The reign of poison, part I: Rachel Carson strikes back

The modern environmental movement owes a great deal to biologist Rachel Carson, best known as the author of A Silent Spring. When you think of Silent Spring you probably think of the pesticide DDT; what few realize, however, is that Carson had a broader goal. She wanted to rethink not only DDT but all modern insecticides.

“[These] sprays, dusts and aerosols are now applied almost universally to farms, gardens, forests and homes — nonselective chemicals that have the power to kill every insect, the ‘good’ and the ‘bad’, to still the song of birds and the leaping of fish in the streams, to coat the leaves with a deadly film and to linger on in soil — all this though the intended target may be only a few weeds or insects. Can anyone believe it is possible to lay down such a barrage of poisons on the surface of the Earth without making it unfit for all life?”

The book made the public aware for the first time that DDT persisted in the environment and wrought havoc with wild birds; and thus, DDT was banned in less than a decade after Silent Spring was published. But Carson’s quest to restrict all pesticide use never came to fruition, because these chemicals are so useful we have a tough time imagining modern agriculture without them.

Pesticides are poisons by definition — chemicals designed to kill whoever eats our food or competes with our crops. Ideally they are targeted poisons. The perfect pesticide would meet a number of criteria. It would kill the pest swiftly at even tiny doses, but it would have low toxicity to all other species and especially to crops. It would break down rapidly in the environment, but it would linger long enough that farmers didn’t have to apply it every day. Most importantly of all, it would have low toxicity to humans.

Unfortunately, finding a chemical that simultaneously meets all these criteria is a tough problem. Take insecticides, for example. Most insecticides target the insect nervous system, because there’s no quicker and more lethal way to kill an insect (or any other animal) than by disabling its command and control. Plants, naturally, don’t have nervous systems, so a chemical that targets the nervous system should in theory be harmless to crops.

Trouble is, insects aren’t the only critters with nervous systems, and thanks to evolution the nervous systems of humans, birds, fish and insects are more alike than you might think. So there’s a good chance that a chemical which interferes with an insect nervous system…is going to interfere with the nervous system of other species, as one scientist in Nazi Germany discovered when he accidentally invented the first nerve agent. (See my previous post, “Deadly Cloud: The Science Behind the Syrian Chemical Weapons Attack” for more on how that happened.)

This doesn’t mean insecticides are dangerous to you by default. In fact, many insecticides have low toxicity to humans because of how they’re metabolized. Take malathion, for example:

Malathion. As usual, carbon is black, sulfur is yellow, phosphorus is orange and oxygen is red.

Malathion isn’t deadly until it’s converted to another chemical called malaoxon by replacing that double-bonded sulfur atom with an oxygen. Both humans and insects have enzymes that replace the sulfur with an oxygen, thereby turning malathion into a lethal poison. In the human liver, however, we also have enzymes called carboxylesterases which rapidly break the ester bonds in the molecule, rendering it harmless. Because we break down most of the malathion before it gets converted to malaoxon, malathion has relatively low toxicity to us. Insects, by contrast, don’t have the same enzymatic equipment, so the malathion gets converted to malaoxon inside them and they die like bugs. (Some insects have evolved the ability to produce more carboxylesterases and are now resistant.) Malathion also breaks down fast in soil, especially soil with an alkaline or basic pH.

Before you conclude it’s environmentally friendly, however, just remember this: it’s highly toxic to many species of fish, not to mention amphibians. And this is the problem with pesticides: almost inevitably, a chemical that poisons pest species has the ability to poison somebody else. It’s just very difficult to design a chemical poisonous only to pests.

Pyrethroids (found in Raid spray!), for example, typically break down fast in the environment and have very low toxicity to humans, but through some quirk of evolution they’re very toxic to fish and cats. (If a pet flea product says it’s for dogs only, it probably contains a pyrethroid. Some owners have accidentally poisoned their cat this way.) Many of the organophosphates and carbamates are highly toxic to birds; the Fish and Wildlife Service says that pesticides in the US probably kill well over 72 million birds a year. The pesticide Imidacloprid belongs to a class of insecticides called neonicotinoids that resemble nicotine and act in much the same way. Like other neonicotinoids, it has very low toxicity to mammals (hence its popularity in flea treatments) and is much less dangerous to birds, but it’s extremely toxic to bees; in the European Union it’s been restricted thanks to its status as a suspect in honeybee colony collapse disorder (CCD). And so on.

Clearly, pesticides aren’t all created equal; some of them are more benign than others. By their very nature, however, they tend to cause environmental damage. Even the most benign of them take out species we don’t want to kill. And let’s not start on the really nasty pesticides like DDT, which hangs around forever in the environment, or carbofuran, a carbamate pesticide that’s unbelievably lethal to birds, or parathion, an organophosphate that’s basically a weak nerve agent, or methyl bromide, which destroys the ozone layer just like CFCs. As long as pesticides are the weapon of choice in our war with pests, we’re going to have accidents where farm workers get hurt, and yeah, we’re going to damage the environment. And to make matters worse, insects and weeds evolve resistance, so we need to keep deploying new chemicals in the arms race with pests.

Are agricultural pesticides a necessary evil? or can we opt out of the arms race and farm without them? The organic farming movement insists we can. Some organic farmers do use pesticides, although they rely on so-called “natural” agents like pyrethrins. Like artificial pesticides, these tend to be dangerous to somebody or another aside from the targets. The pyrethrins, for example, are the natural version of pyrethroids, and just like pyrethroids they’re highly toxic to fish. But from what I understand many organic farmers eschew even these “natural” pesticides, relying instead on other methods of pest control like crop rotation and insect traps.

The key question is does this work? and the answer depends on which crop you’re talking about. A widely-cited meta-analysis (a compilation of dozens of individual studies) in the journal Nature last year found that on average organic farming is 25% less productive, meaning you get 25% less food per acre, but there’s a lot of variability between crops. For many fruits, organic farming was nearly as productive as conventional farming (an average difference of just 3%). Cereals and veggies are where organic farming got trashed, with yields averaging 26 and 33% lower than conventional agriculture.

So the data suggests that switching to organic farming for specific crops like various fruits might be a smart move; the slight loss in productivity would be a small price to pay for the environmental benefit. Yet for other crops like corn, wheat and soybeans, organic farming would cut yields substantially. You could argue we grow more corn than we need, of course, because a lot of our corn goes to ethanol (thanks, Congress) and cattle (which is a crazy-inefficient use of corn, BTW — the ratio of corn consumed to beef produced is terrible). But the fact remains that organic farming is not the universal solution its advocates claim it to be. It looks like a good solution for a variety of specific crops, but we probably can’t do all our farming organically, because for many other crops it just doesn’t have anywhere near equivalent productivity.

If organic farming can’t replace conventional agriculture across the board, what are the alternatives? We could try to reduce pesticide use by employing other techniques wherever available; and this is the idea behind the philosophy called integrated pest management or IPM. Rather than trying to eradicate pests, IPM argues you should try to control them by understanding their biology, using tactics like releasing sterile males or rotating crops or encouraging natural predators wherever possible, then turning to pesticides as a last resort. When you do use pesticides, you should chose the most benign one that will do the job.

Integrated pest management is certainly smarter than the senseless carpet-bombing Carson decried, but it doesn’t eliminate pesticides, just reduces our use and prioritizes the more benign among them. At the end of the day, even if everybody is living the IPM philosophy (which they aren’t), we’ll still spray insecticides and herbicides. And kill birds. Or fish. Or bees. Or something we don’t want to kill.

Is there another way to realize Carson’s dream? There is at least one possibility — and it’s based on something Carson mentioned in her book, oddly enough. For in the latter half of Silent Spring she devotes some space to a soil bacterium that slaughters insects by drilling holes in their gut. The bacterium was called B. thuringiensis, although this name is so unwieldy that nowadays everyone just calls it Bt.

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Coming up next week in part II: How to genetically engineer a plant, and why genetic engineering is neither “good” nor “bad” (as usual, the truth is more complicated).