Plant-herbivore asynchrony necessitates augmentative releases of the exotic beetle, Zygogramma bicolorata, to enhance the biological control of Parthenium hysterophorus.
Systematic information on the quantitative impact of Zygogramma bicolorata on the biology of Parthenium hysterophorus is crucial as the seeds of this weed continue to germinate from the accumulated soil seed bank throughout the year in the form of different germinating flushes, while the activity of the beetle ceases during winter as it enters diapause. Therefore, plant-herbivore interactions need to be explored to develop predictions of the overall impact of the introduced beetle on the weed. The findings revealed that defoliation by Z. bicolorata had a significant impact on the plant height, density and flower production in flushes F3, F4 and F5, but not in F1 and F2 that exhibited longer periodicity, profuse branching, a longer flowering period and maximum flower production and contributed mostly to the existing seed soil bank. Therefore, total depletion of the existing soil seed bank was not possible. Consequently, the effect of augmentative field releases of laboratory-reared beetles was explored on F1 and F2 in February for three consecutive years (2011-2013). Before initiating the trial, random soil samples were taken from the plots that were assigned to the paired treatments (i.e. with the beetle and without the beetle [insecticide-treated]) and it was found that the seed bank in those samples did not differ. The single release of Z. bicolorata adults at five per plant at the six-leaf stage significantly reduced the soil seed bank, compared to without the biocontrol agent, irrespective of the flushes at the end of the season.