The airplane was substantially damaged when it impacted trees and terrain during an agricultural application pass. The pilot stated that he had completed one pass and was turning back to make another when he encountered a "thermal." The airplane was approximately 150 feet above ground level in a 45-degree "plus" banked turn at the time. He noted that as he started to rollout, the airplane began to "sink in what seem[ed] to be a thermal." The airplane was about 40 feet above and 250 feet in front of a tree line at the time of the encounter. The pilot stated: "I added full power [and] lowered the nose, [but] was unable to recover from the thermal." The airplane contacted the trees and "with all lift gone and more trees ahead," he elected to land in the cornfield. The pilot reported no failures or malfunctions associated with the airplane prior to the accident.