruciferous vegetable growers who want to use them.

Source: This article was originally published in French in the May–June 2021 issue of Coopérateur magazine.