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Abstract: The role of spotted knapweed phenology on the attack rate of two seed-head insects Urophora affinis and Larinus minutus was assessed in a series of field studies at four study sites in south-eastern British Columbia, Canada. Slow or later developing knapweed plants had more seed heads that contained only single or multiple U. affinis whereas early or faster developing plants had more seed heads containing L. minutus alone or in combination with U. affinis. L. minutus did not distinguish between seed heads with or without U. affinis larvae when laying eggs. However, seed heads with multiple U. affinis present, produced fewer L. minutus adults than expected. The probability of single or multiple U. affinis galls being present increased with seed-head diameter but was not affected by seed-head height. Attack by L. minutus increased with seed-head diameters >5mm and was lower at plant heights above 50cm. These results demonstrate two mechanisms that enable U. affinis to successfully coexist with L. minutus: differences between the species in their response to the developmental phenology of knapweed heads, and increased survivorship of U. affinis in heads with multiple U. affinis galls through niche interference competition. These mechanisms provide a possible explanation for the persistence of U. affinis populations on spotted knapweed, in spite of high levels of within seed-head mortality that have been observed with increasing L. minutus populations. [Copyright &y& Elsevier]

The physiological responses of plants to variable levels of root and shoot herbivory in the field may yield valuable insights regarding potential compensation or tolerance responses for herbivory. In an infestation of Centaurea stoebe (spotted knapweed) located in the Colorado foothills, we measured physiology, biomass, and flower production of individual plants undergoing a natural range of herbivory by the above- and belowground biological control insects, Larinus minutus and Cyphocleonus achates. Over the growing season, net carbon assimilation rate, transpiration, stomatal conductance, and intercellular leaf [CO2] (C i) all decreased, while water use efficiency increased. The decrease in these physiological traits was due to an increase in the intensity of L. minutus damage over time; effects of C. achates root damage to plant physiology, including transpiration were only marginally significant. The effects of these two species on plant physiology were not interactive, and Larinus minutus was found to exert larger negative effects on this invasive plant in terms of plant physiology and potential reproductive output than C. achates. While previous studies have shown C. achates to have significant negative effects on population densities of spotted knapweed, the addition of Larinus minutus to the suite of insects used in biological control of spotted knapweed should facilitate continued or enhanced reduction in densities of this noxious weed. [ABSTRACT FROM AUTHOR]

The article evaluates the responses of Centaurea stoebe knapweed to herbivores. The growth of this plant is greatly reduced by nitrogen limitation and competing vegetation while Centaurea stoebe's seed production in low-nitrogen soils is reduced by Larinus minutus, a seed feeder. The results show that plant growth responses to herbivory can be generated. It was also found that insect herbivores increase the dominance of such invasive plant.

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 treat- ments (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. [ABSTRACT FROM AUTHOR]

Host plant patch size plays an important role in the distribution and densities of many insect species. Understanding how the host plant patch size influences the population of a biological control agent is necessary to monitor the success of a biological control programme. Urophora quadrifasciata (Dipt. Tephritidae) was released in the early 1970s to control the spotted knapweed, Centaurea biebersteinii (Asteraceae) in North America. The studies reported here investigated the influences of spotted knapweed patch size and stem density on U. quadrifasciata larval populations overwintering within capitula in the Upper Peninsula of Michigan, USA. An inverse relationship was found between the percentage of capitula with U. quadrifasciata larvae and spotted knapweed patch size in 2002 from Houghton county, MI. In 2003, an inverse relationship was also found in the south region of the study area in Houghton and Keweenaw counties, Michigan. The percentage of capitula with U. quadrifasciata larvae and spotted knapweed patch size were positively related for the central region. No relationship was found for the North region in 2003. Dispersal of U. quadrifasciata is directed north-east along the Keweenaw Peninsula, Michigan. With this directed spread, areas with well-established populations of U. quadrifasciata would provide source individuals for new colonization. Spotted knapweed patch size may play an important role in the overwintering populations of U. quadrifasciata as a result of dispersal. [ABSTRACT FROM AUTHOR]

Interspecific competition has been suggested as an explanation for the failure of some insects as biological control agents for weeds. Enclosure and exclusion cages were used, in southern British Columbia, Canada to evaluate the importance of interspecific competition between a seedhead weevil, Larinus minutus , and a gall-inducing fly, Urophora affinis , two biocontrol agents released against spotted knapweed in North America. At the seedhead scale, U. affinis , which is an inferior biological control agent based on knapweed seed mortality, was the superior competitor. Larinus minutus attack rates were significantly lower in the presence of U. affinis compared to release treatments where L. minutus was attacking alone. Reduced L. minutus attack rates were apparent in seed heads expected to contain both species, assuming insect distributions were random, but instead only contained U. affinis . L. minutus did not significantly affect U. affinis density. Although overall attack rates on knapweed seedheads were higher when both species were together at a site, the consequence of the antagonistic interaction is that overall seed destruction was not as high as it could have been if the weevil were attacking on its own. These results support minimizing the number of biocontrol agents released that use similar resources on the target weed, to avoid negative interactions between control agents and potential reductions in biocontrol efficacy resulting from competitive exclusion. [ABSTRACT FROM AUTHOR]

Abstract: Cyphocleonus achates, the knapweed weevil, is an effective biological control agent of the invasive weed, Centaurea maculosa Lam. A meridic diet was developed and tested for the rearing of the larval stage of this insect. Using this diet, C. achates was reared for over three generations, with the adults being offered knapweed plants for feeding and oviposition in greenhouse conditions. Slight or no differences were seen between insects reared on a standard meridic diet formulation and one containing knapweed tissues. The following life history parameters were monitored over the three generations: percent egg hatch (ranging from 42.9 to 59.1%), time to egg hatch (20.0–23.2 days), time to adult emergence (52.0–54.1 days), adult weights 3 days post-eclosion (101.9–117.0mg), percent adult emergence (48.3–58.6%), and percent mortality/deformity in the different stages (with mortality occurring primarily in the early larval stages). Additionally, a study involving low temperature and short day conditions suggested that C. achates could be maintained for longer periods of time in larval diet cells when placed in growth-retarding conditions, although percent adult emergence was lower. External morphology was also studied in order to distinguish between the sexes to ensure that each adult cage had a similar ratio of females to males. Abdominal features were found to be the most dependable characteristics for use when determining the sex of adult C. achates. [Copyright &y& Elsevier]

Release of exotic insects as biological control agents is a common approach to controlling exotic plants. Though controversy has ensued regarding the deleterious direct effects of biological control agents to non-target species, few have examined the indirect effects of a ”well-behaved” biological control agent on native fauna. We studied a grassland in west-central Montana infested with spotted knapweed (Centaurea maculosa) to examine the effects of knapweed invasion and two gall flybiological control agents (Urophora affinis and U. quadrifasciata) on the native deer mouse (Peromyscus maniculatus). Stomach-content analysis revealed that Urophora were the primary food item in Peromyscus diets for most of the year and made up 84–86% of the winter diet. Stomach contents indicated that wild-caught mice consumed on average up to 247 Urophora larvae mouse–1 day–1, while feeding trials revealed that deer mice could depredate nearly 5 times as many larvae under laboratory conditions. In feeding trials, deer mice selected knapweed seedheads with greater numbers of galls while avoiding uninfested seedheads. When Urophora larvae were present in knapweed seedheads, deer mice selected microhabitats with moderately high (31–45% cover) and high knapweed infestation (≥46% cover). After Urophora emerged and larvae were unavailable to Peromyscus, mice reversed habitat selection to favor sites dominated by native-prairie with low knapweed infestation (0–15%). Establishment of the biological control agent, Urophora spp., has altered deer mouse diets and habitat selection by effecting changes in foraging strategies. Deer mice and other predators may reduce Urophora populations below a threshold necessary to effectively control spotted knapweed. [ABSTRACT FROM AUTHOR]