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Propagation ofCyphocleonus achates(Fahraeus) (Coleoptera: Curculionidae) for Biological Control of Spotted Knapweed: Procedures and Cost
BIOLOGICAL CONTROL ARTICLE NO.
7, 167–171 (1996)
0080
Propagation of Cyphocleonus achates (Fahraeus) (Coleoptera: Curculionidae) for Biological Control of Spotted Knapweed: Procedures and Cost J. M. STORY, L. J. WHITE,
AND
W. R. GOOD
Montana Agricultural Experiment Station, Western Agricultural Research Center, Corvallis, Montana 59828 Received September 15, 1995; accepted April 26, 1996
Cyphocleonus achates (Fahraeus) is a root-feeding weevil introduced from Eurasia for biological control of spotted knapweed, Centaurea maculosa Lamarck, in Montana and other areas of the northwestern United States. Because C. achates is flightless and univoltine, distribution of the weevil throughout the vast infestations of spotted knapweed in Montana and the northwestern United States will be extremely slow. To hasten the distribution of this promising weevil, a propagation effort using field corrals was initiated at the Western Agricultural Research Center in Corvallis, Montana. Procedures for propagating the weevil in field corrals are described. The direct cost of rearing 1000 C. achates adults for field release was $1634, an average of $1.63 per insect. Labor comprised 83% of the total direct cost. The described field-corral approach is effective and the cost appropriate for providing C. achates for initial colonization in the western United States. r 1996 Academic Press, Inc. KEY WORDS: Cyphocleonus achates; Centaurea maculosa; spotted knapweed; insect propagation; biological control; cost analysis; corral.
INTRODUCTION
Spotted knapweed, Centaurea maculosa Lamarck, is an introduced perennial weed infesting an estimated 1.8 million hectares of rangeland in Montana (Lacey et al., 1992). A Eurasian, root-feeding weevil, Cyphocleonus achates (Fahraeus) (Coleoptera: Curculionidae), has been introduced into North America for biological control of the plant. The adult weevil is about 14 mm long and flightless. The biology, host specificity, and potential impact of the weevil were described by Stinson et al. (1994). The weevil overwinters as a larva in the root. Following a 2-week pupation in the root, adults emerge from mid July to October, with peak emergence usually occurring in mid August. Adults regularly feed on spotted knapweed leaves throughout their 8- to 15-week life span. Each female lays 1 to 3
eggs per day throughout her adult life. Eggs are laid singly in a notch excavated by the female on the root crown, just below the soil surface. Larvae hatch in 10–12 days and mine into the root cortex. Feeding by older larvae causes considerable damage, especially to small plants or plants containing multiple larvae. Also, feeding by third and fourth instar larvae often results in the formation of conspicuous root galls, about 2 to 4 cm long and 1 cm in diameter. The weevil has one generation per year. The niche separation of C. achates and other knapweed root insects was reported by Mu¨ller (1989) and Mu¨ller et al. (1989). Stinson et al. (1994) reported that, of the root insects on knapweed in Eurasia, C. achates is one of the most damaging and, therefore, should be a very valuable addition to the knapweed biocontrol effort in North America. The first United States release of the weevil was made in Montana in 1988 followed by the first field recovery in 1989. The weevil is now established in moderate numbers at seven sites in Montana (Story, unpublished data). Due to low populations of C. maculosa in Eurasia, field collections of the weevil in Eurasia in 1988 and 1989 by the CAB International Institute of Biological Control (Dele´mont, Switzerland) yielded less than 100 adults/year. Because of the difficulty in collecting the weevil in Eurasia and the fact that the weevil is flightless and univoltine, distribution of the weevil throughout the vast spotted knapweed infestations in Montana (1.8 million hectares; Lacey et al., 1992) and other areas of northwestern United States will be extremely slow. To hasten the distribution of this promising weevil, a propagation effort using field corrals was initiated at the Western Agricultural Research Center in Corvallis, Montana, in 1988. Only a few efforts to propagate phytophagous insects for control of exotic weeds have been reported (Surles and Kok, 1976; Frick and Chandler, 1978; Parrella and Kok, 1979; Stoyer and Kok, 1986; Story et al., 1994). The rearing procedures and cost for the propagation of a root moth, Agapeta zoegana L., (Lepidoptera: Cochylidae), introduced for control of spotted knapweed was
167
1049-9644/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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reported by Story et al. (1994). This paper describes the rearing procedures and the cost for the propagation of C. achates. MATERIALS AND METHODS
Propagation of the weevil was conducted at the Western Agricultural Research Center at Corvallis, Montana, during 1991 through 1993. Because C. achates adults are flightless, propagation of the weevil was conducted in large corrals, rather than in cages. The sequence of procedures related to the propagation effort is shown in Table 1. To initiate the propagation effort, medium-sized spotted knapweed rosettes (roots about 5 to 10 mm in diameter) were dug by hand in April 1991 from a knapweed-infested site and transplanted into a cultivated 5 3 7.5-m plot at the Research Center (Table 1). Approximately 200 plants were transplanted into the plot in 10 rows, using a between-row and within-row spacing of 30 cm. A 90-cm aisle was established between the outermost row and the edge of the plot. The plants were watered at the time of planting and daily thereafter for approximately 1 week. In mid June, the plot was enclosed by a wall made of aluminum flashing (25 cm in height), forming a corral (Fig. 1). The flashing was held in place by 60-cm wooden stakes placed every 1.2 m around the outside edge of the corral. The lower 5 cm of the flashing was inserted below the soil surface. The upper 5 cm of the flashing was folded downward toward the inside of the corral to prevent weevil escape. A 15-cm-wide shade cloth, supported by 20-cm pieces of lath attached perpendicular to the stakes, was suspended over the flashing to prevent excessive solar heating of the flashing and the nearby soil. Upon introduction of the weevils into the corral, 2.5-cm-mesh bird netting was stretched over the top and sides of the corral to prevent weevil predation by birds. The netting was suspended approximately 60 cm above the ground using wires strung between stakes positioned on the TABLE 1 Schedule for Rearing C. achates in Field Corrals in Western Montana during 1991 through 1993 Date Year 1 April 1–10 May 1–November 15 June 15 August 1 August 1–30 November 15 Year 2 May 1–September 30 July 15 July 15–September 30
Task
Prepare plots; transplant Maintain plots Install corrals Install bird netting Introduce adult weevils Remove bird netting Maintain plots Install bird netting Collect adult weevils
FIG. 1. Diagram of 7.5 3 5-m rearing corral. A, aluminum flashing; B, wooden stakes and lath; C, shade cloth; D, detail of inward fold on flashing under shade cloth.
outside of the corral. The bird netting was removed from the corral in mid November. No provision was made to protect the plants in the corral from winter conditions. In 1992, another field corral was established using the same procedures. Initial stocking of the two corrals was accomplished by releasing 16 pairs of adult weevils into each of the corrals in early August of 1991 and 1992, respectively. Collection of newly emerged weevil adults from the two corrals began in early August of 1992 and 1993, respectively, about 2 weeks after the first weevils were seen. Collections occurred 3 times/week and generally occurred in late morning or afternoon when the weevils were active and more readily found. The process of collection involved careful searches of the basal foliage of every knapweed plant as well as the ground in the corral. The collection was done solely by hand due to the difficulty in locating the weevils and the tenacity with which they clung to substrate. Collected weevils were placed in 1-liter paper cans containing spotted knapweed foliage. The plants were pruned twice during the summer to aid collection of the adult weevils and to increase soil temperature for acceleration of larval and pupal development. The areas in and around the corrals were weeded four times during the summer. Direct costs (labor and materials) for rearing 1000 C. achates to the adult stage were estimated at 1992 prices (U.S.$). RESULTS AND DISCUSSION
Rearing Results This first-ever use of a field corral to propagate an insect was successful. A total of 1102 weevils was recovered in 1992 from the field corral established in 1991 and 1003 were collected in 1993 from the corral established in 1992. A wide plant spacing (30 cm) and regular pruning of plants was critical to weevil production and time of emergence. The cooler soil temperatures under narrowly spaced plants delayed weevil development and emergence by up to 3 months. Soil
PROPAGATION OF Cyphocleonus achates
temperatures under dense knapweed canopies were 4 to 11°C cooler than areas under more open canopies. The corrals were not recolonized with weevils during the second year (i.e., the first year of production) because, based upon prior experience, such attempts result in greatly reduced weevil numbers (up to 93% reduction) in the third year (Story, unpublished data). Likely reasons for the reduced weevil numbers in the third year probably include: (1) knapweed mortality directly due to weevil feeding, (2) escape of the adults (those used for restocking) from the corrals during the previous fall, (3) rodent predation, (4) knapweed mortality caused by the fungus, Sclerotinia sclerotiorum (Lib.) de Bary, (5) added stress to the plants by larvae of Agapeta zoegana L., a knapweed-attacking root moth established in the area (Story et al., 1994), (6) the late date at which the corrals were recolonized (i.e., late September) due to the need to defer recolonization until weevil collection was terminated, and (7) possible overstocking of the weevils through oviposition by uncollected females. A combination of the stresses to the knapweed plants applied by the weevil, the moth, and the fungus and the late recolonization date were probably the primary causes for the reduced weevil numbers. The combined insect–fungus stress hypothesis was supported by the occurrence of numerous dead or dying knapweed plants in both corrals in the spring of the second production year. Subsequent efforts have shown that the problem of weevil population decline can be avoided by simply not using the corral for more than 1 year. Thus, to maintain consistent weevil production levels from year to year, a new corral plot must be started each year. The annual preparation of a new plot can occur within the frame of the ‘‘retired’’ corral or it can be started in a new area. The latter approach would probably reduce the problem caused by S. sclerotiorum, but it would require the construction of a new corral each year. Rodent predation, based upon sightings of severed weevil elytra and other body parts, was not a serious problem but did occur, especially late in the fall. Rodents entered the corrals at night, apparently by climbing the support stakes and jumping into the corral. The process of locating and collecting the weevil was very slow because of the weevil’s cryptic coloration, its tendency to reside within the tangle of leaves and stalks at the base of the plant, and the tenacity with which it clung to foliage. The use of lights, pheromones, and other possible attractants to facilitate collection was not attempted, but warrants serious consideration. The objective of this field-corral rearing approach was to produce, in a short time, substantial numbers of C. achates to hasten the rate of the weevil’s introduction throughout the knapweed-infested areas of Montana. Although rearing the weevils in field corrals was
169
labor-intensive and expensive (as discussed later), we chose this method over an indoor insectary approach to insure the weevils’ cold hardiness and phenological synchrony with knapweed. These objectives have been appropriate and effective since colonies of the weevils, originating from individuals reared in the corrals, are now well established at a minimum of seven sites in Montana (Story, unpublished data). Augmentative efforts could also be achieved by collecting from established field colonies if weevil populations were large, but a waiting period of at least 5 years will be likely required before the field populations are large enough to permit such collections. The field-collection method would be considerably less expensive; the only cost would be the labor required to make several 4-h collections at a site. However, field collection of weevils would be much slower and less productive since the weevils would not be concentrated. The field-collection approach will likely be the logical and most costeffective way of augmenting C. achates populations once the weevil is established in large numbers at numerous sites. Until that time, the urgency in slowing the knapweed invasion in Montana and northwestern United States will likely require that propagation efforts on this flightless insect be continued for the indefinite future. This is the second report describing procedures for propagating a root insect for biocontrol of spotted knapweed. Story et al. (1994) described the procedures for propagating the root moth, A. zoegana. Due to the different behavior and biology of these two insect species, the rearing methods were notably different. Both rearing efforts involved outdoor enclosures but the A. zoegana rearing effort was necessarily conducted in numerous 3.6 3 1.8-m cages rather than large corrals. The A. zoegana effort also required daily collections for a 3-month period, different plant spacing, and different collection techniques, and involved much larger insect numbers due to the moth’s greater reproductive capability. Rearing programs will likely be needed for other univoltine, root-feeding species recently introduced against spotted knapweed in Montana and other areas of the northwest United States. C. achates shows great promise as a biocontrol agent. Preliminary studies indicate that weevil populations of about 6 adults per mature knapweed plant will cause up to 50% knapweed mortality within 3 months. In contrast, the impact of the introduced root moth, A. zoegana, on knapweed is not evident until after 2 or 3 years of constant attack (Story, unpublished data). The impact of C. achates on knapweed will likely be complemented by that of A. zoegana and other introduced insect species once all of the species are well established.
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Cost Analysis
TABLE 3
The total direct cost of rearing 1000 C. achates was about $1634, an average of $1.63 per insect (Table 2). This cost included one part-time technician for 3 months, materials, and general supplies. Indirect costs such as utilities, facilities maintenance, storage, shipping, transportation, initial rearing-related research costs, and administrative overhead were not included in the estimate. Labor, at $1351 (83% of the total cost), was the most costly item of the rearing effort. Labor demands were high because of the weevils’s lengthy (2-month) emergence period. Labor allocation by task is shown in Table 3. In general, labor requirements were as follows: plot/corral preparation (12%), corral installation (15%), plot/corral maintenance (17%), weevil collection (32%), shipping preparation or field release (14%), and shipping coordination/record keeping (10%). Spotted knapweed is causing an estimated $4.5 million in annual forage losses in Montana (French and Lacey, 1983) and is expected to reduce livestock revenue by $155 million if knapweed acreage increases in the state (Bucher, 1984). In view of these substantial economic losses, the cost of the field-corral approach for rearing C. achates is appropriate and justified for providing this promising insect for initial colonization. The cost of the field-corral approach could be decreased substantially (up to 25%) by reducing the time spent on weeding and weevil collection. The procedures and costs described in this paper reflect a rearing effort focused on collecting all, or nearly all, of the adult weevils in the corral during a 4- to 5-h period on each collection day. This was accomplished by carefully inspecting every plant in the corral on that day. In addition, effort was taken to routinely eliminate all weeds to aid in detection and collection of the weevils. However, weeding is not essential to the success of this TABLE 2
Labor Allotment by Task and Time per Corral Task
Man hours
Plot preparation Corral installation Plot maintenance Weeding Plant pruning Predator removal Insect collection Shipping preparation or field release Shipping coordination/record keeping Total
16 20 23 (16) (1) (6) 44 20 14 137
rearing approach. Similarly, it is not imperative that every weevil adult in the corral be gathered on the collection days. Instead, by exploiting the weevil’s tendency to wander during warm temperatures, collections could be directed solely at weevils that have accumulated along the corral walls. This 21-h procedure could be done each day. Although many weevils would be overlooked with less intensive collections, most would probably be collected eventually due to their long lives and tendency to wander. The downside of this approach is that some of the females would be old and less fecund when collected. ACKNOWLEDGMENTS We thank Kathleen Welty, Miles Nelsen, and John Pape for their insightful contributions and Brian Story for the drawing. This work was funded, in part, by the Montana Noxious Weed Trust Fund, USDA/ARS, USDI/Bureau of Indian Affairs, Missoula County, the Rocky Mountain Elk Foundation, and various county weed districts. This report is based, in part, on research conducted and supported as a part of SAES Western Regional Research Project W-84. This is contribution No. J-4039 from the Montana Agricultural Experiment Station.
Cost Estimate for Rearing 1000 C. achates REFERENCES Quantity Labor a One part-time technician (3.0 months) Materials Nonrecurring costs Aluminum flashing Shade cloth Bird netting Hedge trimmer Hardware Recurring costs Paper cans Miscellaneous
a
Including benefits.
Total ($)
1351
Two 15-m rolls 3.6 3 3.6 m 9 3 12 m 1
1 (case of 250)
35 37 22 50 19 70 50 1634
Bucher, R. F. 1984. Potential cost of spotted knapweed on range. Montana Cooperative Extension Service MontGuide 8423. Montana State Univ., Bozeman. French, R. A., and Lacey, J. R. 1983. Knapweed: Its cause, effect and spread in Montana. Montana Coop. Ext. Service Circular 307. Montana State Univ., Bozeman. Frick, K. E., and Chandler, J. M. 1978. Augmenting the moth (Bactra verutana) in field plots for early-season suppression of purple nutsedge (Cyperus rotundus). Weed Sci. 26, 703–710. Lacey, C. A., Lacey, J. R., Fay, P. K., Story, J. M., and Zamora, D. L. 1992. Controlling knapweed on Montana rangeland. Montana Coop. Ext. Service Circular No. 311. Montana State Univ., Bozeman. Mu¨ller, H. 1989. Structural analysis of the phytophagous insect guilds associated with the roots of Centaurea maculosa Lam.,
PROPAGATION OF Cyphocleonus achates C. diffusa Lam., and C. vallesiaca Jordan in Europe: Field Observations. Oecologia 78, 41–52. Mu¨ller, H., Stinson, C. S. A., Marquardt, K., and Schroeder, D. 1989. The entomofaunas of roots of Centaurea maculosa Lam., C. diffusa Lam., and C. vallesiaca Jordan in Europe. J. Appl. Entomol. 107, 83–95. Parrella, M. P., and Kok, L. T. 1979. Oidaematophorus monodactylus as a biocontrol agent of hedge bindweed: development of a rearing program and cost analysis. J. Econ. Entomol. 72, 590–592. Stinson, C. S. A., Schroeder, D., and Marquardt, K. 1994. Investigations on Cyphocleonus achates (Fahr.) (Col., Curculionidae), a potential biological control agent of spotted knapweed (Centaurea
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maculosa Lam.) and diffuse knapweed (C. diffusa Lam.) (Compositae) in North America. J. Appl. Entomol. 117, 35–50. Story, J. M., Good, W. R., and White, L. J. 1994. Propagation of Agapeta zoegana L. (Lepidoptera: Cochylidae) for biological control of spotted knapweed: Procedures and cost. Biol. Control 4, 145– 148. Stoyer, T. L., and Kok, L. T. 1986. Field nurseries for propagating Trichosirocalus horridus (Coleoptera: Curculionidae), a biological control agent for Carduus thistles. J. Econ. Entomol. 79, 873–876. Surles, W. W., and Kok, L. T. 1976. Pilot studies on augmentation of Rhinocyllus conicus (Col.: Curculionidae) for Carduus thistle control. Environ. Entomol. 5, 901–904.
7, 167–171 (1996)
0080
Propagation of Cyphocleonus achates (Fahraeus) (Coleoptera: Curculionidae) for Biological Control of Spotted Knapweed: Procedures and Cost J. M. STORY, L. J. WHITE,
AND
W. R. GOOD
Montana Agricultural Experiment Station, Western Agricultural Research Center, Corvallis, Montana 59828 Received September 15, 1995; accepted April 26, 1996
Cyphocleonus achates (Fahraeus) is a root-feeding weevil introduced from Eurasia for biological control of spotted knapweed, Centaurea maculosa Lamarck, in Montana and other areas of the northwestern United States. Because C. achates is flightless and univoltine, distribution of the weevil throughout the vast infestations of spotted knapweed in Montana and the northwestern United States will be extremely slow. To hasten the distribution of this promising weevil, a propagation effort using field corrals was initiated at the Western Agricultural Research Center in Corvallis, Montana. Procedures for propagating the weevil in field corrals are described. The direct cost of rearing 1000 C. achates adults for field release was $1634, an average of $1.63 per insect. Labor comprised 83% of the total direct cost. The described field-corral approach is effective and the cost appropriate for providing C. achates for initial colonization in the western United States. r 1996 Academic Press, Inc. KEY WORDS: Cyphocleonus achates; Centaurea maculosa; spotted knapweed; insect propagation; biological control; cost analysis; corral.
INTRODUCTION
Spotted knapweed, Centaurea maculosa Lamarck, is an introduced perennial weed infesting an estimated 1.8 million hectares of rangeland in Montana (Lacey et al., 1992). A Eurasian, root-feeding weevil, Cyphocleonus achates (Fahraeus) (Coleoptera: Curculionidae), has been introduced into North America for biological control of the plant. The adult weevil is about 14 mm long and flightless. The biology, host specificity, and potential impact of the weevil were described by Stinson et al. (1994). The weevil overwinters as a larva in the root. Following a 2-week pupation in the root, adults emerge from mid July to October, with peak emergence usually occurring in mid August. Adults regularly feed on spotted knapweed leaves throughout their 8- to 15-week life span. Each female lays 1 to 3
eggs per day throughout her adult life. Eggs are laid singly in a notch excavated by the female on the root crown, just below the soil surface. Larvae hatch in 10–12 days and mine into the root cortex. Feeding by older larvae causes considerable damage, especially to small plants or plants containing multiple larvae. Also, feeding by third and fourth instar larvae often results in the formation of conspicuous root galls, about 2 to 4 cm long and 1 cm in diameter. The weevil has one generation per year. The niche separation of C. achates and other knapweed root insects was reported by Mu¨ller (1989) and Mu¨ller et al. (1989). Stinson et al. (1994) reported that, of the root insects on knapweed in Eurasia, C. achates is one of the most damaging and, therefore, should be a very valuable addition to the knapweed biocontrol effort in North America. The first United States release of the weevil was made in Montana in 1988 followed by the first field recovery in 1989. The weevil is now established in moderate numbers at seven sites in Montana (Story, unpublished data). Due to low populations of C. maculosa in Eurasia, field collections of the weevil in Eurasia in 1988 and 1989 by the CAB International Institute of Biological Control (Dele´mont, Switzerland) yielded less than 100 adults/year. Because of the difficulty in collecting the weevil in Eurasia and the fact that the weevil is flightless and univoltine, distribution of the weevil throughout the vast spotted knapweed infestations in Montana (1.8 million hectares; Lacey et al., 1992) and other areas of northwestern United States will be extremely slow. To hasten the distribution of this promising weevil, a propagation effort using field corrals was initiated at the Western Agricultural Research Center in Corvallis, Montana, in 1988. Only a few efforts to propagate phytophagous insects for control of exotic weeds have been reported (Surles and Kok, 1976; Frick and Chandler, 1978; Parrella and Kok, 1979; Stoyer and Kok, 1986; Story et al., 1994). The rearing procedures and cost for the propagation of a root moth, Agapeta zoegana L., (Lepidoptera: Cochylidae), introduced for control of spotted knapweed was
167
1049-9644/96 $18.00 Copyright r 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
168
STORY, WHITE, AND GOOD
reported by Story et al. (1994). This paper describes the rearing procedures and the cost for the propagation of C. achates. MATERIALS AND METHODS
Propagation of the weevil was conducted at the Western Agricultural Research Center at Corvallis, Montana, during 1991 through 1993. Because C. achates adults are flightless, propagation of the weevil was conducted in large corrals, rather than in cages. The sequence of procedures related to the propagation effort is shown in Table 1. To initiate the propagation effort, medium-sized spotted knapweed rosettes (roots about 5 to 10 mm in diameter) were dug by hand in April 1991 from a knapweed-infested site and transplanted into a cultivated 5 3 7.5-m plot at the Research Center (Table 1). Approximately 200 plants were transplanted into the plot in 10 rows, using a between-row and within-row spacing of 30 cm. A 90-cm aisle was established between the outermost row and the edge of the plot. The plants were watered at the time of planting and daily thereafter for approximately 1 week. In mid June, the plot was enclosed by a wall made of aluminum flashing (25 cm in height), forming a corral (Fig. 1). The flashing was held in place by 60-cm wooden stakes placed every 1.2 m around the outside edge of the corral. The lower 5 cm of the flashing was inserted below the soil surface. The upper 5 cm of the flashing was folded downward toward the inside of the corral to prevent weevil escape. A 15-cm-wide shade cloth, supported by 20-cm pieces of lath attached perpendicular to the stakes, was suspended over the flashing to prevent excessive solar heating of the flashing and the nearby soil. Upon introduction of the weevils into the corral, 2.5-cm-mesh bird netting was stretched over the top and sides of the corral to prevent weevil predation by birds. The netting was suspended approximately 60 cm above the ground using wires strung between stakes positioned on the TABLE 1 Schedule for Rearing C. achates in Field Corrals in Western Montana during 1991 through 1993 Date Year 1 April 1–10 May 1–November 15 June 15 August 1 August 1–30 November 15 Year 2 May 1–September 30 July 15 July 15–September 30
Task
Prepare plots; transplant Maintain plots Install corrals Install bird netting Introduce adult weevils Remove bird netting Maintain plots Install bird netting Collect adult weevils
FIG. 1. Diagram of 7.5 3 5-m rearing corral. A, aluminum flashing; B, wooden stakes and lath; C, shade cloth; D, detail of inward fold on flashing under shade cloth.
outside of the corral. The bird netting was removed from the corral in mid November. No provision was made to protect the plants in the corral from winter conditions. In 1992, another field corral was established using the same procedures. Initial stocking of the two corrals was accomplished by releasing 16 pairs of adult weevils into each of the corrals in early August of 1991 and 1992, respectively. Collection of newly emerged weevil adults from the two corrals began in early August of 1992 and 1993, respectively, about 2 weeks after the first weevils were seen. Collections occurred 3 times/week and generally occurred in late morning or afternoon when the weevils were active and more readily found. The process of collection involved careful searches of the basal foliage of every knapweed plant as well as the ground in the corral. The collection was done solely by hand due to the difficulty in locating the weevils and the tenacity with which they clung to substrate. Collected weevils were placed in 1-liter paper cans containing spotted knapweed foliage. The plants were pruned twice during the summer to aid collection of the adult weevils and to increase soil temperature for acceleration of larval and pupal development. The areas in and around the corrals were weeded four times during the summer. Direct costs (labor and materials) for rearing 1000 C. achates to the adult stage were estimated at 1992 prices (U.S.$). RESULTS AND DISCUSSION
Rearing Results This first-ever use of a field corral to propagate an insect was successful. A total of 1102 weevils was recovered in 1992 from the field corral established in 1991 and 1003 were collected in 1993 from the corral established in 1992. A wide plant spacing (30 cm) and regular pruning of plants was critical to weevil production and time of emergence. The cooler soil temperatures under narrowly spaced plants delayed weevil development and emergence by up to 3 months. Soil
PROPAGATION OF Cyphocleonus achates
temperatures under dense knapweed canopies were 4 to 11°C cooler than areas under more open canopies. The corrals were not recolonized with weevils during the second year (i.e., the first year of production) because, based upon prior experience, such attempts result in greatly reduced weevil numbers (up to 93% reduction) in the third year (Story, unpublished data). Likely reasons for the reduced weevil numbers in the third year probably include: (1) knapweed mortality directly due to weevil feeding, (2) escape of the adults (those used for restocking) from the corrals during the previous fall, (3) rodent predation, (4) knapweed mortality caused by the fungus, Sclerotinia sclerotiorum (Lib.) de Bary, (5) added stress to the plants by larvae of Agapeta zoegana L., a knapweed-attacking root moth established in the area (Story et al., 1994), (6) the late date at which the corrals were recolonized (i.e., late September) due to the need to defer recolonization until weevil collection was terminated, and (7) possible overstocking of the weevils through oviposition by uncollected females. A combination of the stresses to the knapweed plants applied by the weevil, the moth, and the fungus and the late recolonization date were probably the primary causes for the reduced weevil numbers. The combined insect–fungus stress hypothesis was supported by the occurrence of numerous dead or dying knapweed plants in both corrals in the spring of the second production year. Subsequent efforts have shown that the problem of weevil population decline can be avoided by simply not using the corral for more than 1 year. Thus, to maintain consistent weevil production levels from year to year, a new corral plot must be started each year. The annual preparation of a new plot can occur within the frame of the ‘‘retired’’ corral or it can be started in a new area. The latter approach would probably reduce the problem caused by S. sclerotiorum, but it would require the construction of a new corral each year. Rodent predation, based upon sightings of severed weevil elytra and other body parts, was not a serious problem but did occur, especially late in the fall. Rodents entered the corrals at night, apparently by climbing the support stakes and jumping into the corral. The process of locating and collecting the weevil was very slow because of the weevil’s cryptic coloration, its tendency to reside within the tangle of leaves and stalks at the base of the plant, and the tenacity with which it clung to foliage. The use of lights, pheromones, and other possible attractants to facilitate collection was not attempted, but warrants serious consideration. The objective of this field-corral rearing approach was to produce, in a short time, substantial numbers of C. achates to hasten the rate of the weevil’s introduction throughout the knapweed-infested areas of Montana. Although rearing the weevils in field corrals was
169
labor-intensive and expensive (as discussed later), we chose this method over an indoor insectary approach to insure the weevils’ cold hardiness and phenological synchrony with knapweed. These objectives have been appropriate and effective since colonies of the weevils, originating from individuals reared in the corrals, are now well established at a minimum of seven sites in Montana (Story, unpublished data). Augmentative efforts could also be achieved by collecting from established field colonies if weevil populations were large, but a waiting period of at least 5 years will be likely required before the field populations are large enough to permit such collections. The field-collection method would be considerably less expensive; the only cost would be the labor required to make several 4-h collections at a site. However, field collection of weevils would be much slower and less productive since the weevils would not be concentrated. The field-collection approach will likely be the logical and most costeffective way of augmenting C. achates populations once the weevil is established in large numbers at numerous sites. Until that time, the urgency in slowing the knapweed invasion in Montana and northwestern United States will likely require that propagation efforts on this flightless insect be continued for the indefinite future. This is the second report describing procedures for propagating a root insect for biocontrol of spotted knapweed. Story et al. (1994) described the procedures for propagating the root moth, A. zoegana. Due to the different behavior and biology of these two insect species, the rearing methods were notably different. Both rearing efforts involved outdoor enclosures but the A. zoegana rearing effort was necessarily conducted in numerous 3.6 3 1.8-m cages rather than large corrals. The A. zoegana effort also required daily collections for a 3-month period, different plant spacing, and different collection techniques, and involved much larger insect numbers due to the moth’s greater reproductive capability. Rearing programs will likely be needed for other univoltine, root-feeding species recently introduced against spotted knapweed in Montana and other areas of the northwest United States. C. achates shows great promise as a biocontrol agent. Preliminary studies indicate that weevil populations of about 6 adults per mature knapweed plant will cause up to 50% knapweed mortality within 3 months. In contrast, the impact of the introduced root moth, A. zoegana, on knapweed is not evident until after 2 or 3 years of constant attack (Story, unpublished data). The impact of C. achates on knapweed will likely be complemented by that of A. zoegana and other introduced insect species once all of the species are well established.
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STORY, WHITE, AND GOOD
Cost Analysis
TABLE 3
The total direct cost of rearing 1000 C. achates was about $1634, an average of $1.63 per insect (Table 2). This cost included one part-time technician for 3 months, materials, and general supplies. Indirect costs such as utilities, facilities maintenance, storage, shipping, transportation, initial rearing-related research costs, and administrative overhead were not included in the estimate. Labor, at $1351 (83% of the total cost), was the most costly item of the rearing effort. Labor demands were high because of the weevils’s lengthy (2-month) emergence period. Labor allocation by task is shown in Table 3. In general, labor requirements were as follows: plot/corral preparation (12%), corral installation (15%), plot/corral maintenance (17%), weevil collection (32%), shipping preparation or field release (14%), and shipping coordination/record keeping (10%). Spotted knapweed is causing an estimated $4.5 million in annual forage losses in Montana (French and Lacey, 1983) and is expected to reduce livestock revenue by $155 million if knapweed acreage increases in the state (Bucher, 1984). In view of these substantial economic losses, the cost of the field-corral approach for rearing C. achates is appropriate and justified for providing this promising insect for initial colonization. The cost of the field-corral approach could be decreased substantially (up to 25%) by reducing the time spent on weeding and weevil collection. The procedures and costs described in this paper reflect a rearing effort focused on collecting all, or nearly all, of the adult weevils in the corral during a 4- to 5-h period on each collection day. This was accomplished by carefully inspecting every plant in the corral on that day. In addition, effort was taken to routinely eliminate all weeds to aid in detection and collection of the weevils. However, weeding is not essential to the success of this TABLE 2
Labor Allotment by Task and Time per Corral Task
Man hours
Plot preparation Corral installation Plot maintenance Weeding Plant pruning Predator removal Insect collection Shipping preparation or field release Shipping coordination/record keeping Total
16 20 23 (16) (1) (6) 44 20 14 137
rearing approach. Similarly, it is not imperative that every weevil adult in the corral be gathered on the collection days. Instead, by exploiting the weevil’s tendency to wander during warm temperatures, collections could be directed solely at weevils that have accumulated along the corral walls. This 21-h procedure could be done each day. Although many weevils would be overlooked with less intensive collections, most would probably be collected eventually due to their long lives and tendency to wander. The downside of this approach is that some of the females would be old and less fecund when collected. ACKNOWLEDGMENTS We thank Kathleen Welty, Miles Nelsen, and John Pape for their insightful contributions and Brian Story for the drawing. This work was funded, in part, by the Montana Noxious Weed Trust Fund, USDA/ARS, USDI/Bureau of Indian Affairs, Missoula County, the Rocky Mountain Elk Foundation, and various county weed districts. This report is based, in part, on research conducted and supported as a part of SAES Western Regional Research Project W-84. This is contribution No. J-4039 from the Montana Agricultural Experiment Station.
Cost Estimate for Rearing 1000 C. achates REFERENCES Quantity Labor a One part-time technician (3.0 months) Materials Nonrecurring costs Aluminum flashing Shade cloth Bird netting Hedge trimmer Hardware Recurring costs Paper cans Miscellaneous
a
Including benefits.
Total ($)
1351
Two 15-m rolls 3.6 3 3.6 m 9 3 12 m 1
1 (case of 250)
35 37 22 50 19 70 50 1634
Bucher, R. F. 1984. Potential cost of spotted knapweed on range. Montana Cooperative Extension Service MontGuide 8423. Montana State Univ., Bozeman. French, R. A., and Lacey, J. R. 1983. Knapweed: Its cause, effect and spread in Montana. Montana Coop. Ext. Service Circular 307. Montana State Univ., Bozeman. Frick, K. E., and Chandler, J. M. 1978. Augmenting the moth (Bactra verutana) in field plots for early-season suppression of purple nutsedge (Cyperus rotundus). Weed Sci. 26, 703–710. Lacey, C. A., Lacey, J. R., Fay, P. K., Story, J. M., and Zamora, D. L. 1992. Controlling knapweed on Montana rangeland. Montana Coop. Ext. Service Circular No. 311. Montana State Univ., Bozeman. Mu¨ller, H. 1989. Structural analysis of the phytophagous insect guilds associated with the roots of Centaurea maculosa Lam.,
PROPAGATION OF Cyphocleonus achates C. diffusa Lam., and C. vallesiaca Jordan in Europe: Field Observations. Oecologia 78, 41–52. Mu¨ller, H., Stinson, C. S. A., Marquardt, K., and Schroeder, D. 1989. The entomofaunas of roots of Centaurea maculosa Lam., C. diffusa Lam., and C. vallesiaca Jordan in Europe. J. Appl. Entomol. 107, 83–95. Parrella, M. P., and Kok, L. T. 1979. Oidaematophorus monodactylus as a biocontrol agent of hedge bindweed: development of a rearing program and cost analysis. J. Econ. Entomol. 72, 590–592. Stinson, C. S. A., Schroeder, D., and Marquardt, K. 1994. Investigations on Cyphocleonus achates (Fahr.) (Col., Curculionidae), a potential biological control agent of spotted knapweed (Centaurea
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maculosa Lam.) and diffuse knapweed (C. diffusa Lam.) (Compositae) in North America. J. Appl. Entomol. 117, 35–50. Story, J. M., Good, W. R., and White, L. J. 1994. Propagation of Agapeta zoegana L. (Lepidoptera: Cochylidae) for biological control of spotted knapweed: Procedures and cost. Biol. Control 4, 145– 148. Stoyer, T. L., and Kok, L. T. 1986. Field nurseries for propagating Trichosirocalus horridus (Coleoptera: Curculionidae), a biological control agent for Carduus thistles. J. Econ. Entomol. 79, 873–876. Surles, W. W., and Kok, L. T. 1976. Pilot studies on augmentation of Rhinocyllus conicus (Col.: Curculionidae) for Carduus thistle control. Environ. Entomol. 5, 901–904.