Pest Update: Cabbage Maggot

 

With the warm weather finally arriving, it's important to pay attention to when insect pests will become active so that control measures can be implemented. Cabbage maggot (Delia radicum) is a particular challenge in brassica crop production. It overwinters in the pupal stage, emerging in the spring, usually coinciding with the bloom of yellow rocket and serviceberry. From a growing degree day perspective, emergence begins around 161 Growing Degree Days (GDD) at a base of 4°C, with peak flight occurring around 250 GDD for the first generation of cabbage maggot. As of Thursday May 16, 2024, 172 GDD (base 4°C) have accumulated in Brooklyn Corner. If you would like to check how many GDD have accumulated in your area go to Perennia’s Farm Data Tools website. Once there, you can make a free account and access the Farm Weather tool. From the Farm Weather tool map, select a station close to you, which will bring up the GDD calculator as well as current weather condition at that station.

Cabbage maggot feeding on roots. 

Other Delia species, Delia platura (seedcorn maggot), Delia antigua (onion maggot) and Delia florilega (bean seed maggot) are also on the move in the Valley. Once the first generation of adults has emerged in the spring, they take flight and lay their eggs. It is important to know when peak flight is taking place so that you have the opportunity to alter planting times or deploy control measures, such as insect netting, accordingly.  For scouting purposes, eggs can usually be found small clumps on the soil around the base of your seedlings.

Pests: Time to get out and scout

 

I hope you all are having a good start to the season! As plants are emerging out in the field, and transplants growing in greenhouses are being set out, nefarious forces are emerging too. It’s time to start scouting and making a plan for the inevitable diseases and pests, with all their inbuilt hunger for your precious produce. Here’s a quick list of some common things to watch out for this time of year.

Leek moth was first trapped two weeks ago in the Annapolis Valley. Their first flights should be occurring throughout the province now. They are nocturnal so you may never notice them until the larvae are creating window-pane like damage to your onions, leeks, and garlic. They are difficult to control once they establish themselves, as they live inside of the leaves of onions or will bore their way into the stems of garlic and leeks, making the plants unmarketable. Their presence is relatively new in Nova Scotia but they have quickly established themselves. They generally have two flights here, one early in the spring and another later in the summer before garlic harvest.

Leek moth larval feeding damage on garlic. (Photo: Agriculture and Agri-Food Canada.)

Onion, cabbage, and seed corn maggot. These each overwinter in the soil then emerge in the spring, with cabbage maggot flies appearing first, followed by onion maggot flies, then seed corn maggot flies. We can expect the cabbage maggot flies to start their first flights in a couple of weeks (middle to late May). Emergence happens roughly at the same time as yellow rocket starts to bloom. Generally you’ll first notice the presence of these maggots when transplants start wilting on hot days. This is due to the larvae feeding on roots; plants will eventually die if left unchecked. On a small scale, physical barriers (such as row cover, insect netting, etc) are effective. On a larger scale chemical controls are available to help manage these pests.

Onion maggot damage (Ontario CropIPM)

For those growing in tunnels or seed-starting greenhouses you will start to see thrips, springtails, aphids, and flea beetles emerging soon, or already emerged depending on the specifics of your tunnel setup. Inspect your transplants for feeding damage such as shotgun-like holes on your brassicas in the case of flea beetles, or tiny white spots on leaves in the case of thrips.

Onion thrip damage (Cornell University)

Flea beetle damage (UMN Extension)

While we’ve had relatively good weather for seeding and putting out transplants, the recent cold, dreary, and wet weather has slowed soil microbial metabolism, meaning that overwintering plants like garlic might be showing yellow leaf tips right now. This is most likely due to a slowdown in nitrogen mineralization as soil temperatures reduced. This can be remedied by applying a plant available nitrogen source such as a liquid fertilizer or urea. In addition to reduced mineralization rates, the cold wet weather was an excellent environment for promoting fungi such as Pythium, Rhizoctonia, and Smut.

Damping-off (Pythium spp.) beetroot seedling, (Penn State University Extension)

This is not an exhaustive list by any means, but hopefully gives you a good push to get out and start scouting your crops!

Please reach out to me to discuss any of these pests, or if your crops are suffering for other reasons.

 Happy seeding and transplanting!

Tim (tmorcom@perennia.ca)

Time to keep an eye out for cabbage maggot

With a cool start to the spring this year, insect pests have been a little later getting out and about than we would typically see. Now that there are warmer days upon us, it's important to pay attention to when those insect pests will be active so that control measures can be taken in a timely fashion.

Cabbage maggot (Delia radicum) is a particular challenge in brassica crop production. It overwinters in the pupal stage, emerging in the spring, usually coinciding with the bloom of yellow rocket and serviceberry. From a degree day perspective, emergence begins around 161 Degree Days (DD) at a base of 4°C, with peak flight occurring around 250 DD for the first generation of cabbage maggot. As of Friday May 22, 2020, 175 DD (base 4°C) had been accumulated in Kentville. Other Delia species, Delia platura (seedcorn maggot) and Delia florilega (bean seed maggot) are also on the move in the Valley.

Cabbage maggot larvae in soil. Photo UMass Extension.

Once the first generation of adults has emerged in the spring, they take flight and lay their eggs. It is important to know when peak flight is taking place so that you have the opportunity to alter planting times or deploy control measures, such as insect netting, accordingly.  For scouting purposes, eggs can usually be found small clumps on the soil around the base of your seedlings. For more information on cabbage maggot, including concerns about pesticide resistance, check out Perennia’s fact sheets “Cabbage Maggot” and “Chloropyrifos Resistance in Cabbage Maggot”.

Field Day: What's the Deal with Delia?

Join us on a field tour on Thursday, July 9th, 2015 from 8:15 am to noon!  

Come learn more about onion maggots (Delia antigua) and other Delia species that attack onions, corn, beans, and Brassicas such as cabbage, broccoli, cauliflower, radishes, kale and Brussel sprouts.

Figure 1. Onion maggot damage.

AAFC is currently conducting research to determine which Delia species are present in Nova Scotian onion fields and to determine if surrounding vegetation or field history can influence Delia populations and subsequent damage.

Netting and floating row cover can be powerful tools to combat insect pests such as cabbage root maggot (Delia radicum). Perennia and AAFC are currently conducting research into the efficacy of different netting mesh sizes. A Wondermesh Netting/Row Cover Retriever will be on hand to demonstrate technology that makes using netting and row covers easier!

Figure 2. Wondermesh Netting/Row cover Retriever.

Carrot weevils (Listronotus oregonensis) are sporadic pests of carrots. Current AAFC studies are evaluating the carrot weevil’s movement within the field and identifying overwintering sites.

Please call Gail at (902) 678-7722 or visit www.perenniaregistrations.com
to register!

Coffee and muffins at 8:15 AM at the Kentville Research Station - bus leaves at 8:30 AM sharp!

Cabbage Maggot Observed Recently

 Cabbage maggot at base of wilted plant.

Cabbage maggot pressure continues to be very high this growing season.  Growers need to apply insecticide at cole crop transplanting and then within 2-3 weeks.  It is important to ensure that PHI is met.  Aside from chemical control, crop rotation contributes to keeping populations low.  Fall tillage to bury crop residues and to expose over-wintering pupae helps reduce overwintering populations.

Floating row covers and insect nettings provide an effective barrier against this pest. Place the cover as soon as the transplants are set.  Do not use where the same crop family -- brassicas or onions -- were grown last year, as flies left in soil could emerge under the cover. Place cover over the crop at the time of seeding or transplanting and seal the edges with soil.

Cabbage Maggot

It seems as though the 2014 growing season is finally underway despite a slow start and quite a bit of cool, wet weather.  The last few seasons have been warmer than average, so development is later this year in comparison to recent years.

Regardless of the cold, wet weather insect development continues and cole crop growers will need to treat for cabbage maggot (click on cabbage maggot in the category list on the right hand side of the page to see earlier posts on the subject). It has been observed that the highest activity of cabbage maggot coincides with full bloom of the common weed, Yellow Rocket.  The picture below was taken in Kings County earlier this week and is a good example of Yellow Rocket approaching full bloom.


For information on treatment options please consult the Guide to Pest Management in Cole Crops 2014.

Insect and Floating Row Covers

Insect covers provide an alternative pest control option for growers.  Insect covers come in different sizes and are suitable for control of a whole host of insects such as carrot rust fly, cabbage maggot, and flea beetles.   Insect covers will create a physical barrier between the crop and flying insects.  It has to be applied tightly so insects are unable to get under the netting.

 

Insect cover on rutabaga

 

Insect covers do not trap heat and therefore are used on cole crops, carrots etc where extra heat might be undesirable.  On the other hand crops such as cucurbits, lettuce and peppers may benefit from extra heat, so floating row covers are more beneficial.

 

Floating row cover on lettuce

 

Cabbage Maggot Control

Cabbage maggot is one of the most destructive pests of cole crops in the Maritimes. The first generation is the most important to control because plants are small and very susceptible to damage.

The best control method consists of preventing the flies from laying eggs. Physical barriers such as row covers or netting of fine screening can be used for this purpose. The research shows that these can be as effective as insecticide application as long as it is placed over the crop immediately after the seeding (rutabaga) or transplanting (cabbage, cauliflower and broccoli).  In order to be effective, there must be no gaps or tears in the material, the material must be placed over the crop before or immediately upon crop emergence or transplanting, and no hosts of the pests should have been grown on that site the previous year. It is also important to bury the edges of the netting so that  cabbage maggot flies cannot get under the edges, and to allow the cover to be a little loose so the plants have room to grow.

Large commercial planting will have to be treated with insecticide for cabbage maggot. For more information please visit: http://www.agrapoint.ca/Pest%20Management%20Guides/Vegetables/2012/Cole%20Crops%202012.pdf

Given that the first generation of cabbage maggot was strong in 2012 it is fair to expect that the second generation which is less severe might be significant.

Rutabaga grown under netting.