Whitefly attraction to rosemary (Rosmarinus officinialis L.) is associated with volatile composition and quantity

Abstract

Whitefly (Bemisia tabaci) is an important insect pest, causing severe damage to agricultural crops. The pest was recorded in a commercial rosemary (Rosmarinus officinalis, Lamiaceae) field, colonizing rosemary variety (var.) '2', but not '11'. A series of field and controlled laboratory choice bioassays confirmed the observed phenomenon. Mature potted plants of the two varieties were randomly organized in a lemon verbena (Lippia citrodora) and lemon grass (Cymbopogon spp.) fields. Seven days later var. '2' was significantly more colonized by whiteflies than var. '11'. Under lab conditions, whiteflies were significantly more attracted to var. '2' plantlets than to var. '11' following choice bioassays. Furthermore, cotton plants dipped in an essential oil emulsion of var. '2' had significantly greater colonization than cotton plants dipped in the essential oil emulsion of var. '11'. Similar results were obtained in 'plant-plant', 'plant-no plant' as well as, 'essential oil—essential oil' choice bioassay designs. Analyses of the essential oils of the two varieties identified a set of common and unique volatiles in each variety. Among these volatiles were β-caryophyllene and limonene, two compounds known to be associated with plant-insect interactions. The attraction of B. tabaci to pure (>95%) β-caryophyllene and limonene using a range of concentrations was examined in vitro by choice bioassays. The compounds were attractive to the insect at moderate concentration, but not at the lowest or highest concentrations used, where the insect was not attracted or repelled, respectively. Limonene attracted the insects at rates that were 10-fold lower than β-caryophyllene. The results emphasized the role of host plant volatiles in shaping the structure of B. tabaci populations in nature and in agricultural systems, and provided insights into the factors that contribute to the development of insect populations with unique characteristics. The results could also serve for future development of bio-pesticides and in breeding programs.

Prospects for management of whitefly using plant semiochemicals, compared with related pests

“…Principal component analysis of the volatile collections may be helpful to identify putative semiochemicals within a mixture . Various techniques for collecting plant volatiles are available, such as solvent extraction, steam distillation or air entrainment . The latter is preferred because headspace collections represent actual released quantities of naturally occurring compounds.…”

“…Overall, whitefly repellents or attractants are evaluated based on the proportion of responding whiteflies in relation to the solvent used or clean air. The degree of the semiochemical effect on whitefly behaviour is also dose‐dependent . The terms repellency and attractancy should be used with caution and always in context.…”

“…4,23 Various techniques for collecting plant volatiles are available, such as solvent extraction, steam distillation or air entrainment. 5 The latter is preferred because headspace collections represent actual released quantities of naturally occurring compounds. In addition, this non-destructive sampling method does not risk extracting compounds formed by damaged plant tissue.…”

“…The degree of the semiochemical effect on whitefly behaviour is also dose-dependent. 5,6 The terms repellency and attractancy should be used with caution and always in context.…”

“…However, increasing numbers of studies have shown that whitefly behaviour is affected by plant‐emitted volatile organic compounds (VOCs). In behavioural bioassays, whitefly preference varies between potential host plants and even among host plant varieties, cultivars and accessions . Furthermore, whiteflies can discriminate between different qualitative conditions in host plants, for example, differences in nitrogen fertilization, leaf position, aphid colonization and virus infection .…”