Editing Centaurea (section)

==Ecology==
Certain knapweeds have a tendency to dominate large stretches of landscape together with a few other plants, typically one or two grasses and as many other large herbaceous plants. The [[common knapweed]] (''C. nigra'') for example is plentiful in the [[Mesotrophic grasslands in the British National Vegetation Classification system|mesotrophic grasslands]] of England and nearby regions. It is most prominently found in pastures or meadows dominated by [[cock's-foot]] (''Dactylis glomerata'') as well as either of [[crested dog's-tail]] (''Cynosurus cristatus'') and [[false oat-grass]] (''Arrhenatherum elatius''). It is also often found in mesotrophic grassland on [[rendzina]]s and similar [[calcareous soil]]s in association with [[glaucous sedge]] (''Carex flacca''), [[sheep's fescue]] (''Festuca ovina''), and either [[tor-grass]] (''Brachypodium pinnatum'') and [[rough hawkbit]] (''Leontodon hispidus''), or [[upright brome]] (''Bromus erectus''). In these [[grasslands]], [[greater knapweed]] (''C. scabiosa'') is found much more rarely by comparison, often in association with [[red fescue]] (''Festuca rubra'') in addition to cock's-foot and false oat-grass.

Due to their habit of dominating ecosystems under good conditions, many ''Centaurea'' species can become [[invasive weed]]s in regions where they are not native. In parts of North America, [[diffuse knapweed]] (''C. diffusa''), [[spotted knapweed]] (''C. maculosa'') and [[yellow starthistle]] (''C. solstitialis'') cause severe problems in [[agriculture]] due to their uncontrolled spread. The [[seed]]s are typically transported by human traffic, in particular the [[tire]]s of [[all-terrain vehicle]]s. The two knapweeds are harmful mainly because they are strongly [[allelopathic]], producing powerful [[toxin]]s in their [[root]]s that stunt the growth of plants around them not adapted to this.<ref name = hcv0304>Hierro & Callaway (2003), Vivanco ''et al.'' (2004).</ref> Yellow starthistle, meanwhile, is inedible to most [[livestock]] due to its spines and apparently outright [[poisonous]] to [[horse]]s and other [[equine]]s. However, efficient methods of [[biological control]] by insect [[pest (organism)|pest]]s of these weeds have been developed; the knapweeds can also exploited to their detriment by [[targeted grazing]]. [[Controlled burning]] may also be used, though the timing is important to avoid the plants having seeded already, and neither allowing sufficient time for them to regrow from the rootstock.<ref name = emerygross2005>Emery & Gross (2005).</ref>

Yet other species of ''Centaurea'' &ndash; mostly ones that occur between Italy and the [[Caucasus]] &ndash; are [[endemism|endemics]] of a single island or valley, and some of these are [[endangered]]. The [[Akamas Centaurea]] (''Centaurea akamantis'') of [[Cyprus]] is almost [[extinct]], while the western Caucasus endemics ''[[Centaurea leptophylla|C. leptophylla]]'' and ''[[Centaurea straminicephala|C. straminicephala]]'' are at least very rare and ''[[Centaurea hedgei|C. hedgei]]'' and ''[[Centaurea pecho|C. pecho]]'' from the same region are certainly not abundant either. The last four species would be adversely affected by the proposed [[Yusufeli Dam]], which might actually destroy enough [[habitat]] to push the two rarer ones over the brink of extinction.

[[File:Melitaea phoebe 1815a.jpg|thumb|right|[[Knapweed fritillary]] (''Melitaea phoebe'').<br/>This butterfly can spend their entire lives living off a patch of [[brown knapweed]] (''C. jacea'').]]
''Centaurea'' are copious [[nectar]] producers, especially on high-lime [[soil]]s. The high nectar yield of the genus makes it very attractive to [[insect]]s such as [[butterflies]] &ndash; including the [[endangered]] [[Karner blue]] (''Plebejus melissa samuelis'') which visits introduced spotted knapweed &ndash; and day-flying [[moth]]s &ndash; typically [[Zygaenidae]], such as ''[[Zygaena loti]]'' or the [[six-spot burnet]] (''Z. filipendulae''). The [[larva]]e of some other [[Lepidoptera]] species use ''Centaurea'' species as food plants; see List of Lepidoptera that feed on ''Centaurea''. Several of these are used in biological control of invasive knapweeds and starthistles.

[[Larva]]e of several [[true weevil]]s (Curculionidae) of the [[subfamily]] [[Lixinae]] also feed on ''Centaurea''. Some [[genera]] &ndash; such as ''[[Larinus]]'' whose larval food is flowerheads &ndash; have many species especially [[adaptation|adapted]] to particular knapweeds or starthistle and are used in biological control too. These include the [[yellow starthistle flower weevil]] (''L. curtus'') for yellow starthistle, [[lesser knapweed flower weevil]] (''L. minutus'') for diffuse knapweed and [[blunt knapweed flower weevil]] (''L. obtusus'') for spotted knapweed. [[Broad-nosed seedhead weevil]] (''Bangasternus fausti'') larvae eat diffuse, spotted and [[squarrose knapweed]] (''C. virgata'' ssp. ''squarrosa''), while those of the [[yellow starthistle bud weevil]] (''B. orientalis'') do not seem to live on anything other than yellow starthistle and occasionally [[purple starthistle]] (''C. calcitrapa''). But perhaps most efficient in destroying developing yellow starthistle seedheads is the larva of the [[yellow starthistle hairy weevil]] (''Eustenopus villosus''). [[Knapweed root weevil]] (''Cyphocleonus achates'') larvae bore into the roots of spotted and to a lesser extentely diffuse knapweed, sometimes killing off the entire plant.

Also used in biological control are [[Tephritidae]] (peacock flies) whose larvae feed on ''Centaurea''. [[Knapweed peacock fly]] (''Chaetorellia acrolophi'') larvae eat spotted knapweed and some other species. The [[yellow starthistle peacock fly]] (''C. australis'') has an initial [[generation]] each year which often uses [[cornflower]] (''C. cyanus'') as larval food; later generations switch to yellow starthistle. The flies are generally considered less efficient in destroying the growing seedheads than the weevils, but may be superior under certain conditions; employing flies and weevils in combination is expensive and does not noticeably increase their effect.