Factors Affecting the Mortality of the Grasshopper, Zonocerus variegatus, in Southern Nigeria

(1) Causes of mortality of Zonocerus variegatus (L.) were investigated, mainly in insects from one locality near Ibadan, Nigeria during the 1975-76 dry season. (2) Egg mortality and mortality of newly hatched insects were negligible. (3) The fungus, Entomophthora grylli Fres. caused extensive mortality of late stage nymphs and immature adults following heavy rain. On some occasions 10% of the population died in one night. (4) Predation by vertebrates was negligible, possibly associated with the aposematic appearance of the insects. First and second instar nymphs were eaten by a range of invertebrate predators, but in these observations their total impact was slight. Army ants sometimes caused dispersal of groups of grasshoppers. (5) Blaesoxiphafilipjevi (Bois.) was found to parasitize large numbers of mature adults in every locality and in every year of observation. Many adults also died without any obvious cause after the first mating and oviposition. It is suggested that the stress of reproduction in relatively unfavourable conditions causes this premature death. (6) Good agreement was obtained between mortality estimated from caged insects caught in the field at 5-day intervals and estimates of changes in the numbers of insects present in the field. Entomophthora caused most deaths before mating and oviposition; reproductive stress and Blaesoxipha accounted for the later mortality.