8342937074

MUHAMAD AWANG, RJTA MUHAMAD AND SYED AZELAN SA. HASSAN

in ihe plants (Malhotraand Khan 1984). However, the possible impact cannot be fully assessed without a thorough understanding of the efFect of pollutant exposure on the plant responses such as peak concentration, duration and frequency of expo-sures, besides the stage development of the insects and plants involved. Moreover, the concentration and time are not directly implicated hence the curves are not proportional to any threshold dose curves for metabolic alterations resulted by the effect (MacLean 1983).

In this study, attempts were madę toassessthe

effects of S0₂ and NOg, either singly or in combi-nation, on the growth of cocoa mirids (Helopeltis theobromaeMiller) utilizing young cocoa shoots as sourcesofnutrient. This insect pest isconsidered to have major ecological and economic impor-tance in cocoa plantations. The concentrationsof S0₂ and NO_t) selected for the study were typical of naturaloccurrence in thisarea (Azman 1990).

MATERIALS AND METHODS

Plant Mateńal

In this study, cocoa seedlings (Theobromae cacaoh.) of the Sabah hybrid variety were used as plant materiał. The seeds were collected from the UPM farm andgerminated in tin tailingsandfor lOdays and then transferred into black polythene bags containingsoil mixturesbasedonJohn Inne^łsPot Composition (J1SC) No. 2 (3:2:1 - soil, organie matter, sand). The seedlings were nurtured for 3 months in a controlled chamber protected from directsunlightduringearlydevelopment. Uniform sized seedlings at 5-6 leaves stage were selected for theexperiment.

Fumigation System

The fumigation chamber used (0.91 m x 0.76m x 0.91 m) was madę of perspex with a wooden base and an iron framework. This chamber was slightly modified from that of Darrall (1982), Horsman and Wellburn (1975) and Ashenden and Mansfield (1977). The temperaturę, relative humidity and movementofairin the chamber was controlled b\ an air exchangeand coolingsystem to simulate the microclimate, outside of the chamber (Muhamad et al. 1987). Air pollutants were introduced and monitored at a controlled ratę to give a constant concentration during the course of the experiment. Mean SOg and NOg concentrations were periodically analysed using the Pararosaniline method and the Golour Developing Reagent method respectively (USEPA 1971).

Treatment

Newly flushing shoot seedlings were placed in a specially designed perspex and nylon net cage. Prior to the treatment in the fumigation chamber, 5nym

at instar 3 from a rearing stock (Rita and KJioo 1983) were placed on to the leaves in the cage. Three typesof treatment were employed, i.e. non-fumigated seedlings with insects (NFIL) as eon troi, fumigated seedlings together with insects (FIL), and non-fumigated insects added after 3 days of seedling fumigation (NFIFL). Each treatment consisted of five replicates and the experiments were repeated three times. Concentrations ofSO_t) and N0₂ either singly or/and in combination were 0.2-0.3 ppm, 0.3-0.5 ppm and 0.03 ppm + 0.03 ppm respectively. Fumigation was conducted for two hours each day. The weight of the cocoa mirids was monitored daily prior to fumigation for relative growth rates analyses (Dohmen et al. 1984) until theyemergedasadults. Thisusually takesabout 10 days.

DataAnalysis

The data were analysed using the factorial experiment in Completely Randomized Design (CRD) in 4 replicates. F-tests were employed to test for significant differences at 5% and 1% levels. Individual significant difference was tested using theDuncanNewMultiple Rangę Test (DNMRT) at P = 0.05.

RESULTS

Fig. 1 shows the progress curves of mean cumulative weight of nymphs subjected to SO_c, and related treatments. Nymphs exposed to SÓ₉ and fed on fumigated leaves (FIL) showed substantial growth compared to the control (NFIL) and non-fumigated nymphs fed on fumigated leaves (NFIFL) at the 0.05% level after two days of treatment. After five days, all the nymphs emerged as adults with cumulatwe mean weights of 0.175 mg, 0.208 mg and 0.155 mg for NFIL, FIL and NFIF respectively. The data clearly indicate that exposure to SC>₂ resulted in a significant inerease in the overall growth of the nymphs. No foliar injury wasobserved on the fumigated shoots. Nonetheless, the relative growth rates (RGR) were not significantlydifferent between treatments throughout the experimental period (Fig. 2). Large initial differences in RGR

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PERTANIKA VOL. 14 NO.l, 1991