Mike Pitcairn is with the California Dept. of Food & Agriculture (mike.pitcairn@cdfa.ca.gov). Lincoln Smith (link.smith@usda.gov) and Patrick Moran (patrick.moran@usda.gov) are with USDA Agricultural Research Service.

 

One of the main reasons why exotic weeds can invade native plant communities is the absence of effective natural enemies (insects or pathogens that attack it). Biological control is a weed control method in which host specific natural enemies of a target weed are transferred from their area of origin to the invaded range. A recent success story is the control of Dalmatian toadflax, (Linaria dalmatica, Plantaginaceae) (Figure 1), following the introduction of the stem weevil, Mecinus janthiniformis. Dalmatian toadflax is an exotic perennial forb that has invaded rangelands, roadsides, and natural areas throughout California. It is native to areas in southern Europe and western Asia near the Mediterranean Sea. It has been found in California since 1920 and is currently reported in 41 counties statewide. Local control measures have eliminated or substantially reduced many of the infestations throughout southern California, however, a very large infestation of Dalmatian toadflax has persisted near Gorman in the Tehachapi Mountains. The infestation is over 1000 acres and occurs on property managed by the US Forest Service and the State of California's Hungry Valley State Vehicular Recreational Area (HVSVRA).  

Two Mecinus weevils, M. janthiniformis and M. janthinus, have been introduced into North America. Mecinus janthiniformis strongly prefers Dalmatian toadflax, whereas M. janthinus, prefers yellow toadflax (Linaria vulgaris). The biology of both weevils is similar. The adult female deposits eggs in the flower stems, and the growing larvae feed by tunneling within the stems. When fully developed, the larva pupates within the stem and waits until the following spring when it emerges as an adult (Figure 2). There is one generation per year. The larval feeding damage reduces the number of flowers and seeds produced by a plant. At high population levels, dozens of eggs can be deposited within individual stems and the resulting feeding damage can substantially reduce or completely suppress seed production.

In 2008, over 1,000 adult M. janthiniformis were released at the HVSVRA to provide biological control of Dalmatian toadflax. The weevils quickly established at three release points and their numbers increased exponentially. In four years, 100% of stems within 15 meters of each release location were infested, and weevils were found over 550 meters away from their release point. Unfortunately, the Grand Fire in May 2013 burned through the toadflax population at the HVSVRA, destroying the above ground vegetation, killing any weevils remaining in the stems and destroyed potential oviposition sites for any adults that may have survived. The toadflax resprouted later that same year from perennial roots and most plants survived the fire.

Based on the encouraging results of the weevils establishing and increasing prior to Grand Fire, over 1,000 adult M. janthiniformis were released in April and June 2014. Both vegetation and weevil abundance were monitored annual from 2014 through 2019. Vegetation was monitored by estimating plant cover along permanent transects. Weevil abundance was monitored by collecting stems in late autumn or spring and dissecting them under a microscope in our laboratory and counting all immature and adult weevils.

The stem weevil re-established at three release sites. By 2017 100% of stems were attacked at five of six monitoring sites (one site was 71.4%), including three sites at which they were not released. Vegetation cover of Dalmatian toadflax decreased from 41% in 2015 to 7% in 2017 and to less than 1% cover (or trace levels) by 2019, a decrease of 99% from peak levels (Figure 3).

Concurrently, the cover of annual grasses increased consistently by 2018 (to 73% cover) but decreased in 2019 (37%). Annual forbs and annual grasses were generally codominant. Perennial grass cover was less than other vegetation classes but increased from 5% in 2014 to 16% in 2019. Perennial forbs (2.4%) and shrubs (0.4%) showed little change over time.

Weed control can take many years, even decades, because the plants can return through germination of seed from the seed bank.   As a result, successful reduction of a target weed is not observed until the seed bank is exhausted. At the HVSVRA in southern California, Dalmatian toadflax cover decreased by 99% in five years following release of the stem weevil which is a fairly short time period for weed biocontrol projects.  

For additional information:

Smith, L., D.M. Woods, M.I. Wibawa, V. Popescu, P.J. Moran, B. Villegas, M.J. Pitcairn and C. Hon. 2021. Release and establishment of the weevil Mecinus janthiniformis for biological control of Dalmatian toadflax in southern California. Biological Control. 104633.115. DOI 10.1016/j.biocontrol.2021