Biocontrol backfires again

May 28, 2002

Biocontrol advocates claim that releasing non-native insects to control non-native plants is safe for native species -- but the number of "exceptions" keeps growing. The latest is a weevil intended to control a non-native thistle. New research shows that the weevil prefers a native thistle and can reduce its seed set by 98%.

"Ecological risk was severely underestimated," say Svata Louda of the University of Nebraska in Lincoln and Charles O'Brien of Florida A & M University in Tallahassee in the June issue of Conservation Biology.

The weevil (Larinus planus) is from Eurasia and is being released in the western U.S to control Canada thistle, which despite its name is also from Europe. Canada thistle is an aggressive weed and may threaten large areas of range and crop land. The weevil damages thistles in two ways: the adults eat the leaves and the larvae eat -- and so destroy -- developing flowers and seeds. Since a 1990 study suggested that the weevil preferred Canada thistle to native ones, the weevil has been widely released in the U.S., notably in western national parks, forests and monuments. However, Louda and O'Brien reanalyzed the 1990 results and found that the weevil fed equally on Canada and native thistles in laboratory tests.

While doing another study, Louda and O'Brien unexpectedly found that the weevil also feeds on a native thistle in the wild: Tracy's thistle, an relatively uncommon species found only in western Colorado and eastern Utah. In 1992 and 1993 the U.S. Forest Service released the weevil on the edge of Gunnison National Forest, which is near Almont, Colorado. In 1999 the researchers collected 30 Tracy's thistle flower heads from a roadside stand near the weevil release site. In 2000 the researchers double-checked their surprising find by collecting 185 Tracy's thistle flower heads from the same stand and 166 from another stand that was further away, as well as 375 Canada thistle flower heads from three nearby stands.

Louda and O'Brien found that the weevil fed extensively on Tracy's thistle. More than 75% of the flower heads either contained weevil larvae or had signs of larval damage. Worse, the weevil reduced seed set in infested flower heads by 98%: infested flower heads produced about one viable seed each while undamaged flower heads produced about 45 seeds each. Overall, the weevil reduced the seed production of the Tracy's thistle stands by two-thirds.

Moreover, the researchers found that the weevil had little effect on the non-native thistle it was supposed to control. In fact, there was no evidence of weevil feeding in any of the three stands of Canada thistle studied. This is striking because Tracy's thistle is sparse while Canada thistle is relatively common in the study area.

Louda and O'Brien call for re-evaluating the release of non-native insects to control non-native weeds in natural areas. "Current practices involving such introductions of exotic insects into nature reserves and national parks rely on incomplete assessments of ecological risk," they say.
CONTACT: Svata Louda (402-472-2763, NOTE: she will be in the field until June 6
  • Charles O'Brien (850-399-3149,

    For PDFs of papers, contact Robin Meadows:

    Society for Conservation Biology

    Related Native Species Articles from Brightsurf:

    Native Hawaiian tiger cowries eat alien invasive species
    Researchers at the University of Hawai'i at Mānoa's Hawai'i Institute of Marine Biology, have discovered the Hawaiian tiger cowrie is a voracious predator of sponges.

    New native Hawaiian land snail species discovered, first in 60 years
    Auriculella gagneorum, a small candy-striped snail from Oahu's Waianae Mountains, represents the first new species of a living Hawaiian land snail described in 60 years.

    Native bushland's fertility secret
    In hotter, dryer conditions with climate change, a secret agent for more sustainable agricultural production could lie in harvesting the diverse beneficial soil microbiome in native bushland settings, scientists say.

    Two new species of parasite discovered in crabs -- discovery will help prevent infection of other marine species
    Two new species of parasite, previously unknown to science, have been discovered in crabs in Swansea Bay, Wales, during a study on disease in the Celtic and Irish Seas.

    About 94 per cent of wild bee and native plant species networks lost, York study finds
    Climate change and an increase in disturbed bee habitats from expanding agriculture and development in northeastern North America over the last 30 years are likely responsible for a 94 per cent loss of plant-pollinator networks, York University researchers found.

    Native bees also facing novel pandemic
    There is growing evidence that another ''pandemic'' has been infecting bees around the world for the past two decades, and is spreading: a fungal pathogen known as Nosema.

    Viewing dopamine receptors in their native habitat
    A new study led by UT Southwestern researchers reveals the structure of the active form of one type of dopamine receptor, known as D2, embedded in a phospholipid membrane.

    Marine species are outpacing terrestrial species in the race against global warming
    Global warming is causing species to search for more temperate environments in which to migrate to, but it is marine species -- according to the latest results of a Franco-American study mainly involving scientists from the CNRS, Ifremer, the Université Toulouse III-Paul Sabatier and the University of Picardy Jules Verne -- that are leading the way by moving up to six times faster towards the poles than their terrestrial congeners.

    New native grass species have been discovered on the Iberian Peninsula and Menorca
    The new species belong to the genus Aira, delicate herbaceous plants, which enjoy their greatest diversity in the Mediterranean Region.

    Directed species loss from species-rich forests strongly decreases productivity
    At high species richness, directed loss, but not random loss, of tree species strongly decreases forest productivity.

    Read More: Native Species News and Native Species Current Events
  • is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to