Toadflax, fire, Mecinus janthinus, and compensatory growth
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Dalmatian toadflax is a noxious weed of the western United States. In western Montana it invades in the foothills zone where bunchgrasses (Agropyron spicatum and Festuca idahoensis) meet low forests (Pinus ponderosa and Pseudosuga menziesii). Our results show that wildfire strengthens toadflax, probably at the expense of native grasses. The stem boring weevil, Mecinus janthinus, is being tested as the most likely biocontrol agent for the weed. On our toadflax infested sites in the Agropyron spicatum/Pinus ponderosa zone Mecinus established, survived, and spread slowly (2-4m/yr) for two years following introduction. At the low initial densities Mecinus induced compensatory growth in toadflax i.e. stem density, plant height, branch number, and seed capsule number all increased. In a garden experiment, we exposed plants to Mecinus at increasingly higher densities (0-32 insects per plant) to measure the effect of a higher attack rate. With exposure rates of 2-4 insects per plant compensatory growth occurred. With exposure rates greater than 8 insects per plant total biomass and especially flower biomasses were reduced. If Mecinus densities in the field exceed eight weevils per plant this may indicate eventual exhaustion and decline of the weed.