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)

WEEK 7 –

  • Seed corn maggot  – Poor emergence may be a sign of underground feeding by seed corn maggots, which are the immature stage of Delia platura, the bean seed fly. Plants are most susceptible at seedling stage. Host plants include: green and broad beans, onion, brassicas, peas, pepper, potato, spinach, and beet.
    • SCM is especially attracted to newly-tilled soil with high organic matter / manure inputs.
    • They have multiple, overlapping generations per year. This image by U. of Illinois highlights how adults, eggs, and maggots may all be present at the same time.
    • If emergence is low, scout 2-ft row sections for seed damage and white, tapered maggots that look very similar to cabbage maggot. Both species favor cool conditions for egg-laying, but D. platura are more active as adults in warm weather.
    • There is a fascinating biological (fungal) control for SCM that alters the fly’s behavior:  It causes the flies to settle on tips of grain stems or high-up flowers and die, which increases dispersal of the fungus to spread farther.
  • Seed bugs – There have been recent complaints of high numbers of ‘small, flying insects’ in both urban and rural areas since mid-April. The bugs are 3-4mm with elongate bodies and wing covers with 4-5 veins. Experts agree that the taxonomy of this group is in need of a major revision, so they are usually referenced to genus level only.
    • More than half of all known Nysius species are from Hawaii, including the endemic wēkiu bug, that migrates to the summit of Mauna Kea each year.
    • Nysius spp. are seed predators and tend to be less selective then other, related  Lygaeidae. Extensive damage can occur in wheat, quinoa, canola, and sorghum. Occasional feeding can occur on ornamentals, other cereals, and tomatoes.
    • Similar to boxelder bugs, they are attracted to large, sunny, white buildings, which has led to nuisance reports by homeowners. Various Ask-an-Expert questions have been submitted, one of which I was able to identify last week as Nysius, probably N. raphanus. The high numbers we are seeing now is likely the result of overwintered nymphs maturing into active, winged adults. There are 4-7 generations per year.
WEEK 24 – Cabbage maggots are one of the most challenging pests for brassica growers. They tunnel through root tissue and increase the risk of exposure to  plant pathogens Read this cabbage maggot page, which includes more info on biology and how to sample for them. Another late season pest is diamondback moth. Many sites are listed as "n/a" this week, because fields have been harvested and traps are being removed.

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  • Traps are being set this week. Stay tuned for a full report next Friday.
 

  • Adult rootflies are abundant; they love this cool rainy weather! If possible, protect seedlings with row covers. (Cabbage maggot info)
  • Did you know that winter cutworm is also a problem in Spring?! Visit this blog post for more info:
    http://bit.ly/2p9mC2Q

VegNet is an insect pest monitoring program funded by the Oregon Processed Vegetable Commission and managed by the Oregon State University Department of Horticulture. To add your name to this newsletter, please click the ‘subscribe’ button on the homepage.

 

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