Response of the Weed Parthenium hysterophorus (Asteraceae) to Defoliation by the Introduced Biocontrol Agent Zygogramma bicolorata (Coleoptera: Chrysomelidae)

Effects of defoliation by the introduced biocontrol insect Zygogramma bicolorata Pallister (Coleoptera: Chrysomelidae) on the annual Asteraceae weed Parthenium hysterophorus were studied in greenhouse and field cages. Feeding by Z. bicolorata caused damage to meristems, resulting in shorter height of the primary stem and changed branching pattern. With no water stress, defoliation caused the plant height to be reduced by 13–56% and flower production by 25–45%, and the reductions were more significant when defoliated at early stages of plant growth. However, there were no reductions in leaf production and plant biomass in defoliated plants, irrespective of the plant stage at which defoliation was imposed. Under water stress, defoliation caused the plant height to be reduced by 10–31%, flower production by 31–75%, leaf production by 23–51%, and plant biomass by 2–9%. In water-stressed plants, the negative effects of defoliation on flower production, leaf production, and plant biomass were more significant when defoliated at flowering stage than at early stages of plant growth. The increase in the negative effects due to synergism between defoliation and water stress was only marginal. Flower production and root and shoot biomass declined with increase in the duration of defoliation. Defoliation for 74 days without water stress reduced flower production by 99%, shoot biomass by 67%, and root biomass 80%. In field-cage, Z. bicolorata caused 92% defoliation in about 90 days and reductions in plant height by 27%, root length by 56%, root biomass by 69%, shoot biomass by 81%, flower production by 83%, and soil seed-bank by 73%.