Cabbage rootfly (CRF-Delia radicum) is a well-known foe for brassica growers in this region. Normally, we do not expect them to be an issue mid-summer because

    • There are presumably two distinct peaks of CRF activity 1:
      • Spring – newly emergent adults; ~330 GDD = Mar 11th, 2018
      • Fall – breeding flight; ~ 2400 GDD = Jul 14th, 2018
    • CRF prefers cool weather and activity tends to diminish during the summer heat.

HOWEVER: Summer activity is measured by pan traps, and

Eggs were detected 4WAP and continued to be evident throughout the summer. There was a clear and steady increase in root damage starting at 3WAP in summer. Eggs were present in the fall as well, but the level of root injury was more gradual. (EGG count = top graph each season; ROOT damage rating = bottom) FROM: S.V. Joseph, J. Martinez / Crop Protection 62 (2014).

the authors of the model above agree that summer activity might have been underestimated because of ‘visible competition’ and attractiveness of blooming crops and weeds vs. yellow traps. The spring generation can be extended up to 3 weeks or more, depending on how long rainy, cool weather conditions persist. Also, we know that there are overlapping generations of CRF, and a study from California suggests that egg-laying behavior and subsequent damage during summer months is markedly different than fall:

 

ANOTHER FACTOR is that Delia radicum is actually part of a much larger ‘rootfly complex’, and different species have different ecological niches, behavior, and activity periods. This table explains some of those differences. Identifying rootflies is hard enough when they are adults, and nearly impossible as maggots and pupae. Thus, they are referred to as a pest complex that can affect growers year-round.

This puparium was found 27-Jul-18, suggesting that rootfly activity continues yearround in the PNW. It may be seedcorn maggot, radish maggot, or turnip maggot, as all are known to infest brassica roots.

 

1According to a regional model (Dreves 2006), and current 2018 data (Agrimet station CVRO)

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