RESEARCH
Successful Biological Control of the Musk Thistle
in Oklahoma Using the Musk Thistle Head Weevil
and the Rosette Weevil
M. Roduner, G. Cuperus, P. Mulder, J. Stritzke,
and M. Payton
ABSTRACT Rhinocyllus conicus Froelich, the musk thistle head weevil, was released in northeastern Oklahoma beginning in 1991 for
biological control of Carduus nutans L., the musk thistle. By 2001, weevils had been released in 34 counties. Trichosirocalus horridus
(Panzer), the rosette weevil, was released in six counties in 1998; and additional introductions in 2000 and 2001 brought the total number
of county releases for this species to 29. Release areas were surveyed in 2000 and 2001 to determine the level of weevil establishment. Head
weevils were recovered from 30 of 34 counties. Thistle populations were considered well infested if 30% of the heads had >4 larvae or pupae
present. In 63% of the counties, ≥25% of the sites were well infested. Thistle densities had been reduced by 25–90% in 13 counties in
Oklahoma where weevils had been released. Rosette weevils were recovered in three of the six original release counties and in one county
where no releases were made until 2001. A combination of head and rosette weevils in the rosette weevil recovery areas provided a synergistic
reaction with thistle density reductions occurring faster than in sites where only head weevils were released. Head weevils are established in
Oklahoma and are effectively reducing thistle infestations. Rosette weevils also are established in several of the1998 release areas.
C
arduus nutans L., the musk thistle, is an exotic noxious
weed that was accidentally introduced into the Unites States
during the 1860s from Europe. Musk thistle has been present
in Oklahoma since the 1940s. By 1960, it had spread through 29
counties in northeast and central Oklahoma (Stritzke et al. 1999);
and by 2001, musk thistle was reported in 32 additional counties,
thus 61 of 77 counties in the state were infested (Fig. 1). Thistle
infestations are most frequently found in pastures, ditches, and abandoned areas. It displaces all other forms of herbaceous vegetation,
reducing pasture yield. One musk thistle plant on 1.49 m2 (6,711
plants/ha) competing for space, light, and nutrients can reduce pasture yields by 23% (Trumble and Kok 1982). In addition, cattle
refuse to feed on thistle plants because of the thorny stems and
leaves (Rees 1991).
The Oklahoma state legislature declared musk thistle a locally
noxious weed (Northeastern Oklahoma) in 1994. In 2000, the legislature upgraded that status to encompass the entire state and provided specific rules on regulating density and control of the plants
(Oklahoma Noxious Weed Law Rules 2000). Under these guidelines, a heavy density of musk thistle is 24 plants/ha, with a requirement that landowners prevent thistles from producing seeds. Landowners are considered out of compliance if there are >24 plants/ha or
the plants are producing seed, or both. Once a landowner is determined to be out of compliance, fines are set at $1,000 per day. The law
also gives counties the authority to spray herbicides on private property and apply those costs to a landowner’s property taxes.
Because of unsatisfactory long-term control using chemical, cultural, and mechanical methods, the Oklahoma Cooperative Extension Service searched for new ways to control musk thistle. Kansas,
Missouri, Texas, and Nebraska have successfully used biological
112
control (Fick and Peterson 1995, Wilson et al. 1996, Smith 2001,
Jackman 2002a) to combat musk thistle infestations. Reductions in
thistle stands in Missouri using Rhinocyllus conicus Froelich, the
musk thistle head weevil, led to its introduction in Oklahoma during
1991. Subsequently, Trichosirocalus horridus (Panzer), the rosette
weevil, was introduced into seven Oklahoma counties in 1998 to
enhance control of musk thistle.
In Virginia, Kok and Pienkowski (1984) used head weevils for
long-term control of musk thistle, following thistle infestations from
head weevil release to thistle stand collapse. Their 12-yr study in the
same location showed a distinct pattern of thistle reduction. Head
weevil larvae infest musk thistle seed heads and consume developing
seeds and receptacle tissue. Each year less seed is produced, and
fewer seeds are added to the soil seed bank. About 6 yr after weevil
release, head weevil numbers were very large, and thistle stands collapsed in 10 yr (Kok and Surles 1975, Kok and Pienkowski 1984).
Large numbers of weevils per head are not needed to significantly
reduce seed production. One or two larvae per head causes some
seed reduction (Surles and Kok 1978); four to five larvae per head
leads to a 55% reduction in mature seed; and an increase to nine
larvae per head can reduce mature seed by 98% (Rees 1991).
Rosette weevils do not directly compete with head weevils. The
rosette weevil larvae tunnel into plant roots feeding on the meristematic tissue causing sublethal damage. Feeding causes a black mass of
frass and necrotic tissue in the rosette center; apical dominance is
broken; and multiple short stems are produced with a reduction in
heads and seed production. (Kok et al. 1986) (Fig. 2). Reduction in
stands occurred in as few as 3–4 yr (Kok et al. 1986).
Since 1991, when head weevil and rosette weevil were first released in Oklahoma (Stritzke et al. 1999), the only available inforAMERICAN ENTOMOLOGIST • Summer 2003
Fig. 1. Musk thistle locations in Oklahoma verified by survey. 1990
locations verified by Bill Stacey, Oklahoma Cooperative Extension
Service; 2000 and 2001 locations verified by Oklahoma Cooperative
Extension Service surveys and this study.
mation pertained to where and when releases occurred. Although
no data were taken initially on thistle stand size or density during
release, there was abundant anecdotal evidence that head weevils
were reducing thistle stands in the earliest release sites. The purpose of
this study was to provide a detailed account of thistle infestations, and
the population levels of head weevils and rosette weevils in 2000 and
2001. In this article, we compare survey data obtained during these 2 yr
and anecdotal information with the thistle stand data of Kok and
Pienkowski (1984) from Virginia. This information will help to establish a baseline for future musk thistle biological control in Oklahoma.
Materials and Methods
During the summers of 2000 and 2001, 36 counties were surveyed . Counties were chosen on the basis of head weevil releases
that occurred from 1991 to 1999. Assessments followed several
steps. We checked records from previous head weevil releases, and
we contacted county Extension Educators for updated release lists.
In areas where Extension Educators could not supply information,
we contacted county road crews and Department of Agriculture
employees for specific thistle information. We sent letters to landowners who had made releases to obtain permission to check thistle
infestations on their land, and ≥50% responded.
Musk Thistle Infestation. Musk thistle infestations were recorded
by location using a gazetteer with the official Oklahoma road numbering system (DeLorme 1998). Levels of thistle infestation were
compared with the levels in Oklahoma’s Noxious Weed Law Rules
as follows:
(1) Light infestation: <2 plants/acre (5/ha)
(2) Medium: 2–9 plants/acre (5–24/ha)
(3) Heavy: 10 or more plants/acre (25+/ha)
(Oklahoma Noxious Weed Law Rules 2000). Any site >1 ha
with musk thistle present was considered a large site, and we estimated the thistle density. These estimates were made with two transect
lines used to count the number of plants per 305 m2.
Head Weevil Detection and Density. At each location, 50 thistle
heads were removed randomly throughout a thistle patch that constituted the sample area. Thistle patches ranged in size from 0.5 to
50 ha. Samples were bagged and dated, and location was noted. We
also noted unusual situations that were encountered at each site
including damage from other arthropods, wind, cattle, and any tillage or mowing operations. After visiting known release sites, we
checked the surrounding area (within a 5-mile radius) in all directions by driving adjacent roads looking for additional thistle plants.
Thistle heads were collected from these adjacent areas to assess the
spread of weevil populations. Where information was not available
Fig. 2. Rosette weevil
damage, Rogers
County, Okla., Bell
Ranch. (top left)
Rosette weevil damage
in musk thistle rosette
with necrotic tissue in
the plant center.
(bottom left) Musk
thistle plants stunted by
rosette weevils, ~550
mm; note multiple
stems caused by larval
damage, May 2002.
(right) Normal musk
thistle plant, ~1,450
mm. Photo of normal
height plant located in
Payne County, OK
(May 2002) provided
for comparison with
rosette weevildamaged plants. There
were no normal plants
in the Bell Ranch
pasture during 2001 or
2002 to use for
comparison.
AMERICAN ENTOMOLOGIST • Volume 49, Number 2
113
Fig. 3. Oklahoma counties with reductions in individual musk thistle stand
densities ranging from 25 to 90%. Counties listed are those surveyed in
2000 and 2001. Numbered counties are referred to in the text.
about weevil releases, or in counties bordering release areas, we
traveled roads in a grid pattern to cover as much territory as possible.
In 2000, 14 counties in northeastern Oklahoma were surveyed
(Fig. 3). Digital photos were taken of known thistle densities and
used to estimate densities in large pastures and fields. Photos and
detailed maps were also used to verify thistle infestations. Large
thistle infestations with head weevil infestations were rechecked in
2001. In 2001, 22 additional counties were surveyed, most in central and western Oklahoma.
Both years’ thistle heads were brought back to the laboratory
and frozen until they could be processed. Each head was measured
in millimeters at the receptacle base, and the number of larvae, pupae, or pupal cases counted. In 2001, weevil-infested thistle heads
were assessed based on the percentage of physical damage and the
number of larvae, pupae, or pupal chambers recorded; 8,375 thistle
heads were dissected in the laboratory, and 4,962 were infested with
one or more larvae, pupae.
All heads with four or more larvae and/or pupae per head were
considered “well infested”. The percentage of well-infested heads in
Fig. 4. Head weevil-infested thistle heads. Physical damage increases
dramatically when numbers of pupae reach six and higher in all head
diameters. (4,962 thistle heads analyzed by Sigma Plot [SPSS 1986–
1987])
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each sample of 50 heads was calculated. A site was designated as well
infested if 30% of heads at a site contained four or more larvae and/
or pupae. The number of well-infested sites per county were divided
by the total number of sites sampled to give the percentage of wellinfested sites in each county.
Percentages of physical damage compared to weevils per thistle
heads were analyzed using Sigma Plot (SPSS 1986–1987) to visualize the changes in damage levels (Fig. 4). This plot is a response
surface plot of the means of the data reflecting the number of pupae
and diameter of the thistle head. It is not based upon any model;
therefore, no assessment of fit is given.
These numbers were used to assess weevil establishment in each
county. Any additional insects observed to cause damage to thistle
seeds also were noted. The effects of these additional arthropods on
thistle seed and/or plant production were assessed by counting the
number of insects per plant or head and ultimately determining the
damage to seed heads.
Rosette Weevils. In 2001,we surveyed counties with rosette weevil releases using the same procedure as for head weevils. In early
spring, before the flower stem elongated (bolted), plants were dug
up and brought back to the laboratory to count the number of
larvae in the meristematic tissue. Any rosette weevil adults captured
outside known release areas were preserved as voucher specimens in
the Oklahoma State University Research Collection to verify the
spread of rosette weevils from previous release sites.
Results
During informal conversations, landowners and growers discussed their impressions of using head weevils for thistle control.
Satisfaction with head weevils appeared to depend on the growers’
initial expectations. All growers who stated they were unable, physically or financially, to use herbicides were willing to allow weevils
time to reduce thistle stands. Anecdotal reports were received that
weevils began reducing thistle stand density in as little as 5 yr. Growers who were impatient or did not understand the thistle–weevil
interactions, mowed thistles out of phase with plant and weevil
growth, or applied herbicides after bolting and subsequently complained of slower reductions in their thistle stands.
Musk Thistle Density. Musk thistle infestations in Oklahoma in
1990 covered 29 counties. In 2001, 61 counties were infested with
this noxious weed. For this 2-yr study, initially, thistle density was
counted as the number of mature (bolted) plants per 0.01 ha (≥305
m2). In 90% of the sites visited, thistle densities were greater than the
numbers established as a heavy infestation (24 plants/ha) under the
Oklahoma’s Noxious Weed Law. Some sites had thistle densities
approaching 50,000 plants/ha (20,243 plants/acre). We made notations of areas with extremely heavy plant densities.
Sites with heavy infestations of weak thistle plants (i.e., thin
stemmed, few leaves, few seed heads, etc.) in 2000 either did not exist
in 2001, or their densities were reduced dramatically. Reductions in
thistle densities were observed, from as high as 20,000 plants/ha (2
plants/m2) in 2000 to as low as 10 plants/ha in 2001 (Fig. 5).
Counties that experienced reductions in thistle density had 25–90%
fewer plants.
In three counties where rosette weevils were recovered, there was
an additional reduction in thistle plant vitality. Thistle plants damaged by rosette weevils were short (average <0.5 m) compared with
healthy plants (average 1.5m). Larvae consumed the main growth
point, which broke apical dominance, causing plants to produce 3–
6 thin stems instead of one main stem. Damaged plants also had
fewer heads (0–3) than healthy plants (5–35).
Head Weevils. Large numbers of head weevil larvae were supported by thistle heads. When 15–25 larvae or pupae were present
in a single head, it often appeared dry, shriveled, and brown (Fig. 6).
AMERICAN ENTOMOLOGIST • Summer 2003
The percentage of physical damage increased rapidly when weevil
levels exceeded 6 per head. In addition, 15 or more weevils caused
100% damage in heads 20 mm and larger (Fig. 4).
Weevils were considered established at a release site (Fig 7A) if
larvae, pupae, or empty pupal chambers were found inside thistle
heads. Of the 36 counties surveyed (Fig. 7B), 2 had no present
record of musk thistle, and 3 counties had thistle but no head weevils. All thistle heads checked in 7 counties had fewer than four
individuals per head. Four counties had up to 24% of well-infested
sites; in 6 counties 25–49% of all sites were well infested. Ten counties had 50–74% of well-infested sites, and 75–100% of sites in 4
counties were well infested. Thirteen counties had reductions of 25–
90% in thistle density between 2000 and 2001 (Fig. 5). Head weevils have been in these areas for a minimum of 8 yr. Thistle density
reductions were especially dramatic in Okfuskee, Nowata, Craig,
and areas of Cherokee, Delaware, and Adair Counties (Fig. 3).
In 2000, nine sites in Okfuskee County (Fig. 3) were surveyed.
Thistle densities were 2,500–7,500 plants/ha with >75% of the sites
well infested with weevils. In 2001, these sites were sampled again,
and 50–90% reductions in thistle density (3,750–250 plants/ha)
were noted.
The eastern border of Nowata County and western border of
Craig County along US Hwy 60 (Fig. 3) contains several reclaimed
mines. The reclamation land and surrounding areas have been heavily
infested with musk thistle. Landowners describe a thistle infestation
Fig. 5. Reduction of musk thistle at the Peabody Coal mine reclamation
area, Craig County, OK, U. S. Hwy 60 and CR 210. (top) June, 2000.
Plants weak with few blooms. (bottom) April, 2001, ~9 yr after head
weevil infestation. Few plants or rosettes present.
AMERICAN ENTOMOLOGIST • Volume 49, Number 2
Fig. 6. Comparison of uninfested and head weevil infested musk thistle
heads. (left) uninfested with all seeds released (right) heavily infested,
no seeds, head did not develop.
>4,100 plants/ha from the 1980s into the early 1990s. Head weevils
were released in this area in 1991 and, according to landowners,
thistles have not been sprayed or mowed because of the excessive
costs. Musk thistle density subsequently declined up to 90% in
these areas. Thistle plants present in 2000 were thin and had only
3–4 heads per plant; rosettes were present in low numbers (Fig. 5A).
In 2001, thistles were difficult to find at all of these sites (Fig. 5B).
A large area encompassing the southern border of Delaware
County and the northern borders of Cherokee and Adair Counties
(Fig. 3) had thistle infestations of 24,700–37,000 plants/ha as reported by landowners. Head weevils were first released in Cherokee
County in 1991 and Adair County in 1992, and then they were
routinely released in subsequent years. In 2000, >50% of these sites
were well infested by weevils. By 2001, reductions in the density of
thistles from those recorded in 2000 varied from 30 to 75% by site
with low numbers of rosettes present.
Rosette Weevils. By 2001, 29 counties are known to have had
rosette weevil releases (Fig. 8A.). In 2000 and 2001, rosette weevils
were recovered from three of the seven counties where initial releases
were made (Fig. 8B).
During 1998, rosette weevils were released within 1 mile of Jay in
Delaware County. During 2001, adult weevils were recovered in
thistle patches from pastures along SR 127, at sites 0.5 and 1.5 miles
west of US Hwy 59 (Fig. 8B). Approximately 60% of plants were
short with multiple stems and reduced head numbers.
In 1998, the first rosette weevil release in Rogers County was
made at the Bell Ranch, north and east of Claremore (Fig. 8B). This
site, ≥810 ha of reclaimed mine land, was infested at a thistle density
of >50,000 plants/ha. In 1991, head weevils were released and provided good control. Rosette weevils were released in 1998 to augment the head weevil population. In 2000, ≥40% of the weevils
collected for redistribution were rosette weevils. In 2001, ≥80% of
the total weevils collected were rosette weevils. Adult weevil densities
of up to 100 per plant were noted during collection. By 2001, the
thistle-infested area was reduced to ≥16 ha with almost 100% of
plants showing severe damage from rosette weevils (Fig. 2A).
Adair County did not have an official rosette weevil release until
2000; however, adults were collected 2 miles south of Chewey, Okla.
(Fig. 8B), several weeks before the rosette weevil release. Head weevil
releases conducted in 1991–1993 were done with weevils collected
in Missouri. An occasional container of head weevils had one or two
adult rosette weevils, and this site likely received one of the contaminated containers (W. Stacey, personal communication, Oklahoma
Cooperative Extension Service; Area Extension Entomology Specialist). If this is true, a significant rosette weevil population developed over 10 yr from relatively few weevils. The original release site
has significant weevil damage, and the weevils are present (damaged
115
Fig. 7. (top) Head weevil release sites from 1991 through 2001.
(bottom) Head weevil recovery sites in 2000 and 2001, showing the
percentage of sites in a county with 30% or more of the heads per site
infested with >4 larvae or pupae (L & P) per head.
plants and adults recovered) in surrounding pastures within a 1mile radius.
Other Insects. Three other insect orders were frequently observed
in thistle heads. These species damaged thistle heads and affected
seed production. In 2000, thistle heads from Cherokee and Delaware Counties had empty dipteran puparia. Several sites in each
county had up to five pupae per head. The puparia were clear, blue
or black, and empty, making identification impossible. Only occasional diptera puparia were present in 2001. During 2000, larvae of
Homeosoma electellum (Hulst), the sunflower moth, (Lepidoptera:
Pyralidae) were present in thistle heads in small numbers (1–2 larvae
in 2–3% of heads). These numbers increased greatly in 2001, with
100% of secondary and tertiary heads infested in thistle patches in
Payne County. The larvae fouled heads with their webbing and frass,
making seed release difficult (Fig. 9A). Some of the smaller heads (8–
10 mm) were totally consumed.
Euphoria sepulchralis (F.), flower scarab beetles, (Coleoptera:
Scarabaeidae) were present in 2000–2001 in new blooms. Thistle
heads were carefully cut apart to avoid disturbing the feeding beetles.
Beetles were observed biting off the tops of developing seed. Previously damaged areas turned black and shriveled (Fig. 9B). Five to
seven beetles were present in large thistle heads, buried head downward and actively feeding. Thistle heads with 5–7 beetles feeding had
up to 75% of the seeds damaged.
Discussion
Head weevils have been successful in reducing musk thistle populations in several other states. Head weevils have been present in
Virginia since the early 1970s with studies following the patterns of
weevil increase and thistle reduction (Kok and Pienkowski 1984).
Musk thistle in western Missouri has been reduced to the point that
weevil collectors from Oklahoma were unable to find enough weevil
numbers for redistribution (W. Stacey, personal communication).
Missouri, Nebraska, and Kansas have active biological control programs collecting head weevils and releasing them in new areas. All of
116
these states have areas with successful control of the musk thistle
(Fick and Peterson 1995, Wilson et al. 1996, Smith 2001).
In Oklahoma, head weevils are associated with reduced musk
thistle densities where they have been established for longer than 6
yr. Since the release program began, 13 counties have experienced
reductions of thistle stands (Fig. 3). Weevils were recovered from 30
of 34 release counties and 25 counties have sites that are considered
well infested (Fig. 7B). Head weevils have spread from their original
release sites infesting thistles in surrounding areas, generally following the prevailing winds. This pattern is similar to that observed in
Virginia (Kok and Surles 1975). Counties with only one or two
known release sites had weevils scattered throughout the county. In
Payne County, for example, head weevils were released on two sites
in the early 1970s and are now established throughout the county.
Thistle infestations were conspicuously absent in areas with dense
grasses, healthy pastures, or heavy shade. The majority of infestations occurred in ditches, pastures with weak stands of grass, disturbed or abandoned areas, and hard-red winter wheat fields. Land
with gullies or water courses that either facilitated seed movement or
trapped it had extremely heavy thistle infestations.
Reductions in thistle size and vitality, with head weevil infestation
and dense pastures were observed in previous studies done in Virginia. Researchers followed thistle seed consumption by head weevils. The highest quality seed was consumed first, leaving lower quality seed to enter the soil seed bank. As the vitality of thistles decreased, pastures were able to recuperate, competing well with thistles
for space and nutrients. This competitive effect appeared to be synergistic resulting in a “crash” of the thistle population (Kok and
Pienkowski 1984, Kok et al. 1986).
Locations in Oklahoma where weevils have been present for 6–
10 yr resemble this scenario. Thistle plants in these areas were thinner and had fewer heads than thistle plants in areas where weevils
were not present.
Although no data on thistle density were taken at the time of
weevil releases, by comparing what landowners reported as thistle
density at the time of these releases with the reductions observed
Fig. 8. (top) Rosette weevil release sites from 1998 to 2001. (bottom)
Rosette weevil recovery sites from 2000 to 2002. Numbered counties
are referred to in text.
AMERICAN ENTOMOLOGIST • Summer 2003
Fig. 9. Damage from other insects: (left) Webbing and frass caused by
the sunflower moth fouling a thistle head. (right) Individual musk thistle
seed damaged by the flower scarab beetle.
from 2000 to 2001, the time frame and pattern of thistle reduction
follow the results from Virginia reported by Kok and Pienkowski
(1984). Head weevil populations increased, and the thistle populations collapsed about 10 yr after the weevils were introduced. Low
numbers of rosette weevils follows the pattern of seed reduction
outlined by Surles and Kok (1978) and Rees (1991) because fewer
seeds were available to germinate in the soil seed bank.
Rosette Weevils. Adding rosette weevils to locations already infested with head weevils provides additional control (Cartwright
and Kok 1985, Kok et al. 1986). Recovery areas for rosette weevils
in Adair, Delaware, and Rogers Counties all had head weevils present
first. When rosette weevil populations began to increase, thistle populations dropped faster than with head weevils alone, verifying the
results of Cartwright and Kok (1985) and Kok et al. (1986). Because the weevils occupy different feeding niches, there appeared to
be no direct competition.
Rosette weevils were first collected in northern Kansas in 1998
for release in Oklahoma. In 2000, weevils were collected from the
state of Kansas and Rogers County, Okla., and were released in 22
counties in Oklahoma. In 2001, weevils were collected in Rogers
County and distributed to additional locations in counties that had
earlier releases and at least three additional counties. Rosette weevils
increased so rapidly that one site supplied adequate numbers for
new release locations without reducing effectiveness in that area.
Personnel from several counties in western Oklahoma shared weevils with other counties.
Although no data on thistle stand density or size were taken
during these initial releases, in the three counties where they have
become established, rosette weevils in combination with head weevil
populations appear to have caused musk thistle decline faster than
in pastures with head weevils alone. The Bell Ranch in Rogers County
provides overwhelming evidence of this phenomenon. The landowner reported thistle densities of ≥50,000 plants/ha in 1991 when
the first head weevils were released. In 1998, he estimated that thistle
plants had been reduced by >50% (<20,000 plants/ha). This was
the same year that rosette weevils were first released into the site. In
2002, 4 yr after releases of rosette weevil, thistle density stood at
<15 plants/ha, and those plants remaining did not develop viable
seed heads.
The county Extension Educator reported that all rosettes examined during the spring of 2002 contained several hundred rosette
weevil larvae. Surviving plants averaged <0.3 m tall and had multiple
stems and few heads (Fig. 2B). Rosette weevils have spread into
neighboring areas and were collected in large numbers from these
sites for redistribution. In 2003, the Bell Ranch along with most of
Rogers County, Nowata County, and Okfuskee County are no
longer viable locations for collection of weevils for the Oklahoma
AMERICAN ENTOMOLOGIST • Volume 49, Number 2
Musk Thistle Roundup because of the low population of thistles
throughout the area (Pratt 2003).
Other Insects. Three other insects were found in thistle heads
that had some effect on thistle seed production. Dipteran larvae
destroyed some receptacle tissue and associated seeds. Sunflower
moth larvae, although pests of commercial sunflower fields, also
provided thistle seed reduction. Infestations of sunflower moths
have been noted in other areas of the country, and their effectiveness
was related to infestation levels (Goyer 1978). Population densities
of sunflower moths vary from year to year, and they can be considered beneficial, if they do not affect nearby cash crops (McCarty
1982). Flower scarab beetles also were present in thistle heads in
numbers large enough to be noticeable.
In addition to reducing the amount available for pollination and
consuming nectar and pollen (Hayes 1929), all three additional insect species fed on newly developing seeds, causing severe damage
and preventing seed maturation. Although anything that destroys
developing seed could be considered beneficial, damage on the plant
did not appear to be consistent enough to provide reliable control.
Additional studies will be needed to determine if any of these insects
are viable and reliable candidates for biological control releases.
Need for Vigilance. Actual musk thistle densities are much higher
than those described in the Oklahoma Noxious Weed Law Rules.
According to these criteria, the legal definition of high thistle density
is 10 plants/0.4 ha (1 acre) or 1 plant/404.6 m2 (4,356 ft2). Thistle
density in several locations was as high as 50,000 plants/ha (20,243
plants/acre) or 1 plant/0.2 m2 (2.2 ft2). Grower education about
musk thistle growth patterns and its ability to reproduce explosively
is of paramount importance. It is easy to discount densities as low as
those in the Noxious Weed Law, but thistles must be controlled
when populations are low to prevent heavier infestations that require increased time for beneficial organisms to assume a regulatory
role.
Head weevil population increases are rapid (Kok and Surles 1975,
Rees 1991), so sites that were considered well infested (30% of
heads with >4 larvae or pupae per head) would be heavily infested
with weevils within 1 or 2 yr. Rosette weevils have experienced a
greater rate of increase than head weevils, especially at the Rogers
County site. Counties with head weevils and rosette weevils can
anticipate rapid reductions of thistle infestations.
This snapshot of musk thistle biological control taken during
2000–2001 shows a pattern similar to the one observed by Kok
and Pienkowski (1984) during their long-term studies of biological
control in Virginia. Although previous data for Oklahoma is unavailable, anecdotal evidence throughout the state is consistent. Musk
thistle densities were much higher before the introduction of head
and rosette weevils, and in the 10 yr since the release program began,
thistle densities are now lower.
Landowners who have given weevils a chance to work are not
only pleased with the results but are telling others about their success. Requests for weevil releases are increasing each year as landowners see the results in neighboring areas.
The head and rosette weevil release program has been a resounding success in northeastern and central Oklahoma. With 1 musk
thistle plant on 1.49 m2 (6,711 plants/ha) reducing pasture yields by
23% (Trumble and Kok 1982), and many thistle stands three to
four times more dense, significant thistle reduction will decrease
costs associated with land management.
Economic Benefits. Based on a survey of Oklahoma growers, the
average amount of improved pasture for each producer ranges from
16 to 64 ha (New 1997). The average cost of controlling musk
thistles for a year using 2,4-D herbicide is estimated at a minimum of
$1.85/ha for 0.39 liters of material per ha (not including application
costs) (Medlin et al. 2003). There are about 7.1 million acres of
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improved pastures in Oklahoma. Therefore, the statewide cost of
controlling musk thistle with herbicides for 10 yr, if all improved
pastures were infested, would exceed $131 million.
Substantial savings are possible when musk thistle weevils are
integrated into a musk thistle management system. Spraying of pastures would be phased out after 3 yr, and no annual border spraying would be required. Costs associated with an integrated approach
(still relying on herbicide application for the initial 3 yr) using weevils would be $5.55/ha for spraying over the 3 yr and $3.45/ha
associated with trips to collect weevils. This represents a savings of
at least $67 million over the first 10 yr, while at the same time reducing the amount of herbicide broadcast into the environment.
Up to this point, the estimated costs ($3.45/ha) associated with
weevil collections in Oklahoma have been provided by Extension
Educators and the Integrated Pest Management Program at Oklahoma State University at no cost to participating producers.
Not without Controversy. There is currently, some controversy
in adjoining states about the use of head weevils for thistle control.
This controversy relates to several species of Cirsium thistles being
attacked by the head weevil. These thistles include the Platte thistle
(C. canescens), Pitcher’s thistle (C. pitcheri), and wavyleaf thistle (C.
undulatum) (Louda 1998, 1999). In addition, head weevil infestations have been recorded from the milk thistle (Silybum marianum;
Jackman 2002b). Presently, from this list of thistles, only the wavyleaf
thistle is found in Oklahoma (Stritzke and Tyrl 2000). Wavyleaf
thistle heads were checked during this study when they appeared to
be at an appropriate stage to support head weevils. Little to no
survival of weevils was recorded on this species of thistle. It is unknown whether the hotter, drier climate in Oklahoma affects head
weevil survival in nonhost species.
Thistle stands in Oklahoma can be so extensive (up to 50,000
plants/ha and 100+ ha in size) that all other plants are excluded, and
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AMERICAN ENTOMOLOGIST • Summer 2003
the land becomes unusable. With infestations of this magnitude and
only one other possible host present, the use of head weevils for
biological control has not been a serious issue. Members of the release
teams and the Oklahoma Cooperative Extension Service are aware of
these concerns; however, presently the benefits of using head weevils
for musk thistle control far outweigh the concerns. They have also
suggested that this question be revisited if other Oklahoma thistle
species become more common and are shown to be in danger.
In summary, the head weevil, has become established in Oklahoma, especially where it has been in place for at least 4 yr. During
that time, musk thistle infestations have been reduced in density and
plant vigor. Furthermore, the addition of the rosette weevil is increasing the speed of that reduction in thistle infestation. Efforts must not
stop at this point. Thistle populations in the western half of Oklahoma have had weevils for only 1 or 2 yr and need time to establish.
Continued releases of head weevil and rosette weevils and grower
education to manage them will reduce thistles in the remainder of the
state, allowing growers profitable use of their land.
Acknowledgments
The authors thank all the landowners and extension personnel
who provided information about thistle infestations. We extend our
sincere gratitude to laboratory technicians Tererai Nyamanzi, Audrey
Sheridan, and Penny Potter for processing thousands of thistle heads.
We are grateful to USDA–APHIS for all the survey and weevil release
work they have supported in previous years that brought us to this
point. We also thank our preliminary reviewers Patricia Bolin and
Thomas Phillips of the Department of Entomology and Plant Pathology at Oklahoma State University for their helpful comments
when reviewing this manuscript. This work was approved for pub-
lication by the director of the Oklahoma Agricultural Experiment
Station, and supported in part under project OKLO2173. This
manuscript is part of a thesis submitted by M. Roduner to Oklahoma State University.
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Mary A. Roduner has completed her degree requirements for a Mas
ters of Science in Entomology at Oklahoma State University and iscurrently located in Saba, Netherlands Antilles in the Caribbean,
where her husband is attending medical school. Gerrit W. Cuperus is
currently on disability leave from Oklahoma State University where
he serves as Professor and IPM Coordinator for the Division of
Agricultural Sciences and Natural Resources. His research interests
are related to IPM in stored products. Phillip G. Mulder, Jr. is Extension Entomologist and Associate Professor in the Department of
Entomology and Plant Pathology at Oklahoma State University. His
extension/research interests focus on IPM in alfalfa, fruit trees, pecans, peanut, soybean, grapes and 4-H youth programs. In addition, he serves as Extension Coordinator for the entomology portion of the department and President-Elect of the Southwestern
Branch of the ESA. Jimmy F. Stritzke is Professor Emeritus within
the Department of Plant and Soil Sciences at Oklahoma State University. His research/extension interests are in the area of management of native and improved pastures. He retired from the Oklahoma Cooperative Extension Service in June, 2001. Mark E. Payton,
is a Professor within the Department of Statistics at Oklahoma State
University. He currently teaches and serves as statistical consultant
for the Division of Agricultural Research and Natural Resources.
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