Jim M. Story

Influence of Seed Head–Attacking Biological Control Agents on Spotted Knapweed Reproductive Potential in Western Montana Over a 30-Year Period

Abstract Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6–93.8% lower in 2004–2005 than 1974–1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m2) was 95.9–99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m2 compared with 19 seeds/m2 at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m2), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5–46.9%; the addition of Larinus spp. further reduced seed numbers 84.2–90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.

Relationship Among Growth Attributes of Spotted Knapweed (Centaurea maculosa) in Western Montana1

Spotted knapweed is an important weed of rangeland in the northwestern United States. A study was conducted near Corvallis, MT, during 1992 to 1994 in order to assess the relationship among the growth attributes of spotted knapweed to identify a minimum set of measurable plant characteristics that are representative of spotted knapweed vigor. Spotted knapweed growth attributes that were examined included plant age, root diameter, plant height, number of stems per plant, aboveground biomass, number of capitula (seed heads) per plant, and number of capitula per stem. Spotted knapweed age was positively correlated with root diameter, number of stems per plant, aboveground biomass, and proportion of bolted plants. Most spotted knapweed plants did not bolt until the third or fourth year. Although plant age is not measured easily in the field, it may be useful as a covariate in an analysis of experiments involving plant competition or nonlethal biological control agents. Root diameter can be used as a nondestructive measure of approximate plant age, especially for the first 5 yr of growth. Root diameter was also highly correlated with many growth measurements, including number of capitula per plant and aboveground biomass, which are most relevant to assessing overall plant vigor. Plant height was positively correlated with aboveground biomass, number of capitula per plant, and mean number of capitula per stem. Number of stems per plant was positively correlated with plant height, aboveground biomass, and number of capitula per plant. Aboveground biomass was positively correlated to number of capitula per plant and mean number of capitula per stem. Measurements of root diameter, plant height, and number of stems are easy to perform and should provide a good indication of plant vigor. Nomenclature: Spotted knapweed, Centaurea maculosa Lam. #3 CENMA. Additional index words: ANCOVA, analysis of covariance; growth–attribute relationships.

Effects of nitrogen fertilization on spotted knapweed and competing vegetation in western Montana.

The effects of N fertilization on spotted knrpweed (Cmtourccr umcuforu Lamarck) and competing vegetation were studied at 2 site-s in western Montana during 1981 through 1984. The N was applied 1 time at each site at rates of 56,112,224, and 448 kg/ha. Spotted ltnapweed biomass showed a significant, positive yield response to N at all rates at both sites during the year of applieation. The only response by spotted knapweed to N In succeeding years was in the second year at Site 1 where a significant response was detected at the 448 kg N/ha rate. Competing vegetation at Site 1 (primarily quackgrass, Agropyron repens (L.) Bauv.) in the first year showed a significant, positive response to N, while competing vegetation at Site 2 (primarily crested wheatgrass, Agropyron crisLotum (L.) Gaertu.) did not respond to N in the first year. No response by competing vegetation to N in succeeding years at either site was detected. There was a significant relationship between percent knrpweed and N rate at both sites in the year of N application, but not in succeeding years. These results suggest that N fertilization, by i&If, as a cultural control approach to knapweed may be impractical, and could contribute toward the increase of knapweed when used in some of the plant communities normally assodrtedwithspotted~pweedon~ge~dinw~~Monhna.

Effect of Summer Drought Relief on the Impact of the Root Weevil Cyphocleonus achates on Spotted Knapweed

Abstract A recent decline in spotted knapweed, Centaurea stoebe L. subsp. micranthos (Asteraceae), has been observed in parts of western Montana. The release of the biological control agent Cyphocleonus achates (Fahraeus) is thought to contribute to the decline, but persistent drought since at least 1999 may be an additional factor. We conducted outdoor plot experiments to test the relative impacts of C. achates weevils and summer drought relief on spotted knapweed survival and growth. Groups of spotted knapweed transplants were assigned to one of four weekly water addition treatments (no added water, and 0.25, 0.5 or full recovery of plant water deficit, where “deficit” refers to potential evapotranspiration minus rainfall) in May to August 2004 and June to August 2005 and to either exposure to or protection from C. achates. In June of each subsequent year (2005 and 2006), plants were harvested and growth attributes that reflect plant vigor were measured. Drought indices showed that throughout the time of the study until January 2006, western Montana was in drought alert or severe drought. Summer drought relief had no effect on aboveground biomass and plant height of knapweed plants in subsequent years, but feeding by C. achates larvae reduced these two measures of plant vigor. Knapweed plants resuming growth after the drought ended in spring 2006 were significantly larger than those resuming growth under drought conditions in spring 2005. Spring drought may reduce knapweed growth, but C. achates reduced knapweed growth regardless of drought conditions.

Comparison of Larval Development and Overwintering Stages of the Spotted Knapweed Biological Control Agents Agapeta zoegana (Lepidoptera: Tortricidae) and Cyphocleonus achates (Coleoptera: Curculionidae) in Montana Versus Eastern Europe

ABSTRACT Larval development of insects introduced for biological control of invasive weeds may be constrained if the new climate is more extreme than in their native range. We surveyed larval development in Agapeta zoegana L. and Cyphocleonus achates (Fahraeus), two species of biological control insects introduced from eastern Europe against spotted knapweed in western North America. We dissected spotted knapweed roots collected from five sites in western Montana over 6 yr either in late fall or early spring and measured larval head capsule size to determine the overwintering instar stage. Development of A. zoegana was estimated equally well with late fall or early spring root collections, but C. achates rate of development may be underestimated using fall samples. The larvae of neither species entered diapause in as advanced an instar in western Montana as reported for their native range. Most A. zoegana larvae reached the third (26%) or fourth (20%) instar at diapause, with only 15% reaching sixth instars, as they typically do in their native Eurasia. Almost all (94%) C. achates overwintered as first instars, with most of the remaining (4%) being eggs, some of which were viable in the spring. Only a small number (2%) of C. achates larvae overwintered as second instars, the common overwintering stage in their native range. Slower development may explain, in part, why A. zoegana only has one generation per year in Montana compared with two to three generations per year in Europe.

Overwintering Mortality of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on Spotted Knapweed: Effects of Larval Competition Versus Exposure to Subzero Temperatures

Abstract The overwintering survival of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) was examined in a 5-yr (1989–1993), multiple-site field study in western Montana. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Significant differences in overwintering mortality of U. affinis and U. quadrifasciata larvae (whether occurring conspecifically or heterospecifically within the same capitulum) were found among sites. Overwintering mortality of U. affinis and U. quadrifasciata (whether occurring conspecifically or heterospecifically within the same capitulum) showed no relationship with site elevation. Overwintering mortality of U. affinis and U. quadrifasciata showed no relationship with the densities of conspecific or heterospecific galls found within the capitula. Extended periods of extremely cold temperature were found to explain most of the variation in overwintering mortality of both fly species. We conclude that periods of extremely cold temperature is the most important mortality factor affecting overwintering survival in U. affinis and U. quadrifasciata.

Use of Picloram to Enhance Establishment of Cyphocleonus achates (Coleoptera: Curculionidae)

Abstract Herbicidal control of spotted knapweed, Centaurea maculosa Lamarck, is rarely cost-effective, and sustainable control may require an integrated approach. Cyphocleonus achates (Fahraeus) is a flightless root-feeding weevil of Eurasian origin that has been introduced into North America for biological control of spotted knapweed. We hypothesized that reducing the density of spotted knapweed using reduced rates of picloram would improve the establishment of C. achates. At two sites in western Montana, three adult weevil densities (none, three, and six weevils m−2) were released in the fall (1995 at site 1 and 1996 at site 2) in 2-m2 plots encircled with enclosures to prevent weevil escape. Six picloram rates (0, 0.03, 0.06, 0.09, 0.12 or 0.15 kg ha−1) were applied the following spring before weevil emergence in a randomized complete-block design with four replications (18 treatments per replication). Spotted knapweed density and spotted knapweed and grass cover were sampled in July each year following treatment. Weevil numbers were counted annually in August. By 1998, picloram rates of >0.09 kg ha−1 had reduced spotted knapweed density from ≈500 to ≈175 plants m−2. Cover was reduced from ≈60 to10% at these rates. There was no detectable impact of weevils on spotted knapweed. Weevil numbers in plots treated with picloram >0.09 kg ha−1 were about half (0.5 weevils per plot) the numbers found in the other picloram treatments (1.25 weevils per plot). Weevil numbers in plots treated with 0.03, 0.06, and 0.09 kg ha−1 were similar to the unsprayed control. Weevil numbers were highest when spotted knapweed cover was between 30 and 70%. Results show than reduced rates of picloram do not limit weevil establishment.

Revised methods for the mass-rearing of the spotted knapweed biological control agent, Cyphocleonus achates (Coleoptera: Curculionidae), in field corrals

Abstract Cyphocleonus achates (Fahraeus), a root-feeding weevil introduced from Eurasia, is an effective biological control agent against spotted knapweed, Centaurea stoebe L. ssp. micranthos. Because C. achates is univoltine and does not fly, distribution of the weevil has been slow. To hasten the weevils distribution, a rearing effort using field corrals was initiated at a facility in Corvallis, Montana. Procedures for mass-rearing the weevil in field corrals are described, with an emphasis on improvements over earlier methods. The described field-corral approach is effective and appropriate for producing C. achates for distribution in the western United States.