SOME STUDIES ON THE BIOLOGY OF THE CITRUS STINKBUG RHYNCHOCORIS SERRATUS DON

Abstract

The feeding ecology, gut structure and digestive enzymes of Rhynchocoris serratus Don, were studied in relation to its special association with its host plant, Citrus microcarpa Bunge. General aspects of the biology of both the host plant and the insect pest is first presented. There are five larval instars of which all except the first instar attack maturing fruits for the seeds. First instars feed from the leaf-lamina and have very short stylets. From the second instar, very long stylets are already developed and although gross length does increase slightly in later instars , the second instar is already able to reach the seeds. In adults also, a significant proportion of non-fruit feedings (eg. on young stem and leaf midrib) occurs and were traced to xylem feeding. In fruit feeding, all bug stages prefer large fruits except second instars which prefer medium-sized fruits. Feeding is always from fruits with fully developed embryos and the fruit size association may only reflect the slightly shorter stylets of the smaller larvae. Rhynchocoris is thus established as primarily a seed feeder. The gut contents of wild fed individuals always contain a slurry of embryo material, and the feeding mechanism is highly adapted to reducing tissues to a suspension for suctorial feeding. Seed feeding is little documented in the Pentatomidae and the assumption is often made that the biology should show special adaptations for a dilute and nitrogen deficient diet such as watershunting and potentially nitrogen-yielding gut symbionts. The histological and histochemical evidence obtained in this study only partly supports this hypothesis, Other evidence eg, the gastric caeca having no luminal continuity with the posterior midgut and the marked difference in pH values between that of Citrus fruit juice and those for optimum activities of gut digestive enzymes do not support previous assumptions of predominant feeding on fruit juice. These results are now explained by the fact that R. serratus while being principally seed-feeding, is also, in biology, transitional between a pure sap-feeder and an extreme dry seed feeder. Higher feeding activity occurs during mornings and evenings but continues throughout the day and night. More time is spent in feeding by second, third and fourth instars and least by adult males. These are no immediately obvious signs of bug attack on the fruits but severe attacks result in uneven yellowing and in abscission after a few days. These fruits had a high proportion of spoilt seeds. Digestive enzymes present in the, gut include protease, ectcrane, lipase, alpha-glucosidase, alpha-galactosidase, beta-glucuronidase and amylase. At least amylase, alpha-glucosidase and protease are present in the larval gut. The presence of a salivary protease in all stages is significant and allows external digestion of the relatively solid, protein-rich food. Polyembryony and the presence of seed 'caps' in Citrus are both interpreted as selective response to seed predation. The behaviour and mouthparts of R. serratus show adaptations to avoid the seed cap. Citrus and Rhynchocoris may thus be seen as an example of co-evolution which explains the strict monophagy of the bug.