> Could the levels fall below the 3.0 pbb and still have caused the kill?

Nope.

An LoD of 3 ppb should have caught a fatal amount of that pesticide.
Perhaps a cocktail of several things was being sprayed.
I'd send more bees in from the freezer and get a full pesticide run done,
but only if there is a clear liable party that can be billed for the
damages.
 
This paper lists many of the listed LD50s for pesticides, and is a handy
reference:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0077550
http://tinyurl.com/opu26z9

In the below, a bee can be assumed to weigh 100mg each, and:
1ppm = 1 x 10^-6
1ppb =  1 x 10^-9
So 
1ppm = 1000ppb, and 
1 gram = 1,000,000 µg 

So ppm is equal to "µg a.i. per gram of insect mass", so each µg / bee
equals 10 ppm.
("a.i." meaning "active ingredient" rather than "artificially inseminated",
don’t let that throw anyone off)

Note in the below that the contact toxicity is higher than the oral toxicity
for Indoxacarb.

EPA Contact LD50 	0.118 µg  / bee  ~ 1.18 ppm ~    1,180 ppb
EPA Oral LD50    	18.52 µg / bee  ~ 180 ppm ~ 180,000 ppb

Agritox Contact LD50 	 0.07 µg / bee ~  0.7 ppm ~     700 ppb
Agritox Oral LD50    	0.194 µg / bee ~ 1.94 ppm ~ 1940 ppb

The paper notes in the chart: "For indoxacarb, contact and oral LD50
information from both the US EPA and Agritox are presented because they were
substantially different."



 	

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