Linda J. White

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.

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.

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.