cabbage maggot « VegNet

After this morning’s invited speaker talk (OPVC Annual Grower Meeting), I thought it might be helpful to assemble some existing resources we have about Delia spp. in the PNW. Browsing these links could help you better prepare for the planting season. Also feel free to leave a comment with any specific concerns or impact you’ve experienced. Thanks to Dr. Nault for a great presentation this morning!

** VegNet alert – late June 2020 – Seedcorn maggot issues reported/confirmed in snap bean and parsnip
** Pest profile page – Seedcorn maggot species complex, literature
** Pest profile page – Cabbage maggot overview, ID, management
Temporal trends – late July 2018 – Temporal trends and species complex info
Scouting report – mid Oct 2020 – Maggot complex found in stored onion
Research report (pdf) – 2016 maggot trial in direct-seeded radish, chlorpyrifos alternatives
VegNet alert – May 2018 – Poor emergence, general info seedcorn maggot
U. of Mass. factsheet – Seedcorn maggot
Dr. Brian Nault, lab site – Cornell Entomology (pdf) – Delayed planting to help manage onion maggot

Root maggots are creamy white to yellow, opaque, and legless. They are tapered; blunt posterior end. Determining species is difficult and requires examination with a microscope.

Root Maggots of the PNW – Overview

Delia species (Diptera: Anthomyiidae). Small (5-8mm) flies – black, brown or grey – the immature phase are called maggots – they feed on root and sometimes stem tissue – identification technical and difficult^* – often referred to as a rootfly ‘complex’. Adults do not cause damage. Eggs are laid at the base of plants. Maggots tunnel into tissue which causes direct damage and also increases the risk of infection by plant pathogens.	.
D. radicum: Cabbage maggot Our most familiar regional issue. Adults prefer cool weather and maturing (4-7 leaf) brassica plants to lay eggs. Flight period well-documented and can be useful for predicting timing of egg-laying pressure. HOSTS: weedy mustards, broccoli, cauliflower, etc.	.
D. platura: Seedcorn maggot Attracted to organic matter and decay. Sometimes worse in fields that have been cover cropped to increase N. Often a secondary pest (invades after initial decay of tissue due to other factors). Active earlier than other species. HOSTS: many, but especially large seeded vegetables like corn, peas, dry beans, snap beans	.
D. florilega: Bean seed maggot Nearly indistinguishable from D. platura, often occur together. HOSTS: association is broad, but mostly a problem in turnips, radish, canola	.
D. planipalpis: Western radish maggot Similar in appearance to D. radicum, but different leg hair arrangement. HOSTS: radish and canola (verified in literature); also probably other crucifers
D. antigua: Onion maggot A major problem in onion production. Many good resources available. HOSTS: onions, garlic, chives, etc.	.
D. floralis: Turnip maggot Similar in appearance to other species; different leg hair arrangement. HOSTS: turnip and radish

^*Link to ID guide (Savage et al. 2016) but beware, it involves counting and measuring hairs on adult fly thoraces and legs – good times!

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 activity¹:
  - 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 seedcorn maggots, which are the immature stage of Delia platura. Plants are most susceptible at seedling stage. Host plants include: corn, 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.`

Read the full report here: http://bit.ly/VNweek24 and subscribe on our homepage to receive weekly newsletters during field season.

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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

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Category Archives: cabbage maggot

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