Spring Pest Watch: Seed Corn Maggot, Cabbage Root Maggot and Related Fly-family Pests
Cultural controls such as hilling vulnerable plants to better protect roots from maggot damage generally only work with established crops already showing good growth and vigor ahead of CRM or SCM infestations, as described here:
The only organic / OMRI listed pesticide product labeled and showing any reported efficacy against maggot pests is Entrust SC, and only moderate control was achieved after three soil drenchings that began in greenhouse trays at the seedling stage. This is probably a more expensive and less effective strategy than would be worthwhile for most Brassica and other vegetable crops on most small farms in the region, but details on Entrust SC usage for CRM control can be seen here:
For organic growers seeking other options to control Seed Corn Maggots and Cabbage Root Maggots in the field, that leaves beneficial nematodes as the main other product that could be sprayed or watered-in over an infested crop. This great resource from MOFGA notes that Steinernema feltiae (Sf) Nematodes can be effective in wet soils at temperatures down to the 40’s F. or ~10* C. Still, with cold and snow in the forecast for typical NC High Country March and even April days, using nematodes before the middle of April or so could be an expensive experiment in pushing those limits. Nematode efficacy can also be affected by soil moisture conditions (more is better), the health of the beneficial nematodes following shipping, the time of day they are released (late evening is best), and other factors.
If you have already-infested Brassica or other crops out in your fields, it may be smartest to till-under those particular plants if you happen to have more in the greenhouse or could easily purchase them from another grower. Once CRM adults find a Brassica patch and lay their eggs, rescuing those particular plants becomes difficult. If you are able to re-plant with more from a greenhouse, you could consider drenching such plants with Entrust SC while they are still in trays, and then at transplanting as well. But probably the cheapest and most effective strategy for a replant (or better yet, your first planting!) is to forego the Entrust, and just immediately cover the Brassica planting with hoops and row cover material such that CRM adults are not able to reach the new plants. Of course, for this strategy to work, your spring Brassica field cannot be in the same location as any Brassica crops from last fall, where CRM pupae will be emerging from soils. Also, the edges of your row cover material should be securely buried or weighted-down such that CRM adults cannot reach the young Brassica plants. MOFGA does a good job providing further details on the use of row covers to protect Brassica crops from CRM as well.
One more note about Cabbage Root Maggots, in particular, is that most references state that this particular fly pest species only infests plants in the Cabbage family. However, the Massachusetts link above notes that CRM have been observed on carrot crops (which are also vulnerable to the carrot rust fly, Psila rosae), and the Ohio link above adds celery and beets to the list of crops that CRM could potentially infest. So to err on the safe side, be on the lookout for wilting and other CRM symptoms on any Apiaceae / Umbel or Chenopodiaceae / Goosefoot family crops you may have planted out as well. Since distinguishing larval stages of SCM from CRM in the field is basically impossible, this point is basically academic, and root maggots of one species or another can be observed on most vegetables when conditions are particularly favorable for these pests.
Seed Corn Maggot (SCM)is the main maggot species in NC where adults would lay eggs in disturbed soils ahead of any actual plants emerging (Bean Seed Maggots also do so). That means they do not wait to find plants, but can lay eggs tilled or disked field, where the seeds and roots of many weeds and nearly any young crops can serve as their food. Being mindful of fresh organic matter in soils, and giving soils where manures or cover crop residues have been incorporated some time to “digest” can help to reduce Seed Corn Maggot pressure on direct-sown crops and transplants. The ideal window between fresh organic matter incorporation and direct-sowing or even transplanting into high-residue soils is 4 or more weeks. Some local success has been seen with only three weeks of interval between fresh organic matter incorporation and planting, but allowing less time than increases the risk of Seed Corn Maggot damage. Unfortunately, this also means that using row cover fabric and row cover materials will not protect crops against Seed Corn Maggots, as the eggs and larvae may be in the soils even ahead of crop sowing or transplanting.
So what is a small-farm organic grower to do to reduce Seed Corn Maggot damage in future years’ Spring crops? While no organic strategy is yet thoroughly proven, one idea may be to plant your next year’s Spring crop ground into summer cover crops such as millet and cowpeas this coming July or August. According to NC State University Professor of Entomology Jim Walgenbach, “No-till is a less hospitable habitat for seed corn maggots.” Tilling, fertilizing, and seeding your planned Spring crop ground the summer prior with cover crops that will frost-kill (such as millet, soybeans, and/or cowpeas) can give you an easily-planted no-till mulch the following Spring that may help to reduce Seed Corn Maggot populations in that ground. This takes a lot of planning and preparation, but in the absence of organic pesticides and other options for SCM and CRM control, such an advanced cover cropping strategy that is already proven to reduce weeds and improve soil quality may also help to reduce maggot populations in Spring Crops that are transplanted into such frost-killed mulch residue.
For more information about these and other High Country vegetable and fruit pests of concern, contact N.C. Cooperative Extension of Watauga County by emailing firstname.lastname@example.org or call 828-264-3061.