The distribution, control techniques and impact of Parthenium hysterophorus on flower visitors and soil chemical properties in Meru district, Tanzania

Abstract

In Tanzania, the invasive plant Parthenium hysterophorus threatens natural and semi–natural environments. Little is known about how this species affects plant–pollinator interactions and soil–chemical properties. Roadside survey was conducted to assess P. hysterophorus invasion status within and outside Arusha National Park (ANP), Tanzania. Soil samples were also collected in invaded and uninvaded plots to investigate the impact of P. hysterophorus on soil–chemical properties. To test alternative natural–based management strategies, the study examined the bio–herbicide potential of Desmodium uncinatum leaf (DuL) crude extract and the competitive ability of fodder legume plant species (Lablab purpureus, Desmodium intortum and Medicago sativa) to suppress P. hysterophorus in pot and plot experiments. Results showed that ANP is currently uninvaded with P. hysterophorus, but some adjacent villages i.e. King’ori, Maleu, Napoco, Ngongongare, Ngurdoto, Oligilai and Sakila have already invaded by this invasive. Parthenium hysterophorus invaded site was more acidic with lower electrical conductivity, less calcium and phosphorus, and high cation exchange capacity. The DuL extract suppressed P. hysterophorus growth vigour, particularly at higher concentrations (>70%). At these concentrations, the invasive seedling stem height was >30% shorter, and seed germination was inhibited by >55% compared to seedlings sprayed with lower concentrations. When P. hysterophorus was grown in combination with all three test plants, its seedling stem heights and total fresh biomass were reduced by >60% and >59% in pots, and >40% and >45% in field plots respectively, compared to when grown alone or in mixture with just D. intortum or M. sativa. Moreover, surveys conducted on invaded and uninvaded sites to investigate the impact of P. hysterophorus on plant–pollinator interactions when two common target plants (Ocimum gratissimum and Ageratum conyzoides) were present showed that flower visitation rate to target plants was significantly lower in invaded plots than in uninvaded plots. This implies that P. hysterophorus may be disrupting pollen flow. The study recommends that the use of bio–herbicide and suppressive plant species to control the invasive should be promoted. However, it emphasizes the use of native suppressive plant species because non–natives may turn into invasives in the future.