Biocidal action of silver oak (Grevillea robusta) leaf extract on the termite Heterotermes indicola Wasmann (Blattodea: Rhinotermitidae)

The subterranean termite Heterotermes indicola (Blattodea: Rhinotermitidae) is a serious pest to wood in service causing significant annual economic loss in Southeast Asia. In the present study, silver oak (Grevillea robusta A.Cunn.) (Proteaceae) leaf extract was investigated for its termiticidal properties against the termite H. indicola in laboratory tests. An ethanol-solvent system was used to remove the extract from dry leaf powder via Soxhlet apparatus. A dose-dependent foraging response and mortality was observed after continuous exposure to a dose range of 1-20 mg/mL of extract based on dry extract weight. Results indicated that maximum ( > 90%) termite mortality occurred at 20 mg/mL along with a significant reduction in termite tunneling activity. An LC so was calculated at 5A7 mg/mL using the concentration series data. In filter paper antifeeding and repellency bioassays, a significant reduction in the total population of gut protozoa compared to untreated and negative controls (solvent) was observed. After 2 weeks of no-choice continuous feeding on filter paper treated at 20 mg/mL, hindgut protozoan population was reduced by 62.90%. The extract conveyed resistance to the non-durable cotton wood and chir pine after vacuum pressure treatment. At the highest concentration tested, 100% mortality was recorded after exposing termites to cotton wood and chir pine treated with silver oak leaf extract. HPLC-DAD-ESI-MS results of the extract showed high level of phenolic compound, alkyl-resorcinol. Results indicated that silver oak leaf extract has termiticidal properties and could be of value in the development of botanical insecticides for termite control in wood products.