Some of my favorite things that I grow in my garden are root vegetables because of their surprise effect. For many other common plants, you can often see their progress through frequent visual inspections. Those that grow underground feel a sense of mystery as only their foliage gives any clue as to what is below the surface. In addition to pulling out triumphant parsnips after their long growing season, I've also experienced some nasty surprises, including the damage done to the carrot fly and the discarding significant portions of my carrot crop due to this pest.
The carrot fly is a common pest for over 100 umbellifers, which belong to the Apiaceae or celery family. Umbelliferae are named after their umbel-shaped collection of small flowers. Carrots are semi-annual plants so it is not as common to see their flowers when you are growing them from seeds as you will likely harvest the roots before the second year. Plants belonging to the carrot family, such as celery, celeriac, and parsnips, are also susceptible to carrot flies. Flies can also feed on weed species such as the wild carrot.
Carrot flies are not very common in urban areas, but are more common in agricultural and rural areas. The carrot root fly is one of the most economically harmful carrot pests. As with many other pests, the larvae of the carrot fly cause most of the damage to plants. Interestingly, a study from Switzerland has shown that climate change and the rise in global temperatures could actually reduce their population as they are better suited to cooler climates.
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Carrot fly overview
Chamaepsila rosae, the carrot fly, or the carrot rust fly. Source: Rasbak
The carrot fly Chamaepsila rosae (formerly known as Psila rosae) is a pest found in all temperate regions of the world. It is also often referred to as a carrot rust fly because of the rust-colored grass or the excrement the larvae leave behind as they tunnel through the roots. The tunnels themselves not only make the vegetables unsightly, they also encourage secondary rot. Adults are approximately 6-8 mm long with a shiny black belly and rib cage, an orange head, and yellow legs. In contrast to the curved shape of male flies, female flies have an abdomen that ends in a pointed tip. Both males and females have dark, transparent wings and usually fly no more than 18 inches above the ground. The eggs of the carrot rust fly are white and oval; the larvae of the carrot fly are small and yellow.
Carrot fly life cycle
Depending on the temperature and climatic conditions, there can be 1-3 generations of carrot flies per year. Carrot root flies overwinter in the ground and hatch from their pupal stage in May and June. They then mate and the female carrot flies lay groups of 1 to 3 eggs on the surface of the soil near their host plants. They are guided to their hosts by sight and smell. Each female can lay around 40 eggs in her lifetime. Eggs hatch into larvae within six to ten days and begin to bore and eat their way through the roots. It takes approximately 25 days for the larvae to pupate and start the cycle again.
Common living spaces
Hibernating pupae hatch as first generation adult flies in early spring near susceptible hosts. Low-flying female flies can be found near a host plant, usually no more than 18 inches above the ground. These flies lay their eggs just below the surface and young larvae are in the root surface. Many carrot growing areas have at least two generations.
What do carrot flies eat?
Carrot flies typically eat the roots of umbellifers. Larval damage is usually found on the lower part of carrot roots, but on the top or shoulder of parsnips. As the larvae mature, so does the damage they cause. If there is a large carrot fly infestation in an area, those flies can also infect the lower stems of celery plants. Since there can be several generations per year, the total damage over the course of the growing season adds up, starting with the smaller spring generation.
How to fight carrot flies
The larvae of the carrot flies bore easily through the roots. Source: Rasbak
Despite its name, the carrot rust fly can feed on many other host plants and are not carrot specialists. Their food supply makes them difficult to control as they do not depend on it. In fact, many market farmers growing a range of different susceptible crops have stopped growing carrots because they could not find an inexpensive way to control this pest for their carrot harvest. Because of the economic importance of carrots, there is an ongoing research effort to examine integrated pest control techniques, companion planting, crop rotation, the use of floating row covers, and more.
Organic or chemical control
Because of the behavior of this pest, insecticides are difficult to use on carrot flies. Adults do not linger near their hosts after laying eggs, and their larvae are found underground. Pyrethroids are not effective against eggs or larvae, but they can reduce the population of adult flies by spraying them, which is not an ideal method of control.
Environmental control
Yellow sticky traps can be placed near carrots and other hosts to monitor the adult carrot fly population. Row covers can be effective on a small scale as they provide a physical barrier to keep adult flies from laying eggs near susceptible plants. The effectiveness of row covers depends on their being firmly secured around these plants once the seedlings emerge, before the adult fly population peaks in mid-June. Crop rotation can be effective on a larger scale, with at least 1000 meters between the previous carrot plot and the future carrot plot. Because of the spacing required, the crop rotation method is likely only suitable for farmers, not home gardeners.
Adult carrot flies identify their hosts by sight and smell. There have been studies recommending mixing carrots with a non-host plant to confuse the flies. Some studies have shown that alfalfa could be a great option for planting as it also increases the number of predatory insects that can help manage the fly population.
Prevent carrot flies
The timing of sowing carrot seeds can affect carrot flies. Since the first generation of flies are active in May and June, delaying sowing of carrots until after the first generation dies without a sufficient source of food for their first eggs can limit the size of the second generation.
Where you grow these plants can also affect the number of carrot rust flies. Since this pest is a weak flier with limited vertical reach, it may be beneficial to plant affected plants in an elevated planter at least 18 inches above the ground. A study in Newfoundland, Canada found that carrots planted in exposed areas with high winds also suffer less damage, likely because those flies are unable to lay eggs and fly stably.
There are currently several varieties that are resistant to carrot rust such as Flyway and Sytan. It's best to plant a non-resistant patch of carrots next to resistant ones to serve as a trap plant for this pest.
frequently asked Questions
Carrot rust fly damage includes black tunnels through the roots. Source: Rasbak
Q: Can you eat carrots with a carrot fly?
A: Some slightly damaged root vegetables can be saved by cutting away the rusty brown tunnels. While this is the case for plants grown in your own vegetable garden, it is not the case for commercial growers or farmers who might not be able to sell contaminated vegetables as carrot rust fly damage opens the root to secondary infection and rot.
Q: Where can you find carrot flies?
A: They occur in most of the temperate regions of the world with mild climates and in more rural areas.
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