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Leaf Miner, Rust Fly & Maggots

Just what to do about leaf miner in Swiss chard, rust fly in carrots, and maggots in green onions.
by Art Drysdale
by Art Drysdale

email: art@artdrysdale.com

Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at http://www.artdrysdale.com


June 17, 2007


Here I go boring you again with shots from my own garden, taken just this Friday afternoon. Above are two shots of a beautiful pink/white semi-double climbing rose that I have yet to identify. Below, a little wooden sculpture along with the prolific flowers on some of our firethorn (Pyracantha), and two Japanese iris in bloom on the little island within our large pool. Author photos.

While I was at the Meadowville Garden Centre in Guelph, Ontario on May 12, Mary Ann Waring talked to me about the major problem she was having with a leaf miner attacking virtually all of her Swiss chard. I told her there were a number of approaches being taken to attack that insect, as well as related insects which ruin green onions, carrots etc. I promised her a response, and unfortunately, I only came across her card this week. It is rather late now, but the same question may be on the minds of other growers, so the information may be handy for next spring, or for late-seeded crops.

Well, Mary Ann and most other vegetable growers (this problem is not only a problem with Swiss chard but also onion maggots and carrot rust fly, and maggots ruining cabbage, broccoli and other brassicas as well) there is no easy answer now that the insecticide Diazinon is gone from our marketplace in Canada and the U.S.A. There simply is not an easy-to-use substitute. Let me explain first how these pests ‘operate’. That may help.

Shiny-green, yellow-headed flies (6 mm) lay eggs on soil surface near plants in late spring, early summer. The emerging larvae (maggots) dig down and attack tender roots. They pupate in the soil and emerge as adults in August or September. These mate and lay eggs which hatch into larvae that over-winter in the soil or in the roots (they are found in stored carrots).

The one characteristic these pests have in common is that the initial contact is made by the adult fly. The ‘old’ idea of putting the granular Diazinon into the soil was to get it into the area where the roots would develop and thus it would kill the larvae before they could do any damage to the roots.

One of the prime methods of preventing attack is moving the plantings of these vegetables to entirely different sections of the garden each year. Then, there are no lingering larvae to infest the young plants. However, that is often virtually impossible in small home gardens.

Now, there may still be an opportunity to use a similar method to that of the old granular Diazinon, only now using Doktor Doom House and Garden Insect Spray which contains 0.25% Permethrin, the water based chemical insecticide that was formulated as a copy of the naturally-occurring Pyrethrins. This product, like all Doktor Doom products, does not bear any ‘Poison symbol’ on its label and is safe to use.

In talking with the good Doktor Grigg, he told me that after checking various university Websites, he is suggesting that since the larvae mature on the soil surface at the base of the planted crops, there is a good possibility that Doktor Doom House & Garden Spray could take the place of the granular Diazinon. Apply the aerosol spray to the soil be-fore planting the rows and treat the areas around the plant stems immediately after planting. “Given we have a resi-dual in the soil and that the plants are treated early in planting--this should last for up to a month and it should work.”

Doktor Grigg emphasizes that you should be certain “to make the application to dry soil and stir it in after it is applied--and immediately follow up with planting (or seeding) and then apply at the base of the plants once they are in the ground, or if seeding, as soon as the young plants emerge.

You can be sure that this method has been tried by many last season but I have had no reports aye or nay!

Now to some other ‘solutions’. Just as planting in areas ‘clean’ of larvae from previous years is helpful, it is likewise important to eliminate host-weeds like wild carrots (Queen Anne's Lace), lamb’s-quarters (Chenopodium album), wild parsnips and hemlock in the neighbourhood. Also, seeding early (mid-March) and harvesting in May or delaying seeding until June and harvesting early August will help due to the life cycles of these insects.

Another control that I have learned has been used in the past is to plant carrot rows between rows of leeks, sage or rosemary. These latter will repel the carrot rust fly. Also, in talking with Gord Nickel of the ‘Get Up and Grow’ TV programme, he recalls his father hanging cloths that were dipped in kerosene on sticks along the rows of the vulner-able vegetables. We have no experience with this, but I can well understand it working in much the same way as do the mentioned herbs.

Finally, perhaps the second-best solution, after the use of the Doktor Doom aerosol insecticide, is row covers. My friend Tom Thomson at Humber Nurseries has been suggesting the use of these for several years, because they are an economical, relatively simple, non-chemical solution to insect problems. The material used should be fine cheese cloth or muslin (possibly called re-may-cloth in the U.S.) and it should be applied over each row immediately after seeding, or planting of young transplants. The width of each row cover should be about 45 - 60 cm (18 - 24”) and the edges should be held down with soil to prevent the insects getting into the planted area. Ideally, each row cover should be slightly hooped up using flexible branches or the like (so that there is a clearance of about 15 cm in at the row centre to allow the plants to grow), but even if the material is flat on the ground with lots of spare material to allow the young plants to push it up as they grow, the row covers should work well. They can be removed after about three or four weeks. Keep in mind that cheesecloth does allow penetration of full light and rain or irrigation.

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