1. Strawberries
Some organic growers joke that conventionally grown strawberries are so full of chemicals, you could grind them up and use them as a pesticide. But pesticides are no laughing matter. Sixty-five different pesticides, fungicides and herbicides are registered for use on strawberries in the US.

Strawberries are the most chemically intensive crop in California. Most commercial strawberry growers use methyl bromide, a toxic, ozone-depleting chemical, to eradicate all fungus, nematodes, microorganisms and weeds, effectively killing every living thing in the soil where strawberry plants are grown. For the remaining growth cycle, the berry plants are drip-fed chemical fertilizers. Because methyl bromide can cause poisoning, neurological damage and reproductive harm, the EPA (US Environmental Protection Agency) classifies it as a Toxicity Category I compound, which is a classification reserved for the most deadly substances it regulates.

Nonorganic strawberries are highly likely to contain pesticide residue after harvest. When the PDP (the USDA's Pesticide Data Program) releases its annual list of produce samples with residues that exceed tolerance levels, strawberries appear more often than any other fruit or vegetable.

2. Red and green bell peppers
More than 50 chemicals, including 10 different organophosphates, are approved for use on bell pepper crops in the United States. Bell peppers are typically treated with insecticides two to six times during their growth cycle, as well as sprayed with herbicides, fungicides, fumigants, nematicides and algaecides. Conventional bell pepper growers often fumigate their fields with methyl bromide before planting to kill weeds and insects. Peppers may also get a dose of Gramoxone Extra (a brand of paraquat), a restricted-use pesticide that has greater acute toxicity to animals than most other herbicides.

3. Spinach
Spinach is relatively simple to grow when the right conditions are present – sandy soil; a long, cool growing season; and adequate water. When conditions are not right, spinach is susceptible to aphid infestations, insect damage and mildew. Because spinach is often grown in less than ideal conditions, conventional farmers use significant amounts of pesticides, fungicides and herbicides. More than 60 per cent of the nonorganic spinach tested by the FDA contains pesticide residue, including DDT, permethrin and other highly toxic pesticides. Several organophosphates are used on spinach crops, including chlorothalonil, a probable human carcinogen.

4. Cherries
Cherries, like most stone fruits, are attractive to many insect pests. Aphids, eriophyid mites, tent caterpillars, webworms and western cherry fruit flies are the major invaders that cherry producers try to fend off with pesticides. Cherries are also susceptible to many viruses and fungal diseases. In order to bring bug- and disease-free cherries to market, many cherry growers spray orchards with a series of pesticides and horticultural oils beginning in the dormant stage in early March and continuing until harvest in June or July. As a result, tests of domestic cherries show the presence of more than 20 different pesticide residues.

Organic growers are prohibited from spraying trees with any petroleum- or synthetic-based pesticide, so they have to make a truce with the wild birds that eat insects that plague cherry trees but often consume the cherries, too. Organic cherry orchardist Lise Rousseau of Bigfork, Montana, says, "It's not impossible to grow an organic cherry, but it does take a little more effort." Rousseau believes that maintaining a healthy tree that can fight off viruses and pests is the best way to grow organic cherries.

5. Peaches
Like cherries, peaches attract many insects, fungi and diseases. They are typically sprayed with assorted pesticides and fungicides on a weekly basis from their dormant stage in March until harvest in July or August. The peach tree borer, a persistent and destructive pest, is often eradicated with endosulfan, a highly toxic pesticide with xenoestrogenic properties.

6. Nectarines
Nectarines, like cherries and peaches, are a magnet for insects and diseases. Something about the sweet, juicy flesh of stone fruits attracts bugs like, well, flies to honey. The same routine for other stone fruits – spraying the trees for several months with various pesticides, fungicides and petroleum-based horticultural oils – is followed for nectarine protection.

7. Celery
The celery plant is essentially a water uptake mechanism, absorbing plenty of toxins from the soil and groundwater in the process. Nonorganic celery is a major potential source of exposure to organophosphates, including the probable human carcinogen chlorothalonil. In FDA tests celery is more likely than any other vegetable to contain pesticide residue -- 82 per cent of the samples tested positive.

8. Apples
This fruit is a polished beauty in the grocery store, but it takes a lot of bug spray, fungicide and horticultural petroleum oil to keep apples so pretty and shiny. Apples are attractive to many kinds of moth larvae, aphids, leafhoppers, mites and various other critters, and are often sprayed five to 10 times during the growth cycle. Some apple varieties are also susceptible to apple scab disease, which leaves brown patches on the skin, and many other fungal diseases. As a result, more than 40 pesticides, herbicides and fungicides are approved for use on apples.

9. Pears
It's pretty clear that insects enjoy fruit just as much as humans do, and pears are no exception. Pear orchards are typically sprayed about nine times during the growth-to-harvest cycle to kill mites, moths, scale, fruitworms and fruit flies. Fungicides, herbicides and petroleum oils are also sprayed on pear orchards to control weeds and diseases. More than 50 chemicals, including several organophosphates, are approved for use on pear crops.

10. Grapes (specifically those imported from Chile)
Table grapes like to grow in just the right conditions – a dry, hot climate, with deep rich soil and plenty of groundwater. Humid conditions can lead to mildew and fungus, while cold temperatures can cause damage to grapevines. The fall and winter weather conditions in Chile are ideal for growing both wine and table grapes, but it's easy for aphids, nematodes, Mediterranean fruit flies and other pests to hitchhike in on imported grapes. That's why the US government requires that all grapes and stone fruits imported from Chile be fumigated with methyl bromide when they arrive at a US port.

Also used on strawberry crops, methyl bromide is classified as a Toxicity Category I compound. Under the Clean Air Act and the Montreal Protocol, two political initiatives designed to protect the environment, the production of methyl bromide, a known ozone-depleting chemical, was supposed to be phased out in January 2005. However, the EPA continues to make regular exceptions that accommodate agricultural users, because "there are no technically and economically feasible alternatives." More than 60 per cent of imported raisins also tested positive for pesticide residues, whereas only 30 per cent of domestic raisins had detectable residues.

11. Raspberries
Raspberries are delightful fresh off the bush, soft and juicy. Those same qualities make raspberries highly perishable and very labour intensive to harvest. And the bugs love them – masses of hungry Japanese beetles, spider mites, aphids and fruitworms eat the fruit, leaves and even the woody canes. Raspberries prefer warm, dry days with cool nights, and cultivating them in undesirable climates can trigger mildew or fungus, causing the fruit to rot on the bush.

As a result of these challenges, commercial growers typically turn to several pesticides and fungicides to kill pests, as well as synthetic fertilizers to grow larger berries. Raspberry samples tested by the FDA have tested positive for residues of up to nine different pesticides. Considering that raspberries are a favourite finger food of many toddlers and small children, it makes sense to be cautious about selecting nonorganically grown raspberries.

12. Potatoes
Potato fields are sterilized before planting with a soil fumigant that kills all of the soil microbes and nematodes. When the potato "eyes" are planted, a systemic insecticide is sprayed over the fields to kill any bugs that may eat the sprouts. A month or so after that, the first herbicide is applied to kill any weeds hardy enough to grow. Because most of the soil nutrients have been eliminated, synthetic fertilizers are drippled into the potato rows every week, like an IV drip of chemical nutrients. Midgrowth, many potato fields are sprayed yet again with the highly toxic organophosphate Monitor to kill aphids, potato beetles and other insects. Finally, to control blight before harvest, potato plants receive successive sprayings of a fungicide containing mefenoxam and clorothalonil, both acute toxins. Given this chemically intensive growth cycle, it's not surprising that a majority of potatoes, especially Russets, test positive for multiple pesticide residues.

Related content:
• Pesticides and your health: How to cut your exposure
• The top 25 healthy fruits
• Food and the environment: Make your grocery shopping greener

Editor's note: While the book To Buy or Not To Buy Organic, from which we excerpted here, is aimed at a US market, we felt that the information was still relevant to Canadians, especially as so much produce in our stores is imported from the United States. Please let us know in the comments if you are aware of any ways in which the Canadian situation differs from what is given above.

Excerpted from To Buy or Not To Buy Organic, copyright 2007 by Cindy Burke. Used by permission of Avalon Publishing Group.
All Rights Reserved. No part of this excerpt may be reproduced except with permission in writing from the publisher.