Effects of Food Plants on Development of Spirama retorta (Lepidorptera: Noctuidae)

Development of Spirama retorta (Lepidoptera: Noctuidae) larvae fed on foliage of three Acacia, spp., namely A. mangium, A. auriculiformis and A. crassicarpa, and Paraserianthes falcataria was assessed in the laboratory. The larvae did not survive when fed on either A. crassicarpa or P. falcataria. More than 64% reached pupal stage when fed on A. auriculiformis and A. mangium. The larval period was completed in 22.10 and 24.83 days when the larvae fed on A. auriculiformis and A. mangium foliage, respectively. The average pupal period xuas 10.51 and 1132 and, the resulting adults lived for 36.51 and 37.94 days on A. auriculiformis and A. mangium, respectively. Even though the overall development variables were not significantly different, females from larvae fed A. auriculiformis had a significantly higher fecundity than those females from A. mangium. A total of 412 eggs// was recorded from those fed A. auriculiformis as compared to 255 eggs// on A. mangium. This study thus shows that foliage of A. auriculiformis and A. mangium provided a suitable diet for S. retorta larvae. As such, these species may serve as alternative food resources important in the population dynamics of the moth in the absence of indigenous host plants. INTRODUCTION The declining supply of timber from natural forests has led many tropical countries including Malaysia replenish their timber resource by adopting a reforestation program involving planting of fast-growing exotic species. In Malaysia, about 500,000 ha of unproductive forest has been alienated for establishment of forest plantations. To date, approximately 100,000 ha of forest plantations have been established. Ninety percent of this was planted with Acacia mangium. While Gmelina arborea and Paraserianthes falcataria were planted on a smaller scale. A. mangium is a fast growing leguminous tree indigenous to Northern Australia, Papua New Guinea and Irian Jaya (Anon. 1983). The AHMAD SAID SAJAP and ABD. KARIM ABD. SAMAD tree can grow up to 30 m high with a straight bole measuring 40 cm in diameter at breast height. It can be harvested for pulpwood in five to seven years or sawlog production in 12 to 15 years. In addition to A. mangium, A. auriculifarmis and A. crassicarpa have also been the subject of many researches in Malaysia. Results from provenance trial plots indicated that these trees have the potential to be grown for commercial plantations (Nor Aini et al. 1994; Kamis et al. 1995). Even though trees like Acacia often perform very well when grown as exotics, they are, however, prone to attack by diseases and insects. To date, many indigenous insects have been reported to be associated with these Acacia and some could pose serious threats to the plantations (Abe 1983; Hutacharern 1993; Chey 1996, Sajap et al 1997). One of these insects was a rare moth, Spirama retorta (Lepidoptera: Noctuidae). The larvae of this insect were found in an outbreak where they defoliated a one-year old A. mangium stand in an area of 800 ha at Gunung Besaut Forest Plantation, Sungkai, Perak. The biology of this insect was described by Sajap et al. 1996. Apart from A. mangium, no other host plant has yet to be associated with this insect in Malaysia. Albizzia lebbek was the only recorded host plant elsewhere (Beeson 1961). In this study, we examined the suitability of three Acacia spp. and P. falcataria for the development of S. retorta. Paraserianthes falcataria was included as it was related to the reported host plant, A. lebbek. This information is pertinent in determining the host range of the insect in view of its becoming a potential pest of Acacia spp. MATERIALS AND METHODS Insect Rearing A colony of S. retorta was established from larvae collected from A. mangium plantation at Gunung Besaut, Perak. The larvae were kept in 11 x 15 x 25 cm plastic boxes provided with fresh A. mangium foliage. The foliage was changed everyday. When the larvae reached the fifth instar, vermiculite which acted as a substrate for pupation, was added into the box. The pupae were collected, sexed and surface-sterilized with 1% sodium hypochlorite. Five pairs of male and female pupae from the same cohort were held in a cage for adult emergence, subsequent mating and oviposition. The oviposition cage consisted of a cylindrical wire mesh, 9 x 12 cm, internally lined with a netting cloth which was used oviposition site. Ten percent honey solution in a cotton-plugged vial and a slice of very ripe papaya were placed in the cage and served as food sources. Eggs collected from the colony were kept in 9 cm petri dishes for hatching.