pdf, 57.71Mb - Entomological Society of Canada

Pest Status
Biological Control Studies
Discussion
Chapter 44
Verbascum thapsus L., Common Mullein
(Scrophulariaceae)
M.O.MAW
Verbascum thapsus L. is a biennial or, rarely, an annual of Eurasian origin. It was
probably introduced into North America several times as a medicinal herb (Gross &
Werner 1978). It is found over most of the United States and southern Canada, but is rare
on the Canadian prairies, North Dakota, and Montana (Gross & Werner 1978. Maw
1980). It is most vigorous on well drained gravelly or stony soils in rough pastures,
fence rows, roadsides, railway yards, and waste places. It is not a major weed, not
allergenic, not poisonous to humans, and not a problem on cultivated land. It does,
however, harbor insects that may be vectors of diseases of economic plants. For example,
the mullein leaf bug, Campy/omma verbasci Meyer, whose normal plant food is mullein,
may cause damage to apple or pear fruit and potato leaves (Pickett 1939) and may also be
a vector of fire blight, Bacillus amy/ovorus (Burr.) Trev. (Leonard 1965).
The most effective control agents of mullein in Europe appear to be a weevil, Gymnaetron
letrum F .• and the mullein shark moth, CucuJ/ia verbasci L. (Noctuidae) (Miotk 1973).
The weevil is already well established in Ontario, British Columbia. and the United
States from New England to Arkansas and the Pacific states (Blatchley & Leng 1916,
Hatch 1971). The adults eat mullein leaves, and the larvae and adults destroy up to 50%
of the seeds (Gross & Werner 1978).
C. verbasci was screened (Maw 1980) and although there was some feeding on all
Scrophulariaceae tested, and there was some nibbling on tomato, Nicoliana sp. and
Brass;ca napus, sustained feeding and development occurred only on Verbascum spp.
V. thapsus is an early colonizer on abandoned agricultural fields with the tall flowering
stocks usually appearing the second year after abandonment. Populations are usually
scattered but locally abundant, and the weed may cover entire fields where the ground
has been left undisturbed. This situation is, however, relatively short-lived. In a study
by Gross & Werner (1978) it was found that in a barley field that had been abandoned for
three years, the density of V. thapsus was 1.0 plants per mI. In an adjacent field that had
been abandoned for 12 years, the density of V. thapsus had declined to O.17/m l • Since
seeds can be dormant but viable for 35 years, the typical pattern seems to be one of
ephemeral adult populations and long-lived seed pools (Gross & Werner 1978).
Although the mullein leaf bug. Campy/omma verbasci, may damage apple and pear
fruit, the bug is only partially phytophagous. It cannot complete its life cycle without
insect prey and feeds on several species of mites and insects that attack the fruit crop
(Gross & Werner 1978).
211