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196 P. Harris. A. T. S. Wilkinson and J. H. Myers
of some maritime stands of the weed following cinnabar defoliation to the frost sensitivity
of the regenerating plants. Several studies (Green 1974, Campbell 1975, Myers 1976,
Myers & Campbell 1976a, 1976b) have investigated the importance of ragwort spacing
on cinnabar egg distribution and the success of larval transfer between plants. They
showed that one requirement for explosive increases of the moth was dense stands of the
weed. Dempster (1971) reported a higher mortality of cinnabar eggs and larvae on
ragwort rosettes than on flowering stems and Myers (1980) found that cinnabar populations
in areas with a high density of closely spaced rosettes tended to be less stable than
on those with low rosette density. A high nitrogen content in the plant increased both
moth fecundity and egg mass size in the following generation. Myers & Post (1981)
argued that this would tend to destabilize the cinnabar population by over-exploitation
of the food resource. In a study by Lakhani & Dempster (1981), the number of eggs had
a rather small effect on the subsequent moth numbers because high egg density was
often associated with larval starvation. In a dune habitat, Meijden (1979) described the
cinnabar population as "walking on ice floes" because small stands of ragwort became
extinct one or two years after attack by the moth and then reappeared after the moth
emigrated to other stands. Predators were important in some instances (Wilkinson 1965,
Myers & Campbell 1976c, Meijden 1979). In 1980 at Nanaimo, British Columbia, most of
the pupae were destroyed resulting in a low population in 1981 in which only 3% of the
plants were attacked.
Philogene (1975) found that the day length and temperature during larval development
did not affect the obligatory pupal diapause. However, the moth has adapted its temperature
threshold for emergence in the spring so that in the various regions of North
America larval feeding remains synchronized with ragwort flowering (Myers 1979).
Richards & Myers (1980) found that maternal moth size and temperature requirements
for moth emergence were heritable and Myers (1978) found that the present populations
of the moth have regional differences in their enzyme systems that are irrespective of
their origin. Thus in a few years since release, the moth has adapted to its new habitat
with the result that its behaviour now is not necessarily the same as that of the stock
released.
A model by Lakhani & Dempster (1981) showed that for both Nanaimo, British
Columbia, and Weeting Heath in Britain, the changes in the density of the weed closely
conformed to the amount of spring and early summer rainfall. The model showed that at
these sites the cinnabar population merely tracked the changes in the density of the
weed. Myers (1980) also concluded that population fluctuations after an initial reduction
of plant size by introduced cinnabar moths have a large environmental component. One
indication that different factors are important in different sites is that the model did not
fit the results from a more moist site in Oregon. In a more generalized model Roff &
Myers (unpublished) found that depending on the degree of larval dispersal, the cinnabar
population could be cyclic, stable, or chaotic. The gamut of these conditions
occurs in nature. There are several ragwort habitats that are not utilized by the moth: the
weed tends to be avoided when growing in partial shade; the cinnabar pupae are not able
to overwinter in wet sites (Dempster 1971) so the moth tends to be absent in Europe from
pastures on river flood plains, which often have dense ragwort stands; the moth is rare in
the Swiss Jura on the widely separated plants or small clumps of plants. It is a weak flier
and has dispersed less rapidly in California than the beetle Longitarsus jacobaeae
(Waterhouse).
In many habitats the cinnabar moth has not reduced the density of tansy ragwort, but
plants defoliated annually tend to be smaller so there has been some reduction in ragwort
biomass. Also for about two months in the summer there is little ragwort foliage in the
pastures or hay fields so that availability of the toxic foliage to cattle is reduced. The
level of control is not satisfactory as a high density of the weed remains on many sites for
much of the year, so the cinnabar moth needs to be supplemented by additional agents.