Increased weed and pest pressure associated with longer growing seasons and warmer winters will be an increasingly important challenge; there are
already examples
of earlier arrival and increased populations
of some insect pests such as corn earworm.64 Furthermore, many
of the
most aggressive weeds, such as kudzu, benefit more
than crop plants from higher atmospheric carbon dioxide, and become more resistant to herbicide control.72 Many weeds respond better
than most cash crops to increasing carbon dioxide concentrations, particularly «invasive» weeds with the so - called C3 photosynthetic pathway, and with rapid and expansive growth patterns, including large allocations
of below - ground biomass, such as roots.73 Research also suggests that glyphosate (for example, Roundup), the
most widely - used herbicide in the United States, loses its efficacy on weeds grown at the increased carbon dioxide levels likely to occur in the coming decades.74 To date, all weed / crop
competition studies where the photosynthetic pathway is the same for both species favor weed growth over crop growth as carbon dioxide is increased.72