A Study of Population Changes in the Grasshopper, Zonocerus variegatus, in Southern Nigeria

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Abstract
Estimates of the Z. variegatus (L.) population occurring in a site near Ibadan, Nigeria were made from 1972-1975. The site was approximately 2 ha in area and comprised a patchwork of cultivated plots, usually containing cassava, and fallow plots. It was relatively isolated from other areas in which large numbers of Zonocerus occurred. Observations were made in other similar sites. Only populations present during the dry season are considered. Most of the insects hatched in Nov. and became adult in Feb. or March, although the precise timing varied. Oviposition occurred in late March and April. The duration of nymphal development was relatively constant. The total population of the site increased 10 .times. from 1972-1973 to 1974-1975, but declined in 1975-1976. In all seasons loss of reproductive potential occurred, mainly through death of the adults early in their reproductive period. This loss is not the key factor regulating variation in numbers from season to season. Mortality during the late nymphal and immature adult stages was critical in determining seasonal variation in numbers. Mortality was mainly due to the fungus Entomophthora, the key factor regulating annual variation. There is no practical value in developing a model to predict the size of hatching populations. Damage is caused mainly by the later instars and the pressure on crops depends on the availability of alternative food sources. Early rain reduces this pressure and increases mortality due to the fungus so that the size of a hatching population is not necessarily directly related to the economice damage it causes.

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