ASTERACEAE

The Asteraceae or daisy family comprises some 30,000+ species of herbs, shrubs, and trees of cosmopolitan distribution. It is probably the largest flowering plant family to date (along with Orchidaceae). Asteraceae plants bear a distinctive flower structure called the capitulum, which is a flower-head comprising of many small flowers (even the ‘petals’ are individual flowers). Many Asteraceae genera including Ageratum, Chromolaena, Gynura and Senecio contain toxins called pyrrolizidine alkaloids. The pyrrolizidine alkaloids are anti-herbivorous because they induce chronic liver damage and cancer. Grazing animals are most at risk of pyrrolizidine alkaloid toxicosis. Humans are poisoned when they consume pyrrolizidine alkaloid as contaminant of crops or medicinal herbs. The genus Ageratina is also especially poisonous.  The white snakeroot (Ageratina altissima) contains a metabolic poison called tremetol, which can be concentrated in cow’s milk. Ingestion of tremetol-contaminated milk is fatal to humans. A. adenophora and A. riparia contain toxic terpenes called ageraphorones, which can induce chronic lung disease and death to grazing animals. The Siberian cocklebur (Xanthium strumarium) and the Mediterranean distaff thistle (Atractylis spp.) are also deadly poisonous. This is due to carboxyatractyloside (diterpene), a metabolic poison. Carboxyatractyloside prevents cells from using energy. The victim suffocates and dies of  multi-organ failure. Siberian cocklebur is a valuable traditional Chinese medicine, but it must be used with caution. The most harmful Asteraceae plant is undoubtedly the Santa Maria feverfew (Parthenium hysterophorus). It is an invasive species that causes epidemics of skin allergy and asthma. It can even poison humans via the air!

Many South American Asteraceae are pioneer species which are quick to colonise disturbed habitats, particularly those created by humans (wasteland, roadside, agricultural fields etc). To ward off competitors, they secrete toxic chemicals from the root that can kill or inhibit the growth of surrounding plants. This phenomenon is called allelopathy and it is prevalent in Asteraceae. Thus, the following list contains some of the most destructive invasive plants known to humans.

The devil weed (Chromolaena odorata) is an invasive species that was introduced from America to tropical Asia. It contains pyrrolizidine alkaloids which causes epidemics of poisoning to livestock.

Chromolaena odorata, close up of composite flower-head.

The Siberian cocklebur (Xanthium strumarium) is a traditional Chinese medicine often used to treat ear diseases. It contains a highly toxic kaurene glycoside called carboxyatractyloside. Instances of poisoning and death due to consumption of the Siberian cocklebur is well-known.

Santa Maria feverfew (Parthenium hysterophorus) is one of the harmful poisonous plants known to humans. It causes severe allergic dermatitis on skin contact, and asthma if the dried hairs are inhaled. It is one of the most dangerous and destructive invasive weeds of the world. 

Parthenium hysterophorus, close up of flower.

Parthenium hysterophorus, leaves.

The mistflower or croftonweed (Ageratina riparia) is another highly toxic species with devastating invasive potential. As its name suggest, this species favours riparian habitat, often near water. A. riparia produces toxins called ageraphorons (sesquiterpene), which can cause lung lesion and multi-organ damage. It is implicated in poisoning of horses called 'Numinbah horse sickness'.

Absinthe wormwood (Artemisia absinthium) contains a convulsant monoterpene toxin called thujone. This species was widely but wrongly implicated in the once illegal and 'addictive' psychoactive drink absinthe. Absinthe does not contain thujone, hence non-addictive to humans.

The billygoat weed (Ageratum conyzoides) is introduced to nearly all tropical and subtropical regions of the world. A. conyzoides contains pyrrolizidine alkaloids (lycopsamine, echinatine and clivorine), which are suspected to cause thousands of cases of human liver disease in rural parts of Africa (Northern Ethiopia).

Ageratum conyzoides, close up of leaf and flower. All parts of the plant emit a fetid odour when crushed.

The bluemink (Ageratum houstonianum) is similarly toxic as A. conyzoides. However, this species is often planted as an ornament due to its showy blue flowers.

Ageratum houstonianum, close up of flower.