# Conium maculatum

**Poison Hemlock**

**Apiaceae**

Poison hemlock, *Conium maculatum*, is an herbaceous weed of European
origin found throughout many parts of the world. In southern California,
poison hemlock reproduces only from seed, both as a biennial and winter
annual, and occasionally as a short-lived perennial. It has smooth,
purple-spotted, hollow stems, 0.5 to 3 m tall, which bear large
pinnately decompound leaves, which are largely petioled (though sessile
above). The flowers are small, white, and born in large, open, compound
umbels. Achenes are oval, 2 to 2.5 mm long, and prominently ribbed
(Robbins et al. 1970, Munz 1974).

Poison hemlock is native to Europe, but is extensively naturalized in
temperate North America, as well as other parts of the world including
Australia, New Zealand, and South America (Parsons 1976, Holm et. al
1979). This species is commonly found in waste places and along roads,
ditches, and cultivated fields, and it occasionally invades riparian
forests and flood plains (Goeden and Ricker 1928). It is currently
distributed throughout most of the continental United States (Reed 1970)
and southern Canada (Montgomery 1964). In California, *C. maculatum* is
listed by the California Exotic Pest Council as a \"List 2\" species: a
wildland weed of secondary importance characterized by a localized
community (WWW 1995). This weed is generally considered to be noxious,
and all aerial parts are poisonous to both humans and livestock (Sperry
et al. 1964, Painter and Keeler 1989). The toxicity of *C. maculatum* is
due to the production of relatively high levels of conin and piperidine
alkaloids, including methylconiine, coniceine, and conhydrine. In
addition to the alkaloids, the foliage and flowering parts of this
species contain small quantities of furanocoumarins, phototoxic
compounds that are toxic or deterrent to both vertebrate and
invertebrate herbivores (Berenbaum 1981, Murray et al. 1982). These
plants also produce polyacetylenes (Bohlmann 1971), which have been
implicated in livestock poisoning by other apiaceous plants (Panter et
al. 1988).

Relative to other introduced weed species, poison hemlock is attacked by
few insect herbivores (Berenbaum 1982, Goeden and Rickers 1982). In an
extensive survey of poison hemlock in southern California. few insect
species or individuals were found on this species. Sixteen of 20 (70%)
of the phytophagous insects found on poison hemlock in this survey were
rare and were only encountered as a few individuals at one or two sites
(Goeden and Ricker 1982). Of the relatively few native insect species
that have colonized the species throughout its range, most are species
that feed generally on native and introduced plants in the Apiaciae,
including *Papilio zelecaon* Lucas (Goeden and Ricker 1982). *Papilio
polyxenes asterius* Stoll (Feeney et al 1985)(Lepidoptera:Papilionidae),
and *Euleia frateria* Loes (Diptera: Tephritidae)(Berenbaum 1981). In
California, the most abundant insect associate with poison hemlock is
the aphid, *Hyadaphis foeniculi* (Passerini), which was introduced from
Europe, where it also feeds on *C. maculatum*. In recent years, another
European specialist on poison hemlock, a leaf-rolling oecophorid
caterpillar (*Agonopterix alstroemeriana*) has been reported feeding on
flowers and foliage of poison hemlock in New York (Berenbaum and Parsons
1983), as well as in Oregon, northern California, Utah, and mesic areas
of Washington, Idaho, and Colorado (Powell and Passoa 1991). This
species was also recently detected in association with *C. maculatum* in
Illinois (Berenbaum and Harrison 1994), where it seems to be undergoing
very rapid habitat expansion. These authors further report the use of
*C. maculatum* as a host species for several other species of
caterpillars, including *Eupithecia miserulata* Grote (Geometridae),
*Trichoplusia ni* Hubner (Noctuidae), and *Spilosoma virginica* F.
(Arctiidae). Of these species, *E. miserulata* and *T. ni* have been
previously reported on poison hemlock in California (Goeden and Rickers
1982). Poison hemlock seems to be a newly recognized host for *Spilosoma
virginica*, but this species is broadly polyphagous and is associated
with a variety of other toxic plants as well (Tietz 1972).

**References**

Berenbaum, M. 1981. Furanocoumarin distribution and insect herbivory in
the Umbelliferae: plant chemistry and community structure, Ecology 62:
1254-1266.

Berenbaum, M, and S. Passoa. 1983. Notes on the biology of *Agonopterix
alstroemeriana* (Clerck), with descriptions of the immature stages
(Oecophoridae). J. Lepid. Soc. 37: 38-45.

Berenbaum, M, and T.L. Harrison. 1994. *Agonopterix alstroereriana*
(Oecophoridae) and other lepidopteran associates of poison hemlock
(*Conium maculatum*) in East Central Illinois. Great Lakes Entom. 27:
1-27.

Bohlmann, F. 1971. Acetylenic compounds in the Umbelliferae. pp.
279-291. In Heywood, V.H., ed. The Biology and Chemistry of the
Umbelliferae. Academic Press, New York.

California Exotic Pest Council. 1995. Draft list of California exotoc
pest plants of greates concern. The Nature Conservancy: Conservation
Science. http://www.consci.tnc.org/src/weeds/worst.htm

Goeden, R.D., and D.W. Ricker. 1982. Poison hemlock, *Conium maculatum*,
in southern California - an alien weed attacked by few insects. Ann.
Entomol. Soc. Amer. 75: 173-176.

Holm, L., J.V. Pancho, J.P. Herberger, and D.L. Plucknett. 1979. A
geographic atlas of world weeds. John Wiley and Sons, New York.

Montgomery, F.H. 1964. Weeds of the northern United States and Canada.
Frederick Warner and Co. Inc. New York.

Panter, K.E., and R.F. Keeler. 1989. Piperdine alkaloids of poison
hemlock (*Conium maculatum*). pp 109-132. In Cheeke, P.R., ed. Toxicants
of plant oprigin. Vol.1. CRC Press, Baca Raton.

Parsons, W.T. Noxious weeds of Victoria. Inkata, Melbourne.

Powell, J.A., and S. Passoa, 1991.Rapid colonization of the western
United States by the palearctic moth, *Agonopterix alstroemeriana*
(Oecophoridae). J. Lepid. Soc. 45: 234-246.

Reed, C.F. 1970. Selected weeds of the United States. U.S. Dept. Agric.
Res. Serv. Agric. Handbk. No. 366: 1-463.

Sperry, O.E., J.W. Dollahite, G.O. Hoffman, and B.J. Camp. 1064. Texas
plants poisonous to livestock. Texas A & M Agric. Expt. Station Publ.
10M-12-64. p. 19.

Tietz, H.M. 1972. An index to the described life histories, early
stages, and hosts of the macrolepidoptera af the continental United
States and Canada. A.C. Allyn, Sarasota.