Onion and Seed Corn Maggot Concerns in a Cold, Wet Spring
Crystal Stewart-Courtens, Extension Vegetable Specialist
Eastern New York Commercial Horticulture
Last year we saw a lot of problems with root feeding damage from maggots early in the season, which has growers on edge about pest pressure this year. Seed corn maggots can damage a variety of crops, and in previous years have been observed on everything from sunflower shoots in the greenhouse to pea seedlings and onion transplants in the field. Onion maggots are more particular, and will only feed on allium hosts.
Both seed corn and onion maggot flies are attracted to and will lay eggs in fields with large amounts of decomposing organic matter. Large applications of compost and decomposition of robust cover crops benefit cash crops, but also have this unforeseen side effect. For this reason, planting early crops into fields with lower organic matter is a best practice if maggots are a concern on organic farms. Conventional growers will often choose to use seed treated with an insecticide to protect early plantings.
In small scale plantings it's also possible to exclude adults by applying insect netting or row cover during flights. Onion maggot flights are tracked on the NEWA website, which tracks growing degree days (GDD): http://newa.cornell.edu/index.php?page=onion-maggot Seed corn maggot flight is also predicted by growing degree days, although NEWA doesn't use a model.
When will the maggots arrive? Right now we have accumulated 83 GDD at a base of 40 F in Clifton Park (Saratoga County) and 148 GDD in Montgomery (Orange County). Seed corn maggots emerge at 360 GDD base 40, and onion maggots emerge at 400 GDD base 40. We have a ways to go, with GDD accumulating faster the higher the temperature climbs above 40. If using exclusion netting, get it on prior to the flight starting. Another option, if feasible, is to hold plants in a protected environment until after the flight has concluded.
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