Galinsoga parviflora (gallant soldier)

G. parviflora is a cosmopolitan fast-growing annual herb with the capacity to invade agricultural and other disturbed areas in most temperate and subtropical regions of the world (Holm et al., 1979). It is highly competitive and can spread quickly, often being the dominant species in a field. G. parviflora generates considerable economic impact on crop systems, greenhouses, gardens and nurseries. It is listed as an invasive and noxious weed in North America, Europe, Asia, Australia, and on islands in the Pacific Ocean (see Distribution Table for details). 

The genus Galinsoga is named after M. M. Galinsoga, Head of Madrid Botanical Garden in 1880. The name Galinsoga parviflora is derived from the Latin parvu (little) and flore (flower). A revision of the genus Galinsoga (Compositae: Helianthae) was made by Canne (1977).

Although most authorities treat the closely related G. ciliata as a separate species, these two species are almost certain sometimes to be confused in the weed science literature. This account deals primarily with G. parviflora in the stricter sense, but some records or observations may relate more correctly to G. ciliata. See Similarities to Other Species for their distinguishing features.

G. parviflora is herbaceous, erect and 20-80 cm tall, depending on growing conditions. Leaves are simple-opposite, the lower leaves with petioles, the upper ones without petioles. Leaf blade oval to oblong with sharp apex. The inflorescence consists of typical Compositae/Asteraceae composite flowers, each 5-8 mm across, borne on long acillary peduncles. Each 'flower'/capitulum bears two types of flower: ligulate female white flowers at the margin and tubular hermaphrodite yellow flowers in the central disc (Kissmann and Groth, 1993). Two types of achenes are present: peripheral (1.5-2 x 0.4-0.7 mm) and central (1.1-1.5 x 0.4-0.5 mm). Both types are black. The dispersal units are achenes bearing pappus or parts of flower structures (involucral bractea and the two paleas) that can easily be transported by wind or animals.

G. parviflora is an American weed, and its centre of origin is considered to be the mountainous area of Mesoamerica (i.e., Mexico and Central America; Vibrans, 2009). Because this species was first described from material collected in South America (a cultivated plant from Peru), it has been erroneously cited as native to this part of the world (Vibrans, 2009). Currently, G. parviflora is a cosmopolitan species widely distributed in North America, Europe, Africa, Asia and Australasia. It is considered a weed in at least 40 countries, mostly temperate and sub-tropical but also at higher altitudes in many tropical countries (Holm et al., 1979). 

G. parviflora spreads by seeds. Seed viability is usually high (90%). One plant can set as many as 30,000 seeds (achenes), but it is common to find 5000-10,000 seeds/plant (Kissmann and Groth, 1993). Seed output and dry-matter yield increases with population density, although seed output is reduced at very high densities (Rai and Tripathi, 1983). Achenes can be dispersed by wind, animals, or water. They can also be dispersed by human activities, such as movement of soil or plants. In Finland, the most effective means of dispersal is believed to be seedlings inadvertently grown in nurseries or commercial greenhouses.

G. parviflora is commonly confused with G. ciliata. The latter differs in being much more hairy, with many glandular hairs 0.5 mm or longer on the peduncles, whereas G. parviflora has only few, less than 0.5 mm long. Also G. ciliata has aristate (long-pointed) pappus-scales on the seeds; those of G. parviflora are not long pointed. G. ciliata may also be more shade-tolerant than G. parviflora (Rai and Tripathi, 1986b).

Many of the natural enemies mentioned here are polyphagous species. Batra (1979) comments that host-specific natural enemies should be sought in the area of origin of the weed in the mountainous regions of Latin America.

Fungi: G. parviflora is a host to Plasmopara yunnannensis (Tao and Qin, 1987) and seeds have been found bearing more than 15 fungal species (among them Alternaria, Ascochyta, Penicillum, Phoma, Drechslera, Trichoderma, Fusarium and Botrytis sp.) (Lorenzi, 1986; Prete et al., 1986). Ten fungal species were identified infecting G. ciliata and G. parviflora, the most notable being Glomerella cingulata, the causal agent of anthracnose in G. parviflora (Gasich, 1997).

Viruses: Tomato spotted mosaic wilt virus on G. parviflora plants growing in nearby fields of vegetable crops has been reported in Belgium (Gofflot and Verhoyen, 1990) and Argentina (Gracia and Feldman, 1989). Cucumber mosaic virus was isolated in G. parviflora plants near a celery and lettuce field in the USA (Bruckart and Lorbeer, 1976). G. parviflora was reported as a new host of Turnip mosaic virus in Zimbabwe. Myzus persicae, Brevicoryne brassicae and Aphis fabae transmitted the virus from infected to healthy cabbage plants (Chivasa et al., 2002). Tomato spotted wilt virus infection was detected in the Czech Republic where the main vector Frankliniella occidentalis was also present (Mertelik and Mokra, 1998). A virus disease of sunflower caused by Sunflower mosaic virus had a very narrow host range, infecting only one weed host, G parviflora (Nagaraju et al., 1997).

Mites: several mite species have been found on G. parviflora in the USA (Batra, 1979).

Insects: Agrotis ipsilon, a serious pest of maize, sorghum, soybean, cabbage and rape crops in Brazil, develops its first-instar larvae in G. parviflora plants (Link and Severo-Pedrolo, 1987). Other noctuids, such as Heliothis, Plutella and Pieris spp., feed on G. parviflora (Rai and Tripathi, 1985). Thrips tabaci, the vector of tomato spotted wilt tospovirus, is usually found in G. parviflora plants in tobacco fields of Poland (Wegorek and Lipa, 1979). The host plant for the larvae of Dioxyna bidentis or Paroxyna bidentis is G. parviflora in southern Norway (Greve, 2001). G. parviflora was reported as a supplementary or alternative food for Arion lusitianicus (Kozlowski and Kozlowska, 2000).

In an extensive survey in the USA, 122 insect species were found on G. parviflora plants, among them 22 crop pests, including three virus vectors (Batra, 1979). Orthezia insignis, a serious eucalyptus pest in India, may be found feeding on G. parviflora (Srikanth et al., 1988).

Nematodes: species of the genus Meloidogyne (arenaria, thamesi, hapla, javanica and schachtii) and Globodera rostochiensis, that produce serious root damage in Hungary, Brazil (Lordello et al., 1988) and the Philippines (Zorrilla and Davide, 1983), have been found in G. parviflora.

Bacteria: results indicated that G. parviflora can maintain Ralstonia solanacearum (race 3 biovar ) in the absence of potato as a source of infection in the following years (Barchend and Schmidt, 2002).

Parasitized aphids were observed on G. parviflora in Chile (Russo et al., 2000).

Kranz et al. (1982) found that the critical periods of weed competition for beans were at initial crop development and flowering/pod formation. Leaf development, plant height, pod number per plant, seed number per pod, seed production and harvest index were all reduced by weed competition; seed production was reduced by 51%. Other than G. parviflora, other weeds present were Cyperus rotundus, Bidens pilosa and Brachiaria plantaginea.

In a maize crop, in which the weed flora was dominated by G. parviflora (80% of the total weed dry matter), weed competition reduced growth, yield and harvest index of the maize crop (Hegewald, 1982). Grassy weeds including G. parviflora offered maximum competition to a maize crop reducing grain yield by 77.4%, followed by non-grassy weeds (44.2%) and sedges (38.4%) (Pandey et al., 2002).

Wheat crops with no weed control over the whole growing season gave significant lower yields than wheat crops with weed control from 13-55 days after sowing. G. parviflora populations ranged from 241-1907 plants/m². These weed populations reduced the number of ears of wheat/m², but showed little effect on grain weight or number (Thomas et al., 1978).

Competition for more than 30 days by G. parviflora, at a density of 241 plants/m² and Chenopodium album at 5 plants/m², significantly reduced wheat grain yields in a season where a frost occurred (Thomas and Schwerzel, 1979).

A 70% cover of G. parviflora and Amaranthus hybridus caused reduction in grain and shoot dry-matter yields of Eragrostis tef and wheat, which in turn promoted weed growth. The losses in E. tef could be partially recovered with nitrogen fertilizer (Pulschen, 1992).

In areas of Latin America (Mexico, Colombia), young stems and leaves are eaten raw or cooked. The same is reported in Tanzania, where G. parviflora exists in abundance and it is consumed as a leafy vegetable. Plants can be dried, ground into a powder, and then used as a flavouring in soups. In Africa, the species is also reported to be used as a medicine for treating nettle stings by rubbing it on the skin. In Uganda, it is reported as a traditional herbal drug used for treating bleeding (Damalas, 2008). 

Cultural Control

The critical period of competition between lettuce and G . parviflora, Bidens pilosa, Amaranthus hybridu s and Raphanus raphanistrum, depends on transplanting date. For maximum crop production the crop must be kept weed free only during the first week after transplanting. If the lettuce crop is transplanted later the weed-free period must be extended to 2 weeks ( García Blanco, 1983 ).

Increasing Stevia rebaudiana cv. BPP72 density, plus 0.08 mm black polyethylene sheeting as mulch, achieved significant reductions in G. parviflora populations ( Baltazar et al., 1980 ).

Solarization with 0.11 mm transparent polyethylene sheeting is effective in reducing viability of G. parviflora seed; microwaves have been found to affect G. parviflora seed viability ( Barker and Craker, 1991 ). It is possible that higher temperatures under the sheeting (41.5°C) as opposed to the control (35.2°C) caused lethal injuries in embryo tissues of the seed.

Mechanical control with pre-emergence harrowing achieves a degree of control in maize, peas, beans and other row crops.

Host-Plant Resistance

Among a total of seven varieties of Phaseolus vulgaris studied, differences were observed in ability to compete against G. parviflora ( Senesac et al., 1979 ).

Biological Control

Vulgamycin, produced by a new strain of Streptomyces sp., demonstrates some control of G. parviflora ( Babczinski et al., 1991 ). Although no pre-emergence applications of vulgamycin showed any control of G. parviflora, post-emergence applications resulted in good control of several weeds, including G. parviflora . Cotton, barley and maize showed 10% damage, whereas sunflowers demonstrated no tolerance to the compound.

Chemical Control

Chemical control may be achieved with isoproturon, bromoxynil, 2,4-D, MCPA and dicamba in winter and spring cereals.

Rimsulfuron, a sulfonylurea herbicide, gives adequate control of G. parviflora in maize and potatoes, as do atrazine and related herbicides in sorghum.

Urea derivatives are applied to cotton, celery, onion and carrot crops. Metribuzin has been successfully tested in tomatoes.

Imazaquin, fomesafen and acifluorfen are used for G. parviflora control in soybeans.

Desmetryn, nitrofen and chloroxuron are used for G. parviflora control in cabbage.

In flower crops (chrysanthemum, gerbera, gladiolus, freesia and carnation), oxadiazon and napropamide are used.

Chemical control of G. parviflora in orchards may be achieved with terbacil, simazine and diuron.

Prolonged herbicide application using metamitron resulted in development of resistance in G. parviflora ( Schmidt, 1986 ).