Sensory and nutritional effects of amino acids and phenolic plant compounds on the caterpillars of two Pieris species

The relationships between caterpillars of Pierisbrassicae L. and Pierisrapae L. (Lepidoptera: Pieridae) and a common host plant Brassicaoleracea L. were studied using chemosensory and nutritional techniques. Attention was focussed on amino acids, which are in part essential nutrients, and on phenolic and flavonoid derivatives of two aromatic amino acids, that are products of the secondary metabolism in the host plant.An electrophysiological study of amino acid gustation showed that in both species 14 out of 22 amino acids were stimulants to a receptor cell in a maxillary sensillum. The nutritionally essential amino acids were generally stronger stimuli than dispensable ones. A correlation analysis provided indirect evidence that the amino acid receptor possessed four sites, one less specific and three or possibly four specific ones. A comparison of data on free amino acid concentrations in B.oleracea with dose-response relations of the amino acid cell showed that this cell can quantitatively sense foliar amino acids.Phenolic acids and an anthocyanin that naturally occur in B.oleracea elicited neural responses from two to three maxillary gustatory cells. Chlorogenic and protocatechuic acids, both carrying ortho-substituted hydroxyl groups on the aromatic ring, were the most effective stimulants. A steep increase in responsiveness was occurring with increasing concentrations in the range 0.2 - 5.0 mM. P.rapae was the less sensitive of both species. Flavonols were ineffective. The predominant anthocyanin in B.oleracea , cyanin, evoked neural activity in some cells but inhibited the activity in gustatory cells sensitive to sugars, amino acids and glucosinolates in P.brassicae . Chemosensory responsiveness was reflected in preference behaviour. Naturally occurring levels of phenolic acids in B.oleracea as found in phytochemical studies are able to affect sensory processes in the caterpillars (Chapter 3).Assessment of possible metabolic effects of dietary phenolics and some other dietary variations was performed using a flow-through respirometer. This was designed to monitor continuously the gas exchange of feeding caterpillars during the complete final instar. The results of these measurements were compared to the results obtained using standard gravimetric techniques that make use of the measurement of food intake to calculate metabolic efficiency. Respirometric results yielded small effects on the energetic efficiency of growth, which was in contrast to gravimetric results. The causes of the discrepancies between both methods and the consequences of these findings for studies on insect food utilization in general are discussed (Chapter 4).The nutritional utilization of amino acids and nitrogen was studied comparatively for caterpillars of both species on an artificial diet and on B.oleracea . Food consumption in the final instar was lower on the artificial diet. More food was consumed when leaf amino acid content was lower. Relationships were found between food consumption and the absorption efficiencies of most of the essential amino acids. Absorption efficiencies for glycine, cystein and serine were lower on the artificial diet, differences for other amino acids were small between the diets. Amino acid utilization patterns were similar for both species. Balance sheet calculations showed that an extensive conversion from phenylalanine to tyrosine occurred. For both species indications were obtained that tyrosine and cystein may become limiting for growth when dietary protein levels are low (Chapter 5).Phenolic acids (caffeic and chlorogenic acids) and flavonoids (oenin and quercetin-3-rutinoside) inhibited survival, development and growth when larvae of both species were continuously exposed to these compounds present in an artificial diet. P.brassicae was distinctly more sensitive at lower levels of the compounds (0.4 and 1.0 mM). Final instars of both species were much less sensitive than earlier instars. Growth inhibition in final instars was primarily due to reduced food consumption. The results suggest a potential role of phenolic acids and flavonoids, normal constituents of leaves of B.oleracea , in defence against Pieris caterpillars (Chapter 6).Seven cultivars of B.oleracea were offered as food to study their suitability as a host plant for larvae of both caterpillar species. Parameters of larval performance showed differences between cultivars. Highperformance liquid chromatography was used to analyse leaf tissues of five cultivars with respect to concentrations of phenolic acids and flavonoids. Each of the cultivars was found to have its own quantitative pattern of these compounds. Unidentified polar flavonoid components were detected in highly variable amounts. These preliminary results further support a potential role of phenolic and flavonoid compounds in resistance of B.oleracea against Pieris (Chapter 7).