Insects on common reed (Phragmites australis):: community structure and the impact of herbivory on shoot growth

In this paper, community structure of endophagous insects attacking common reed (Phragmites australis (Cav.) Trin ex Steud.) and impact of herbivory on shoot growth was analyzed in south west Germany. All the 26 phytophagous insects were monophagous, i.e. restricted to their host plant P. australis, which appeared to be due to the predictability and apparency of the large and ancient Phragmites stands. Habitat types were clearly separated due to the mean shoot diameter ('dry' and 'wet' reed) and the mean level of shoot damage (high and low abundance of Archanara shoots). The stem-boring moth Archanara geminipuncta greatly influenced structure of the insect community, due to heavy shoot damage, and could be classified to be the key species of the reed ecosystem. In contrast to common expectations of negative effects, Archanara shoot damage enhanced populations of many insect species, so insect guilds of 'primary attackers' (on the main shoot) and 'secondary attackers' (on damaged shoots) could be separated. Primary attackers either killed the growing paint or attacked the apical, medium or basal internodes, while secondary attackers were facilitated by shoot damage and could also be subdivided in four guilds: Inquilinous species living between the wilting leaves of damaged shoots, attackers of the side shoot growing in response to the destruction of apical meristems, saprophagous species feeding on Archanara droppings, and the successor community of bees and wasps nesting in empty galls or broken shoots. There is a continuum from evident plant damage, e.g. by stem-boring moths and leaf-sucking aphids, to enhanced shoot growth due to the parasitic influence of gall makers, e.g. the significant shoot elongation by the gall midge Giraudiella inclusa. In Europe, conservationists are concerned about the die-back of P. australis, while in North America and Australia, P. australis is considered to be an invasive weed. Classical biological control with the introduction of European insects (like the moth A. geminipuncta) is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.