The variegated grasshopper, Zonocerus variegatus (L) (Orthoptera: Pyrgomorphidae) is widespread in Western and Central Africa. It is primarily associated with the forest regions, but extends into the savannah, where it is restricted to riverine habitats (Chapman et al., 1986). Its polyphagous habits and typical group behaviour contribute to its status as a pest (Chapman et al., 1986).  Z. variegatus is a polyphagous insect and its food plants include a wild range of wide and uncultivated plants, though it exhibits preferences.

 

However, not all the food plants eaten by Z. variegatus are adequate for survival and development. For example cassava, Manihot esculcuta (Crantz) (Bernays et al., 1975; McCaffery et al., 1978; Tamu, 1990) and Vernonia amygdalina (Shreb) (Tamu, 1990) have been shown to support growth and development of Z. variegatus. On the other hand, Citrus spp only support growth while Chromolaena odorata and Aspilia africana (L) do not support growth of insects (Bernays et al., 1975; McCaffery et al., 1978). Z. variegatus thrives very well on C. odorata from the 1st to the 4th nymphal instar. The early instars do not eat cassava in the field, although they may roost on it ( Bernays et al., 1975); it is the later instars that are responsible for damage. Bernays et al. (1975) also found that insects reared on cut leaves of cassava in the laboratory fed readily on the cut leaves, development was more rapid and the resultant insects were heavier than on other food plants.     

 

The purpose of this study was to determine the impact of the different food plants associated with the population of Z.variegatus in the University of Agriculture, Abeokuta on its growth, development and fecundity.

 

 

Collection and maintenance of insects

 

Populations of Z. variegatus resident in grounds of University of Agriculture, Abeokuta were studied for 3 years. Oviposition sites were marked in the previous seasons and the population was monitored from hatching until moulting to 5th nymphs instar. Newly moulted 5th nymphal instars of Zonocerus were collected and maintained in wire mesh cages (47 X 30 X 30 cm) in the laboratory under ambient temperature (29±2⁰C) and relative humidity (79 to 85%). Four cages, each containing 150 5th instar nymphs, were fed different diets of standard food plants, namely cassava (Manihot esculenta), pawpaw (Carica papaya) and acalypha (Acalypha wilkesiana) and mixed diet (a mixture of pawpaw,cassava and acalypha), respectively. Insects were starved for 24 h before being fed their various diet regimens. The growth and development of nymphs were monitored until fledging. Upon fledging, adult insects from different cages were marked with indelible ink for easy identification in accordance with the experimental designs described below. Mated females were separated into oviposition cages with the different tags bearing the name of the respective food plants. The number of eggs laid per individual female were counted and recorded.

 

 

Measurement of body parameters

 

The length of the body, elytron, antennae, femur and the length of the entire leg as well as the body weight of adult Z. variegatus were recorded 18 days after emerging as adults (n=50).

 

 

Dissection of insects

 

Adult Z. variegatus of 18 days old (n=10) from the different cages were dissected for the reproductive structures. The right and left rows of ovarioles and testicular follicles (female and male respectively) of individual insects were counted under a dissecting microscope. The lengths of ovarioles and testicular follicles, including the median ligaments (males), terminal-filament (female), and vagina were measured.

 

 

Haemocytes estimation

 

Thin smears were made from the haemolymph of the adult insects (male and female) obtained by cutting the antennae. These were air dried and treated with Leishman’s stain.  Identification of the cells was made as described by Chapman (1982).

 

 

Proximate analysis

 

Proximate analysis for crude protein, fat, moisture and carbohydrate content of the adult Z. variegatus fed on different food plants was determined by standard methods (AOAC, 1980). The moisture content of food plants was also determined.

 

 

Insect feeding assays

 

From the different cages, 5 adult instars (8 to 12 days old) were removed, starved for 24 h, weighed and placed in small cages with 20 g excised leaves of each of the food plants described above. The insects were observed for 3 h between 10 am to 1 pm, which coincided with the peak feeding period of Z. variegatus (Idowu and Akinsete, 2000).  Insects and plants were then re-weighed and the difference between the initial and final weight of the insects and plants was recorded as weight gained and food consumed by the insects.

 

Developmental period

 

The duration of nymphal development of Z.variegatus fed on the different food plants from 5th instars to adult ranged between 5 to 23 weeks.  Insects fed on pawpaw and acalypha required significantly longer period to develop and to moult to adults than insects fed on cassava and mixed diets. Observation showed that during the period of developing from 5th to 6 h and finally to adult, mortality value of 10, 12, 46 and 50% were recorded in cages containing cassava, mixed diet, pawpaw and acalypha, respectively. Hatching into adult adult instar was first observed in the 5th week in cages containing insects fed on cassava and mixed diets. By the 7th week of feeding, 57 and 33% respectively of 5th instar fed on cassava and mixed diets, respectively, had moulted to adult. Whereas, moulting has just started with only 7 and 1% for insects fed on pawpaw and Acalyhpa, respectively.  Moulting was completed by the 16th week for insect fed on cassava and a mixed diet, while that of pawpaw and acalypha was at the end of the 23rd week. Percentage mortality of emerged adults just before copulation was 2.2, 9.3, 24.1 and 45.7 for cages housing insects fed on cassava, mixed diets, pawpaw and acalypha, respectively. In all overall mortality of grasshopper fed on pawpaw and Acalypha was very high compared to those fed on cassava and mixed diets. Insects fed on excised cassava leaves had the highest percentage food consumption (14.8%) with highest weight gained (8.04%) while insects fed on pawpaw had the least (0.4% and 1.47% for food consumption and weight gained respectively). Mixed diets had 7.8% for food consumption and 7.87% for weight gained, while acalypha had 5.8% and 3.51% for food consumption and weight gained, respectively. Observation during this study also showed that cassava leaves were most preferred even among the mixture of food plants.

 

 

Morphology