What about Brassica pests and diseases?

A very common question we receive about using forage radish as a cover crop in vegetable systems is: what about Brassica pests and diseases?

Harlequin bug adult

Harlequin adult. Photo credit Russ Ottens, University of Georgia.

The biggest pest of concern in the mid-Atlantic is harlequin bugs, and this post is devoted to them. Harlequin bugs are voracious eaters of Brassica crops, and it is with due cause that farmers are concerned about providing overwintering habitat for them. As with many insects, they are an even greater concern for organic farmers because organically approved insecticides are relatively ineffective at reducing their populations. This article from the Journal of Pest Management (2011) has a nice summary of the life cycle and habits of harlequin bugs. Some take home points that are relevant to using forage radish as a cover crop:

  1. Harlequin bugs prefer mustard, turnip, kale, rutabaga, and Chinese cabbage over cabbage, cauliflower, broccoli, collards, Brussels sprouts, radish, and kohlrabi.
  2. If desperate, harlequin bugs will even turn to non-Brassica wild hosts such as pigweed and lambsquarters.
  3. Trap cropping can be an effective management tool, using knowledge of preferred hosts.

SARE has published a helpful factsheet on trap cropping that discusses harlequin bugs, among others.

Trap cropping- do’s and don’ts and how does forage radish fit in?

These harlequin bugs came over from a rape cover crop that was mowed.

These harlequin bugs came over from a rape cover crop that was mowed.

The only “nightmare” story about forage radish hosting harlequin bugs that we’ve heard (and people do turn to us for all things forage radish, for better or worse!), was a case of infestation resulting from (inadvertent) trap cropping gone awry.

The farmer had planted a mixed cover crop for an NRCS project that included rape (Brassica napus). Harlequin bugs love rape. He mowed that cover crop mix, essentially evicting the harlequin bugs and sending them packing for a new home, which they found on his field of forage radish. If this situation had gone differently, the rape could have served as a trap crop, either with the farmer spraying or even vacuuming the rape plot to eliminate the concentrated harlequin bug population. (This would be a problem if the rape plot were large- an ideal trap crop is not grown on a huge area). However, mowing the rape just caused the harlequins to seek a new home.

Most recommendations that I’ve read for cultural controls of harlequin bugs (and nearly all bugs, really) recommend plowing under all crop debris to eliminate habitat/overwintering sites for bugs. I understand from an insect control perspective why this is the recommendation, but leaving the soil plowed and bare for the winter is such an erosion risk, and a nutrient leaching risk, and it means sacrificing all that carbon that you could be adding to the soil via a cover crop.

So what’s the answer on forage radish and Brassica pests?

I haven’t given a conclusive answer for “what about Brassica pests and diseases?” because we really only have anecdotes about pest pressure in Brassicas following a forage radish cover crop. The anecdotes indicate that farmers haven’t noticed an increase in pests, but anecdotes are not data!  Researchers continue to look into this issue. One thing they are investigating is how soil temperature under different regimes might affect overwintering. Because of the relative lack of residue from forage radish, the soil is not insulated as it is after a high-residue cover crop like oats. This allows faster warming in spring, but also deeper freezes in winter.

In the mean time, if you are concerned about including yet another Brassica in your rotation, but you want to to have a low-residue winterkilled cover crop, we suggest trying phacelia. Seeds right now are expensive, but can be purchased in larger quantities at a much lower cost than other seed sources from Walnut Creek Seeds ($5.11/lb as of 2016–there may be other sources, too, but you’ll have to search the web a bit). We have some plots planted currently, and look forward to reporting more about weed pressure and nutrient release after phacelia this spring.

Phacelia has been used extensively in Europe as an insectiary when planted in spring, but it has also been used as a cover crop prior to no-till corn.

Maize planted into phacelia cover crop. Notice the minimal residue. Photo credit: Francesco Vidotto, TOPPS photo gallery.

Maize planted into phacelia cover crop. Notice the minimal residue. Photo credit: Francesco Vidotto, TOPPS photo gallery.

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