Combinations of Microbial and Insect Biocontrol Agents for Management of Weed Seeds

Important roles for biotic agents in integrated weed management include preventing seed production and weed emergence from the seed bank. Seed-attacking microorganisms have been described for a limited number of economically important weeds and serve as examples illustrating the potential for reducing weed seed production. Innundative releases of seed-feeding insects have also successfully reduced viable seeds produced by specific weeds and subsequently reduced the viable weed seed content in soils. There are some indications that seed attacked by either microorganisms or insects carry over detrimental effects in soil and result in reduced weed seedling emergence in the following growing seasons. Combinations of different biotic agents could enhance efficacy of weed management over that observed with either agent alone. For example, a selective seedfeeding insect, Niesthrea louisianica, combined with seed-attacking fungi (Fusarium spp.) significantly decreased velvetleaf (Abutilon theophrasti) seed viability and seedling emergence compared to either the insect or fungi alone. Insect-attacked, fungal-infected velvetleaf seeds survived at very low rates in soil (ca. 2% viable seeds at 24 months.) and in plots where the seedbank was not replenished, no viable seeds were retrieved from soil after 24 months. Integration of compatible biotic agents can be an effective method for reducing seed viability before and after entry into the seed bank. Prevention of seed production is considered the foundation for successful weed management — effective insect + microorganism combinations are key in building this foundation in biologically-based weed management.