Science apples that are actually very safe

Published on June 21st, 2011 | by Steve Savage

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Apple Crop for 2009: Safest On Record

The Environmental Working Group (EWG) just published a new version of their criminally misleading, “Dirty Dozen list” based on data from the 2009 crop that the USDA released this spring.  Apples got the questionable honor of topping that list.

I know a lot about apple growers and the Integrated Pest Management techniques that they employ to counteract their formidable, pest foes.  I was pretty sure that EWG had gotten this wrong, so I pulled down the data from the USDA-AMS, Pesticide Detection Program (PDP).  They buy produce samples from commercial sources, and analyze them for pesticide residues.  The EWG got their data from this same source, but EWG didn’t make the effort to look at the raw data (which is fully transparent and available for downloading).  Only by doing that can you see exactly what chemicals were found and at what levels.

What The Data Actually Shows

I was able to find 12 years of apple data between 1992 and 2009, and as I suspected, it shows a drop in the average, tiny, risk associated with the residues (The average “safety factor” is how many times your body weight you would have to eat to consume a toxic dose was ~400,000). The data graphed as a distribution of five year averages (see above) also shows an overall shift towards even lower risk categories.  Note that the largest subset of detections in the 2005-9 data (red line) is at a 10-billion-fold safety factor (you would need to eat 10 billion times your body weight of apples to get a deadly dose).  Note also that there were very few detection in the 10,000 to 10 million-fold safety factor range.   Apples were still plenty safe in 1992 when safety factors as low as 100 were seen, but they are even far safer now.  The US produced 9.7 billion pounds of Apples in 2009.  That would only be enough to deliver a toxic dose of pesticide to 163 people.  Does that sound like the top of the dirty dozen list!

But What About the Really Toxic Pesticides?

The chart above is based on all the categories of chemicals, including those which are not hazardous at any dose.  What about for those insecticides that can represent a risk if consumed at a high enough dose?  When one hears the term “Pesticide,” what comes to mind are the 40-60 year-old cholinesterase inhibitors – the organophosphates and carbamates.  These are neurotoxins that kill insects at very low rates, but they are also rather toxic to animals (some have LD50s as low as 1 to 10 mg/kg).  Its hard to understand, but these products looked like a good option in the 1960s because, unlike DDT, they break down quickly and don’t “bio-accumulate” in the food chain.

Why Cholinesterase Inhibiting Pesticides Were Used On Apples

My grandfather used to say, “its not so bad when you bite into an apple and find a worm.  The problem is if you bite and find half a worm”.  He was referring to the worst pest of apples, the Coddling Moth.  The adult moth lays eggs on the young, developing fruit.  When the larvae hatch they immediately bite a hole in the skin of the apple, spit out that bite, and then begin to mine their way through the fruit as seen in the image above.  This means that to protect the fruit, the insecticide has to be so potent that it can kill the larvae with the tiny dose from the one bite of skin.

In the bad old days, farmers sprayed every 21 days to maintain a continuous coating of insecticide on the apples.  Over time they learned that you can trap adult male moths in traps that contain the female sex pheromone as an attractant.  In this approach you wait until you find moths in the trap and then make a spot spray only in the part of the orchard where you saw the “hot spot.”  Far less insecticide is needed over the season which saves the farmer money.  This IPM system is actually what makes it possible to grow a small percentage of the crop (~5%) as Organic. If the moth ever gets a toe-hold in the Organic section, there is no way to stop it, but if the control program is excellent in the conventional trees surrounding the Organic patch, then the small Organic section can be protected using Pheromone Confusion Technique.  Fortunately, companies are finally discovering safer options to use on the hot spots, so they no longer have to rely so much on the old Cholinesterase inhibitors.

The Chart below shows how the safety factors associated with two of the less toxic Cholinesterase inhibitors over time (The safety factor is the LD50 of the chemical or how toxic it is in mg/kg, divided by the level of the residue, also in mg/kg.  The safety factor is effectively the number of times your body weight you would have to eat to consume a toxic dose).

The chart below shows the safety trends for six other cholinesterase inhibitors.  Note that the average residue for nearly all of them have shifted to a safer level with the exception of Dimethoate which is mainly used to control mites.

 

So, the Environmental Working Group was completely wrong about apples.   They owe the apple industry a big apology, and they should both retract their lists and give back the money it helped them raise.

You can email me at savage.sd@gmail.com.  My website is Applied Mythology.  Apples image from Muffet. Codling Moth picture from the USDA.  Graphs by me.




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About the Author

Born in Denver, now living near San Diego. Agricultural scientist for 30+ years with a Ph.D. in Plant Pathology. Have worked for Colorado State University, DuPont and Mycogen and for the last 13 years consulting for all sorts or companies, universities and grower groups. Experience in biological control, natural products, synthetic chemicals, genetics, GMOs and agronomic practices. Have given multiple invited talks on the interaction between agriculture and climate change (both ways)



  • http://Web Marc Ballat

    Steve,

    one minor comment : organic apple growers have three tools to limit the damage caused by the coddling moth.

    1. Granulosis virus. It must be present on the eggshell that the larvae will consume. Over time, populations have developped resistance to the original strain (discoverd AFAIK in New-Mexico). There are a number of other strains available or in development.

    2. Pheromone confusion : it does not need to be enclosed in an IPM orchard (i.e. an orchard where synthetic pesticides are used). AFAIK, the larger the plot, the better the results. An organic colleague of mine uses it with success on a plot of about 20 acres.

    3. Nets : it does not seem very practical to me and may have other impacts but you can enclose your orchard completely in nets that prevent the coddling moth from entering into it.

    FYI, I have recently seen a film showing nematodes that feed on the wintering coddling moth.

    Kind regards, Marc

  • http://appliedmythology.blogspot.com Steve Savage

    Marc,
    What you describe are some fairly weak tools that “limit damage.” A commercial conventional grower would not qualify for the lucrative international market with that level of control. I would consider 20 acres a small plot. Still, all but the net are part of an integrated solution.

    • http://appliedmythology.blogspot.com Steve Savage

      Eleanor,
      That is right – you would have to eat 3.5 times more. I know that graphs that go up intuitively look like they are getting worse, but what is being graphed is the safety factor so more is good

    • http://Web Marc Ballat

      Steve,

      virus and confusion are what most commercial growers successfully use in Western Europe. I don’t but some of them grow for export and as far as I know they do well financially (better than many of our colleagues in IPM). For the first time in my 4 year experience, I have managed coddling moth control using granulosis virus and have noticed till now less than 1% damaged fruit.

      As you mention it yourself, they are used in IPM as well : growers start with synthetic pesticides and switch to virus later on in order to limit residues at harvest time. If they were that weak, I guess they wouldn’t spend 50-60 $ per acre per spray for the virus and 200 $ per acre for confusion.

      20 acres is the average size for organic fruit farms in Western Europe although it seems that recently existing businesses tend to grow in size. Keep in mind that organic plots are more labor-intensive than their IPM counterparts (thinning, grass removal).

  • http://www.plannedresilience.net/ Eleanor

    So, just to make sure that I understand these last two graphs. Back in ~2003, I would have been able to eat only 100 times my body weight in apples to reach a toxic dose of Methomyl. But now, I would have to eat almost 400 times my body weight in apples to reach a toxic dose of Methomyl. Thus, the amount of Methomyl in apples has decreased substantially over the past several years. Is that how it works?

    For the average person, this might be confusing. If you don’t read this carefully, it looks like the amount of pesticides used has increased over time, not decreased.

  • http://Web Amy

    Steve, you’ve got a very interesting point buried in here: a certain percentage of organic apples can be grown because the vast majority of acres use conventional pest control. Do you think you could explore this “herd immunity” topic further, maybe in another post? I think it’s an important part of the organic vs conventional debate.

    I heard an apple breeder speak a while back who mentioned that growers in Washington are concerned that if the percentage of organic orchards grows much more, it will be difficult for all growers to control apple pests. I’d love to hear your thoughts on this!

    • http://Web Marc Ballat

      Amy,

      have you ever heard of urban myths ? This must be the countryside version ;-) I have read about this chemical-sustained-organic-agriculture once in the past on a site that was obviously pro conventional agriculture and against organic.

      It is in the very interest of an organic grower to keep populations of insects low on their plot in order to preserve yields. An infested organic plot is simply commercially not sustainable.

      In my experience, the number one problem of all (IPM and organic) growers is resistance when it comes to two major pests : apple scab and coddling moth. Mind you, conventional growers have been able to select resistant strains well before the surface devoted to organic agriculture reached 0.01 %.

      Finally, if synthetic pesticides are as efficient as the industry claims and if the percentage of land devoted to organic agriculture is so small, it shouldn’t be a problem, should it ?

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