Use of nanoemulsions of plant essential oils as aphid repellents

It is believed that climate change will greatly impact the relative importance of pests. The bird cherry-oat aphid, Rhopalosiphum padi L. is probably the major pest of temperate cereal crops on a world scale, it attacks all the major cereals and pasture grasses. The organic sector is in need of alternative aphicides or products that can repel this pest. In spite of the properties of plant volatiles that allow them to act as insect repellents, there is a lack of such products on the market for the agricultural sector. In this work, we tested a group of essential oils and pure compounds in a laboratory choice bioassay with R. padi (20 replications per product) and the repellency index (R.I.) was computed after 24 h. At 0.15 μl/cm2, aniseed, peppermint and lemongrass essential oils were repellent for apterous females. trans-Anethole and caryophyllene exhibited volatile toxicity to the insects (LD50 = 0.11 μl/cm2). R.I. values ranging from 68.8 to 100 were obtained using farnesol, geraniol, cis-jasmone, citral, linalool, estragole, pulegone and caryophyllene. Water emulsions of the active products were obtained (nanoemulsions with oil droplets less than 100 nm via ultrasounds for 10 min) and applied at increasing volumes using a computer-controlled spraying apparatus for the bioassay, and a dose response was obtained. Some products were active: carvone increased mobility, whilst cis-jasmone repelled R. padi at a very low dose (0.02 μl/cm2 of the treated leaf). Zetasizer measurements indicated that the smaller the particle size within the nanoemulsion, the higher the activity. Using lecithin (1:2) or lecithin plus glycerol (1:2:1) in addition to a bioactive produced larger negative Z-potential values and therefore more stable formulations without any evident effect on activity.