The present study investigated the effect of dietary changes on differential and total haemocyte
counts (THCs) and midgut histopathologiocal changes of S. gregaria injected with Bacillus thuringiensis
israelensis (Bti). THCs of Bti-injected adults significantly decreased in all insects fed on all experimental
plants. In Bti-injected locusts, prohaemocytes (PRs) percent decreased in insects fed on grass and quietly
disappeared in other treatments. plasmatocytes (PLs) percent increased, while the granulocytes (GRs)
decreased in all tested insects fed on sorghum, sesban, grass and clover than those of the control. The
percentages of oenocytoids (OEs) increased in all treatments except in insect fed on sesaban. The results
indicated that insects fed on clover had the highest number of haemocytes, while those fed on grass had the
lowest number. Certain pathological symptoms in the midgut of Bti treated adults were observed after 48 hr
post injection, such as (1) disorganisation, disintegration and vacuolization of epithelial cells, (2) lysis of the
basement membrane and surrounding muscles and (3) detachment of microvilli from epithelial cells. The most
deformed group to Bti was grass fed locust. Subsequently, the host plant could be a major factor that affect on
the response of herbivorous insects to pathogens.