Effects of the gall midge Asphondylia borrichiae, simulated herbivory, and nutritional status on survival, flowering, and seed viability in Borrichia frutescens, the sea oxeye daisy

Although herbivory and other types of plant damage typically are viewed as detrimental to plant fitness, vigorous regrowth and fitness benefits may be possible for plants experiencing certain types of herbivory and damage, especially removal of the apical meristems, or actively growing terminal regions of stems. Damage to the apical meristem releases apical dominance, activates dormant lateral buds, and enables lateral shoots to grow in some plant species. Since each stem may bear a flower, removal of the apical meristem may result in stem bifurcation and ultimately increase the number of flowers and seeds, thereby increasing plant fitness. Overcompensation in response to apical meristem damage caused by simulated herbivory and the gall midge Asphondylia borrichiae Rossi and Strong (Diptera: Cecidomyiidae) was investigated in the native coastal halophyte, sea oxeye daisy Borrichia frutescens (L.) DC. (Asteraceae). The numbers of galls and stems were counted on 1,000 plants and analyzed using linear regression. Gall count on a plant was positively correlated with stem count. Seventy-five additional single-stemmed B. frutescens were assigned to one of three experimental treatment groups (galled, clipped, or intact), and the presence of bud break and distance from the apex to the uppermost bud break was recorded after six weeks. Another experiment involved assigning 300 single-stemmed B. frutescens to one of six treatment groups (galled/fertilized, galled/unfertilized, clipped/fertilized, clipped/unfertilized, intact/fertilized, intact/unfertilized), with a second control group of 20 intact/unfertilized plants covered with mesh bags to protect them from galling. Plant height, stem count, gall count, and flower count were recorded over six months, and seeds were counted, weighed, and germinated. Although these experiments are incomplete, preliminary results suggest that sudden removal of the apical meristem increases stem mortality and releases lateral buds more quickly than gall stem tips that experience gradual loss of apical dominance.