An unwelcome visitor in soybean fields
By by Madeleine Baerg
Seed corn maggots took a costly bite out of many south-central Ontario soybean fields this spring. In Wellington County, some fields were decimated to the point of needing complete replanting. While seed corn maggots are not a new problem, their unpredictable occurrence, limited control options and sometimes devastating consequences make them a major and costly headache for producers.
“Are seed corn maggots the kind of pest that wipes out 50 per cent of Ontario’s soybean acreage? No. The overall per cent is relatively small. But, if you are a grower that gets hit with maggots, it’s very significant and very costly for you,” says Horst Bohner, soybean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs.
Seed corn maggots are small, light yellow maggots that feed on germinating soybean and corn seeds. Because adult flies will only lay eggs in moist, rotting vegetation and larvae need time to do maximum feeding, seed corn maggots are most damaging in cool, wet, slow springs.
Most outbreaks tend to be fairly regional. That said, predicting an outbreak remains extremely difficult.
“The biggest problem with predicting when seed corn maggots will be a problem is that we don’t have a handle on when there will be large numbers of adults. We understand that if seed comes out of ground slowly there is more time for larvae to feed. But why there were a large number of females at one time in a specific region this year, I don’t think anyone knows that. So many of these insects just cycle,” Bohner says.
Once seed corn maggots hit a crop, there is not much a farmer can do but wait to assess damage. Seed corn maggots are difficult to counter because it is virtually impossible to scout for adult flies, and there are no post-seeding pesticide treatment options.
While seed treatments tend to be effective against the larvae, gaining approval for their use can prove to be a chicken or egg scenario: to be approved, one must prove damage has caused a loss of 30 per cent or more of the stand. However, once need is identified it is much too late to counter the maggots and the benefits of a neonicotinoid treatment can only be seen in a replant.
One maggot countermeasure every producer should follow is prioritizing speedy germination and seedling emergence, Bohner says. “Think about proper planting depth, good seeding timing, adequate nutrition and disease management, good residue control. You want to do everything you can to get that seed out of the ground as fast as possible.”
Like many flies, adult seed corn flies are attracted to the odour of decay. Seed corn flies lay their eggs in freshly tilled soil, decaying crop residues, and manured fields. As such, farmers concerned about seed corn maggot infestation may want to consider no-till management. At the very least, farmers should seriously consider opting not to till under cover crops or manure within three weeks prior to seeding.
Farmers who suspect seed corn maggot infestation in their fields should look for widespread and fairly consistent damage across the field, rather than patchy or localized damage. Then, dig up seed to look for obvious physical damage and/or the telltale yellow maggots.
Because the seed corn maggot’s entire lifecycle can occur in as little as three weeks, be aware that a new generation of maggots may be primed and waiting for a second planting of seeds. If seed corn maggots are verified in a field and damage warrants a replanting, consider planting insecticide-treated seed.
It is very difficult to estimate the cost of damage inflicted by seed corn maggots on Ontario fields.
“What typically happens in Ontario is that we have considerable acreage that needs to be reseeded each year, but it’s hard to always know why it needs reseeding. It could be soil borne diseases, insects, cold stress, soil crusting. More often than not, it’s a combination of factors. Seed corn maggots are just part of the overall picture. Replanting costs money and reduces yield potential, but calculating exactly how much of that is due to seed corn maggot is almost impossible,” Bohner says.
“But, I’ll tell you this,” he adds. “Seed corn maggots are frustrating and they are costly. I’ve been doing soybean trials for 15 years. This year, we had a large experiment completely wiped out because of seed corn maggots. So I do know exactly what the farmer goes through when he sees his hard work destroyed by a hard-to-manage pest like seed corn maggot.”