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Preventing Onion Maggots & others w/o Diazinon

Now that granular Diazinon is gone from our midst, what to do to prevent onion maggots and carrot rust fly from attacking!
by Art Drysdale
by Art Drysdale


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

May 15, 2005

Last week it was Abkhazi Garden in Victoria that I visited; this week it was the Finnerty Garden on the campus of the University of Victoria. Above, three shots of parts of the campus, including a gorgeous native Pacific madrone or Arbutus tree (Arbutus menziesii) in full bloom. The next four shots taken near the entrance include 1) two Rhododendron augustinii ‘Electra’, 2) a lovely yellow/orange combination with a couple of giant camas (Camassia leichtlinii) on the right-hand side, 3 )a close-up of a tree peony, with flowers almost dinner-plate size, and 4) ‘Summer Snowflake’ Viburnum plicatum. The next four shots are: 1) azaleas and rhododendrons along with a large number of giant camas all in front of a Magnolia sieboldii, 2) just one of the many ponds that set off the plants in all seasons, 3) an extremely rare evergreen tree (with white flowers in July), Eucryphia x intermedia which I am about to plant (a smaller one!) in my own garden here; Milner Gardens and Woodland being one of the few sources of this fine tree, and 4) a white azalea which stands out like a beacon. The final five shots include: 1) a close-up of the flowers on the Enkianthus campanulatus ‘Palabinii’, 2) a fairly large Choisya ternata in full bloom; an evergreen shrub barely hardy here, but I have planted one of the cultivar ‘Sundance’ in our garden, 3) a delightful purple Rhododendron, and 4) two shots of a large Dove tree or handkerchief tree (Davidia involucrata).

Author photos.



When Alice Bates of “zone 3 in Alberta” wrote with this question, I knew there would be thousands of other gardeners wondering what they were going to do: “I grow a vegetable garden every year and have very good success with growing carrots and onions. I always have to add a granular insecticide to the rows of seeds when I plant them. If I didn't, I wouldn't get any vegetables. Now I'm told that I can no longer get this substance so what on earth am I to do? There must be some thing else that will take its' place. Could you please advise me on this? Thank you.”

Well, Alice, and most other vegetable growers (it’s not just a problem with onion maggots and carrot rust fly, but also 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 before planting the rows and treat the areas around the plant stems immediately after planting. “Given we have a residual 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 will be being tried by many this season.

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), 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 vulnerable 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 allows penetration of full light and rain or irrigation.

So, there is a rather lengthy response to what is likely to be a very common question this year.

Finally this week, this question from Candace, out here on the West Coast (I think) needs far less words as a reply. “Is there something I can do to help eliminate the Wolf Spiders in the basement? I of course vacuum and I spray spider control stuff. My kids hate spiders.”

My response to spider questions is always the same: use Doktor Doom Residual Insecticide Spray (in the bright yellow-labelled can). I have been using it for over four years, and it works very well. The residual effect is at least two months, and it is safe to use. Check for a dealer near you.

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