Effects of gall damage by the introduced biocontrol agent Epiblema strenuana (Lep., Tortricidae) on the weed Parthenium hysterophorus (Asteraceae)

Effects of gall damage by the introduced moth Epiblema stremiana on differe nt growth stages of the weed Parathenium hysterophorus was evaluated in a f ield cage using potted plants with no competition and in naturally regenera ted populations with intraspecific competition. Gall damage at early stages of plant growth reduced the plant height, main stem height, flower product ion, lear production, and shoot and root biomass. All galled, potted plants with no competition produced flowers irrespective of the growth stage at w hich the plants were affected by galling, but lesser than in ungalled plant s. Gall induction during early growth stages in field plants experiencing c ompetition prevented 30% of the plants reaching flowering. However, 6% of t he field plants escaped from gall damage, as their main stems were less vig orous to sustain the development of galls. Flower production per unit total plant biomass was lower in galled plants than in ungalled plants, and the reduction was more intense when gall damage was initiated at early stages o f plant growth. In potted plants with no competition, the number of galls i ncreased with the plant vigour, as the gall insects preferred more vigorous plants. But in field plants there were no relationship between gall abunda nce and plant vigour, as intraspecific competition enhanced the negative ef fects of galling by reducing the vigour of the weed.