From Thomas Jefferson Encyclopedia
In Jefferson's time, the grain weevil was found only south of the Potomac River, and it attacked the wheat of Virginia and North Carolina shortly after harvest, unless the crop was immediately threshed and stored with its chaff. Hand threshing was slow and treading with horses in the open field was dependent on the weather, so getting the wheat crop threshed fast enough was tricky and it also took time away from preparation of the next season's crop.
No reference to the weevil has been found after 1796, when Jefferson erected his first Threshing machine, so that presumably the efficient thrashing of the crop allayed the problem.
Primary Source References
1793 April 12. (Jefferson to Thomas Pinckney). "The threshing out our wheat immediately after harvest being the only preservation against the weevil in Virginia, the service you will thereby render [sending a model of the threshing machine] that state will make you to them a second Triptolemus."
1793 August 14. (Thomas Mann Randolph to Jefferson). "It [threshing machine] has probably been of servie in checking the Weevil, which appeared very early, but has scarcely increased fast enough to give alarm."
'1793 September 1. (Jefferson to James Adair). "A very peculiar circumstances in all the country South of the Patowmac, the finest wheat country in America, renders such a machine as valuable as the discovery of the grain itself. If wheat is not threshed out there within 3. or 4. weeks after it is cut, it is destroyed all of a sudden by the weavil. If threshed immediately after harvest and kept in it's chaff, it is secure against that insect."
1795 January 22. (Jefferson to Martha Jefferson Randolph). "...the thermometer has been once as low as 10 degrees and only once in the whole time as high as the thawing point...So much the better for our wheat, and for the destruction of the weavil."
1795 September 8. (Jefferson to Thomas Pinckney). "But those of corn and tobacco are much injured, indded almost ruined, by such continual floods of rain as were never before known. This circumstance too, preventing our treading out our wheat, which is generally done in the open air, exposes that much at this moment to the weavil."
1795 September 12. (Jefferson to George Washington). "The weavil is completely possessed of all the wheat in this neighborhood which is not yet got out, or which is got out & cleaned. I have avoided loss by keeping mine in the chaff, but I will never meet another harvest without a threshing machine."
1795 September 22. "Finish treading wheat at Shadwell. No weavil yet to do injury."
1795 October 4. (George Washington to Jefferson). "I am sorry to hear of the depredation committed by the weavil in your parts. It is a great calamity at all times, and this year, when the demand for wheat is so grat, and the price so high, must be a mortifying one to the farmer."
1815 June 13. (Jefferson to Charles Willson Peale). "Our threshing machines are universally in England fixed with Dutch fans for winnowing, but not with us, because we thresh immediately after harvest, to prevent weavil, and were our grain then laid up in bulk without the chaff in it, it would heat & rot."