Ambrosia trifida
(giant ragweed)

A. trifida is an annual herb native to temperate North America which is now present in a number of countries in Europe and Asia. The primary means of spread of A. trifida occurs accidentally as a contaminant of seed or agricultural equipment. This species readily colonises disturbed areas and is often one of the first plants to emerge in early spring. As a result it has an initial competitive advantage and can therefore behave as a dominant species throughout the entire growing season. A. trifida is a particular problem for cultivated agricultural and horticultural crops where it can significantly decrease yields. Like many species of Ambrosia, A. trifida produces pollen which is allergenic and can induce allergic rhinitis, fever, or dermatitis. A. trifida is extremely competitive and can also decrease native biodiversity. A. trifida is a declared noxious weed in its native range in California, Delaware, Illinois and New Jersey, USA and is also a quarantine weed in Poland and Russia.

A. trifida is found primarily in temperate areas around the world and is native to eastern North America; it is found throughout much of the USA and southern Canada. In Canada, this weed ranges from Nova Scotia westward to the Northwest Territory. Within the USA, it is most common in the mid-Atlantic states and the Ohio and Mississippi River valleys.

A. trifida is not currently present in Africa (CJB, 2016), South America or Australasia (Council of Heads of Australasian Herbaria, 2016).

Further spread of A. trifida is highly likely, due to the risks of accidental movement seeds with agricultural equipment (locally) and as a contaminant of crop seeds. A. trifida is considered a noxious weed in many areas due to its intrusive nature and is a quarantine weed in Poland and Russia.

A. trifida is most prevelant in disturbed sites such as, damp soils, wastelands, uncultivated and cultivated lands (field crops), garden, ditches, roadsides, field margins, constructions sites, etc. (Uva et al., 1997). It may also be found colonising riverbanks, pastures and grasslands (Abul-Fatih and Bazzaz, 1979; Megyeri, 2011). A. trifida is common in flood plains and rare in low rainfall areas.

A. trifida is a weed of cultivated agronomic and horticultural crops such as species of Sorghum, Glycine max (soybean), Helianthus annuus (sunflower), Phaseolus spp (beans), Zea mays (maize), species of Triticum (wheat) and Gossypium hirsutum (cotton) (EPPO, 2014). It is extremely competitive and is especially difficult to control in many broadleaved crops. Experiments carried out by Baysinger and Sims (1991) in Missouri, USA, found that over two years dense populations of A. trifida reduced soybean seed yields by about 50%. A 55% reduction in corn yields in Michigan have also been recorded (Michigan State University, 2016).

Genetics

A. trifida is a taxon with 2n = 24, 48 (Strother, 2006). A hybrid was described with A. artemisiifolia (A. × helenae Rouleau) (Strother, 2006). Hybrids were found to have somatic chromosome numbers ranging from 2n = 27 to 2n = 33 (Bassett and Crompton, 1982).

Reproductive Biology

Each A. trifida plant can produce up to 10,300 seeds, while in soybeans and corn crops one plant can produce 1900-5500 seeds (Michigan State University, 2016). Pollination is mainly performed by wind, the pollen being small (20-30 µm), tricolporate, sphaerical, with 60-65 spines (Bassett and Crompton 1982). It has been estimated that one plant can produce 10 million pollen grains per day and one billion pollen grains during its lifetime (Johnson et al., 2007). The stigmas protrude and are receptive prior to the shedding of pollen in the male flowers (Bassett and Crompton, 1982).

Physiology and Phenology

In the native range in the USA, seeds generally begin to mature in mid-August (Uva et al., 1997). Most seeds fall near the parent plant, but some can disperse long distances with water, animals, and human activities. Most germination in the field occurs early to mid-spring and at soil depths to 16 cm, optimal 2 cm (Bassett and Crompton, 1982). Germination rarely occurs on the soil surface. Seedlings emerge in mid to late spring. Seedlings emerging from shallow depths are most likely to survive as they compete with other species for light. Mature seeds are usually dormant and require a cold, moist period to germinate. Seeds can germinate in a wide range of temperatures (8-41°C), but with an optimum range of 10-24°C (Abul-Fatih and Bazzaz, 1979).

Detailed study on seed germination of A. trifida by Abul-Fatih (1977) found that A. trifida usually produced seeds of various sizes. Large seed was able to germinate under relatively cold temperatures, a wide range of moisture regimes and soil depths. Small seed, which have smaller energy reserves, must germinate and establish rapidly during favourable periods if the seedling is to survive longer than other competing summer annuals.

A. trifida plants produce new stems and inflorescences when cut in late July and August.

Environmental Requirements

A. trifida occurs in a wide range of rainfall zones, but generally prefers moister soil with a summer rainfall regime. Mean monthly temperatures can range from 5°C to 30°C and it is fairly indifferent to soil types.

Leaf-spot diseases (Cercospora spp.) are common in the field in the USA, but rarely have significant impact on the competitiveness of A. trifida plants. In Louisiana, A. trifida is noted as a host of Papaipema nebris (Alvarado et al., 1989) and stem galls are caused by Protomyces gravidus (Holcomb, 1995).

A number of other natural enemies have been identified as A. trifida is the subject of a biological control programme. Potential agents include Euaresta bella, E. festiva (Batra, 1979), Zygogramma suturalis (Xie et al., 2000) and the pathogens Puccinia xanthii f.sp. ambrosid-trifidae and Pseudomonas syringae pv. tagetis (Batra, 1981).

Materials

• Essential oils

Medicinal, pharmaceutical

• Source of medicine/pharmaceutical

A. trifida can be easily distinguished from the other annual species belonging to the genus Ambrosia by the leaves which are palmate, while in the other related taxa (A. bidentata, A. acanthicarpa, and A. artemisiifolia) the blades are pinnate. The flowering heads and fruit of A. artemisiifolia are similar but smaller than those of A. trifida. The leaves of A. artemisiifolia are usually twice divided and not 3-5 lobed, and the overall plant size is generally much smaller.

When a large number of the leaves of A. trifida are unlobed, it can appear similar to Xanthium strumarium or Helianthus annuus; however, the leaves of these species are mostly alternate, whereas those of A. trifida are opposite.

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.

Control

Mechanical Control

Tillage is effective for control of seedlings of A. trifida because of their early emergence in relation to many other summer annual weeds (tillage is less effective in populations where plants are larger). Under moist soil conditions, plants may be 'transplanted' and begin growing in another area. Hand-pulling can be effective for seedlings, especially in gardens and other small plots. Repeated mowing will effectively reduce seed production but will not eliminate A. trifida. If allowed to approach maturity it pays to hand-pull the weed, because the stout, woody stalks dull and break the blades of mowers and harvesters, and clog threshing machines. Control of A. trifida using electrical discharges proved to be good to excellent (Rasmusson et al., 1980).

Biological Control

Surveys of potential biocontrol agents of A. trifida began in California in 1967 (Goeden, 1978). Euaresta bella and E. festiva have been released in Russia from 1976 (Batra, 1979) but are not now known to have established (Julien and Griffiths, 1998). Puccinia xanthii forma specialis ambrosid-trifidae was found to be species-specific and a potential biocontrol agent (Batra, 1981). Limited research has been made on biological control with Zygogramma suturalis (Coleoptera: Chrysomelidae); introduced into China from Canada and the former USSR (Xie et al., 2000). The bacterial pathogen Pseudomonas syringae pv. tagetis is the causal agent of a disease characterized by apical chlorosis on several members of Asteraceae, and this potential biological control continues to be researched.

Biocontrol programmes are in place for a closely related Ambrosia species. Introductions of biological control agents have been made against A. artemisiifolia in Australia, China, Georgia, Russia and Yugoslavia and against A. pilyostachya in Kazakhstan and Russia (Julien and Griffiths, 1998). No doubt some of the successfully introduced agents also attacked A. trifida if it was present, as most the majority are specific only to the genus.

Chemical Control

Application of atrazine, chlorimuron, and imazaquin are effective pre-emergence herbicides for control of A. trifida. The post-emergence herbicides acifluorfen, bentazone, dicamba, glyphosate, imazethapyr and 2,4-D have been recommended (Weed Science Society of America, 2003).

Glyphosate will control plants up to six inches in height. Tank mixing 2,4-D with glyphosate is also useful in controling A. trifida. However, biotypes of A. trifida have been found to be resistant to ALS (acetolactate synthase)-inhibitor herbicides in the USA since 1998 (Weed Science Society of America, 2003).

Further research is required to investigate the molecular differences between native vs. alien populations of A. trifida. In addition to this detailed phytosociological studies on non-native areas in which A. trifida is naturalized/invasive could be conducted.

22/11/2016 Updated by:

Duilio Iamonico, University of Rome Sapienza, Rome, Italy