Can silicon complement mutation-derived resistance to cowpea aphid?

This study investigated whether silicon applied as a drench to cowpea (Vigna unguiculata L.) could complement mutation-derived resistance to cowpea aphid (Aphis craccivora Koch). Lutembwe is a cowpea variety widely grown in Zambia but is susceptible to aphids. Genotypes LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12 are cowpea mutants derived from Lutembwe using gamma irradiation and have been identified as having partial resistance to cowpea aphid. Silicon accumulation capacity of cowpea genotypes and aphid performance parameters including colony growth, mean relative growth rate and feeding behaviour were assessed. Where silicon was applied to plants significantly higher silicon concentrations were recorded in LT 3-8-4-1, LT 4-2-4-1, LT 11-3-3-12 and Lutembwe genotypes compared to untreated plants. Silicon application on Lutembwe resulted in slower aphid colony growth and lower mean relative growth rates compared to untreated plants. Electrical penetration graph recordings of aphid feeding on silicon-treated Lutembwe plants showed that silicon-derived aphid resistance is mediated by phloem-based resistance factors. Silicon application to LT 3-8-4-1, LT 4-2-4-1 and LT 11-3-3-12, however, did not enhance aphid resistance. This may be due to the partial resistance of mutation-derived genotypes masking any benefits from silicon application to these plants. These results indicate that silicon application or use of mutation-derived genotypes may be effective tools with which to manage aphids on cowpea, but there appears to be little benefit of combining these approaches. This has important implications for developing an integrated pest management framework for cowpea aphid.