Crops: Crop Scene Investigation 11 Solved: What was behind Sam's uneven soybeans?

Tuesday, June 3, 2008

by CLARE KINLIN

It turns out that seed corn maggots caused Sam's soybean stand to be thin and uneven.
Don't let the name deceive you. Seed corn maggot is not only found in corn. It's a common pest in soybean and vegetable crops throughout the province.

Damage caused by seed corn maggot can be difficult to distinguish from other insects and disease, but two features made it an easy diagnosis in Sam's field – spring-applied manure and cotyledon feeding.

Seed corn maggots tend to be most predominant in fields with decaying organic matter, such as manure. Even though Sam only applied a light coat, it still attracted the seed corn maggot adults (flies), which prefer to lay their eggs in moist soils that give off an odour of decay. Cover crops, crop residue and weeds also provide environments conducive to egg laying.

After hatching, the small yellowish-white, headless and legless larvae (maggots) use two tiny black hooks to feed on germinating seeds and cotyledons. Although they shareabove-ground symptoms, below-ground cotyledon feeding sets seed corn maggots apart from white grubs and wireworms. Another key difference is that grubs and wireworms usually affect patches of soybeans, whereas seed corn maggot damage tends to cover a large portion of a field.

There's a good chance Sam's fields have played host to seed corn maggots before. They went unnoticed, however, because the soybeans emerged before the pest could pose a threat. Damage is more likely in cool, wet soils, when the seeds are slow to germinate but the insects are actively feeding. Because Sam's field received a significant amount of rain after planting, the soybeans struggled to emerge and the seed corn maggot damage made the plants vulnerable to even more pests and disease.

There is no rescue treatment for a severely damaged field. The only option is to replant. In Sam's case, he lost 30 per cent of what he planted. His stand was thick enough to leaf, and the beans eventually outgrew the maggots.

Prevention is the best approach to managing seed corn maggot infestations. The use of good quality seed treated with an insecticide is a great strategy to get your crop off to a quick and healthy start.

Congratulations to: Willy Ann Kennes, Parkhill for providing the correct answer.
CSI returns in October. Watch the August/Sept. issue of Better Farming for the solution to CSI 12 which appeared in the May issue of Better Farming. BF
 
Clare Kinlin, CCA, is a Field Agronomist for NK Brand, Syngenta Seeds. He is based near Prescott.