Observations on Aggregation, Migration, and Feeding Habits of Zonocerus variegatus in Ghana (Orthoptera: Acrididae)

Abstract

Zonocerus variegatus L. has only 1 generation a year in Ghana. Nymphal period and adulthood cover approximately 7 months. Development took 1–2 months longer in small cages than in a large cage, probably because the nymphs normally require a fairly large area for activity in nature. Females became sexually mature and mating occurred about 40 days after emergence. In males, spermatozoa were found 20 days after emergence. Females produced 2–4 egg pods, averaging 37 eggs per pod. Zonocerus nymphs roost from evening to next morning, preferably on shrubs with thick and broad leaves; the older the nymphs, the taller are the plants chosen for roosting. Population density, types of vegetation, and microclimatic conditions in an area are important factors for nymphal migration. Zonocerus nymphs were attracted by moving objects within a very short distance and reacted by following them. Once migration was started, mechanical body contact between the resting and marching grasshoppers stimulated the whole migrating crowd to move onward. As a rule, the nymphs walked rather than hopped when migrating. The speed of migration was directly proportional to the number of individuals involved. Within a cassava field, the number of individuals per plant depended on the density of nymphal population on the ground, which in turn was determined by various microclimatic factors. The pattern of upward movement on tall cassava stems was like that of migration on the ground. Adults are solitary and were never seen migrating in masses; thus gregariousness and mass migration were observed only in nymphs. The nymphs were attracted by objects standing vertically within a distance of about 7 cm, but did not discriminate between palatable and non-palatable plants even at this distance. They were also capable of distinguishing the form of leaves from that of stems. The grasshoppers are polyphagous on various forbs, but certain plants were preferred to others. Maximum feeding activity took place between 25° and 30° C, during daylight hours only. In dietary experiments cassava, alone or in combination with false rubber (Funtumia elastica Stapf), gave the best results as to body weight, developmental period, and adult fecundity and longevity; the poorest results were obtained with insects fed Justicia flava. A diet of Gramineae alone produced sterile females, although males were fertile; adults of both sexes reared on such a diet were shorter lived than those reared on forbs.