Development of bio-assays and screening for resistance to beet armyworm (Spodoptera Exigua Hubner) in Allium cepa L. and its wild relatives

The beet armyworm (Spodoptera exigua Hubner) is the most important pest in tropical Allium cultivations. All shallot ( Allium cepa L. group Aggregatum ) cultivars are susceptible to this pest. Therefore accessions from three w ild Allium species, namely A. galanthum Kar. et Kir., A. fistulosum L. and A. roylei Stearn, next to A. cepa L. were used to screen for resistance. Fi rst of all, a reliable bio-assay had to be developed. To this end transpare nt plastic cages with in total 5 plants of one accession per cage were plac ed on perlite in a heated greenhouse. Five 3-day old larvae were inoculated on each plant. Eight days after inoculation the number of surviving larvae per cage and the mean fresh weight per larva was determined. The lowest la rval survival (36%) was found on A. roylei. This was not, however, signific antly different from other Allium accessions. Significant differences were found in the fresh weight per larva fed on different Allium accessions. The larvae survived on A. roylei had a very low fresh weight (10.3 mg per larv a), while those on an accession of A. fistulosum had the highest fresh weig ht (45.1 mg per larva). The larval fresh weight on A. roylei was lower than all the other accessions except from the tropical shallot cultivar Bawang Bali. To check whether or not a toxic compound was involved in the resistan ce present in A. roylei, ten accessions from four Allium species were scree ned. Five 3-day old larvae were inoculated on regularly replaced leaf mater ial of each accession of Allium species. No significant differences were fo und in mean fresh weight per larva and mean survival of larvae among differ ent accessions. There were also no significant differences in pupal weight and developmental time. All larvae became pupae 10 days after inoculation. The data indicate that there is no toxic compound present in A. roylei. The se results are underlined by the observation in the greenhouse bio-assay th at A. roylei plants were equally damaged by the beet armyworm compared to o ther Allium species. The results obtained so far therefore suggest that int roduction of resistance to S. exigua via the exploitation of variation for resistance to the beet armyworm in A. roylei is unclear and that genetic en gineering using Cry sequences could provide a way forward.