Glucosinolates on the leaf surface perceived by insect herbivores: review of ambiguous results and new investigations

Abstract

Herbivorous insects identify their host plants either by structural features, chemical cues, or a combination. Some insects probe the host leaf prior feeding or oviposition, other species use olfactorial cues or compounds somewhere on the surface. Insects attacking Brassicaceae are no exception, some are attracted and stimulated by volatile isothiocyanates (ITC), many others depend fully on the non-volatile glucosinolates (GS) for host-plant recognition and acceptance. Since most insects have no access to the leaf interior investigators concluded that GS must be present on the leaf surface and ITC in the headspace. However, peelings of mechanically removed surface waxes were devoid of measurable amounts of GS, whereas solvent surface extractions revealed a correlation between stomatal conditions and GS concentrations. Both observations lead to the conclusion that the presence of GS on the top leaf surface is rather unlikely. In the experimental part we show that a chloroform/methanol/water (2:1:1 vol/ vol/vol) solvent leaf extract contains GS and, in addition, thia-triaza-fluorenes (TTF), other oviposition stimulants of the cabbage root fly, Delia radicum. Electrophysiological investigations showed that both, GS and TTF stimulated specific receptor neurones of the fly. We suggest that these compounds probably originated from deeper leaf layers and that herbivorous insects may penetrate the wax layer and perceive the stimulating compounds in deeper layers or through the stomata.

Surface Waxes as a Plant Defense Barrier Towards Grain Aphid

“…The layer of the epicuticular waxes may contain aliphatic components, sugars and amino acids (Eigenbrode and Espelie, 1995;Niemietz et al, 2009;Yin et al, 2011;Haliński et al, 2012), as well as secondary metabolites (Schoonhoven et al, 2005;Städler and Reifenrath, 2009;Supapvanich et al, 2011). The roles of primary and secondary com-pounds on aphid feeding and probing behaviour have been studied for decades.…”

“…Plant epicuticular waxes are complex mixtures of long chain aliphatic and cyclic components including fatty acids, hydrocarbons, alcohols, aldehydes, ketones, β-iketones and esters, as well as low levels of terpenoids, sterols, flavonoids, and phenolic substances. This layer may also contain sugars, amino acids and secondary plant substances such as glucosinolates, furanocumarins and alkaloids (Eigenbrode and Espelie, 1995;Schoonhoven et al, 2005;Städler and Reifenrath, 2009;Haliński et al, 2012;Supapvanich et al, 2011). The morphology as well as the composition of EW vary widely between species or cultivars and are also affected by the plant age and certain environmental factors, such as heat, humidity and irradiance levels.…”

“…It is also a protective barrier against water loss and loss of organic and inorganic compounds by leaching from the interior of the plant tissues. The external layer of EW may have other ecological functions including shielding of UV-B, protection against pathogen invasion (bacteria and fungi) and influencing insect behaviour by functioning as allelochemicals (Müller and Riederer, 2005;Städler and Reifenrath, 2009;Yin et al, 2011;Niemietz et al, 2009).…”

Fecundity and Survival of Grain Aphid at Various Growth Stages of Waxy and Waxless Triticale

“…Volatiles emanating from plant surface waxes can act as attractants or repellents [6,7]. The layer of the epicuticular waxes may contain aliphatic components, sugars, and amino acids [8][9][10][11][12], as well as secondary metabolites [13][14][15][16].…”

Choice of feeding sites, growth and survival by Crocidolomia pavonana (F.)

“…Oviposition does not damage leaves in the same way as feeding (which liberates odours) but, since glucosinolates are readily available near the leaf surface these are easily detected by moths upon landing (Städler & Reifenrath, 2009). This supposition is supported by the study of Marazzi and Städler (2004), where the P. xylostella moths were found to prefer to oviposit on Brassica napus that received sulfur fertilization, rather than on plants that had not received additional sulfur.…”