Received 18-II-2000. Corrected 10-X-2000. Accepted 10-X-2000
Abstract
The volume of secretion obtained from adult Zonocerus variegatus (Orthoptera: Pyrgomorphidae)was influenced by the type of food plants. Insects fed on leaves of cassava Manihot esculenta, bitter leaves Vernonia amygdalina, and a mixture of M. esculenta and Acalypha wilkesiana gave a good volume of secretion while Chromolaena odorata, Elaeis guinensis, Aspilia africana and Citrus sinensis did not favour secretion production. No significant difference was recorded in the volume of secretion obtained from Z. variegatus from the two seasons irrespective of the food plant. Similarly, food plants gave no significant difference on the volume of secretion between the two seasons.
Keywords
Zonocerus variegatus, secretion, food plants.
Zonocerus variegatus (L) (Orthoptera : Pyrgomorphidae) has a repellent gland which is situated in the 1st and 2nd abdominal segments. It is present in all instars (Idowu 1995). The gland stores secretion in its lumen (reservoir) and discharge of the secretion following any disturbance is a function of the size and volume of the gland lumen (Idowu 1996). The secretion which is ejected in the form of a jet-like spray, is colourless to whitish, volatile, soluble in water, rich in chemical constituents (Idowu and Modder, 1998) and has a very strong odour which is unrelated to the odour of the insect`s food plant (Idowu 1997). Reports showed that the odour of the secretion can be percieved by man from a distance of about 40 cm. Thus, the grasshopper avoid being preyed upon by vertebrate and invertebrate predators (Idowu 1997). ]]>
Z. variegatus is a polyphagous insect (Modder 1986). Nevertheless, it exhibits distinct preferences. However, preference does not necessarily reflect survival. Bernays et al. (1975) and Tamu (1990) have shown that not all the food plants eaten by Z. variegatus are adequate for survival and development. Its food plants include wild plants and cultivated plants (Chapman et al. 1986). For example cassava, Manihot esculenta (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 supports growth while Chromolaenaodorata and Aspilia africana(L) do not support the survival of the insects (Bernays et al. 1975, McCaffery et al. 1978).The possible effect of food plants eaten by dry - and wet - season adult Z. variegatus on the production of its repellent secretion was examined.
Materials and Methods
Newly hatched 1st instar nymphs of Z.variegatus were collected at the uncultivated land adjoining the lake at the International Institute of Tropical Agriculture (IITA), Ibadan. The insects were reared in wire cages (30 x 30 x 45 cm), and fed daily fresh leaves of Acalypha wilkesiana (L) and M. esculenta. The period of collection and rearing was from November to March for the dry season population and April to October for the wet season population.
At the start of the experiment, newly moulted and already milked (emptied gland) adult Zonocerus were picked from the stock cages containing the nymphs of the grasshopper and divided into cages. For this experiment, the dry-season population were fed with M. esculenta (Crantz), A. wilkesiana (L), mixture of A.wilkesiana and M. esculenta while the wet-season population were fed with M.esculenta, A. wilkesiana, A. africana (Thuars), Vernonia amygdalina (Schreb), mixture of A. wilkesiana and M. esculenta, C. odorat (L), Citrus sinesis (L), Elaeis guinensis (Jacq) and Jussiciea abysinica (L). The insects were fed daily on fresh excised leaves of the plants while some were left without food. The choice of food plants was based on their availability in the field during the seasons and their ability to support growth and development as reported by Bernays et al. (1975), McCaffery et al. (1978) and Tamu (1990).
The number of grasshoppers in each of the cages at the start of the experiment was 100. On each day of the experiment, three insects were randomly picked from the different cages, milked and the volume of secretion obtained recorded. At the same time, the number of dead insects in the cages was recorded only for the wet-season population. After the duration of the experiment, the data obtained were analysed statistically using the analysis of variance (ANOVA). Duncan`s multiple range test was also used.
Collection of secretion: A pipette, calibrated in microlitres( m l), was used for collecting the secretion ("milking") from the repellent gland, through the orifice on the dorsal surface of the insect.
Results ]]>
The initial trial effect of plants on secretion using dry-season adult Z. variegatus showed that the plants: M.esculenta, A. wilkesiana and the mixture of M.esculenta and A. wilkesiana all supported secretion production (Table 1). No secretion was obtained on the first day of the experiment. From days 8-12, Z. variegatus fed on M. esculenta recorded the highest mean daily secretion followed by those fed on M.esculenta and A. wilkesiana together while others fed on A. wilkesiana had the least for all the days. However, there was no significant differences in the mean daily secretion of dry-season adults fed on the three sets of plant except on days 6 and 9. Starved Z.variegatus secretion was low; inconsistent and there was no record for secretion from day 7 because of mortality of the starved insects. No death was recorded in the cages of insects with food plants.On the other hand, there was relatively large variation in the volume of secretion of wet-season adult Z. variegatus fed on different plant species (Table. 2). Thus, the plants could be placed into two broad categories (i) those that supported secretion namely M. esculenta, A. wilkesiana, V. amygdalina, M. esculenta + A. wilkesiana and J. abysinica, and (ii) those that did not favour secretion production: C. odorata, A. africana, E. guinensis and C. sinenisis. No secretion was obtained on the first day of the experiment Variation was recorded in the volume of secretion obtained from Z. variegatus fed on plants that supported secretion production as from day 8 with M. esculenta having the highest recorded volume of secretion, A.wilkesiana and J. abysinica had the lowest while V. amygdalina and M. esculenta + A. wilkesiana fed Z. variegatus were intermediate (Table 2). There was a consistent pattern in the volume of secretion,which gradually increased and reached a peak on the 10th day (Table 2). No secretion could be recorded for starved Z. variegatus from day 7-12 and those fed on C. odorata, A. africana, E. guinensis and C. sinensis on days 11 and 12 due to mortality (Table 2). No mortality was recorded in population of grasshopper fed on M. esculenta, V. amygdalina and the mixed plants while mortality recorded for A. wilkesiana and J. abysinica was lower than those recorded for C. odorata, A. africana, E. guinensis and C. sinensis.
A comparison of mean volume of secretion obtained from adult Z. variegatus fed on M. esculenta, A. wilkesiana and mixture of M. esculenta and A. wilkesiana in both dry-and wet-seasons was carried out by means of ANOVA. Statistical analysis showed that there is no significant difference (P>0.05) in the volume of secretion of grasshopper of the two seasons fed on the different food plants except on days 8 and 9. Also, there was no significant difference in the volume of the daily secretion of adult Z. variegatus from wet- and dry seasons fed on the same set of plants when subjected to T-test analysis (P>0.05). Also season and food plants interaction on secretion was not significant (P>0.05).
Discussion
In the present study, a differential effect of food plants on the volume of secretion obtained from adult Z.variegatus was observed. This is in line with the findings of Bernays et al. (1975), McCaffery et al. (1978), and Tamu (1990) that food-plants have differential effect on the growth and post-embryonic development of Z.variegatus.This study has also shown that food plants also affect the production of secretion by the repellent gland of the insect. Plants that supported growth and development of the grasshopper also gave good secretion of repellent fluid, these plants include M. esculenta,V. amygdalina, mixture of M. esculenta and A. wilkesiana; A. wilkesiana and J. abysnica (Table 1,2). On the other hand, the present findings of low secretion and high mortality of Z.variegatus fed on C. odorata, C. sinensis,A. africana, E. guinensis agree with the postulation of McCafferyet.al. (1978) that the survival and development of grasshopper on these food plants used is poor. The results of differences obtained in the volume of secretion from adult Z.variegatus fed on different food plants may be linked to the efficiency of conversion of food materials to body substances. McCaffery et.al.(1978) had earlier reported that Zonocerus fed on M. esculenta had a greater efficiency of conversion of digested food to body substances, particularly during oocyte growth, than similar insects fed with other food plants such as Citrus and Aspilia. The efficiency of conversion of food substances to protein was amply demonstrated by the work of Tamu (1990) who found that Zonocerus fed on M. esculenta, V.amygdalina had more protein in their haemolymph than those fed on A.wilkesiana or C.odorata.
In a related study, the analysis of the repellent secretion of insects fed on different food plants showed that the chemical constituents of the grasshopper`s secretion were similar (Idowu, in press). For example, a major constituents of the secretion, alkaloid was present irrespective of whether the chemical was present or absent in the food plant eaten by the insect. This contrary to the present study in which variation in the volume of secretion by Zonocerus fed on different food plants was observed. This probably indicate that the chemical constitution of the food plants might not have any necessary effect on the suitability of the plants as a favourable food for the production of secretion by thy repellent gland of Z.variegatus.
M.esculenta , A.wilkesiana, C.sinesis and C.odorata are among the commonest plants during the dry- and wet seasons in southern Nigeria. Studies have shown that M.esculenta growing in the field is not an acceptable food for Zonocerus at all developmental stages because of its distasteful hydrogen cyanide content. However, survival of all stages was found to be better, growth was faster and adult produced larger number of eggs within a shorter time when fed on excised cassava (Chapman et.al.1986). Although C.odorata thicket in the field is one of the favoured sites for oviposition by Z.variegatus and the insect is highly attracted to the inflorescenes of the exotic weed (Modder 1984), it is not a suitable food for the growth and the development of the grasshopper (McCaffery et.al.1978). No research work has been done on the suitability of the ornamental plant, A.wilkesiana growing in the field as a food plants of Z.variegatus. Laboratory feeding of Zonocerus on excised A.wilkesiana has shown that the plant supports the survival and the development of the insect (Modder W.W.D., pers.comm.). ]]>
The result of this present work confirms that not only does excised M.esculenta and A.wilkesiana support the survival of the insects, but also they support the production of repellent secretion of Z.variegatus.( McCaffery, et.al. 1978) had earlier suggested that the success of the large dry season population of Zonocerus in southern Nigeria is probably due to the increase in cassava cultivation.There was no difference in the volume of secretion obtained daily from dry- and wet-season adults of Z.variegatus fed on M.esculenta, A.wilkesiana and a mixture and a mixture of the two. The results indicated that the production of secretion by the repellent gland of Zonocerus fed on these food plants is similar whatever the season or the food plants.
The production of secretion by the repellent gland of Zonocerus is an important factor in the survival of the grasshopper as the repulsiveness of the secretion shield the grasshopper from invertebrate and vertebrate predators (Idowu 1997). The rate of recovery of the repellent secretion following a discharge is a key factor in the efficiency of the secretion as a defensive weapon. Thus, it is of a great adaptive significance for the grasshopper to feed on plants such as M.esculenta that will aid the refilling of its resevoir not long after a discharge.
Resumen
El volumen de secreción obtenida de adultos de Zonocerus variegatus (Orthoptera: Pyrgomorphidae) fue influenciado por el tipo de planta de alimento. Los insectos alimentados de hojas de casava Manihot esculenta, hojas ácidas de Vernonia amygdalina y una mezcla de M.esculenta y Acalypha wilkesiana dieron un voulumen bueno de secreción, mientras que Chromolaena odorata, Elaeis guinensis, Aspilia africana y Citrus sinensis no favorecieron la producción de secreción. No se encontró diferencia significativa en el volumen de secreción obtenida de Z. variegatus en las dos estaciones independientemente de la planta de alimento. Similarmente, las plantas de alimento no dieron diferencia significativa en el volumen de secreción entre las dos estaciones.
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1 Dept of Biological Sciences, University of Agriculture, Abeokuta, Nigeria. Fax: 234-39-242700; idowuab@unaab.edu.ng ]]>
