Weed Control News

Canada Thistle Biocontrol Agent Can be Collected in Alfalfa

Hadroplontus litura (formerly Ceutorhynchus litura) is a stem-boring weevil introduced into North America from Europe for the biological control of Canada thistle (Cirsium arvense), a long-lived perennial weed. The host range of the weevil is restricted to C. arvense and some Carduus species, although it does not feed on Carduus nutans (musk thistle). One of the major limitations to the widespread deployment of H. litura as a biological control agent of Canada thistle has been the inability to collect large numbers of adults for redistribution. Both hand collections from the thistles and sweep nets have been used to gather adults. Hand collecting is labor-intensive, causing the commercial cost to be high, at about $2.38 per insect. Sweeping patches of thistle generally yields low numbers of insects. In a Canadian study, only 130 adults were collected in 5500 sweeps taken from 17 to 19 October, and 38 adults in 7400 sweeps from 15 July to 29 September despite relatively high larval populations in the vicinity.

In the summer of 1995, during routine sweep sampling of an alfalfa field, we noticed H. litura in our samples. On 21 Aug. 1995, we collected 25 adults in 30 sweeps, and on 24 Aug. 1995, we collected 52 weevils in 100 sweeps in the same area. Adult weevils were not found in adjacent grassland or wheat fields, but were found in a Canada thistle patch, although in much lower numbers (e.g. 12 adults in 320 sweeps). Although the thistle patch was in proximity to the alfalfa, the field was nearly thistle-free.

After identification of the weevils as H. litura, we initiated a sampling program in 1997 and 1999, to determine temporal occurrence and relative abundance in alfalfa. During 1997, we monitored the same field that had yielded the initial specimens. Two other local fields were sampled in 1999 because management changes at the first field had eliminated the Canada thistle patch. Fields were sampled each week, beginning on 7 May 1997 and 26 May 1999. Two samples were collected at each date, each sample consisting of 25 sweeps with a standard 38-cm net

The first H. litura specimens collected from alfalfa fields were found in late August, and were probably newly emerged adults, based on the scale patterns and state of ovarian development, determined through dissection. During a more extensive survey of alfalfa arthropods, H. litura was not detected when Canada thistle was absent from the vicinity of the alfalfa.

H. litura occurred over a nearly two month period. The greatest numbers of H. litura were present in alfalfa fields between 675 and 800 DDU (calculated as DDU = [(high C – low C) – 10], beginning 1 May). This narrow period of occurrence in alfalfa offers a practical way of timing collection efforts.

The presence of H. litura in alfalfa indicated possible dispersal periods for the adults. Dispersal of adults from Canada thistle may be in part a function of density. Infestation at several initial release sites located in Bozeman was slow to expand in the first few seasons, but after ten years weevils were found 9 km from these releases. Alfalfa fields may form a suitable, albeit temporary, habitat, providing shade and moisture for dispersing adults during the hot, dry portions of the summer. It is unlikely that the weevils were using the alfalfa as a source of food, as they have not been recorded as feeding on alfalfa.

In conclusion, we suggest that monitoring alfalfa fields near Canada thistle infestations known to contain H. litura may provide an easier source of this agent for redistribution efforts. Collection efforts should be timed to occur between 675 and 800 DDU and in alfalfa fields that are in proximity to patches of Canada thistle. Further studies which focus on movement between Canada thistle and alfalfa, and the magnitude of distances traveled will provide needed information for successful deployment of H. litura as a biological control agent of Canada thistle.

- P. M. Denke, S. Blodgett, and J. L. Littlefield, Montana State University

ACKNOWLEDGEMENTS

We acknowledge the assistance of Prairie Big Horn, David Boehm, Matt Carroll, Keri Fleming, P.J. Friede, Damian Higham, Andy Lenssen, Roses Little Owl, Bill Quimby, Amy Warde, Deb Waters, Sherri White, the Montana Minority Apprenticeship Program (MAPs), Greg Johnson, Kevin O’Neill, and Jim Story, and two anonymous reviewers.

Natural Enemies of Tamarisk

Although pretty at certain times of the year, saltcedar is an invasive exotic weed that is harming both agriculture and the environment. Here, it is overtaking native vegetation along the Gila River in Arizona. Photo by Jack Dykinga, USDA.Tamarisk or saltcedar (Tamarix spp.) has been classified as among the 10 worst noxious weeds in the U.S. They were originally introduced as ornamentals and for windbreaks in the early 1800's, but now have become a weed that replaces native vegetation and lowers the water table, and are a target for a biological control effort by the USDA.

Beginning in 1987, exploration for natural enemies was conducted in China, France, Israel, Kazakhstan, and Turkmenistan. More than 200 natural enemies of saltcedar were found in China and the former Soviet Union. A leaf beetle, Diorhabda elongata, and a mealybug, Trabutina mannipara, that feed exclusively on saltcedar were sent to USDA-ARS quarantine labs for further study. Last summer, the beetle was approved as the first biological control agent for saltcedar. The adults and larvae feed on saltcedar leaves, repeatedly defoliating the tree.

However, there is a complication with releasing this beetle to destroy saltcedar. This invasive plant has replaced native willows, and has become an important nesting plant of the endangered southwestern willow flycatcher. In order to protect this bird, the existing vegetation can't be removed too quickly (as the beetles might do) until native plants are restored to provide nesting habitat. Currently the project is entering a 3-year experimental phase, evaluating the beetles in cages first to monitor the rate at which they damage the saltcedar. Eventually they will be released in other critical habitats when their impact can be predicted.

Another potential natural enemy introduction is Coniatus sp., a weevil from France, China, and Kazakhstan. This defoliator may survive better than D. elongata because the weevil pupates on the tree, whereas D. elongata pupates on the ground and is susceptible to drowning in wet areas where saltcedar often occurs.

Source:

Stelljes, K. B. and M. Wood. 2000. Foreign agents imported for weed control. Ag. Res. 48(3): 4-9.


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