"And, of course, at the top of the food chain is US as the food manufacturers have no way of taking the systemics out of what ends up on the shelves."

This statement suggests a total ignorance of the pesticide registration process.  Let me try to help with this understanding.  But, be aware that my explanation is terribly abbreviated as a total explanation would take several hundred pages.

The first step is to determine metabolic fates of the proposed pesticide in soils, water, plants and animals.  These metabolites are tested to determine toxicity.  From such studies you know what compounds the consumer is potentially exposed to when the food is consumed.  You next perform toxicology studies on the parent and metabolites of interest.  Next, you take an actual field crop grown in its normal places and treat it with the proposed pesticide.  The crop is harvested as normal for that crop.  The edible parts are then tested for parent residues as well as residues of any metabolites of toxicological interest.  From tox studies you know the safe daily dose of both the parent and its metabolites.  Then from tox studies the safe dose is divided by some rather large number to make sure you are safe.  Generally that large number is something like 100 or 1000.  Years ago it was always 1000 but recently the EPA has changed this safety factor to a lower
 number in at least some cases.  This is defined as the allowable daily intake (ADI).

From residue testing you know the exposure.  The EPA assumes 100% of that crop will be treated and also knows what the maximum of that crop consumed by a human would be and calculates daily exposures for that crop based on those factors.  For the sake of argument assume that the crop is corn and the exposure used up 3% of the allowable daily exposure.  The manufacturer registers that pesticide on specific crops such as corn, tomatoes, potatoes, green beans, etc.  The daily exposures for each of those crops must be determined as outlined above.  These exposures are added up and once the ADI reaches 100% no further crops may be registered.  Use on any other crop is illegal.  This also applies to imported foods.  Only pesticides registered in the US are permitted to be used on imported foods and residues must be consistent with or lower than those on US grown crops.

Note, this is a very conservative way of treating the data.  For instance no pesticide I know of ever has 100% market penetration in any crop.  Thus the ADI calculated for corn overall is higher than reality.  EPA does the calculation this way for the simple reason that for some consumer 100% of what they eat might be treated.  Also, people vary in what they eat a great deal.  Some would rather starve than eat some foods.  So, it is highly unlikely that any individual would ever eat the maximum amount of every single treated crop giving an added safety factor.

Cancer is a common fear from pesticides.  Congress long ago passed a law that simply says any proposed pesticide that can be shown to cause cancer in any test animal can not be registered unless it can be shown that test animal has a metabolic pathway that is uniquely different from the pathways in humans and this unique metabolic pathway results in cancer in the test animal.  Thus such a cancer is not relevant to human consumption risk.  In my personal experience any hint of a cancer in early testing simply kills the product.  No one is going to spend the money going forward when the chances are nearly 100% it is going to be wasted.  In fact it does not even take conclusive evidence of cancer to cause a product to be killed in early development.  All it takes is enough histological evidence of cell changes characteristic of pre-cancers to kill it.

Overall I personally fear long term toxicity issues and cancer far more from the drugs my doctor prescribes me to make me healthy than I fear those problems from any pesticides that might be on food I eat.  All chemicals are poisons at high enough doses and will kill you.  That includes things like water and oxygen.  The most acutely toxic chemicals known are made by nature, not by man in the lab.  There is nothing that is safe in unlimited doses.  But, the regulations on pesticide registration are such that the consumer has nothing to fear as long as they are used according to label directions.  None of these limitations apply to human drugs.  Take statins as an example.  Probably 20% of the group reading this is taking a statin.  Every statin causes cancer in mice at plasma dosages similar to the plasma level aimed at in human treatment.  Does this mean long term use of statins might lead a few people to get cancer?  Probably.  The judgement is the
 good these drugs do in preventing heart attacks out weighs the potential downside of an uncommon cancer.  If statins were proposed for a pesticide use they could not be registered because of this cancer problem regardless of how much the consumer intake would be.  In fact, it turns out that probably 100% of the people who take statins suffer some adverse side effects that impact quality of life.  I sure did.  Had I stayed on them longer than the 10 or 12 years I took them I would likely be dead today.  If still alive I would be an invalid.  I was well on my way to invalid status.  I have never fully recovered and have been off them for 12 years now.

Dick

" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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