Here are a couple of squirmy questions that have recently come to me from home gardeners:

I grow vegetables in raised beds. Each season, I plant three crops of carrots. The first two are very good, with sweet, well-formed roots. The third crop is always infested with some kind of worm. What am I doing wrong? – Ulla, Camano Island.

Ulla, it sounds as though your late crop of carrots is infested with the larvae of the carrot rust fly, a major pest of carrots that may also attack parsnips, turnips, radishes, celery, parsley and dill. They burrow into roots, causing scars and distortions and allowing plant pathogens to enter, causing decay. Heavy infestations can result in stunted, droopy, discolored foliage.

The adult rust fly is approximately a quarter-inch long, with a metallic, greenish-black body and iridescent wings. Legs, eyes and antennae are yellow. Its offspring (the larvae that do the damage) are yellowish or white maggots about a third of an inch long when mature.

These pests pupate and overwinter in areas where host plants were grown the previous season. There are three breeding seasons that likely coincide with your plantings. The first brood of adults emerges in April and continues through June. Females lay their eggs on or just below the soil surface at the base of host plants. The larvae feed on plant roots for about four weeks, then move to the soil to complete a month-long pupal stage.

Summer brood flies emerge from late June through mid-August, producing a second generation of maggots that chomp on your carrots in August and September. Pupae of this generation are present in the soil from August through October, and some of them may overwinter. The third generation of flies emerges from September through early November. The larvae feed during October and November, and can cause extensive damage to roots left in the soil after mid-October.

Carrot rust fly infestations seem to be greatest in soils high in organic matter. Garden beds with a history of rust fly damage are at increased risk, so be sure to rotate your crops next year, and don’t plant parsnips, turnips, radish, celery, dill or parsley in that particular bed, either.

Researchers have also found that gardens surrounded by brush, hedges or woods increase the risk of rust fly infestation, especially if weed hosts such as wild carrot and nettle are present.

Delaying planting until mid-May may minimize fall damage, because many first-brood adult flies are then unable to deposit eggs near suitable hosts, breaking the cycle. Floating row covers also work very well at keeping adult flies out. Extend the row cover at least six inches on each side of the seed rows.

Losses can also be minimized by frequent plantings (four to six weeks apart) and early harvest. Remove all carrots from the soil during fall cleanup, as well as any “volunteer” carrot plants that come up. Deep digging the planting bed in fall or spring is helpful, as well.

Some home gardeners used to drench planting furrows with Diazinon to control rust fly, but that chemical is no longer commercially available. Pyrethrins (insecticides made from plant extracts) may be helpful when used as a soil drench at planting time.

I have also heard of organic gardeners using beneficial nematodes to control root maggots, but don’t have information on how well they work. You can purchase beneficial nematodes at most full-service nurseries. Follow label directions precisely, even when using these “natural” products.

Soldier flies at work

I’ve been composting for about two years, but this spring, big black flies are all around the bin. There are also gross-looking grayish maggots, which I’m guessing are the flies’ offspring. I want to get rid of these disgusting bugs, but I’m afraid that if I spray the bin I’ll ruin all the good compost I’m making. Any ideas? – B.K., Oak Harbor.

Congratulations, B.K.! From your description, it sounds as though you have compost-dwelling soldier flies and their larvae. Veteran composters are happy to have these creatures, since the maggots have an endless appetite for nitrogen-rich material such as kitchen wastes and manures. They break down decaying matter and recycle the nutrients into rich compost.

The soldier fly is black, about five-eights inch long, with black wings that fold over on its back when not flying. It’s sometimes mistaken for a wasp. The maggot it produces is grayish white, segmented, about an inch long. As it matures, it morphs into a flat, dark-brown torpedo with leathery skin, a small head, and no legs. This denizen of decay is also adorned with hairs, spines, and breathing pores on its rear end.

If you want to keep these beneficial, if ugly, creatures out of your compost, be sure to bury kitchen scraps deeply, and blanket with leaves, shredded paper and dry grass. Cover any openings in the bin with window screen to keep flies from laying eggs on exposed compost.

You’re correct to withhold insecticides from your bin. Not only would the chemicals contaminate your compost, but would also kill red worms and other beneficial organisms that help make great compost. If bugs in your bin are still a bother, put a lid on it!

We Master Gardeners enjoy inquiries from fellow plant lovers, even those about maggots. We may not always have the answers at the tips of our tongues, but we have access to the most up-to-date literature, as well as experts on the subject, from Washington State University Extension.

Master Gardener plant clinics are held each Saturday through September at local nurseries and farmers markets throughout Island County. Another option is to call the Master Gardener Hotline, (360) 240-5527 every Monday (except holidays) from 10 a.m. to 2 p.m. Bring us your tired, your poor, your huddled hostas, even your maggots, and we’ll do our best to help

Mariana Graham writes this biweekly column as a volunteer WSU-Island County Master Gardener. Contact her at artsnflowers@hotmail.com.