Abstract: Present study was conducted to investigate the adaptability of the pest H. nigrorepletus to its host plants with reference to life history statistics under laboratory conditions. Nymphs and adults of H. nigrorepletus were reared on five selected host plants i.e., Oryza sativa, Zea mays, Saccharum officinarum, Desmostachya bipinnata and mixed diet. Feeding on O. sativa (29.85 days), Z. mays (30.48 days) and mixed diet (29.80 days) led to the faster development of nymphs compared to those fed on S. officinarum (41.85 days) and D. bipinnata (42.45 days). Adults laid greater number of egg-pods (3.30`1.82 and 3.30`1.50) per female and production of eggs was also high (78.30`46.19 and 78.40`33.23) on O. sativa and mixed diet, respectively, compared to other host plants tested. H. nigrorepletus showed significant differences in term of duration of life cycle, longevity and fecundity, when raised on various host plants. However, O. sativa and mixed diet proved to be the most suitable host compared to other host plants were tested.
INTRODUCTION
The acridid grasshopper Hieroglyphus nigroreletus I. Bolivar is reported as a major pest of rice, sugarcane, wheat, maize and as a minor pest of millets and fodder crops in Pakistan and India (Roonwal, 1978; Riffat et al., 2007). It was in 1949 that, this pest was reported by Bhatia (1950) to be causing extensive damage to millets in Ajmer Marwara state and in the adjoining areas of Rajastan about 75% of the village covering an area of 112.707 acres of maize and jowar in Ajmer-Marwara state was reported to be infested with this grasshopper and the loss caused by this pest was reported to be six million rupees.
Food plants are known to affect the biology and behaviour of insects including rate of growth and development, survival and fecundity and fertility (Pickford, 1962; Banjerjeet and Haque, 1985; Aslam and Whitworth, 1988). Since grasshoppers are largely phytophagous insects, there have been extensive studies on food selection in grasshoppers and these have been adequately reviewed by Uvarov (1977) and Chapman (1990). It has been reported that plant availability in a habitat is very important in establishing the diet breadth of grasshopper species. However, diet breadth is determined not only by the relative abundance of individual plant species, but also by individual plant species nutritional and pathological condition (Chapman, 1990).
According to Bailey and Mukerji (1976), an insect ability to consume a variety of food plants indicates little with regard to the ability of plants to support growth. Low digestibility of a plant may result in most of the ingested materials being voided in faeces rather than being assimilated. In other words, while a plant must be ingested before growth of the insects is possible, ingestion of the plant does not ensure growth (Fraenkel and Soo Hoo, 1966).
Therefore, the present study is an attempt to record the effects of various host plants on the life history statistics of H. nigroreletus. The results of such studies will be instrumental in understanding and devising population management strategies, which could help avoiding or preventing any possible future outbreak.
MATERIALS AND METHODS
Stock culture: Present study was designed to investigate the effects of various host plants on the nymphal development, fecundity and fertility of H. nigrorepletus. The stock culture consisted of adult specimens mostly collected from cultivated fields having rice, maize, sugarcane and other surrounding vegetation of grasses from different climatic zoon of Sindh province during months of July-September in the year 2005. It was only established to provide regular sources of nymphs and adults for the actual experiment, which was carried out only one generation. The insects were mass-reared in cage measuring (length 16½, width 13 ½ cm) on one selected host plant Zea mays. Green shoots of fresh leaves cuttings were clipped and placed into 50 mL conical flask filled with water.
The cage was kept under laboratory conditions (25°-23/N 68°-24/ E), where temperature fluctuated between the ranged of 28±2°C-39±2°C at morning 9 O’clock but, by the late afternoon the room temperature reached 30±2-41±2°C with the relative humidity ranged from about 26-51 to 28-61% during July to September. These temperature and relative humidity regimes are similar to field conditions. Eggs trays filled with fine sand then water added in ratio of 100:15 sand: water by volume to achieve humidity of 70-80% for optimum oviposition. Nymphs and adults from the egg-pods laid were also used for the subsequent experiments.
Statistics of the nymphal development: Newly emerged (vermiform larva) first instar nymphs were obtained from the stock culture and transferred into glass jars and they were introduced on different host plants such as Oryza sativa, Zea mays, Saccharum officinarum, Desmostachya bipinnata, (as single host plant with ratio of 4.64 g) and Mixed diet (combination of these all host plants with equally ratio of 0.77 g). Thus for each developmental stage at least ten specimens held individually for every host plant. Insects were reared to adult, host plant were obtained from laboratory cultivation, 3-4 weeks old leaves were used for the experiment, these immersed in water and changed daily. Developmental periods were recorded for every instar through to the adult.
Life history statistics of the adult: Newly emerged adult from the stock culture were paired (one male and one female) in glass jars filled with moist sand to one- fifth of the sand capacity to serve as oviposition medium. Ten pairs were treated individually on single host plant and mixed diet (their feeding conditions were identical than that of numphs). Adults were maintained till their death. Pre-oviposition, oviposition period, longevity of insects, egg pods per female and number of eggs per-pod and total production of eggs was recorded.
Statistical analysis: Data obtained from experimental groups were subjected to one-way analysis of variance (ANOVA) (SPSS 10.0 Soft-ware) with repeated measures and significant means were determined using Duncan’s New Multiple Range Test (DNMRT).
RESULTS
Nymphal development: It seems from the Table 1 that as in the first instars of H. nigroreplretus, while feeding on the Z. mays and mixed diet led to the fastest development and was comparable to the stadia of nymphs fed on D. bipinnata however, developmental period of numphs are almost similar when feeding was on O. sativa and S. officinarum as single host plant. In the second instar, developmental period of nymphs was prolonged when feeding was on D. bipinnata and S. officinarum but there was no significantly different in the nymphal duration of hoppers either reared on O. sativa, Z. mays and mixed diet it taken almost equal time to moult in next stage. In third instar, feeding on mixed diet led to a significantly shorter stadium followed by nymphs fed on O. sativa. There was no significant difference in the nymphal development of instars when feeding was on D. bipinnata, S. officinarum and Z. mays.
In the forth instar, feeding on D. bipinnata and S. officinarum led to prolonged developmental period than nymphs were reared on O. sativa and mixed diet followed by Z. mays. While in the fifth instar, nymphs complete their development earlier when fed on O. sativa which is high significant among the nymphs fed on S. officinarum and D. bipinnata. The developmental period also differ significantly among nymphs fed on mixed diet and Z. mays. In the sixth instar, the developmental period did not differ significantly among nymphs fed on either O. sativa or Z. mays than nymphs fed on mixed diet but, it was significantly longer in nymphs fed on S. officinarum and D. bipinnata.
Summing up these findings, it is suggested that in the mean total developmental period no significant differences were observed among nymphs that fed on O. sativa, Z. mays, D. bipinnata and mixed diet. Average duration for nymphal development was found similar on these four host plants but, S. officinarum led to prolonged developmental period. Total nymphal period showed that hoppers complete their development in significantly shorter period when feeding was on O. sativa and mixed diet as single host plant.
| Table 1: | The effects of host plants on nymphal development period of H. nigrorepletus |
| Mean in the same column followed by the same letter(s) are not significantly different from one another at 5% level of probability (DNMRT), ns = Not Significant p≥0.05 | |
| Table 2: | The effect of various host plants on the life history in adults H. nigrorepletus |
| Means in the same column followed by the same letter(s) are not significantly different from one another at 5% level of probability (DNMRT), ns = Not Significant p= 0.05 | |
Life history statistics: Table 2 showed that shorter pre-oviposition period was recorded in the adults of H. nigrorepletus while fed on O. sativa followed by adults kept on mixed diet took (2 days) more. The prolonged pre-oviposition period was calculated in the adults fed on S. officinarum compared with the adults fed on Z. mays and D. bipinnata. Similarly feeding on the O. sativa led to the prolonged oviposition period about (26.10±7.97 days) followed by adults fed on Z. mays and mixed diet while it was significantly reduced when adults fed on S. officinarum and D. bipinnata. This suggested that adult females lived longer when fed on the mixed diet comparable with the adult females housed on the O. sativa and Z. mays. The longevity of females fed on the D. bipinnata was significantly different from those fed on S. officinarum. Adult fed on Z. mays laid the greater number of egg-pods (3.60±1.40) than adults fed on O. sativa and mixed diet. However, the least number of egg-pods laid by females fed on S. officinarum and D. bipinnata. The mean number of eggs per pod was maximum in the adults grow on O. sativa. Adults fed on Z. mays and mixed diet followed this. The least number of eggs per pod laid by female fed on S. officinarum than that of females fed on D. bipinnata. The total eggs production by per female during entire life was greater in the adults fed on O. sativa and mixed diet (78.30) and (78.40), respectively than that of adults fed on Z. mays (58.23) S. officinarum (37.0) and D. bipinnata (48.9).
DISCUSSION
The experiment involving rearing of H. nigrorepletus on various host plants presented singly or as mixed diet, clearly shows that the type of food plant can significantly affect the development of insect. For a single host plant, feeding on O. sativa resulted in the shortest nymphal development for the 5th nymphal instar, while feeding on Z. mays and mixed diet similarly gave the shortest nymphal development period for the 1st and 3rd instars. Contrary to this, D. bipinnata led to the prolonged nymphal development for 2nd, 5 and 6th instars. Thus, variable developmental period could occur in nature, depending on the preponderance of particular food plants in various localities (Nzekwu and Akingbohungbe, 2002). However, in localities where mixed population of food plants these can sever as complements to one another.
McFarlane and Thorsteinson (1980) reported that mixed plant diets are superior for Melanoplus bivittatus (Say), by promoting higher survival larger adults and higher growth indices than any single plant diet. Adams and Bernays (1978) had earlier shown that a range of chemicals having antifeedant properties of nymphs of Locusta migratoria (L.) is additive in its effects. However, Bernays and Bright (1991) provide evidence that individual polyphagous grasshoppers do switch more between dietary items and mix intake more on two different complementary foods, than when the two foods are nutritionally adequate and identical. Fanny et al. (1999) found that in Orya nitidula (Walker), the shortest nymphal periods were obtained when rearing was on Panicum maximum. Adequate diet is expected to reduce nymphal developmental period, which is very important for the fitness and survival of insects (Price et al., 1980).
Experimental results showed that in adults, feeding on the single host plant i.e., O. sativa and Mixed diet led to a shorter pre-oviposition period than when feeding was on S. officinarum, Z. mays and D. bipinnata. This suggests that the former plants have qualities that enable them to promote faster maturation and oocyte development. The mean number of pods per female was highest when the adult females were fed on Z. mays. Lee and Wong (1978) demonstrated that food plants have significant effects on oocyte development in Oxya japonica Willemse, related to the nutritional requirements of the insect as well as the chemical composition and amount of food ingested.
Correct nutrient levels of food play a vital role which triggers the activity in endocrine system for oocyte development. It is generally supposed that the activity with in the endocrine system is generated by the +stimulation of foregut stretch receptors during increased feeding (Hill et al., 1966) during somatic growth fairly large amount of food are ingested in order that fat may develop to a point at which vitellogenic protein synthesis can begin (Mordue and Hill, 1970). Therefore, the optimal amount and quality of food are necessary pre-requisites for the development and production of eggs. McCaffery (1975) reported that ingestion of less than 80 mg dry weight of grass per female per day is insufficient to initiate oocyte development in locusts whose somatic growth was normal.
The total number of eggs per female was greater when the grasshoppers were fed on O. sativa and mixed diet, indicating improved fertility on O. sativa and mixed diet. This shows that the overall nutritional value of the O. sativa and mixed diet in term of adult’s fertility, is superior to that of single host plants.
Significant differences in pre-oviposition, oviposition and post-oviposition period observed in present study might be due to qualitative as well as quantitative differences of food plants used in this investigation. H. nigrorepletus feeding on host plants poor in nutritive value might have delayed the oocyte development resulting in prolongation of pre-oviposition period. The results of present study are correlated with the findings of McCaffery (1975), McCaffery and Hook (1978) and Ullah and Pfadt (1985).
It may be concluded that in H. nigrorepletus, while O. sativa and mixed diet are highly favored for optimum nymphal development and higher fertility certain single host plants could adequately promote adult maturation and egg-pod production. However, in nature, selection pressure may favor habitats with mixed host plants, since these will ensure adequate nutritional requirements for the development and survival of the nymph, a stage that is regarded as the most important with respect to population regulation among grasshoppers (Joern and Gaines, 1990; Lockwood, 1993).
ACKNOWLEDGMENTS
The authors are cordially thanked to Mr. M. Irshad Principal Scientific Officer (NARC) Islamabad for critical reviews of this manuscript. Our sincere thanks are also extending to Prof. Dr. N.T Narejo Department of Fresh water Biology and Fisheries, University of Sindh, Jamshoro for his kind guidance on statistical analysis. This study was financially supported by Pakistan Science Foundation Islamabad for research grants (Project No. PSF S-SU/ Bio 338).