Rhinocyllus conicus
(thistle-head weevil)

Native to Europe and western Asia, R. conicus has been deliberately introduced to Canada in 1968 (Harris and Zwölfer, 1971; Harris, 1984); South America in 1980 (Feldman, 1997); Australia in 1989 (Woodburn and Cullen, 1993; 1995); and New Zealand in 1973 (Jessep, 1975; 1981) as a biological control agent for thistles in the genera Carduus,Cirsium and Silybum. Introduced populations were collected from different hosts and so included populations from both temperate and Mediterranean climates.

North America has a number of species of native thistle in the genus Cirsium that are susceptible to damage from R. conicus. As the impacts of R. conicus on some of them has been quite significant (Louda et al., 1990; 1995; Louda and Potvin, 1995; Louda 1998), and as some of these native thistles were already threatened,  R. conicus is now considered an invasive species within parts of North America (Louda et al., 1997). At the time of its release, the potential impact of R. conicus on native thistles was considered acceptable.

In regions other than North America, no vulnerable native or economically important plants occur within the host range of R. conicus. It is not a regulated pest in any country of introduction.

The known distribution of R. conicus does not perfectly match the distribution of its main host plants in its native range, which suggests some apparent gaps in its native range may simply be because the host has been recorded but the pest has not. R. conicus is likely to have a largely continuous distribution around the Mediterranean corresponding to the distributions of its main host plants (see Hosts/Species Affected). 

Its true range in North America is likely to be greater than what has been recorded. Zwölfer and Harris (1984) stated that R. conicus ‘has been established in most parts of North America with a Carduus nutans problem’; C. nutans is known from nearly all of North America except 5 US states and 4 Canadian Provinces (USDA, 2013).

The risks of the introduction and spread of R. conicus to other regions is limited, as it is specific to a few genera of host plants and has only left its native range through deliberate release as a biological control agent. Further deliberate introductions seem unlikely as there are few other countries where the thistle host plants are serious weeds.

The negative impacts of this insect are restricted to North America as this is the only continent with native (or commercial) plants that are susceptible to attack. R. conicus is probably already in most parts of the USA where its target hosts grow, so risks of further spread within the USA are only to regions with susceptible native thistles but without any of the relevant exotic thistles. To reduce risk of spread within the USA, R. conicus is now banned from inter-state movement.

R. conicus is restricted to the habitats and regions where its host plants are found (see Hosts/Species Affected). The host plants are found in pastures and disturbed sites, like roadsides, in temperate and Mediterranean climates in Europe, North Africa, Western Asia, North America, southern South America, south-eastern Australia and New Zealand.

North America has about 90 species of native thistle in the genus Cirsium (USDA, 2013). Susceptibility of these species to R. conicus is related to a) their proximity to exotic host thistle populations on which this weevil is found, and b) the degree to which flowering phenology is synchronous with the reproductive cycle of R. conicus (Russell et al., 2007). A number of rare and threatened native Cirsium species in North America have been documented: these include C. canescens Nutt. (Louda et al., 1990; Arnett and Louda, 2001), C. undulatum Nutt. (Maw, 1982), C. ownbeyi S.L. Welsh (DePrenger-Levin et al., 2010) and C. hillii (Canby) Fernald (Sauer and Bradley, 2008). Turner et al. (1987) listed another twelve species of native Cirsium thistles in California from which R. conicus has been reared. Maw (1982) also found eggs on the native C. flodmanii (Rydb.) in Canada.

Genetics

The various groups and ecotypes of R. conicus have been described but not been fully understood genetically. Some may be subspecies. Populations from different regions and hosts have distinct host preference profiles and capacities to differentially exploit different thistle hosts (C. nutans, C. pycnocephalus, C. vulgare (Savi) Tenore and Silybum marianum Gaert.) (Unruh and Goeden, 1987). Populations from Onopordumacanthium have also been described (Zwölfer and Harris 1984). More work is needed to better understand the genetic patterns underlying these differences and to see if the different ecotypes show any degree of reproductive incompatibility.

Reproductive Biology

R. conicus is a univoltine to partly bivoltine weevil. Adult females lay about 200 (54-360) eggs which take about a week to hatch. There are four larval instars which feed and pupate internally in the capitulum, in a hard black ovoid cell that resembles a gall. Pupation takes up to two weeks but the immature adults stay in the cells several weeks. Development from egg to adult takes about seven weeks, but may be shorter in Mediterranean climates. Newly emerged adults seek aestivation sites in summer in Mediterranean regions, whereas in temperate regions a second partial generation takes place. In late summer and in autumn the adults seek overwintering sites away from their hosts. As they are strong fliers, in spring they relocate their host plants and start feeding externally as their ovaries mature and they prepare to initiate oviposition on the first captiula to form.

Physiology and Phenology

This weevil only has a partial second generation in more temperate climates (Gassmann and Kok, 2002); however, in Tennessee and Georgia, R. conicus was observed to complete one generation and then not feed on thistles until the following spring (Wiggins, 2013, personal communication). Zwölfer and Harris (1984) considered that day-length determines second generations, so that new adults experiencing increasing daylength will start a second generation on flower heads of their host (mainly Carduus nutans) which developed later in the season. The first generation is very much skewed towards host captiula produced in spring (Cullen and Sheppard, 2012), which are also the largest; this means that later, smaller capitula receive proportionally fewer eggs. The capacity for a second generation is assisted by a longer flowering period of the host in temperate wetter summers.

In the Mediterranean region, where R. conicus also uses the hosts C. pycnocephalus, C. tenuiflorus and S. marianum, the hot dry summer means the hosts have a relatively early, but shorter flowering period, meaning R. conicus starts ovipositing earlier than in temperate regions, but only has time for one generation before their hosts stop flowering and adults need to seek aestivation sites. These differences relate to the different ecotypes by both region and host, and allow this species to persist under a broader range of hosts and climates.

In the exotic range these differences in phenology by ecotype are maintained with little evidence of quick synchronization to new conditions (Cullen and Sheppard, 2012), suggesting the ecotypes are phonologically distinct and not a phenotypically plastic strategy of survival. Effectiveness as a biocontrol agent and the impact of R. conicus on native thistles in the exotic range are determined by the degree of synchrony between the weevil oviposition and flowering of available potential hosts (Goeden and Ricker, 1985). Therefore the native thistles most impacted on by R. conicus tend to be those that flower early in the growing season. As a biological control agent it has proved more effective in temperate regions and this may be in part because the populations of the weevil introduced into these regions have a partial second generation.

Longevity

Adults live up to 12 months.

Activity Patterns

Mediterranean-adapted populations undergo aestivation through the dry summer months. All populations hibernate through the cold months and migration tends to occur in spring, when the adults seek host populations having emerged from their hibernation sites. Once the adults have located host populations they become much more sedentary.

Population Size and Density

Population size is determined by the abundance of their principal host plants, but can range up to tens of individuals per square meter in spring when there are dense populations of host plants.

Nutrition

Nutrition is restricted to known host plants in the genera Carduus, Cirsium, Silybum and Onopordum.

Associations

This weevil is one of a diverse community of insects found in the capitula of its host plants in its native range (Sheppard et al., 1991; 1994). Other insects in this community tend to be active later in the flowering periods, but competition in the captula is quite intense. In the exotic range, the insect community is largely limited to other species that have been introduced as biological control agents in the exotic hosts (e.g. Woodburn, 1996); however, amongst the native thistles R. conicus competes in the developing capitula with native insects, particularly tephritid seed flies, and may locally threaten the population viability of such flies (Tatyana and Louda, 2011).

R. conicus is attacked by many different species of parasitoid in its native range (Zwölfer and Harris, 1984). The impact of these parasitoids on native R. conicus populations can be very significant, with particularly high levels of egg parasitism. Indeed, it appears that R. conicus lays more eggs on early capitula than the individual captulum can support, as in the native range parasitoids often kill a proportion of eggs; however, this over-laying of eggs can mean that capitula in the exotic range may wilt under the attack levels of R. conicus, leaving all larvae dead (Cullen and Sheppard, 2012). R. conicus in its native range can also be quite heavily attacked by Nosema, a microsporidian parasite (Woodburn and Cullen, 1995; Cullen and Sheppard, 2012). In its exotic range R. conicus has also attracted parasitoids (Zwölfer and Harris, 1984; Wilson and Andres, 1986), but at quite low rates.

Environmental

• Biological control

The other four species in the genus Rhinocyllus also feed in the capitula of members of the tribe Cardueae (in the Asteraceae). These are R. oblongus Cap. (found in Spain, Italy, Greece and Turkey), R. depressirostris Boh. (southern Russia), R. remaudieri Hoffm. (Iran) and R. schoenherri Cap. (Caucasus). Of these species, only R. oblongus is sympatric with R. conicus in the latter’s native range (Zwölfer and Harris, 1984), and may indeed be a subspecies (Klein and Seitz, 1994).

A sixth species, R. inquilinus Gyll., described from Finland, appears to be a small individual of R. conicus (Zwölfer and Harris, 1984).

No other species of Rhinocyllus have been introduced as biological agents or are found outside Europe, North Africa and western Asia.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Containment/Zoning

The movement of R. conicus across USA state borders is now prohibited, an attempt by the authorities to contain the impact of this weevil on native thistles.

More work is needed to better understand the genetic patterns underlying ecotypic differences in R. conicus and to see if the different ecotypes show any degree of reproductive incompatibility.

26/03/12 Original text by:

A Sheppard, CSIRO Entomology, Australia