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When adult males of the smaller brown planthopper, Laodelphax striatellus were fed on 2% crude rice (leaf and stem ) extract containing 15% sucrose there were characteristic stylet sheaths deposited on parafilm membrane. Further bioassays with the butanol-soluble fr. of the extract revealed that it is highly effective for the insects. When the butanol fr. was charged on an ODS open column and eluted in sequence with 20, 40 and 100% m ethanol in w ater, the ODS-40% m ethanol fr. was shown as the most effective one. F urther separation of the ODS-40% methanol fr. resulted in six effective components. These components acted to stimulate very high probing response on L. striatellus only when they are combined. This activity was found to be similar to those of the ODS-40% methanol fr. and the original crude rice plant extract. Two of the active components were identified as tricin 5-O-glucoside and tricin 7-O-glucoside, respectively, through spectroscopic analyses.

The feeding behaviour of the aphid Megoura crassicauda Mordivilko (Homoptera: Aphididae), which feeds selectively on plants in the genus Vicia (Fabaceae), was studied. The aphids deposited proteinaceous stylet sheaths intercellularly towards the phloem tissues of host plants. Similar stylet sheaths were formed on a Parafilm membrane when host-specific acylated flavonoid glycosides [two 2 ″ -O -( E )p -coumaroyl esters of quercetin 3-O -diglycosides] present in the extracts of the narrow vetch, Vicia angustifolia L., were supplied in the solution covered by the membrane. In contrast, their corresponding deacyl analogues, present more abundantly in the host plant tissues, were not stimulatory, which suggested specificity in the structural requirements of the probing stimulants. While the aphids imbibed an artificial diet composed of primary nutrients (e.g., sucrose and amino acids) and produced a large quantity of honeydew, acylated flavonoids alone and non-acylated flavonoids supplied with the nutrients more or less suppressed honeydew production. These findings implied that the acylated flavonoids serve as a cue to navigate the stylet sheath towards the phloem prior to sap-sucking, whereas nonacylated flavonoids may serve as a negative stimulus to refrain from sucking during tissue penetration before tapping the phloem, although the distribution of these compounds in the plant tissues remains unknown. Thus, the feeding behaviour of M. crassicauda appears to be controlled by multiple chemical stimuli in the process of the settling on its host plant.