A Scattered Start For Leek Moth In 2019

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Originally published in the September edition of The Grower

Leek moth (Acrolepiopsis assectella) is a destructive pest of Allium species including onions, shallots and chives, but prefers leeks and garlic. All parts of the plant including the leaves, stems, flower stalks, scapes, and bulbs are targeted. Feeding injury in leeks and green onions renders the crop unmarketable while in garlic the moth destroys leaves, bores into the scape and cloves and also creates an entry point for fungi or bacteria.

Leek moth was first detected in Canada near Ottawa in 1993 and has since spread west through Ontario, east towards the maritimes (Québec, New Brunswick, Prince Edward Island), and south into the United States (New York and Vermont). During the 2018 season leek moth was captured throughout Southwestern Ontario and was identified as far west as Lambton and Chatham-Kent counties. So far, leek moth has not been detected in Essex, Elgin, Niagara or Bruce counties.

Managing leek moth once it is established has shown to be difficult. In areas with high infestations, floating row covers are often the most effective pest management strategy. Row covers are generally hard to implement over large acreages and must be applied prior to adult activity in each generation in order to be effective.

The efficacy of insecticide applications is difficult to assess. Leek moth larvae are often found near the plant’s meristem or in the stem or scape which makes it difficult to make contact with an insecticide. As the larvae grows older, it ventures out onto the leaf which makes it more vulnerable to the insecticide application. Not all female leek moths lay their eggs the same day, or in the same week, which makes it even more difficult to time an insecticide application. Population peaks vary based on many factors and this year we found fields that were relatively close to each other had different peaks in the leek moth captures.

The best way to determine the level of leek moth pressure in your field is to monitor using a pheromone trap lined with a removable sticky card (Figure 1). Traps and lures can be purchased from Cooper Mill Ltd (ipm@coopermill.com) and Distributions Solida (info@solida.ca). Order Delta 1 traps with removable liners and mount the trap in the field at the end of April as adult moths emerge from overwintering spots when night temperatures reach 9.5°C. Place at least two traps per field, near the field perimeters, as this is where the highest leek moth numbers are usually observed. Each trap requires at least 14 sticky cards for the season and if the pheromone lure is changed every three weeks, you would require approximately five lures per trap. Extra traps are always a good idea, as traps are sometimes lost or damaged.

Fig1-Adult leek moth on sticky card accented by the tip of a paperclip.
Figure 1. Adult leek moth on sticky card accented by the tip of a paperclip.

This year over 10 conventional and organic garlic and leek fields were monitored in Southern Ontario (Figure 2). There was no clear peak flight of male moths early in the season as there has been in previous years. Many regions in Southwestern Ontario experienced a cool, wet spring which likely influenced the first flight of overwintering adults.

Fig2-LeekMothChart2019.JPG
Figure 2. Leek moth counts from multiple traps in garlic and leek fields in various Ontario counties in 2019. No clear peak flight of male moths early in the season as there has been in previous years.

Insecticide applications are best when targeted at the second generation of leek moth larvae which was between the first week of June to around mid July this year. If the number of overwintering moths trapped was low and minimal damage by first generation larvae was seen in the field, a single insecticide application 10 days after the peak of the second flight would be most effective. If the damage to the crop was moderate to severe the previous year, and/or the trap counts of overwintering moths were high and damage by first generation larvae was observed in the field, two insecticide applications may be necessary. Two applications will target the most larvae if they are applied 3 days after the date of peak moth capture and the second treatment 14 days later. Currently, Matador 120EC, Bioprotec CAF, Delegate WG and Entrust are registered for leek moth management in garlic in Canada. These insecticides are most effective when they make contact with the larvae.

Renfrew County Leek Moth Captures 2019
Figure 3. Leek moth counts at a field site in Renfrew county. With no conventional insecticides applied, the level of captured leek moths doubled in 2019 (purple) compared to 2018 (pink) at a site in Renfrew county.

In 2019 several of the same fields from the previous year were monitored and differences in the adult moth populations were recorded. With no conventional insecticides applied, the level of captured leek moths doubled in 2019 compared to 2018 at a site in Renfrew county (Figure 3). However, exclusion nets at this field site have shown to be quite effective at eliminating insect damage. At a field site in Huron county, two insecticide applications were applied after the second peak in June 2018 corresponding to peak moth counts using sticky cards (Figure 4). Traps counts at this location suggest that the level of leek moths present are much lower here than they were in 2018.

Huron County Leek Moth Captures 2019
Figure 4. Leek moth counts at a field site in Huron county in 2018 (light green) and 2019 (dark green). Two insecticide applications were applied after the second peak in June 2018 corresponding to peak moth counts using sticky cards. Research suggests that timing insecticide applications directly after peak insect captures have the best chance at reducing leek moth populations.

Work is being conducted to evaluate a parasitic wasp as a biocontrol for leek moth. Releases of Diadromus pulchellus in Ontario have been conducted by researchers at Agriculture and Agri-Food Canada (AAFC, Dr. Peter Mason’s lab) over the last decade. Early research shows that D. pulchellus overwinters in Ontario and its impact in reducing leek moth populations in a commercial setting is currently being evaluated.

Always observe a 3 year or more crop rotation and avoid planting near infested areas from the previous season if possible. Collect and destroy any scapes or stems that show any sign of leek moth damage including the pupae (Figure 4). After the crop is harvested, check for small, 2mm-wide holes in the sheath of the garlic bulb and cloves and cull any that you find. Burying plant debris after harvest will also reduce pupae and larvae populations.

If you are a grower of garlic, onions, brassicas, potatoes, carrots, tomatoes, peppers, sweet corn or cucurbits, you might be interested in an IPM Scouting Workshop in Kemptville October 29th and 30th.  Register by calling the Agricultural Information Contact Centre (AICC) at 1-877-424-1300. The garlic portion will focus on production as well as pest management while most of the remaining information will focus mainly on pest identification and management. Agenda with times for the different crops will be released shortly. E-mail travis.cranmer@ontario.ca if you have any questions.

We are also hosting a Garlic Production and Pest Management Workshop this winter in Guelph. Topics include purchasing clean seed, row spacing and crop density, cultivar selection, crop insurance options, choosing the right equipment, crop rotation, processing, leek moth management, garlic viruses as well as information on bulb and stem nematode and other garlic diseases. If you or anyone else you know would like to attend the garlic workshop this December, please register by calling the AICC at 1-877-424-1300.

Garlic Workshop – Production and Pest Management of Ontario Garlic

  • December 4th, 2019 from 9:00 AM – 4:30 PM
  • 1 Stone Rd. West, Guelph, conference rooms 2 & 3
  • Booklet provided; lunch on your own; pay parking available for $12/day
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