by Rosalie Gillis-Madden
Two new vegetable pests have recently emerged in the Maritimes, Swede midge (Contarinia nasturtii) and Leek moth (Acrolepiopsis assectella), and growers are encouraged to be on the lookout.
Leek moth is a pest of Allium crops, attacking garlic, leeks, onions, shallots, and chives. This pest arrived in Canada in the 1990s and has established populations in Ontario, Quebec, and the northeastern United States. Recently, Leek moth has been positively identified in the Annapolis Valley in Nova Scotia, and in New Brunswick near the Quebec border. There are conflicting reports on whether or not this pest is currently in Prince Edward Island.
Based on life cycle patterns observed in the Ottawa Valley, we can expect three generations in the Maritimes, with Leek moth adults first emerging in mid-April. The second flight is from mid-June to mid-July, and the third flight is from mid-July until the end of August.
Females live for approximately 23 days and lay eggs on the lower surface of leaves or near the base of the plant. Eggs are very small (0.4 millimetres) and hard to see with the naked eye. The larva will hatch four to 11 days later and will start to either mine the leaves (causing “windowpaning” in onions) or will bore through folded leaves to the centre of the plant in the case of garlic or leeks.
Larvae are small (1-14 mm) and it is more likely that growers will notice frass (insect feces) and feeding damage before they will spot larvae. This feeding damage and frass on inner leaves can render leeks unsaleable. Larvae that emerge toward the base of a garlic plant will drill down into the stem and attack the bulb, leaving frass and holes in garlic bulbs.
Swede midge is a pest of Brassica crops (also known as crucifer or cole crops). This gall midge was first found in Ontario in 2000, the first appearance of this pest in North America. It is widely distributed in Ontario and Quebec and in parts of the United States.
This pest has been positively identified in New Brunswick near the Maine border, where it caused significant crop loss. In the mid-2000s, it was suspected that Swede midge had landed in Nova Scotia and P.E.I., but the lack of widespread damage in commercial Brassica fields suggests that this may have been a misidentification.
Growers are encouraged to regularly scout their fields to look for signs of this pest. Adult Swede midges are very small and hard to see (1.5-2 mm) and difficult to distinguish from other gall midge species (for example, Apple leaf-curling midge or Dasineura mali). In Ontario, there are four to five overlapping generations per year, and it would be expected to be similar in the Maritimes.
Adults emerge in mid-May to mid-June and are present until mid-October. Females lay eggs around the growing point (apical meristem) of Brassica crops such as broccoli, cauliflower, cabbage, canola, kale, Brussel sprouts, collards, and rutabagas, as well as Brassica weeds such as Wild mustard, yellow rocket, Shepherd’s purse, and stinkweed.
Swede midge eggs are 0.3 mm, and larvae are 0.3-4 mm, requiring a hand lens to detect. Larvae secrete a substance that breaks down cell walls, and so feeding changes the plant’s physiology. Growers are more likely to notice damage caused by this pest than the pest itself, for example, death of the main shoot or growing point resulting in a blind head or, conversely, a multi-stemmed or multi-headed plant.
Other symptoms include distorted and twisted tissue, swollen or disjointed tissue, and brown corky scarring along petioles and stems.
Swede midges are poor fliers, so when scouting, look for them in protected areas such as along field edges or wind breaks. Once this pest becomes established, it is very difficult to control. This pest is most commonly spread through the transportation of transplants so be sure that your transplant supplier has an active pest management strategy.
In Nova Scotia, Perennia is teaming up with Agriculture and Agri-Food Canada, the Nova Scotia agriculture department, and industry to set pheromone traps to monitor these two destructive pests.
Allium and Brassica growers are encouraged to scout their fields often and look for signs of damage. Should you suspect that you have Leek moth or Swede midge, and are in Nova Scotia, please contact your local Perennia specialist (902-896-0277 in Truro and 902-678-7722 in Kentville).
Growers in P.E.I. and New Brunswick should contact their local agriculture department for assistance.
(Rosalie Gillis-Madden is a vegetable specialist with Perennia Food and Agriculture Inc. based in Kentville, N.S.)