Direct effects of insecticides on common ragweed-implications for natural enemy exclusion trials

Experimentally applying pesticides is an important method to assess the e cacy of weed biocontrol agents, but potential direct e ects of the chemicals on plant performance are controversial or unknown. We assessed how three broad-spectrum insecti- cides applied in combination a ect the performance of the widely invasive, crop-yield reducing, allergenic common ragweed (Ambrosia artemisiifolia L.) in an insect-free environment. Spraying insecticides had no signi cant e ects on aboveground dry weight, seed and pollen output or pollen allergenicity, and only explained 1–8% of variation in these parameters. Our insecticide treatment can hence be applied to assess biocontrol impact on biomass and reproductive output of common ragweed. As our in- secticide treatment delayed senescence, however, other methods of insect exclusion should be preferred when studying common ragweed phenology