The history of J. vulgaris in New Zealand has murky beginnings. The earliest account of
“ragwort” in New Zealand was recorded by Fereday (1872) in his response to Bathgate (1870)
and observations therein regarding N. annulata. Fereday (living in Christchurch) commented
on Bathgate’s observations from Otago – in particular the abundance of N. annulata and its
association with “ragwort” (this presumably refers to J. vulgaris, as native Senecio species
and the naturalised S. vulgaris are often referred to as “groundsel” in the written accounts
from this era (e.g. Anonymous, 1889a; Anonymous, 1889b; Grapes, 1896)).
“The moth mentioned as having black wings with white spots, and the abdomen annulated with orange, is Leptosoma annulatum… It is very abundant here, and its larvae are common on a species of ragwort – I have never seen them on any other plant.”(Fereday, 1872, p.218)
However, Thompson (1922) is commonly considered the earliest reliable account of J.
vulgaris invasion. In this retrospective he mentions first noticing J. vulgaris around Dunedin in 1874. Not unimportantly (and admittedly with the benefit of hindsight), he also suggests
that proliferation of J. vulgaris was directly responsible for a dramatic increase in the
abundance of the endemic New Zealand magpie moth, N. annulata.
“Since the vast increase in this weed, there has been a correspondingly enormous increase in numbers of New Zealand magpie moth – Nyctemera annulata – the larvae of which feeds mainly on this plant. In summer the moths are frequently to be seen almost in clouds in the infested districts… and coincidentally with [J. vulgaris]’s spread has been an enormous increase of Nyctemera annulata.” (Thompson, 1922, p. 434)
From the early stages of the invasion, naturalists have provided some evidence that N.
annulata numbers were increasing. About a decade after Bathgate’s (1870) initial observation, Buller (1881) noted the how the moth seemed to be increasingly common:
“This familiar moth occurs plentifully during the summer months in all parts of the colony… it appears to increase and multiply every year, while many of the other common moths are becoming extinct.” (Buller, 1881, p. 238)
A couple years later Smith (1893) published his own observations about N. annulata in which he declared that the apparent rise in moth numbers was a direct result of the invasion of exotic plants – the earliest such account I was able to locate. He went further and also made the
claim that N. annulata larvae develop better on exotic Senecio species and that adult moths
reared on exotic hosts reproduce at a higher rate compared to those reared on native hosts.
“When, however, alien plants of the same order have supplanted the native species, it is interesting to note how several species of insects have adopted the ailen plants as food, and continue to multiply more than when their larvae fed on native plants... The case of N. annulata, a large diurnal moth , may be cited as one of perfect adaptation to new food, by which the species continues to increase annually.” (Smith, 1893, p. 220)
Early in the J. vulgaris invasion, N. annulata was so closely associated with this exotic plant
that many farmers initially suspected moth larvae as the cause of livestock mortality:
“I understand the danger lies in the grubs or worms which are to be found in every stalk of [J. vulgaris], presumably the young of the black moth so pleantiful whever the yellow weed is found.” (Maclean, 1894, p. 11)
“It was at one time thought that a disease which affects cattle grazing in districts overrun by Senecio jacobaea and its allies—the food plants of the larvae —had its origin in the animals inadvertently eating the larva along with its food plant. It is now known that the Senecio itself contains a poisonous substance.”(Philpott, 1907, p. 213)
Around the same time livestock deaths in New Zealand were on the rise from J. vulgaris
poisoning, Meyrick (1889) reported how common N. annulata was is in some areas. He also
gave some idea of the impact that larvae of the native moth could have on their host plants:
[regarding N. annulata:] “Common throughout the North and South Islands.... feeds on various species of Senecio, sometimes entirely stripping the plants. The imago has a curious habit of soaring in the early morning sunshine, soon after sunrise, in calm fine weather. I have seen them in numbers flying round the tops of trees at a height of over 100ft.” (Meyrick, 1889, p. 218)
Observations of high N. annulata population densities were affirmed by Hudson, who also
commented on the ubiquity of the species:
“It is extremely common, especially during the latter end of the summer, when specimens may often be seen flying in all directions.” (Hudson, 1898, p. 3)
A bit further north in Taranaki, local newspapers of the time provide clues that Taranaki was
in the early stages of invasion by J. vulgaris:
“Mr. G. Cliff brought to this office on Friday morning specimens of a weed found growing on his place near Upjohn’s Bridge. The plant was identified by Mr F.
P.Corkill as the ragwort, or golden cushag of the Isle of Man, where it grows freely and is not regarded as a pest. Some two years ago Mr. Corkill wrote to this paper warning settlers, however, against the plant, which he had seen on land near Inglewood. For although not objectionable in the Isle of Man, he foresaw that it would not be a desirable plant to get into this country.” (Anonymous, 1899, p. 2)
About a decade later, Taranaki was in the midst of a N. annulata population boom.
Eventually locals began arriving at the realization that the increase in J. vulgaris in the
landscape was the root cause of the increase in moth abundance:
“There has been a veritable plague of moths in Hawera during the past month… A well- known Ngaere settler is given as the Stratford Post’s authority for stating that a species of black moth specially chooses ragwort plants as a depository for its eggs, and that when the caterpillars emerge therefrom they practically destroy the plant of this noxious weed, upon which they feed. These caterpillars are further said to be unusually numerous in the district this year.” (Anonymous, 1910, p. 4)
Just a couple years earlier, the naturalist James Drummond (1908) wrote a column entitled
“Bird and Insect Notes” in the Wellington daily newspaper, Evening Post. In his column he
made reference to this precise phenomenon:
“The magpie moth, whose scientific name is Nyctemera annulata, is a pretty diurnal moth. One of its most interesting features is the fact that it is a striking illustration of the manner in which some introduced plants help to bring about an increase in the numbers of some species of native insects. The accidental introduction of the ragwort (Senecio jacobaea) brought loss to farmers in Southland, but it also provided an additional and natural food for the larvae of the magpie moth, which has increased prodigiously every year. In Southland, where the ragwort grows in large masses over areas of country, the caterpillars may be seen in their tens of thousands feeding on the luxuriant plants. As a traveller walks through the forests of yellow flowers, the moths rise around him in great numbers, and pass from flower to flower.”(Drummond, 1908, p. 13)
Around the same time, the prolific entomologist Thompson (1909) made his earliest written
acknowledgement that increasing N. annulata numbers seemed to be caused by J. vulgaris
invasion and spread:
“Of late years there has been a very remarkable increase in the abundance of these insects, especially in the southern end of this island, and this is apparently due to the great abundance of ragwort (Senecio jacobaea) which has become such a serious pest in the south. Armies of the caterpillar are sometimes met with during the month of November, followed later by myriads of moths.” (Thomson, 1909, p. 7)
Bathgate (1922) also indicates that there were unusually high densities of N. annulata in areas
of Southland where J. vulgaris was abundant.
“... if I may judge from what I saw in the neighbourhood of Lumsden a few years ago, where these moths were very abundant and the larvae were also numerous, feeding on the ragwort, which evidently afforded a suitable food supply and this resulted in these moths being present in far greater numbers than I had ever seen elsewhere.”(Bathgate, 1922, p. 275)
Thompson (1922) echoed this observation.
“The common magpie-moth (Nyctemera annulata) has certainly become extremely abundant wherever the introduced ragwort (Senecio jacobaea) has become a common pest… Wherever ragwort has spread and become an abundant weed the Nyctemera has also increased enormously, and may be seen rising in vast swarms from plants during the adult moth stage.” (Thompson, 1922, p. 512)
In his reminiscence on the previous 40 years, Thompson (1922) also included an account of a
how invasion of the Waikato area of the North Island by J. vulgaris (almost 40 years after he
watched it occur in Otago) coincided with a surge in N. annulata numbers:
“In June 1913 ragwort was reported as common on the Volcanic Plateau, an area of 5,000 acres lying to the south of Mangatautari and west of the Waikato River, and coincidently with its spread has been an enormous increase in Nyctemera annulata.” (Thompson, 1922, p. 434)
However, as Thompson (1922) noted some time later, the impact of magpie moth on J.
vulgaris was not significant enough to curb its spread and abundance:
“The larva is hairy and distasteful to birds, and there is apparently nothing to check its increase, but it is quite unable to cope with the vast increase of the weed.” (Thompson, 1922, p. 434)
As is the case with population explosions, the dramatic increase in weed and moth
populations was eventually followed by a similarly striking drop in these two species. Back in Otago – at about the same time Thompson (1922) recorded the weed and moth increase in
the Waikato – Philpot (1916) recorded what he saw as a distinct drop in N. annulata
abundance. He indicated that he believed the reason fewer moths were around was because of
decreased J. vulgaris biomass in the landscape as a result of regional control measures:
“[N. annulata] is now less common than formerly, owing to the destruction of much of the food-plant (Senecio jacobaea) of the larva.”(Philpot, 1916, p. 196)
A few years later in a reflective paper, Bathgate (1922) recalled witnessing the same thing in
the area of Dunedin, although he indicated that J. vulgaris was maintaining high population
densities in other areas of the South Island:
“The moth with black wings spotted with white (Nyctemera annulata) was much more commonly to be seen in the neighbourhood of Dunedin than is the case at the present day, which is probably due to the extermination by cultivation of the food plant of the larvae. While this may be the case in this locality, it is very different in the situation where the introduced weed ragwort (Senecio jacobaea) abounds…”(Bathgate, 1922, p. 275)