Onion fly season 2024: Growers limit damage with beneficial nematodes

In Emmeloord, in the heart of the Noordoostpolder, the Koppert and Nemacontrol teams are taking stock of the growing season. The Noordoostpolder is known for the intensive cultivation of onions, which, as a natural consequence, means that the onion and bean seed fly pose significant challenges for growers. Thanks to the use of beneficial nematodes, however, many growers look back on a successful harvest and forward to a future where these pests are well-controlled.

From the earliest signs of the onion and bean seed fly in spring up until the onion harvest, Koppert and Nemacontrol are in constant contact in pursuit of a shared goal: helping growers control these pests with beneficial nematodes. Through precise monitoring and custom-made crop protection protocols, they offer an effective solution.

Their solutions are crucial at a time when growers are losing access to effective crop protection products against the onion fly (Delia antiqua) and bean seed fly (Delia platura), particularly the larvae that cause damage to the onion bulb. As a result, damage from especially the onion fly has gone up in the Noordoostpolder, the Dutch epicentre of the pest problem. In other regions, the bean seed fly was the main culprit, with the onion fly being a close second.

‘The sentiment in the Noordoostpolder was: if this continues, we will no longer be able to grow onions in this region. It is understandable to feel this way when you have lost half of your harvest,’ says Sebastiaan ten Napel, director of Nemacontrol. The company conducts research into various types of nematodes, including potato cyst nematodes and free-living nematodes. As of last year, Nemacontrol also offers monitoring services to detect pests, such as the onion and bean seed fly. Their monitoring services and Koppert’s beneficial (insect-parasitic) nematodes are complementary, working together to control the onion fly and bean seed fly. Subsequently, growers can reduce their use of chemical products, which helps to safeguard the future of their business.

‘The effective control of the onion fly and bean seed fly starts with precise monitoring,’ says Frank Druyff, account manager agri at Koppert. In 2022, following years of research and testing, Koppert launched Capirel, a beneficial nematode product that controls the onion fly and bean seed fly by parasitizing the larvae. These nematodes are highly effective when applied in the right way and at the right time: not too early, not too late, and while the soil is moist. ‘Timing is crucial,’ adds Frank. ‘As a grower, you need to know exactly what is happening in your crop. By conducting high-precision counts, Nemacontrol is able to deliver these insights.’

Their measurements indicated another season of high pest pressure. Early in the sowing period, the bean seed fly population was particularly high. Shortly thereafter, the onion fly population spiked. ‘The onion fly has three flights per season,’ says Laurens Pelgröm, representative and agricultural consultant at Nemacontrol. ‘We can accurately predict the timing of these flights, which is roughly the same each year. What surprised us was the high pest pressure so early in the season, especially given the wet conditions in the spring.’

By measuring early and often, growers gain the insights they need to plan their crop protection strategy. ‘As soon as you miss the first fly, you are already behind the curve,’ says Sebastiaan. ‘When fly pressure is high, you can expect larval pressure to be high about one and a half to two weeks later. The correct application of beneficial nematodes enables you to limit the damage caused by those larvae.’

The initial reactions of distributors and growers affirm the importance of timing. ‘Distributors say: it is not a perfect ten everywhere, but the results are still very solid,’ says Frank. ‘The success stories have one thing in common: the growers began applying beneficial nematodes in a timely manner, before the first fly was caught, so to speak.’

‘Last year, the damage caused by the onion fly reached 50 percent in some fields. This year, despite equally high or even higher pest pressure, we are seeing fields with damage levels as low as eight percent,’ Sebastiaan adds. ‘It shows that real progress has been made. In a number of fields, the peak of the onion fly’s second flight, which by nature is exponentially higher than the peak of the first flight, was much lower than we had anticipated. We will have numbers on the end-of-season population in a few weeks, which is an indicator of next season’s initial population. The outlook is positive.’

The timely application of beneficial nematodes begins with placing the order, explains Frank. ‘This is a living product, with organisms that need to be bred. If you plan to use Capirel next season, get in touch. Make sure that you have Capirel in your fridge by the time you are ready to sow, that way you will always be ready on time.’