Life cycle of Zygogramma bicolorata and its effectiveness as biocontrol of Parthenium hysterophorus in Lamjung, Nepal

The invasive weed Parthenium hysterophorus L. poses a significant ecological and agricultural threat in Nepal, necessitating effective and sustainable management strategies. This study investigates the life cycle and biocontrol efficacy of Zygogramma bicolorata Pallister against P. hysterophorus under greenhouse conditions in Lamjung, Nepal. Detailed observations on developmental biology revealed an average life span of 62–78 days from egg to adult, with females showing longer longevity (43.3±0.72 days) than males (37.0±0.94 days). Each female laid an average of 650±21.45 eggs during its lifespan, with 84.08% hatching success, 81.32% pupal recovery, and 78.69% adult emergence. The beetle exhibited four distinct larval instars, each showing progressive growth and color changes, followed by pupation at 3–5 cm soil depth. Feeding potential varied with the developmental stage and beetle density. Feeding trials revealed that both adults and larvae caused significant defoliation of Parthenium with higher beetle densities leading to faster defoliation. Defoliation was faster at the seedling stage than the flowering stage, with five beetle pairs completing defoliation in 1.13 days and 6.67 days, respectively, compared to 6.67 and 15.67 days with two pairs. Larvae were particularly voracious feeders, contributing more substantially to defoliation than adults. Based on these findings, Z. bicolorata shows strong potential as a biocontrol agent against P. hysterophorus, and further field-based research is recommended to evaluate its establishment and effectiveness across Nepal’s diverse agro ecological zones.