re:  "I wonder why pesticides are all directed at killing insects, rather than making food crops unattractive to them."

Because that's what the suffix "-cide" means?  (See also regicide, infanticide, fratricide.)
 
On a more serious note, because killing the bug is generally more effective than attempting permanent repellence.
 
1) Persistence - If the farmer can kill the pests (or at least kill enough to knock back the population), then in theory it only has to be done once per season.  If he/she attempts to apply repellents, they must either be noxiously persistent or be reapplied repeatedly through the season.  Otherwise, the bugs will stay away for a few days but come back and decimate your crops after the next rain.
 
My wife attempts this strategy with Liquid Fence.  It smells like baby vomit and she's out there almost weekly trying to keep the deer off her flowers.  It has not been an outstandingly successful strategy.
 
2) Resistence - If we can kill a pest, it is out of the gene pool.  Done properly, you knock back the population without driving too much resistence to the pesticide.  Repellants, on the other hand, would require constant exposure.  I've not seen any studies to prove this but suspect that the pests would more quickly evolve to not be repelled.
 
My wife attempted this strategy a few years back with a cayenne-based spray.  The deer apparently developed a taste for spicy flowers.  (Okay, that's not really resistence but I swear those deer liked it better after the spray.)
 
3) Desirability to humans - Some factors that decrease attractiveness to pests such as a harder shell on the grain can also make it less attractive/useful to humans.  There are some pest-resistent tomatoes on the market.  They thrived where little else could.  I thought they tasted like cardboard, though.  But that's the same balancing act we struggle with when trying to decide between more varroa-resistent Russian bees and higher-producing Italian bees.
 
4) Side-effects - Mosquitos are attracted to carbon dioxide.  You can immediately make yourself less attractive to them by not breathing.  The side-effect of that plan (dying) seems worse to me than a few bug bites.  
 
This is a weak point, perhaps.  Pesticides have side-effects, too, some of them quite severe.  I suppose my point is that repellents are not necessarily any better on this dimension.  Depending on the mechanism, repellents can have adverse side-effects just like pesticides.
 
That's not to say that we shouldn't be researching repellent mechanisms.  A number of folks are doing just that for various crops.  There are very significant hurdles for the strategy, though.
 
Mike Rossander

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