One of the requirements for an esrog is that it must be edible. If the esrog contains so much pesticide that it would not be edible, the esrog would likely not be considered kosher.
Most of our esrogim come from California and our farmer, John Kirkpatrick has provided us with the following information:
“Lindcove Ranch practices intensive Integrated Pest Management (IPM) in an effort to minimize use of pesticides. We are in full compliance with the regulations of the California Department of Pesticide Regulation (DPR), the most consumer protective statutes and regulations imposed anywhere on the planet. As such, we can warrant and guarantee that if esrogim obtained from Lindcove Ranch are to be eaten or processed into products such as marmalades or candy, they can used with confidence that they completely safe and in full compliance with civil law and regulations of any jurisdiction. This is in addition to the halachic requirements that are supervised and certified by Rabbi A. Teichman.
In the U.S. of A. all pesticides must be registered and approved for their intended uses by the United States EPA and FDA in compliance with ”FIFRA”, the Federal Insecticide, Rodenticide and Fungicide Act. Information regarding FIFRA regulations can be found at this link: https://www.epa.gov/laws-regulations/summary-federal-insecticide-fungicide-and-rodenticide-act
The California Department of Pesticide Regulation provides for considerably more stringent regulation than is required by FIFRA. We conduct all of our agricultural operations, including use of pesticides, in compliance with the most stringent regulations in the world It is my belief that fruit produced in this regulatory regime is the safest available anywhere. Qualified laboratory analysis of fruit harvested before and after pesticide treatments in our pardes have always returned reports that no detectable pesticide residues could be found. This, BTW is better than organic certification which allows for specified maximum residue of many pesticides.
Established Federal and California pesticide residue tolerances contemplate all possible uses. Laboratory and human trial safety limits are stretched from what is considered to be safe by 10 to 100 orders of safety magnitude (or more) depending on the category of possible negative consequences.
Schnapps made with peel of California fruit can be made with confidence that there is no threat to human and animal health or the environment.
Whether this is true for offshore production, I am unable to say. Those growers and their governments will have to speak for themselves. It is my understanding that some foreign kashrus supervisors are requiring certain management practices and certifying as to compliance.”