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Centaurea solstitialis (yellow starthistle, Asteraceae) is an invasive annual weed in the western USA that is native to the Mediterranean Region and is a target for classical biological control. Aceria solstitialis is an eriophyid mite that has been found exclusively in association with Ce. solstitialis in Italy, Greece, Turkey and Bulgaria. The mite feeds on leaf tissue and damages bolting plants, causing stunting, witch's broom and incomplete flower development. Field experiments and laboratory no-choice and two-way choice experiments were conducted to assess host plant specificity of the mite in Bulgaria. Mites showed the highest degree of host specificity in the field and lowest in the no-choice experiments. In the field, highest densities of mites occurred on Ce. solstitialis and Ce. cyanus (bachelor's button), and either no mites or trace numbers occurred on the other test plants: Ce. diffusa (diffuse knapweed), Carthamus tinctorius (safflower) and Cynara scolymus (artichoke). In no-choice experiments, mites persisted for 60 days on Ce. diffusa, Ce. cyanus, Ce. solstitialis, Ca. tinctorius and Cy. scolymus, whereas in two-way choice experiments mites persisted on 25% of Cy. scolymus plants for 60 days and did not persist on Ca. tinctorius beyond 40 days. The eight other species of plants that were tested in the laboratory were less suitable for the mite. These results suggest that although A. solstitialis can persist on some nontarget plants for as long as 60 days in the laboratory, it appears to be much more specific under natural conditions, and warrants further evaluation as a prospective biological control agent. [ABSTRACT FROM AUTHOR]

We compared nectar secretion rates and bee guilds of yellow star-thistle, Centaurea solstitialis, on Santa Cruz Island (USA) and the Northeast Aegean Island of Lesvos (Greece). This plant species is non-native and highly invasive in the western USA but native to Eurasia (including Lesvos). "Nectar flow" was assessed by measuring nectar volumes in florets of flower heads covered with mesh bags (preventing visitation by bees); "nectar standing crop" data were taken from open (unbagged) flower heads to which all bees could gain access. We censused bees at C. solstitialis during comparable periods on both islands and determined the bee guild composition of the plant on Lesvos. Significant differences in nectar levels occurred between bagged and unbagged florets at each locale, especially during the period that pollinators were most common. Nectar flow and nectar standing crop volumes were lower on Lesvos than on Santa Cruz Island. The bee guild diversity at Lesvos was higher relative to Santa Cruz Island. Surprisingly, however, honey bees were not recorded during our monitoring periods on Lesvos. [ABSTRACT FROM AUTHOR]

Yellow starthistle (YST) (Centaurea solstitialis) is an invasive weed in the USA and a target of classical biological control. However, the precise geographical origin(s) of YST in the USA is not known. This study was conducted to learn whether the origin(s) of YST in California, USA could be determined on the basis of digital image analysis (DIA) of YST reactions to isolates of the rust Puccinia jaceae. One trial was conducted with two isolates of P. jaceae and another trial with only one of these isolates. Six different YST collections were tested. The DIA consistently detected and measured rust pustules on YST leaves. Compared to visual assessment, DIA was a fair predictor of the number of rust pustules per leaf. There were no differences between isolates of P. jaceae in reactions of different YST collections in the first trial. Differences in plant reactions between trials were predominately due to more disease incidence and severity in the second trial, which was probably from the greater amount of inoculum. Based on analyses of variance and canonical discriminative analyses, YST from California appeared most closely related to the YST collection from Erzurum, Turkey. In examination of squared Mahalanobis distances, all collections, except the collection from Turkey, were significantly distant from the California collection in both trials. More studies are necessary to determine the precise area(s) of origin (within Turkey and neighboring countries) of YST in the USA. Digital image analysis of rust reactions can be a powerful tool to do this. [Copyright &y& Elsevier]

Simple Summary: Yellow starthistle (Centaurea solstitialis) is an invasive annual plant that has infested multi-million ha in the western United States, causing multi-billion dollars of losses and management costs. The rosette weevil (Ceratapion basicorne) has recently been approved for biological control of this weed. However, this weevil reproduces only once a year, which hinders mass-rearing for release. Here, we tested whether insect hormones can break the reproductive diapause of female weevils. We found that applying two insect hormones, 20E and methoprene, can consistently terminate diapause of female weevils to enable them to lay eggs. Thus, topically applying insect hormones could be used to induce females to lay eggs at any time of year, which would permit rearing more than one generation per year. This could greatly increase the number of weevils produced per year in a mass-rearing program to accelerate the release of rosette weevils to help suppress yellow starthistle in the United States. These insects would revert to having one generation per year after release in synchrony with their host plant. Classical biological control is an important method for controlling invasive alien weeds. Univoltine insects can be highly effective biological control agents of annual weeds because they are well synchronized with their host plant. However, having only one generation per year makes it difficult and slow to multiply them in the laboratory for initial field releases. If it were possible to terminate reproductive diapause early, then we could rear multiple generations per year, which would greatly increase annual production. We used a recently approved biocontrol agent, Ceratapion basicorne (a univoltine weevil), for yellow starthistle (Centaurea solstitialis) as a model system to study the use of two insect hormones, 20-hydroxyecdysone (20E) and methoprene, to terminate reproductive diapause. Methoprene (1 μg applied topically) terminated reproductive diapause of female weevils, whereas doses of 0.0, 0.01 and 0.1 μg did not. The combination of methoprene and 20E had a stronger effect and induced an increase in eggs (1.51 ± 0.16 eggs/day, mean ± SE) compared with a methoprene only group (1.00 ± 0.13 eggs/day), and a control group (0.21 ± 0.04 eggs/day). Thus, topical application of these hormones should enable us to rear the weevil out of its normal season and produce more than one generation per year, which will increase productivity of mass-rearing it for field release. Once released in the field, the insect would continue as a univoltine agent that is well-synchronized with its host plant. [ABSTRACT FROM AUTHOR]