Sains Ma1aysiana 28: 39-47 (1999)                                                                                                       Sains Hayat/

                                                                                                                                                                        Life Science

Sources of Possible Inoculum for Horizontal Transmission

of Nosema bombycis in Diamondback Moth,

Plutella xylostella (L.)

Idris, A. B

Pusat Pengajian Sains Sekitaran & Sumber

Fakulti Sains & Teknologi Alam

Universiti Kebangsaan Malaysia

43600 UKM Bangi, Selangor D.E. Malaysia

E. Grafius

Department of Entomology

Michigan State University

East Lansing, MI 48824-1115 USA

ABSTRACT

A study to identify possible sources of inoculum for horizontal transmission (within generation) of Nosema bombycis spores in diamondback moth (DBM), Plutella xylostella (L.), larvae were conducted. The body excretions (frass, silk, regurgitate and exuviae) of DBM larvae, and meconium of newly emerged DBM adults reared from Nosema-infected larvae were examined for N. bombycis spores. The presence and relative abundance of spores were observed microscopically. The effect of Nosema-contaminated Diadegma insulare (Cresson) on parasitism of DBM larvae was also investigated. Percentage infection and mortality of DBM larvae were used as evidence for the parasitoid's role in spore transmission among neonate DBM larvae. Results showed that all body excretions and the exuviae had N. bombycis spores or can be sources of Nosema spores. This suggests that these excretions are sources of N. bombycis spores for horizontal transmission. The spores were relatively more abundant in the late DBM instars than in the early instars. Percent infection and percentage mortality of DBM larvae parasitised by the Nosema-infected D. insulare was significantly higher when parasitism occurred in the early instars than in the late instars. Methods of avoiding N. bombycis infection in laboratory rearing of DBM are suggested. The importance of D. insulare as a major source for Nosema transmission and its possible diminishing role as biological control agent of DBM in the field are also discussed.

ABSTRAK

Satu kajian bertujuan mengecam sumber-sumber yang berkemungkinan terlibat dalam pemindahan mendatar (dalam generasi yang sarna) spora-spora Nosema bombycis dalam larva rama-rama belakang intan (DBM), Plutella xylostella (L.), telah dijalankan. Spora Nosema telah diperiksa dalam bahan buagan badan (tahi, bebenang, muntahan dan kulit salinan larva DBM) dan mekonium DBM dewasa yang baru menjelma (di temak daripada larva DBM yang telah dijangkiti Nosema). Kehadiran dan kelimpahan relatif spora-spora N. bombycis telah di lihat menggunakan mikroskop. Kesan parasitisme Diadegma insulare (Cresson) yang dikotori oleh spora Nosema terhadap jangkitan dan mortaliti larva DBM telah juga dikaji. Peratus jangkitan dan kematian larva DBM telah diguna sebagai bokti kemungkinan parasitoid memainkan peranan dalam pemindahan mendatar (antara larva DBM segenerasi) spora-spora N. bombycis. Keputusan kajian mendapati bahawa semua bahan buangan badan larvae dan exuviae mengandungi spora Nosema atau berkemungkinan bertindak sebagai sumber spora Nosema. Ini menunjukkan bahawa bahan buangan badan tersebut adalah sumber spora Nosema yang berkemungkinan bertanggung kepada berlakunya pemindah mendatar spora-spora N. bombycis. Kelimpahan relatif spora-spora adalah lebih tinggi dalam instar-instar akhir DBM jika dibandingkan dengan dalam instar-instar awal larva DBM. Peratus jangkitan dan mortaliti larva DBM yang di parasitkan oleh D. insulare yang dijangkiti oleh Nosema adalah lebih tinggi dan signifikan bila parasitsme berlaku pada instar awal DBM jika dibandingkan dengan instar-instar akhir DBM. Kaedah-kaedah untuk mengelakan berlakunyajangkitan N. bombycis dalam kultur DBM di makmal ada disarankan. Kepentingan D. insulare sebagai sumber utama dalam memindahkan spora Nosema dan kemungkinan berlakunya pengurangan peranannya sebagai agen kawalan biologi DBM di lapanganjuga dibincangkan.

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