Isolation and Identification of the Major Component of Setal Exudate from Corythucha ciliata

Abstract

Developing lace bug nymphs form large aggregations on the undersides of leaves on host plants and, unlike many other gregarious feeding insects, are relatively free of predators and parasites. Various types of processes extend as outgrowths of the body wall of nymphs. Many species have additional setae which are secretory and considered important to defense. In the case of Corythucha Ciliata Say (Hemiptera; Tingidae), potential predators exhibit aversion behavior after tactile engagement with microdroplets of setal exudate. As part of a comparative study of setal exudate material from lace bugs, we have previously reported the presence of 2-alkyl-5-hydroxychromones and the corresponding chromanones and diketones from the azalea lace bug Stephanitis pyrioides ( l ) , and dihydroxyphenyl1,3-diketones and the corresponding chromones and chromones possessing an additional phenolic oxygen from the rhododendron lace bug Stephanitis rboddendrz (2). The presence of these unusual compounds in Sttpbanitij prompted us to explore the genus Corythucba. Generally restricted to the Western Hemisphere, many of the 68 species of Cmythucha are strikingly similar to each other, and each species is limited to one or a few host plants (3). RESULTS AND DISCUSSION Initial samples of setal exudate from the sycamore lace bug (C. Ciliata) were obtained by wicking of the microdroplets onto small, triangular pieces of filter paper that were then extracted with CH,CI,. Compared to previous tingids examined by this wicking procedure (1,2), a much smaller amount of material was available from C. Ciliata. Analysis of this material by gc indicated the presence of six components, the most abundant of which constituted 53% of the total. A threshold effect was observed during gc analysis, which suggested the components were extremely polar. The combination of ei, ci, and deuterium exchange gc/ms established a molecular weight of 336 and the presence of two exchangeable hydrogens for the major component 1. Five less abundant components had molecular weights of 338 -t 336 (chromatographically unresolved; 4% of total exudate material), 362 (9%), 346 (6%), and 362 (28%); each possessed two exchangeable hydrogens except for the compound with molecular weight of 346, which had one. The mass spectra of the five less abundant components resembled 1 in that some ions were in common but differed considerably in intensity. None of the ei mass spectra closely resembled those of compounds we have isolated from other tingids (1,2), nor did the mass spectra of the six unknowns exhibit a high degree of correlaNov-Dec 19871 Lusby et a f . : Component of Setal Exudate 1127 tion with any entry in the data system mass spectral library. Hrms analysis provided an empirical formula of C20H3204 for the molecular ion. In addition to supporting empirical formula assignments listed in the experimental section, hrms also indicated a curious loss of C 2 H 4 0 to yield a weak (3%) ion at m/z 292. Preliminary gc/ms of a much larger sample obtained by whole body rinsing of ca. 10 g of insects and cast molt skins with CH2C12 revealed, not surprisingly, numerous components in addition to the six observed in the initial wicked sample. Many of the spurious compounds were hydrocarbons and probably of cuticular origin as well as derived from leaf material co-aspirated with cast skins. A sample of only stellate hairs, the predominant leaf structure that was co-aspirated, was collected, extracted, and by gc/ms was found not to contain any of the six compounds present in the wicked sample. Only the major component 1 was obtained in sufficient amount for final trapping by gc for subsequent spectroscopic analysis. Bands at 3470, 1667, and 966 cm-' in the ir spectrum of 1 suggested, respectively, the presence of a hydroxyl in a single bridge intramolecular hydrogen bond configuration (4) , an a,P-unsaturated ketone (5), and a trans olefinic bond. Ozonolysis of compound 1 provided the derivative 3,6-dihydroxy-2-( 1 , l O dioxodecyl)cyclohex-2-en1-one, which was demonstrated by eims and cims to have many ions in common with 1, and to have a molecular weight of 296, thereby indicating a loss of a four-carbon fragment from 1. The uv spectrum of 1 with X max at 270 and 232 nm closely resembled that of 2acetylcyclohexane1,3-dione 1272 and 232 nm (6)] and suggested a tri-ketone structure. An examination of the literature for compounds with these characteristics led to Mudd's investigation of 2-acylcyclohexane1,3-diones isolated from larvae of Ephestia kuehnieffa Zeller (7). Among the compounds reported by Mudd was 3,6-dihydroxy-21 1-oxo-9(Z)-octadeceny1}cyclohex-2-en1-one 123. The ei mass spectrum of 2, isolated from E . kuehnieffa and generously supplied by Dr. Mudd, was very similar to that of 1, the major difference being the molecular ion at 392 for compound 2 vs. 336 for compound 1. The ir spectra of 1 and 2 were nearly congruent except for the presence of a bond at 966 cm' in the spectrum of 1 that was absent in that of 2, and minute differences in the regions between 1220-1245 cm-' and 1150-1200 cm-'. H-nmr spectra (Table 1) indicated a single terminal methyl, an isolated double band, and a strongly hydrogen-bonded enolic H (6 18.24 in CDCl,). Chemical shifts of ring protons were in agreement with thosepublished by Mudd (7), and the assignments were confirmed by decoupling experiments. Comparison of the 'H spectrum of 1 obtained in C6D6 to the CDCl, spectrum was interesting; the linear chain methylene groups 3'-8', and 13'shiftedfrom6 1.54-1.27 to 1.38-1.22. Thisanisotropicshiftrevealed underlying signals that permitted assignments of ring protons on C-4 at 6 1.62 (2H, m) and those on C-5 at 6 1.76 (2H, m). Also, the resonance of the side chain 3' 1