BIOLOGICAL CONTROL OF MUSK THISTLE

 DRAFT  Year: 1999

COOPERATORS:

Sam Hill, Landowner

Roy Jacoby, Landowner

Dr. Sid Kunz, Kerrville Livestock Laboratory, USDA-ARS

LBJ Ranch

Jake Landers, Landowner

Ted Masser, Landowner

COORDINATING AGENTS:

Bill Botard, CEA, Gillespie Co.

Dale Brandenberger, CEA, Kimble Co.

Sam Kuykendahl, CEA, Menard Co.

Warren Thigpen, CEA, Bandera Co.

Eddie Holland, CEA, Kerr Co.

John A. Jackman, Professor and Extension Entomology Specialist, College Station

Tom Fuchs, Professor and Extension Entomology Specialist, San Angelo

Allen Knutson, Professor and Extension Entomology Specialist, Dallas

Noel Troxclair, Jr., Assistant Professor and Extension Entomology Specialist, Uvalde

J. W. Stewart, Extension Entomologist (retired), Uvalde

Paul Boldt, USDA-ARS, Temple

Tom Robbins, USDA-ARS, Temple

SUMMARY: The thistle head weevil was first release by USDA-ARS in Texas as a biological control agent for musk thistle at the USDA Livestock Laboratory in Kerrville in1987. The thistle head weevil was found to be established in 1992 and in recent years infests up to 100% of the thistle heads at the Livestock Laboratory reducing the seed production from this weed. This site now serves as a nursery for the weevils and a total of 8445 weevils were subsequently collected and relocated to other sites around the state for biological control of this thistle. The weevil is now established in Dallas, Gillespie, Kerr, Bandera, Kimble, Menard and Deaf Smith counties. No releases were conducted in 1998 or 1999 due to national concern that the thistle head weevil infests related thistles. One of the related thistles that it infests is on the threatened and endangered species list but does not occur in Texas.

PROBLEM: The musk thistle, a member of the Carduus nutans L. (Compositae) complex, is a noxious weed in pastures, rangelands, croplands, along state highways and even in urban areas in many parts of the U. S. It was introduced from Eurasia in about 1853, and is now found in more than 40 states. Musk thistle has become a problem in several counties in Texas primarily in the Hill Country but it is also becoming a problem in the Panhandle and Central Texas.

             Chemical control, although effective, is not usually permanent nor cost effective because the thistle produces large numbers of seeds, the seeds have a long life, the plant competes well with other plants and it has no natural enemies in this country. However, biological control of musk thistle has been successful in other states, particularly with the thistle head weevil, Rhinocyllus conicus (Froelich). The thistle head weevil has recently been established in Kerrville, Texas, but this weevil migrates slowly and manual redistribution of the weevil to other areas is important to effect control in a reasonable time frame.

            Three other biological control agents, a rosette weevil, Trichosirocalus horridus, and a syrphid fly, Cheilosia corydon, that attack the roots and a flea beetle, Psylliodes chalcomera, that feeds on leaves, have also been released by USDA-ARS in Texas. The flea beetle has been recovered from the wild but establishment of these species has not been confirmed.

OBJECTIVE: The primary goal of this project is to increase the use of biological control as a key method to control musk thistle in Texas.

MATERIALS AND METHODS:

Establishment.

            Thistle head weevils were originally introduced into Texas in 1984 at Ellis County by USDA-ARS but this attempt at establishment was not successful. Subsequent releases in 1987 in Kerr County by USDA-ARS resulted in the population that was detected at the Kerrville site in 1992. This site was used as the source for collection and redistribution efforts from 1992 to present.

Redistribution.

            Manual redistribution of thistle head weevils is the primary method of spreading the weevil to new locations. Adults weevils can be collected in the field when thistle heads are blooming. At Kerrville this is about April 15 but it may change in other parts of the state based on location and climate.

            Redistribution efforts were conducted with the following steps:

1. Select a release site. An appropriate site is any dense patch of thistles that will not be disturbed for 3 to 5 years after the release. Select a site where cattle will not be grazing because cattle interfere with thistle head weevil activity.

2. Obtain starter colonies of thistle head weevils. Spring emerging R. conicus can be collected at established sites in April before they complete egg laying. Collect adults directly from the plant heads using an aspirator. Alternatively, use a beating sheet and an aspirator to collect adult weevils as they feed on thistles and lay eggs in the heads.

3. When weevils are received, keep them in a cool place (less than 80 degrees F) because high temperatures will kill them. Paper cartons with tight lids are appropriate for holding and transporting weevils. Release them as soon as possible.

4. Release a minimum of 300 to 500 weevils per site depending on the size of the thistle infestation. Starter colonies are best released when thistles have elongated and buds have formed but prior to flowering. Release adult weevils among dense thistle patches away from traffic and in non-windy areas near trees or shelter. Release weevils in the evening, not during the hottest time of the day. For widespread thistle infestations, release weevils at a number of sites.

5. Monitor weevil establishment. Make additional releases only if weevils fail to become established. Weevil impact on thistles takes at least 3 years and perhaps as long as 7 years after establishment at a site.

Detection Surveys.

Weevils were detected by visual inspection of the plants during blooming. This procedure was augmented by the use of a sweep net or beating sheet which seems to collect more weevils per unit of time. A second method of detecting weevils is by inspection of heads for the presence of egg punctures, larvae in the head or damage after larvae have matured. This procedure can be conducted after the thistles head. A target time for this activity would be May 15 or later at the Kerrville site but this may be adjusted for other sites in the state due to climate or location.

Plant Density.

Reduction of the density of musk thistles through time at a site may be due to biological control agents but may be confounded by climatic factors. Musk thistle populations at weevil release sites were estimated by counting the numbers of musk thistle rosettes in three 30 m by 2 m (60 sq. m) transects per site. All rosettes, >5 cm in diameter were counted. Transects were generally parallel and placed about 3-4 m apart or greater through the center of the release site. Counts were recorded within one meter on each side of the transect providing six counts of rosettes in a 30 m by 1 m area. Musk thistle populations can be estimated in January or February. Later counts may be useful but plant competition may reduce the numbers dramatically.

Infested Heads and Plant Height.

To estimate the percentage of plants infested by the weevil at release sites, the center or primary lateral flowerheads of one hundred plants, twenty-five in each of four quadrants, were examined and the presence or absence of adult weevil feeding or oviposition was recorded. Height of five or more plants per quadrant was also measured as supplemental information but reflects the climatic conditions of the site more than plant damage by the weevil.

RESULTS:

            Musk thistle head weevil was first recovered in 1992 at the Kerrville Livestock Laboratory (Table 1.). Since that time there have been releases at 4 additional sites in Kerr County, 2 sites in Gillespie County, 1 site in Bandera County, 3 sites in Dallas County, 3 sites in Kimble County, 2 sites in Menard County and 1 in Deaf Smith County. Two sites in Kerr County have established weevil populations, and one additional site has occurred by natural migration. Establishment was documented at Dallas and two sites in Gillespie County in 1999 (Table 2). One site in Gillespie County near Harper has a positive detection that occurred by natural migration. The site in Bandera County, one site in Menard County and the site at Hereford all have positive detections. Establishment at Hereford has not recently been confirmed.

            Musk thistle stand counts (Tables 2 & 5) in 1999 were highest at the site in Bandera County at 13.9 rosettes/m². The next highest rosette densities were 11.2/m² at Ted Masser’s shed in Gillespie County and 2.39/m² at the Guadalupe River site in Kerr County. Musk thistle density had declined at several sites like the Kerrville Livestock lab, the race track at Junction and the pasture area at Masser’s probably due to the impact of thistle head weevils in conjunction with plant competition

            In 1999, the percentage of musk thistle heads infested with musk thistle head weevil (Tables 3 & 6) was very high (over 95%) at the Kerrville Livestock lab, the motel site, the Guadalupe River site (all in Kerr County) and the race track at Junction in Kimble County. The Landers’ site in Menard County had 57% of the heads infested and Masser’s pasture had 47% infested heads. The Bandera site had 12% infested heads which is the highest recorded there yet. Grazing of the thistles by goats may be interfering with rapid increase in weevil numbers at that site. The 20% infested heads recorded at the Davids house site in Dallas County was a reduction from last year. However, a reduction after the first year of establishment may be a result of weevil dispersal rather than actual decline in weevil numbers. The infested head rate, 15-17% at the LBJ Ranch does show that the weevils remain established there. Fire ants were observed attacking weevils in the heads at that site which is likely a contributing factor to weevil mortality.

            Plant height (Tables 4 & 7) in most locations in 1999 was quite high with averages over 1 m in Bandera, Kerr, Gillespie and Menard counties. The tall plants suggest that adequate moisture was available for good plant growth in 1999.

Table 1. Summary of Releases and Recoveries of Biological Agents for Musk Thistle Control in Texas

Biological control agent: Rhinocyllus conicus 

First

County            Location                                 Date (number released)                      Recovery*

Ellis                Waxahachie                           May 29, 1984 (600)                            (-)

Kerr                Kerrville Livestock Lab.        May 22, 1987 (750)                            (+)1992

                        Craig LeMeilleur property     May 22, 1987 (750)                            (-)

                        Hunt, Sam Hill's roadside       April 15, 1992 (100)                           (+)1995

                        Hwy 1340 at Guad. R.            April 15, 1992 (100)                           (+)1995

                        4 mi. W Ctr. Pt., Guad. R.      May 1, 1995 (250)

                        B.F. Syfan ranch, Mt. Home  May 1, 1995 (300)  

                        Shoney's Motel                       (migrated)                                           (+)1992

Gillespie         LBJ Ranch, pipe, haystack     April 23, 1992 (575)                           (-)1995

                        LBJ Ranch, pipe, haystack     April 15, 1997 (500)                           (+)1997

                        Masser's Flagg Creek             April 15, 1993 (500)                           (-)1995

                        Masser's Flagg Creek             April 15, 1997 (300)                           (+)1997                        Harper                                    (migrated)                                           (+)1995

Bandera          Warren Thigpen Ranch          April 19, 1993 (175)                           (+)1995

Dallas             UT Dallas                               April 15, 1993 (275)                           (-)1995

                        TAMU Center                        April 15, 1997 (500)                           (+)1997

Kimble            Bobby Chenault                      April 15, 1993 (500) 

                        Juntion - race track                 April 15, 1993 (250)                           (+)1997

                        Paul Henry                              April 15, 1993 (250) 

Menard           Roy Jacoby's                           April 15, 1993 (320)

                        Jake Lander's                          April 23, 1992 (500)                           (+)1997

Deaf Smith     Hereford                                 May 18, 1994 (950)                            (+)1995

Biological control agent: Trichosirocalus horridus

County            Location                                 Date (number released)                      Recoveries

Kerr                Kerrville Livestock Lab.        June 23, 1987                                     (-)1995

                        1 mi. SW Hunt, N. Guad. R.   June 23, 1987

                        0.5 mi. E. Hunt, N. Guad. R.  June 23, 1987

Deaf Smith     waste pond in Hereford         May 22, 1995 (400)

Gillespie         Harper                                     June 10, 1994 (400)                            (-)1995

Menard           J. Landers                               Dec. 4, 1995 (250)

Biological control agent: Cheilosia corydon

County            Location                                 Date (number released)                      Recoveries

Deaf Smith     Hereford, loading dock           May 3, 1995 (135)

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(* - = no weevils detected; + = weevils detected; years are last checked or first detection)

Table 2. Musk Thistle Stand Counts in 1999 in Texas

Table 3. Musk Thistle Head Weevil Recoveries in 1999 in Texas

Table 4. Musk Thistle Plant Height in 1999 in Texas

Table 5. Musk Thistle Plant Stand Counts at Various Locations in Texas from 1992 through 1998

Table 6. Musk Thistle Heads Infested by Thistle Head Weevils at Various Locations in Texas from 1992 through 1998

Table 7. Height of Musk Thistle Plants at Various Locations in Texas from 1992 through 1998

            ECONOMIC ANALYSIS:

Control Costs.

Estimated costs of chemical control per acre were collected from various sources. Total acres treated by individuals or organizations was also recorded. These estimates give an average cost of over $21.00 per acre to control musk thistle.

Cost of chemical control of musk thistles

CONCLUSION: Biological control of musk thistle using the musk thistle head weevil is underway in Texas. The time for the weevils to build up and show impact on the thistles is apparently 7 years or more in Texas which is longer than the 3 to 5 years reported in other states, that is probably due to environmental factors affecting the weevil and thistle. In the future, the thistle head weevil will become more important as a biological control agent to control musk thistle in Texas. At present, musk thistle head weevils are not commercially available in Texas. Sources of these weevils in other states generally provide weevils too late in the growing season for establishment in the Hill County of Texas. Ranchers and others who are interested in controlling or suppressing musk thistle using the release of musk thistle head weevils should contact their local County Extension Agent for additional information.

ACKNOWLEDGMENTS: We thank the USDA-APHIS-PPQ, Cooperative Agricultural Pest Survey Program for partial funding to support this project.

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