Survey and Collect Different Developmental Stages of Zygogramma bicolorata Associated with Parthenium hysterophorus in the Study Area

Parthenium hysterophorus L. is an invasive and highly competitive weed found throughout agricultural and non-agricultural habitats in India. The Mexican beetle, Zygogramma bicolorata Pallister, is an effective biological control agent used against P. hysterophorus. The present study was conducted to determine the seasonal abundance of different developmental stages of Z. bicolorata on P. hysterophorus in different habitats of Kanpur region of Uttar Pradesh and to assess its feeding efficiency at different growth stages of the weed. Field observations were made on randomly selected P. hysterophorus plants in agricultural, fallow, roadside and agroforestry/field habitats during 2024 and 2025. The population of eggs, grubs and adults of the beetle showed marked seasonal fluctuations, with relatively higher abundance during the monsoon period, especially July–September. During the first year, the highest population in agricultural habitat was recorded as 4.60 eggs, 4.20 grubs and 5.60 adults per plant, whereas roadside habitats recorded maximum densities of 6.10 eggs, 5.20 grubs and 6.60 adults per plant. In 2025, roadside habitat again recorded comparatively high densities, with 6.15 eggs, 4.80 grubs and 6.20 adults per plant. The overall mean population in agricultural habitat during 2024 was 1.22 eggs, 0.71 grubs and 1.65 adults per plant, compared with 1.13, 0.63 and 1.51 per plant, respectively, in 2025. Feeding-efficiency studies revealed an inverse relationship between beetle density and the time required for complete defoliation. At the early vegetative stage, one pair required 13.45 days for complete defoliation, while five pairs required only 2.15 days. At the pre-reproductive stage, the corresponding period declined from 21.15 to 3.62 days, whereas at the reproductive stage it declined from 27.36 to 6.01 days. Thus, Z. bicolorata showed greater defoliation efficiency on younger plants and at higher beetle densities. The results illustrate the importance of seasonal beetle abundance and host-plant phenology in maximising the biological control potential of Z. bicolorata against P. hysterophorus.