United States 

Department of 

Agriculture 

Forest 

Service 

Northern 

Region 

200 East Broadway 

P.O. Box 7669 

Missoula, MT 59807 

Forest Health

Protection

Report 03-7 March 2003 

EVALUATING RELEASE SITES OF CYPHOCLEONUS ACHATES

AS POTENTIAL INSECTARIES IN MONTANA AND IDAHO 

Nancy J. Sturdevant, Sandra J. Kegley and Jed Dewey 

Introduction 

Cyphocleonus achates (Fahraeus) is currently 

the most promising introduced biological control 

agent of spotted knapweed, Centaurea 

maculosa. This large root-feeding weevil mines 

the center of the knapweed taproot often 

stunting growth and can even cause plant 

mortality (Corn et al. 2003). Hundreds of 

releases, consisting of 50 to 200 weevils, have 

been introduced across most of the knapweed 

range in Montana and in surrounding states. 

The Western Montana Agricultural Center has 

produced most of the weevils for these releases. 

There, the weevils are reared in corrals made 

from aluminum flashing. Large knapweed plants 

are transplanted into the corrals and weevils are 

introduced. The knapweed is widely spaced, 

clipped and irrigated to favor the growth and 

production of both the knapweed and weevils 

(Story et al. 1996). 

The objectives of this project were to identify 

sites where weevils had been previously 

released that could be used to make future 

collections of weevils for redistribution and to 

evaluate the use of field corrals for collecting 

weevils. This project was funded through the 

National Fire Program. Weevils will be collected 

from insectary sites in future years and 

redistributed to sites that were burned and have 

mature knapweed plants or into other high 

priority areas identified by county or forest weed 

cooperators. We also verified the establishment 

of C. achates, collected information on site 

characteristics that might influence 

establishment, and impact data at most of the 

sites. The site information and impact data will 

be summarized in a later report. 

Methods 

In April and May of 2002, county and USDA 

Forest Service (Forest Service) weed specialists 

were contacted regarding C. achates releases 

within their counties or forests. Initially, 97 sites 

were evaluated for potential as field insectaries 

based on the following characteristics developed 

in 2001: 1) knapweed infestation > 3 acres or 

knapweed infestation with large plants, 2) good 

access, and 3) free from major disturbances 

(Sturdevant and Dewey 2002 ). Seventy-one of 

the 97 sites met the above criteria and were 

further evaluated for weevil establishment. 

Each site was visited between May and July. At 

each site 20 of the largest rooted plants within 

approximately 50 feet of the original release 

point were excavated, examined, and number of 

larvae found were recorded. Adult weevils were 

sampled between August 5 and September 6 by 

sweeping with an insect net over 300 m of 

knapweed surrounding the original release point. 

Global Position System coordinates were 

recorded to ensure accuracy in relocating 

releases in subsequent years. 
0 

During June, weevil corrals, similar to those at 

the Western Agricultural Research station, were 

constructed at six different locations, five in 

Montana and one in Idaho. However, we did not 

transplant, clip or irrigate plants; instead only 

existing field knapweed populations were used. 

Therefore, we do not expect to produce as high 

numbers of weevils as the Western Agricultural 

Research Center. At each of three of the six 

locations, two corrals were built measuring 

either 10 x 10 m or 25 x 25 m (fig. 1). Ten 

female-male pairs of weevils were introduced 

into one corral to augment the existing 

population. At the remaining three locations, 

only one corral was built. Mesh netting was 

placed over the tops of corrals to prevent bird 

predation. Between July 30 and August 28, 

weevils were sampled by searching within the 

corrals. Often, weevils were found congregated 

on the flashing inside corrals (fig. 2). To 

compare the efficacy of collecting in corrals with 

other methods, we also swept the knapweed 

surrounding the insectaries for adults. This 

comparison will be more meaningful when 

repeated in 2003 once weevils have had a 

chance to reproduce inside the corrals. 

Figure 1. Weevil corral located in Lewis & Clark County. 
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Figure 2. Weevils often gather on metal flashing of corral making collection easy. 

Results 

Forty eight of the 97 releases were made by 

weed specialists from 12 counties. Forest 

Service weed specialists made the remaining 49 

releases from 3 National Forests. Table 1 lists 

26 sites that were not suitable for sampling 

either the weevil or knapweed populations 

because the size of the knapweed infestation 

was too small, plants were too scattered or the 

plants themselves were too small for 

colonization by C. achates. Only initials are 

used to represent site names to protect the 

location and integrity of the release. These sites 

were initially considered to have a knapweed 

infestation sufficient to justify introducing C. 

achates and in some case other biological 

control agents as well. Many of these 

infestations have become reduced in size, or no 

longer exist due to 

herbicide spraying, or possibly the effects of the 

biological control agents. Intensive livestock 

grazing excluded other sites from the survey. 

Table 2 lists 47 sites that were suitable for 

weevil establishment but did not meet all of the 

criteria for a field insectary. C. achates larvae 

and adults were sampled at these sites to give 

land managers information on weevil 

establishment (Table 2). 

Table 3 displays C. achates population data 

from 19 sites that have potential as field 

insectaries. The potential field insectary sites 

are located in nine counties and on one National 

Forest. The percent of roots infested with C. 

achates across sites ranged between 0% and 

95% (mean. 45%). Number of adults found per 

minute of sweeping ranged between 0 and 2.5. 

It typically took about 10 minutes to sweep 300 

meters of knapweed. 
0 

This survey also found that at many sites C. 

achates became established after only 1 year 

following the release. Also, there were many 

releases of only 50 weevils that became 

successfully established. This is probably in 

part due to site characteristics and weather 

conditions at the time of release and following 

the release. 

Table 1. Sites Unsuitable for Sampling C. achates or for Use as Field Insectaries 

COUNTY/FOREST SITE (Initials Used) 

Beaverhead County C.C. 

Glacier County AMS R. 

S.B. 

Granite County A.N.R. 

I.R. 

F. #1 

F. #2 

Lewis & Clark County 

W.C./O. B. R. 

Madison County V.C. 

Powell County McQ. P. 

Sanders County B.P. 

McG. P. 

Silver Bow County S.P. 

Stillwater County F.P. 

L.C.#1 

L.C.#2 

S.#6 

S.#7 

S.#28 

Bitterroot National Forest 

S.#13 

Kootenai National Forest R.C. 

B.C.C. 

McC.F. 

M. 

M.C. 

Lolo National Forest 

S.L.#5 
0 

Table 2. Sites Suitable for Weevil Establishment but not Suitable for an Insectary. 

County/Forest Site 

Release 

Date(s) 

Number 

Released 

No. Larvae 

/20 plants 

No. Adults 

/300 m swept 

Broadwater Co. S.C. 1999 100 12 4 

Y.I. 1998/99 100/100 13 1 

Beaverhead Co. B.H. 2001 250 12 0 

Cascade Co. A.R.(Ulm) 1998 100 0 0 

H.P. 1999 75 0 0 

Glacier Co. O.A. 1995 100 0 0 

Granite Co. B.R. 1999 100 2 0 

Lake Co. W.of P. 1993 100 0 3 

Mineral Co. D.C.E. 1999 50 0 0 

R.O. 1997 100 0 0 

D.A.#2 1995 100 0 0 

D.A.#3 1995 75 4 0 

Powell Co. 

H.C. 2001 200 0 0 

Sanders Co. R.G. 1999 50 8 3 

R.P. 1999 100 3 3 

H.S. 1999 150 0 0 

1-90 1995 100 0 0 

Silver Bow Co. 

R.R. 1995 100 17 5 

Park 1995 50 3 1 

P.C. 1995 50 0 0 

Stillwater Co. 

Y.D. 1997 100 2 0 

B.H.R#16 1999 100 0 0 

B.H.R.#23 1999 100 0 0 

B.G. 1999 100 3 0 

G.D. 1999 100 0 0 

S.G.R.#1 2001 100 8 2 

S.G.R.#20 1999 100 5 0 

S.G.R.#21 1999 100 8 3 

T.C.#10 2000 50 0 0 

T.C.#10A 2000 50 2 0 

Bitterroot NF 

T.C.#11 2000 50 2 0 

E.C. 1995-96 300 3 0 

P.C. 1995 100 0 0 

W.P.C.#1 1999 50 9 9 

W.P.C.#2 1999 50 0 0 

W.P.C.#3 1999 50 0 0 

W.P.C.#4 1999 50 0 0 

C.C. 1997 75 4 2 

S.C. 1997 75 14 3 

2.6.M.L.R. 1999 100 7 0 

K.C. 1999 100 3 2 

Kootenai NF 

S.T.M.Y 1997 50 4 0 

P.C. 1999 200 2 0 

F.G. 1998 150 3 0 

K.C. 1999 200 1 0 

S.L.#1 1999 50 0 0 

Lolo NF 

S.F.L.J.C. 1999 200 0 0 
0 

Table 3. Sites Considered as Potential Field Insectaries for Collections of C. achates. 

County/Forest Site 

Release 

Dates 

Total Number 

Released 

#Larvae/ 

20 Plants 

#Adults/300 

 m swept 

Broadwater Co. W. 1999 100 16 14 

Cascade Co. S.C. 1994-98 500 17 8 

Glacier Co. L.P. 1995 100 18 15 

L&C Co. S.C.M. 1996 100 16 18 

Mineral Co. C.C. 1999-2000 150 12 15 

S.A. 1999-2000 300 16 6 

Powell Co. D.A.#1 1995 75 0 5 

 M.M. 2001 50 12 12 

Sanders Co. P.G.P.#1 1996 50 2 25 

P.G.P.#2 1996 50 6 6 

Silver Bow Co. H.#1 2000 60 11 2 

H.#2 2000 60 19 15 

Stillwater Co. E.P. 1994-95 80 9 22 

B.C.#14 2000 100 3 18 

B.H.R.#24 1999 100 2 2 

B.H.R.#25 1999 100 3 6 

R.R.#1 1999 100 3 3 

S.G.R.#2 2001 100 3 4 

Bitterroot NF 

S.G.R.#22 1999 100 3 0 

Preliminary results of weevil surveys at six sites 

where corrals were built are shown in Table 4. 

Weevils were detected in or near corrals July 30 

but the majorities were found in mid to late 

August. Numbers of weevils found by searching 

corrals and sweeping around corrals varied 

considerably. At the most productive site (Silver 

Bow Co.), 120 weevils were found by searching 

one corral for 30 minutes. In 2003, we will 

evaluate the efficacy of both collection methods 

after the weevils have had a chance to 

reproduce in corrals. 

Discussion/Conclusions 

In 2002, we identified 19 sites that will serve as 

field insectaries in 2003 and possibly beyond. 

These sites had high populations of C. achates

with an average root infestation of 45% and a 

range between 0 and 95%, greater than 3 acres 

of knapweed, easily accessible and were free 

from major disturbances. We also collected an 

average of one adult per minute, with a range 

between 0 and 2.5 adults per minute, by 

sweeping knapweed across the release sites. 

During 2003, we will coordinate with county and 

forest weed specialists that manage the 19 sites 

to redistribute the weevils. Late larval 

populations or early emerging adults will be 

sampled to determine if the weevil population 

overwintered successfully. Weevils collected 

from field insectaries can be released into 

knapweed infestations or directly into corrals. 

Releasing weevils into corrals would probably 

increase the number of weevils produced for at 

least the first few years following the collections 

as well as protect the weevils from natural 

mortality factors such as predation. 

Collecting C. achates larvae and/or adults at 

release sites verifies establishment of 

populations of insects at these locations. 

However, finding no C. achates is not definitive 

evidence that the insect is not present. Many 
0 

variables can influence the probability of finding 

this insect in samples. These variables include 

number of insects released, years since release 

was made, condition of plants and weather 

conditions at the time of adult sampling. At 

some sites we found fewer number of adults 

than our larval sampling would have suggested. 

One plausible hypothesis is that the summer of 

2002 was unusually hot and dry throughout 

much of Montana. At many sample sites the 

knapweed bloom was completed and the plants 

appeared dried out by mid-August, prior to 

sampling the adults. Knapweed in this 

condition does not provide a suitable host plant 

for the weevils therefore our adult sampling via 

sweeping plants may have been compromised 

at some sites. Also, some sites had thrifty 

knapweed and larval populations in spring and 

early summer but had few mature plants and 

stems at the time of adult sampling due to heavy 

grazing and herbicide applications. 

Although this survey concentrated on C. 

achates, other introduced biological control 

agents were observed at many sites. These 

insects included the root-feeding moth Agapeta 

zoegana Haw., seedhead feeding weevils 

Larinus spp., and the seedhead flies Urophora

spp. Some of these insects were released at 

the sites surveyed, but the presence of agents at 

other sites can only be attributed to migration, 

and in some instances from long distances. 

The need for the biological control agents of 

knapweed, especially C. achates, often exceeds 

what is available. The weevils are flightless and 

therefore natural dispersal is limited and 

perhaps slower than other insect species. 

Efforts should be continued on the local level to 

redistribute this insect from sites where they are 

prolific. 

During 2003, we will compare the efficacy of 

collecting weevils from field corrals versus 

sweeping. The comparison will include the time 

required for building and maintaining corrals. 

We will work with local managers to take over 

maintenance of these corrals for future 

collections if they prove to be effective. 

References Cited 

Corn, J., Story J. and L. White. 2003. Effects of 

the root weevil, Cyphocleonus achates on 

Spotted knapweed growth and survivorship. 

Montana Weed Control Association Annual 

Conference. 13-16 January 2003. 

Story, J.M., White, L.J., and Good, W.R. 1996. 

Propagation of Cyphocleonus achates 

(Fahraeus) (Coleoptera: Curculionidae) for 

biological control of spotted knapweed: 

procedures and cost. Biological Control 7, 

167-171. Article No. 0080. 

Sturdevant, N.J. and Dewey, J. 2002. 

Evaluating releases of Cyphocleonus 

achates and Agapeta zoegana as potential 

field insectaries and effects of wildfire on 

previous releases. USDA Forest Service, 

Forest Health Protection Report:R1-02-6.