Carrot Fly: Advantageous Umbillifer PestJanuary 13, 2022
Some of my favourite things to grow in my garden are root vegetables because of their element of surprise. With many other common plants, you can often track their progress by frequent visual inspection. Those that grow underground hold a sense of mystery with only their foliage to give some indication of what’s below the surface. Along with tugging free triumphant parsnips after their long growing season, I’ve also encountered some unpleasant surprises, including experiencing the damage of the carrot fly and discarding substantial parts of my carrot harvest due to this pest.
The carrot fly is a common pest for over 100 umbelliferous plants that are part of the Apiaceae, or celery family. Umbellifers are named after their umbel-shaped cluster of small flowers. Carrots are bi-annual plants so it’s not as common to see their flowers if you grow them from seed because you are likely to harvest the roots before the second year. Plants that are in the carrot family such as celery, celeriac, and parsnips are also susceptible to carrot fly attack. Flies can also sustain themselves 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 regions. The carrot root fly is one of the most economically damaging carrot pests. As with many other pests, the carrot fly larvae are what cause the most damage to crops. Interestingly, a study from Switzerland has shown that climate change and the increase in global temperatures might actually decrease their population since they are more suitable for cooler climates.
Carrot Fly Overview
The carrot fly, Chamaepsila rosae (formerly known as Psila rosae) is a pest found throughout temperate regions of the world. It is also commonly called the carrot rust fly for the rust-coloured frass or excrement that the larvae leave behind as they tunnel through the roots. Not only do the tunnels themselves make the vegetable unsightly, but they also encourage secondary rot. Adults are around 6-8mm long with a shiny black abdomen and thorax, an orange head, and yellow legs. Female flies have abdomens that end in a pointed tip as opposed to the curved shape of male flies. Both males and females have dark transparent wings and typically fly no more than 18 inches above the ground. The eggs of the carrot rust fly are white and oval-shaped; the larvae of the carrot fly are small and yellow.
Life Cycle of Carrot Flies
Depending on temperature and climate conditions, there may be 1-3 generations of carrot flies per year. Carrot root flies overwinter in the soil and emerge from their pupal stage in May and June. They then mate and the female carrot flies will lay clusters of 1 to 3 eggs on the soil surface near their host plants. They are directed 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 tunnelling and eating through the root. It takes approximately 25 days for the larvae to pupate and start the cycle again.
Overwintering pupae emerge as first-generation adult flies in early spring from the soil near susceptible hosts. Low flying female flies can be found near a host plant, typically no more than 18 inches off the ground. These flies lay their eggs just under the surface and young larvae are found inside the root surface. Many carrot-growing regions experience at least two generations.
What Do Carrot Flies Eat?
Carrot flies typically eat the roots of umbelliferous plants. Larvae damage is typically found on the bottom part of carrot roots but on the top or shoulder of parsnips. As larvae mature, the amount of damage they cause also increases. When there is a large infestation of carrot flies in an area, these flies may also impact the lower stems of celery plants. Since there might be multiple generations per year, the overall damage compounds over the course of the growing season starting with the smaller spring generation.
How to Control Carrot Flies
Despite their name, the carrot rust fly can feed on many other host plants and are not carrot specialists. Their range of foods makes them difficult to control because they are not dependent on one. In fact, many market farmers who grow a range of different vulnerable crops have stopped growing carrots because they have not been able to find a cost-effective way to control this pest for their carrot crop. Because of the economic importance of carrots, there are ongoing research efforts to study integrated pest management techniques, companion planting, crop rotation, using floating row cover, and more.
Organic or Chemical Control
Insecticides are difficult to use with carrot flies because of this pest’s behaviour. 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 can decrease the adult fly population through broadcast spraying, which is not an ideal control method.
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 because they provide a physical barrier to prevent adult flies from laying eggs near susceptible crops. The effectiveness of row covers depends on them being firmly secured around these plants as soon as 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 separating the previous plot of carrots and future carrots. The distance required means that the crop rotation method is most likely only applicable for farmers instead of home gardeners.
Adult carrot flies identify their hosts through sight and smell. There have been studies that recommend intercropping carrots with a non-host crop to confound the flies. Some studies have shown that alfalfa could be a good option for companion planting as it also increases the number of predatory insects that can help manage the fly population.
Preventing Carrot Flies
Timing the planting of carrot seeds can have an impact on carrot flies. Since the first generation of flies are active in May and June, delaying sowing carrots until after the first generation has died without an adequate food source for their first eggs can limit the size of the second generation.
Where you plant these crops can also impact the number of carrot rust flies. Because this pest is a weak flier with limited vertical range, it might be beneficial to plant affected crops in an elevated planter at least 18 inches off the ground. A study in Newfoundland, Canada, found that carrots that are planted in exposed areas with heavy winds also saw less damage, likely because these flies are not able to lay eggs and fly stably.
There are currently several carrot rust fly-resistant varieties such as Flyway and Satan. It’s best to plant a non-resistant carrot bed next to resistant ones to serve as trap plants for this pest.