Preliminary Results on the Phytophagous Insect Fauna on Onopordum acanthium ( Asteraceae ) in Bulgaria

The Scotch thistle, Onopordum acanthium (Asteraceae) has the Eurasian origin and represents an invasive weed in the USA and Australia. It is a serious problem in pastures, along roadsides, rangeland, etc. The weed is very common in Bulgaria and in 2009-2010 several sites with large populations of O. acanthium have been located. The weekly surveys of some of the sites, mainly in the region of Plovdiv, revealed a significant diversity of phytophagous insect species, some of which had very high population density. More than 30 species have already been identified and observations on the phenology and biology of some of them have been conducted. For some of the species, such as endophagous Larinus latus, Lixus cardui, Eublemma amoena, Trichosirocalus briesei, or ectophagous Cassida rubiginosa etc. found during our surveys, there are reports in the literature describing them as having been already introduced as successful biocontrol agents in Australia. Endophagous species like lepidopteran Myelois circumvoluta and Pyroderces argyrogrammos which feed on the stems and the head, and the tephritid flies Tephritis postica and Chaetostomella cylindrica which feed on the capitula, seem quite destructive for the weed, but need to be further studied regarding host specificity. Species like Vanessa cardui and Brachycaudus cardui were found regularly, but in low population density, mainly due to the high rate of parasitism.


INTRODUCTION
The Scotch thistle, Onopordum acanthium, is a widespread weed, found in nearly 50 countries (CAB International, 2004).It is a major problem in North America as well.Its presence has been recorded in 37 states of the US (USDA- NRCS, 2006) and five provinces in Canada (Moor and Frankton, 1974).In the United States, the weed is most problematic in the arid parts of the Northwest, California and Nevada.Since the late 1970s, there has been an exponential expansion in the number of states in the Northwest that have been infested by this weed (Rice, 2006).It belongs to the sunflower family (Asteraceae) and it is also known as cotton thistle, heraldic thistle, Scotch cotton thistle, and woolly thistle.It is native to Europe and Asia (Cargill et al., 1998) and has been introduced to temperate climates elsewhere, including much of North America (Taylor, 1990) and Australia (Briese et al., 2002).In North America Onopordum acanthium causes significant economic losses to ranchers.It occurs in agricultural areas, range/ grassland, riparian zones, disturbed scrubs/shrublands.Infestations of O. acanthium reduce forage production and prohibit land utilization for livestock.Dense stands of large, spiny plants constitute a barrier to livestock movement, almost totally excluding animals from grazing and access to water (Hooper et al., 1970;Sindel, 1991).It is easily transmitted to new locations by livestock and floating vegetation or debris, or when used for ornamental purposes.Seeds are dispersed locally by wind or by livestock and wildlife (Piper, 1984).
Physical and chemical management has partial effect on the thistle.Australia was the first country to initiate a biological control effort especially targeting Scotch thistle by surveying the insect fauna on Onopordum spp.thistles in Europe and the Mediterranean region (Balciunas, 2007).Briese et al. (1994) identified 129 insect species that fed on Onopordum spp. in Europe.Eventually, seven insect species were released in Australia (Briese et al., 2002;Swirepik and Smyth, 2002).Several biocontrol agents have been introduced in the United States for the biological control of Musk thistle (Carduus nutans) from the same family, including Rhinocillus conicus, Trichosirocalus horridus and Psylliodes chalcomera in 1969, 1974, and 1997, respectively (Gassman and Kok, 2002).In 2007, the BBCA started field surveys in several countries in Europe and Eastern Asia (Turkey, Kazakhstan) for the selection of new biocontrol agents with the aim to introduce them in the Unit-ed States (Cristofaro, 2008).Numerous associated organisms were found and three of them were preliminary selected -Trichosirocalus briesei, Larinus latus, and Psylliodes chalcomerus.The last two have already past preliminary host range tests in quarantine facilities in the USA (Cristofaro, 2008).Recently, Bulgaria has been included on the list of surveyed countries and the objective of the current study was to establish the native insect fauna associated with O. acanthium.

MATERIAL AND METHODS
Immature and mature phytophagous insects were collected from O. acanthium at several locations in the central part of southern Bulgaria -Chernichevo, Stamboliyski, Karadzovo, Plovdiv, Poroyna.The plants were examined once a week from May to mid-November for insect-caused injuries during their early and late vegetative, flowering, fruiting and senescing stages.The leaves, stems, and capitula of at least 20 plants per site were examined in situ and ectophagous insects were collected by hand or by aspiration.The leaves and stems of 10-15 plants were dissected on-site in search of endophagous insects.Larvae and nymphs of unidentified species were collected together with the damaged plant parts and placed in plastic containers in laboratory, for emergence of adults and identification.Relative frequency of insect occurrence was recorded as: rare (found at less than 15% of the sample sites); occasional (found at 15-50% of the sites); and common (found at more than 50% of the sites) (Watts and Piper, 2000).The adults were identified using the identification keys for different species.
For studying the insect fauna in the flower and seed heads, 50 capitula were collected from a location near Karadzovo (42 o 05,45 N, and 024 o 54,35 E) in June.At the time of collection, some of the capitula were still in the bud stage, while some in the same cluster had ripening seeds.Each capitula was placed separately in a 300 ml plastic container covered by muslin on the top.The containers were checked several times later and the emerged adults were recorded.The ratio of the adults of different species was calculated.

RESULTS AND DISCUSSION
A total of 42 insect species was detected, feeding on different plant parts of Onopordum acanthium in the central part of Southern Bulgaria (Table 1), four of which were not identified at all and five -identified only to genus.Inclusion in the table was based on direct observation of feeding-caused damage.The insect fauna was represented by five orders and 28 families.The most abundant were the species from Coleoptera and Lepidoptera.
The number of coleopteran species feeding on Scotch thistle was 15, some of them found in very high population density, like Onopordum capitulum weevil Larinus latus, the stem-boring weevil Lixus cardui, the green thistle beetle Cassida rubiginosa, the flea beetle Psylliodes chalcomerus, and the cigarette beetle Lasioderma serricorne.The first two are specialized and already introduced for biological control of Onopordum spp.thistles in Australia (Briese et al., 2002;Briese et al., 2004, Swirepik et al., 2008).The green thistle beetle C. rubiginosa was found suitable for introduction in New Zealand (Goulray and Hill, 2006) as potential control agent for Californian thistle (Cirsium arvense).After laboratory and field tests it was concluded that it is unlikely the beetle would attack any plant species outside the tribe Cardueae.C. rubiginosa is widely distributed in Europe, North America and northern Asia, and according to Zwölfer and Eichhorn (1966), it is one of the most common insects on thistles in Europe.It has been present in North America for over 100 years and is now commonly found in the Western United States and neighbouring parts of Canada (Ward and Pienkowski, 1978).The flea beetle P. chalcomerus was evaluated as prospective control agent for Yellow starthistle, Centaurea solstitialis (Cristofaro, 2004).Several biological forms of this species were collected on different host plants, including Onopordum, and the studies suggest that each of the established forms is closely connected with the specific plant.
Lasioderma serricorne though very common and in high population density is polyphagous and is considered a pest of stored products like tobacco (Ashworth, 1993).The specialized Rhinocyllus conicus and Ceratapion onopordi were rarely found.The rest of the coleopteran species were polyphagous (8 species) and without influence on the survival of the plants.
From 9 lepidopteran species only Eublemma amoena is specialized, the rest being non-specialized or with unknown specialization.The most common were Myelois circumvoluta (Figure 1 and 2) and Pyroderces argyrogrammos (Figure 3 and 4), the larvae of which were feeding in the stems and the capitula.There is information that the larvae of M. circumvoluta are oligophagous, feeding on different thistles -Arctium lappa, Cirsium eriophorum and C. vulgare (Fitter and Peat, 1994) although no host specificity tests have been performed for either of the two species so far.Six dipteran species were found to feed on Onopordum acanthium: three from family Tephritidae, one from Agromyzidae, one from Anthomyiidae (Figure 5 and 6), and one from Cecidomyiidae (Figure 7), the last three being unidentified yet.The agromyzid fly from genus Melanagromyza was quite common, together with the tephritid fly Tephritis postica.Acanthiophilus helianthi was found only on one occasion, and Chaetostomella cylindrica -on two. A. helianthi is a generalist fly, attacking the flower heads of more than 50 plants (Knio et al., 2002), some of which with economic importance and for this reason it is not considered suitable as biocontrol agent.Tephritis postica is related to one genus only -Onopordum (Knio et al., 2002).It is spread in Europe, Western Asia and Northern Africa (Neuenschwander and Freidberg, 1983;Foote, 1984;Freidberg and Kugnel, 1989).It has been included in the strategy for the control of Onopordum spp.thistles in Australia (Briese et al., 2002).Recent studies by Knio et al. (2007) on the host-race formation in C. cylindrica revealed that the fly is most probably a complex of cryptic and reproductively isolated species and one of them is the Onopordum host race.The species from order Hemiptera found during the survey were polyphagous or in low population density, causing insignificant damage.Psyllid nymphs (Figure 8), which were not identified, were found at one of the sites in Karadzovo.
Of all the species detected in our study, the specialized feeders represent 24% and for 19% the host specialization is not known, or additional host specificity tests are needed (Table 1).In a survey carried out in the United States, Watts and Piper (2000) encountered no specialist insects; as suggested by Goeden and Ricker (1986), the absence of Onopordum endophages there may be attributed to the lack of congeneric thistle species in the region, which would provide to the pre-adapted specialist feeders an opportunity to cross over to the weed.
The percentage of internal feeders found in Southern Bulgaria was quite high -45% of all species, compared to 5% in Australia (Briese, 1989), and no internal feeders in a survey in south-eastern Washington and north-western Idaho (Watts and Piper, 2000).In the latter case, according to Briese et al. (1994), all endophagous feeding niches are open for exploitation by non-indigenous specialist insects.Among the internal feeders in Australia 16% damaged the stem, 32% -the capitulum, 26% -the stem and the capitulum, 16% -the stem and the leaves, 5% -the root and the stem, and 11% -the root and the leaves (Briese, 1989).Among the external feeders, only the green thistle beetle Cassida rubiginosa was of significance.As for the European endophagous fauna, 21% were feeding on the capitula, 9% were stem and root feeders, and 8% stem and leaf feeders (Briese, 1989).
A guild of capitulum feeders could be of great importance in the regulation of O. acanthium, as the effective occupation of this niche could significantly affect plant reproduction and survival (Briese et al., 1994).In our study, the most abundant in the capitula were larvae of M. circumvoluta, L. latus and P. argyrogrammos (Figure 9).The larvae of C. cylindrica and T. postica were found in less than 3% of the capitula.A great number of parasitoid species emerged in the containers, which is one possible reason for the low population density if the capitulum feeders at the place of collection.
Host-specificity testing of several candidate European natural enemies of O. acanthium is being carried out at the Exotic and Invasive Weed Research Unit, USDA-ARS, Albany, CA.Two culculionids (Lixus cardui Olivier and Lixus sp.) ware examined in 1996-1997 by Balciunas (2007) and according to him L. cardui feeds and completes development on several indigenous thistles (Cirsium spp.).The root-infesting weevil Trichosirocalus sp. is still in the process of evaluation.It is hoped that the beetle or other yet-to-be-tested insects will be host-specific enough to be approved for release against Scotch thistle in the Western United States (Watts and Piper, 2000).Botanophila spinosa was identified among four species of Onopordum (Briese et al., 1994) and failed to develop on members of other carduinae thistle genera that were tested.
Among the species damaging the capitula (Figure 9), the most numerous were M. circumvoluta, L. latus and P. argyrogrammos.Besides L. latus, neither of the other two species has been studied.Obviously, HS testing will be necessary for evaluating them as possible biocontrol agents.

Table 1 .
Phytophagous insect species feeding on Onopordum acanthium in Southern Bulgaria