Reversing the tide of the ‘Famine Weed’: how biocontrol using the Mexican beetle offers a safe and sustainable solution for Parthenium hysterophorus in Kenya

Famine Weed, Parthenium hysterophorus L. (Asteraceae), is an invasive plant species of global importance, negatively impacting agriculture, environment, animals, and humans in over 50 countries, including Kenya. Its management has predominantly relied on synthetic herbicides, whose ecological drawbacks, high costs, and increased risk of herbicide resistance have necessitated need for safer alternatives. The use of its co-evolved predator, Calligrapha bicolorata, has been credited with outstanding success in several countries. Following importation of Calligrapha bicolorata Pallister (Chrysomelidae) into Kenya, this study assessed its efficiency against P. hysterophorus, host specificity, and potential risks to non-target plant species. Choice and no-choice bioassays were conducted using P. hysterophorus and 13 non-target plant species. In no-choice bioassays, the number of oviposited eggs was significantly higher (67.3 ± 0.7) on P. hysterophorus, where adult feeding resulted in 100% defoliation, while non-target plants had negligible number of eggs and minimal feeding damage. Similarly, in multiple-choice bioassays, C. bicolorata oviposited significantly higher numbers of eggs (66.0 ± 17.8) on P. hysterophorus, which was significantly damaged (88.9 ± 3.3% defoliation) compared to non-target plant species. Moreover, larval development was only recorded on P. hysterophorus. Computation of risk indices, based on feeding and reproduction performance on P. hysterophorus and three plant species, with positive oviposition and feeding damage, showed that none of the other test plants could support viable C. bicolorata population. The findings support C. bicolorata host specificity and demonstrate its potential role as an effective biocontrol agent of P. hyterophorus in Kenya, paving way for its release in the country.