What a disappointment to begin pulling carrots out of the soil only to discover that a mining operation has been underway and the long orange tubers are now tunnelled beyond hope. Sure, if the damage is not too bad you can cut the bad areas out without losing too much, but it may put your appetite off altogether if you actually uncover the culprit still at work. Even worse is to miss it when washing your veg and find them floating in the pot as you prepare your meal. Ick!
The insect that uses this modus operandi is as common as the carrots you grow. The carrot rust fly is abundant in North America and is often the most limiting factor for gardeners and farmers who grow carrots as a crop.
Origins and Life Cycle
A small, seemingly innocuous fly, the carrot Rust Fly or Psila rosa, was unintentionally introduced to Canada from Europe in the late 1800’s. With an ideal climate, the pest quickly took off. One hundred and fifty years later, it’s widely distributed throughout the north-eastern states and the Pacific Northwest. Preferring cooler, moister climates, the fly is not found in the drier, more arid regions of central Canada or the U.S.
The adult fly emerges from its overwintering stage as a pupa in late May. The female immediately finds a mate and begins depositing small clusters of eggs around the crown of the carrot or other host plants. Carrot rust flies have a keen sense of smell and can source out carrot seedlings when they have only two leaves. Under ideal conditions, these eggs hatch in 6-10 days at temperatures between 60-70 degrees F (15-20 degrees C). This makes coastal areas particularly susceptible. The emerging larva then burrow and begin feeding on the tiny rootlets, and then into the main tuber as they near maturity.
This stage usually lasts 4-6 weeks in the summer. The carrot rust fly can produce three generations per season: the first adults appearing late May, the second generation adults emerging in late July, and if conditions are favourable, a third generation of adults will emerge in late September to lay overwintering offspring.
The Key to Control
The details of the lifecycle are the essential piece of information gardeners need to know when it comes to monitoring and managing carrot rust fly. By timing your plantings in accordance with their emergence you can diminish the first generation significantly. This in turn will make the second generation less than it would have been. For example, a carrot crop planted in the month of May will host the first generation of female rust fly. Each of those females can lay up to 150 eggs in her life time of six weeks. So, if you can delay your plantings of carrots until the peak of the spring generation has passed, you can avoid an explosion of the second generation.
Monitoring Rust Fly Arrival
The most accurate way to determine when carrot rust fly arrive in your own garden is with the use of yellow sticky cards. In early May, place one card at the canopy level every 10 feet of carrot bed. Check these cards twice a week for adult carrot rust fly. As soon as adults are detected, implement row covers or barriers.
Since each garden has its very own ecosystem, the timing of rust fly will vary from location to location. A great indicator is to take note of what is blooming at the time of their discovery. In the following years, you can use this bloom time as your cue to start management practices.
Effective Cultural Controls
If late seeding is not an option, or there is a plan for successive plantings, then a more intensive approach is necessary. First, carrot rust fly overwinters in the soil of the previous year’s crop, so yearly crop rotation is essential to avoid re-infestation. The farther away you can get from the previous year’s bed the better. They have a number of alternative hosts upon which they can survive (see below), so those crops must also be taken into consideration in crop rotation plans. Certain weed species also provide habitat, so eliminating them around your garden will also help with management.
Alternative vegetable and weed hosts of carrot rust fly
Umbelliferous vegetable crops such as celery, celeriac, chervil, parsnips, parsley, caraway, dill, fennel, wild carrot, and water hemlock.
The flight pattern of carrot rust fly females is generally just above the carrot canopy. Using this to your advantage, you can install simple barriers or walls of floating row cover at 24”, disrupting their ability to access crowns. You can also use row cover to create a small hoop house to enclose the bed completely. A hoop house provides room for carrots to grow and avoids the constant maintenance of raising a row cover when simply rested on the bed.
If carrot rust fly was present in the bed in years previous, there is the chance that emerging adults could be trapped inside the hoop house or barrier walls. If there is this risk, an addition of a yellow sticky card within the crop will capture these random adults.
Install 3′ x 2” x 2” stakes every two feet around the perimeter of the carrot bed. Using a 24” wide roll of row cover, wrap the bed perimeter and staple to stakes as you move from one to the next (except the final stake, since this will provide an access point). Here you will use clothespins to affix temporarily. Insert sticky cards at seedling emergence to avoid cards getting dirty with other flies and debris in the period they are germinating.
Using black poly pipe or similar material, insert three foot lengths into each side of the bed every foot, creating a length of hoops. Insert yellow sticky card. Cover the hoops completely with floating row cover and secure at each end with landscape staple or rocks.
As mentioned earlier, the female adult rust flies source their host through scent. The strong smell of carrots luring them in from a good distance away is how they make their way from one garden to the next. This also makes them susceptible to deception. By planting carrots within a perimeter of onions, garlic, or strong smelling annuals like marigolds, it is possible to distract them away from your crop.
Carrot rust fly is one of the most destructive and difficult to combat pests in the garden, but with a little education and effort, it is possible to limit the damage so you can still enjoy a fresh carrot out of the garden.
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