I had a few damaged carrots in my bed last year, but didn’t research it to find out what the problem was. This year, my crop was ruined. I don’t think I had six undamaged carrots in the whole bed. (Not the same bed as last year.)
I discovered that the culprit was the worst carrot pest in western Washington State (and many other places, too), the Carrot Rust Fly (CRF), scientifically known as Psila rosae.
This fly is said to be ¼ to 1/3 inch long, with a shiny greenish-black thorax and abdomen, a reddish-brown head and yellow legs, eyes and antennae. The yellowish or white
maggots are about 1 mm long when mature. The pupal stage is enclosed in a slender brown puparium about 1/5 inch long.
It either pupates over winter in a previously infested bed, or after becoming an adult lays its eggs in the soil at the base of the first carrot leaves in a new bed. After the eggs hatch, the larvae eat the
root hairs, and then tunnel into the carrot to
feed, leaving dark rusty-looking horizontal streaks of excrement in the carrot root, then return to the soil to pupate. The longer you leave the carrots in the ground, the more damage there will be.
There are usually three generations of this fly per year in western Washington. The first brood of adults starts flying in early April through mid-June. Larvae feed in plant
roots for about 4 weeks, then move to the soil to complete a pupal stage that lasts 1 month. The second brood of flies emerges in late June through mid-August, and a third generation emerges in late September through early November. Some of this last generation pupate in the soil.
You can monitor the CRF by placing yellow sticky traps (at a 135° angle) around the perimeter of the bed to monitor the arrival of the adults. Monitor the traps daily. When/if you see the first adult CRF on a sticky trap, you can expect to find the larvae in the carrots 25 days later.
There are 107 species of plants that host the carrot rust fly, all of them in the carrot family, and some are food crops (which they can also damage): celery, celeriac, dill and parsley. The experts say that this fly only approaches the carrots when it wants to lay eggs; the rest of the time, it is in the plants outside the carrot beds. And they go on to say that although crop rotation is a good way to reduce infestation, you
should plant at least 1000 yards from an infested bed. That may be fine for a grower with a lot of good
land, but my acre only has about 250 ft in a straight line in the longest direction. And who knows what my neighbors are growing.
Scientists have discovered that carrots produce a compound called chlorogenic acid when stressed by environmental conditions, and the carrot fly is attracted to it, encouraging them to use those plants as a place to lay eggs.
Organic ways to fight the Carrot Rust Fly
Commercial growers are often advised to only plant carrots every other year to break the CRF cycle, and to delay planting until the first wave of adult CRFs have passed.
Some people say to pellet your carrot seeds by dampening them and then rolling them around in fine
wood ashes just before you plant them. Others recommend dusting a light layer of wood ashes over the bed after planting. Why not both? Carrots like potassium. Others advise dusting the newly planted soil with rock phosphate.
Scientists say the female fly uses both sight and scent to locate an appropriate spot to lay her eggs, and a fragrant cover crop may confuse her scent cue. Washington State University did some trials on this, but said those years were low-CRF years, so their results neither confirmed nor denied the idea. But is still may be viable to Interseed a cover crop to prevent CRF adults from finding carrots when they lay their eggs. Possibly useful cover crops could be white clover, crimson clover, subterranean clover, vetch or medic.
Also the cover crops may result in increased numbers of predatory beetles that feed on CRF eggs. The two main predatory beetles of the CRF are Carabidae and Staphelynidae.
There is some indication that soil that is high in organic matter is more at risk from the fly. And I also ran across a note that said “It [CRF] has not generally been a serious problem in carrots grown on mineral soils in upland areas of New York State.”
So, if the carrots were grown organically and had properly mineralized soil, would this be
enough to prevent the stress that causes them to produce chlorogenic acid? Would the CRF look elsewhere to lay her eggs?
The most common solution offered was to plant your seeds in soil where no carrots had grown the previous year, and cover them immediately after planting. Since carrots don’t need pollination when you’re harvesting for the roots (the flowers and seeds form in the second year if the roots are left in the ground), it is a good idea to cover the beds with either floating row covers or covers made of window screening.
If using floating row covers (Reemay or similar), be sure to bury all the edges completely to seal them from entry by the fly. Either set some frames or arched wires to hold up the covering, or leave a large enough pleat so the plant has room to grow to its maximum height. It is pointless to use row covering in a bed that had CRF last year, as they’re likely still in the soil and you’ll be trapping them under the covering. Check under the row covers for weeds, to pull them before they get too large. You’ll also have to watch the moisture in the soil. When the plants are tall, the rain or sprinkler
water may just run off the rounded top of the row cover and wet the edges, not the bed itself. Drip irrigation may be of value here.
You can also use wooden frames with vinyl or wire window screening stapled on all sides except the bottom. Be sure to mound up the soil along the edges to keep the fly out.
It is said that the CRF flies low, only two feet or so above the ground. Some people have wrapped three-foot-high clear plastic around the sides of their carrot beds (but not the top), supported by stakes, with mounded soil around the base of the plastic to foil CRF entry.
You can apply parasitic nematodes in late spring, using the variety
Steinernema feltiae. This nematode attacks and kills its host by transferring a deadly (to the CRF) bacterium to the host, causing the host to stop feeding and quickly die.
There is also a fungus by the name of Beauveria bassiana that occurs naturally in the soil and attacks soft-bodied insects that can be applied to the soil.
Other advice from home growers:
Avoid planting your carrot crop after sod.
Don’t wait to harvest late in the year, and don’t overwinter carrots in the soil.
If you find damaged roots, discard them into the garbage as soon as possible.
The carrot ‘Fly Away’ from Thompson and Morgan is supposed to be fly-resistant, but most people say it gets damaged almost as much as other varieties. In fact, one person reports that she planted alternate rows of Fly Away and a cheap Nantes, and the Fly Away acted like a trap crop for the CRF, and the Nantes was relatively clear of damage.
In Japan, growers spray their carrot crops with garlic as a repellant. It is not 100% effective, but apparently minimizes the damage and confines it to the outer rows, which they find acceptable.”
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