Farnesol, at 0.06 × 10⁴ ppm concentration was tested against the fifth nymphal instar of Locusta migratoria induced antifeedant activity by 33.6 % and 45 % to males and females, respectively. Ultrastructural studies, using transmission electron microscopy, revealed marked pathological changes in the neurosecretory cells in brain and corpora allata (CA) cells of treated nymphs. The brain neurosecretion was held inside the neurosecretory cells whose axons did not liberate normally, leading to the accumulation of neurosecretory granules inside. These cells appeared ruptured and irregular. Corpora allata cells had hypertrophied nuclei, numerous lysosomes, malformed mitochondria, Golgi bodies and some other intracellular organelles in the gland cells. Multivesicular bodies were scattered and intercellular spaces appeared within the cell matrix. The haemolymph testosterone and progesterone levels were significantly decreased in the fifth nymphal instar treated with farnesol. Moreover, haemolymph and brain superoxide dismutase (SOD) activity were significantly decreased in brains of treated males and females. On the other hand, haemolymph malondialdehyde (MAD) only declined. By contrast, the brain MDA level was significantly increased in farnesol-treated fifth nymphal instars of both sexes. The ultrastructural and biochemical changes reflect the sensitivity of L. migratoria to farnesol as a neurotoxin bioinsecticide. It appears that the changes in the biochemical macromolecules reflect the ultrastructural alterations obtained. Thus, farnesol can be used as a promising biocide against the migratory locusts as it could be mixed in bait traps in the integrated pest management programme (IPM).