Bio-control of Canada Thistle

PROJECT BACKGROUND
This family farm operation has been operated using sustainable practices for nearly 15 years. Reduced herbicide and ridge till have been used extensively. Canada thistle has created a road block to advanced levels of sustainable farming without herbicides as in organic farming.

As indicated in the project proposal, this effort was initiated to address the issue of Canada thistle infestations in cropland in the area of southwest Nebraska. An alternative was being considered to replace spot treating patches of the weed pest in cropland which currently has the high cost of labor and herbicides. The farmland being studied was a 250 acre irrigated tract with a crop rotation including wheat, sunflowers, corn and soybeans.

Releases of insects were made in patches of thistles in grassed areas of rangeland at the perimeter of the cropland being considered. This was done at the suggestion of various persons with experience in this field. Cropland, since it is disturbed with tillage, is not a good place to establish populations of the biological control insects.

PROJECT DESCRIPTION AND RESULTS
The damage to the patches by insects were observed. Two insects were used in the study. The stem mining weevil, Ceutorhynchus litura, and the defoliating beetle, Cassida rubiginosa, were released during the summer of 1996 into thistle patches for observation.

The damage to thistles by these predatory insects was much less than was desired. Much greater control is needed in the perimeter patches, where releases were made, in order to be feasible for control of the cropland thistles. In cropland acres under study here, thistle populations continue to flourish indicating much greater control is needed if these insects are to gain control in cropland areas.

5/15/96. 500 (5 releases of 100 each) of the stem mining weevil were received from Biological Control of Weeds of Bozeman, Montana. They were immediately released at three locations in grassed areas:
- 200 insects at south end of Pond north of house herein called “North-House”
- 100 at northeast side of pond at East well and center pivot, “Northeast-Pond”
- 200 at southeast side of pond at East well, “Southwest-Pond”

6/22/96. 500 of the defoliating beetles (5 releases of 100 each) were received and immediately released in the same proportion and locations as the stem mining weevils.

6/25/96. A count was made of the plants of Canada thistle at the various sites in the study. North-House had 368 plants, Northeast-Pond had 264 plants and Southwest-Pond had 60 plants. These counts were made to compare to later counts to determine if changes were being made. During late summer of this year, serious flooding affected the observation sites, inundating especially the Northeast-Pond site.

7/15/96. observation of the thistle patches where releases were made indicated that the defoliating beetles did excellent work of cutting holes in leaves and causing dead leaves, at least at the lower portion of the plant. However, the upper portions of the plants were not disturbed. This growth had taken place during the past several weeks, approximately, during the same period that rainfall had increased. Later observations also indicated an end to beetle activity. It appears that beetle activity ceases about July 1, allowing for much growth by the plants.

10/15/96. Gary Hein, Extension entomologist from Scottsbluff, NE. visited the sites of this study. He collected various plants from the sites for further analysis of damage by the stem mining weevil. The Southwest-Pond site analysis by Hein showed that 8 in 10 plants showed stem damage with 5 in 8 plants having severe damage. This indicated “fair” establishment. At the North-House site water level limited sampling, but thistle density was high. However, only 5 in 25 plants had damage by the stem miner. Hein says this is not much damage. A desired level is in the 90 to 95% damaged range. The Northeast-Pond site could not be located because of flooding.

1/21/97. An extension of the project was approved to provide for an additional year of observation.

7/9/97. There was evidence of activity of the defoliating beetle at the release sites of the previous year. Several of the beetles were found to indicate that the damage was very likely from the defoliating beetle. However, larvae activity and webbing were also noted within the plants. This may have been from some other “native” species. Stems were also observed and there were indications that stem miners were active within the stems. Observations of the adjacent cropland areas and thistle plants within the fields indicated little insect activity on thistles within the cropped areas.

7/28/97. Plants at the various sites were counted again to compare to the 1996 count. North-House had 401 plants, up from 368 a year earlier. Northeast-Pond had no plants to be found, but this was most affected by high water in late summer of 1996 and also during the summer of 1997. Southwest-Pond had 20 plants compared to 60 in 1996, but this was also affected by high water.

8/21/97. Ken Schneider visited the various release sites. It appeared that there was more damage from the beetles than from the stem miners. After splitting the stems only a few indicated stem burrowing by mining weevils. Some webbing in the leaves was observed with black insect droppings. The next day, thistles in range areas over a mile away from the release areas were examined. Webbing was observed along with damage to leaves similar to that done by defoliating beetles at the release sites. Gary Hein suggests that the webbing is due to the migrating native, the thistle caterpillar.

11/18/97. Gary Hein again examined treated thistles. The North-House patch indicated about a 30% infestation, about the same as a year earlier and not high to provide significant control, according to Hein. He did observe gall like damage at base of infested stems which often causes rot.

OUTREACH
No field days were held since it was felt that results of the insect releases were minimal. However, a copy of this report will be provided to the editor of the Nebraska Sustainable Agriculture Society newsletter where a summary may be published.

CONCLUSIONS
This project has given mixed results about the effectiveness of biological control of Canada thistle by insects in cropland. It is felt that high levels of populations and damage are needed in the rangeland areas to provide for a serious threat to thistles in disturbed cropland. These high levels were not obtained. Although flooding had some impact on the clarity of results, one can conclude that additional levels of insect damage are needed to be effective.

Future Considerations:
Various literature and persons associated with the study of Canada thistle indicate that certain fertility levels of the soils are conducive to the thistle’s infestation of cropland. Some sources suggest that low levels of calcium and humus in the soil (similar to the Project site) provide a good environment for Canada thistle. The operator of this farm hopes to address the fertility issue in coming years. Adequate moisture, such as in irrigation, also promote growth of thistle populations. Dry land crop areas are not nearly as susceptible as irrigated fields.

Another option of control is to moldboard the patches of thistles, and till the patches regularly to dry out the soil and starve the plants of moisture and nutrient. This has been suggested by numerous sustainable and organic practitioners. This would be possible on this farm since wheat is in the rotation. After wheat is harvested in July, several months of fallow tillage are available. Patches would have to be flagged at harvest or earlier. Another suggestion has been to moldboard plow patches in the fall. This would possibly disturb storage of nutrients in the fall and perhaps provide for winter kill of the plants. Any tillage of this type needs to be limited to small patches to avoid serious wind erosion during the winter and following spring, because residue on the soil surface would be eliminated.