Insects and controls

I saw the first Japanese beetle about two weeks ago.  I presumed these were the advance guards of the coming hordes.  But so far I see only the occasional beetle.  A possible explananation is the heavy spring and early summer rains have literally drowned the emerging beetles.  This happened in 2008, and the beetle populations were suppressed in ’08 and the following year.   2010 was a bad beetle year.  If the rains got ‘em this year, I’m not complaining.

As a former medicinal chemist who did research at a drug company, I want to add my observations about pesticides.  A lot of people think organic food means no chemicals but the fact is that living organisms and laboratories both make chemicals.  In fact everything is made of chemicals.  And just because a useful chemical is made by a plant or fungi does not mean that it is not toxic.  So if I don’t use any synthetic chemicals to control pests in the garden, what does that really mean?

First of all here’s a look at the infamous DDT, now banned in most countries.  It was made famous in Rachel Carson’s book , Silent Spring.  DDT is classified as a Persistent Organic Pollutant, meaning it doesn’t decompose, it goes into the fatty tissues of animals, it persists in the food chain and is concentrated as it travels up the food chain, which is why it was so deadly to apex predators like the Bald Eagle.  Here’s the structure: 

For those not familiar with chemical structures, every point where two or more lines intersect or a line ends represents a carbon atom, unless there is a symbol for another atom.  The lines represent chemical bonds.   Notice that DDT has five chlorine atoms.   

Here’s an insecticide made from natural products, pyrethrin.  It actually works on insects in much the same way as DDT, blocking sodium channels in neurons.  Compared to DDT, it has a more complex structure and has no chlorine atoms.  Unlike DDT, pyrethrin is quickly hydrolyzed in the stomach, rendering it nontoxic by ingestion.  Also unlike DDT, it breaks down in a few days when exposed to light or oxygen.  It’s  not completely harmless.  Pyrethrin will kill good insects like bees and bad insects alike.  It is toxic to fish which absorb it through their gills and can be absorbed through the skin if exposed to large quantities.  This is the structure.  The R represents either a methyl group or an ester group.

  

Synthetic derivatives of pyrethrin are more toxic.  Here’s the structure of permethrin.  Notice the chlorine atoms. 

A synthetic insecticide that’s used a lot in gardens is Sevin or carbaryl.  There’s a lot of inconclusive data about carbaryl, and in 2009 it was reclassified by the EPA as a likely human carcinogen based on tumor formation in rats that were fed large amounts.  It’s a neurotoxin and has shown mutagenic effects, as well as indications of birth defects.  It is readily absorbed by the skin and is more toxic by inhalation or skin absorption than by ingestion, where much of it is broken down in the stomach and liver.   A number of countries have banned it’s use by homeowners.  It’s a compound that appears to have nasty cumulative effects but no outright lethality, clouding the issue of regulation that runs counter to economic interests. 

Carbaryl does break down in soil, water, and air, but not especially fast, and some of the breakdown products are highly toxic.  Spraying large quantities of it has resulted in some groundwater contamination.  Can it be used safely by the home gardener?  Possibly, in small quantities, with proper precautions, as a last resort,  it can.  Does the average homeowner take those precautions? 

Which brings me to the last compound, a molecule that has been approved in California for use on strawberries – methyl iodide.  The point I made about DDT is that it has a number of chlorine atoms in it’s structure.  Chlorines are in a group of chemicals called halogens, which also include flourine, bromine, and iodine.  Most organic compounds with halogens are toxic.  Many are very toxic.  Chlorine in particular is a bad actor because organic molecules with chlorine, like DDT, are both stable and toxic.   Molecules with bromine and iodine tend to be more reactive, meaning they will break down, but they also are more toxic. 

Methyl bromide, which is very toxic, was used for years as a fungicide on strawberries, then it was shown to degrade the ozone layer.  It was found that methyl iodide does not break down the ozone layer, and it was approved for use on strawberry fields in California. 

Now I can tell you that as a synthetic chemist working in a lab with fume hoods that constantly pulled air through them, with any protective clothing I needed, that there was a short list of chemicals that I just did not want to use.  Methyl iodide was one of them.  It was always stored in the refrigerator and carefully withdrawn by syringe inside a fume hood.  And now it’s been approved to spray in quantity on fields.    

The good news for the backyard gardener is that natural insecticides like Pyrethrin and Neem are less toxic than older synthetic ones.  They break down in the environment.  Good gardening practices that minimize the need for insecticides and careful applications of the safer products when needed should not pose any hazards to the user.   You can buy all kinds of insecticides off the shelf in the US.  It’s up to the grower to make the judgement on what and what not to use.