CONTROL OF RAGWORT THROUGH INSECTS.

New Zealand Journal of Agriculture, Volume XXXIX, Issue 1, 20 July 1929, Page 9

CONTROL OF RAGWORT THROUGH INSECTS.

EXPERIMENTAL WORK WITH CINNABAR MOTH

DAVID MILLER,

Ph.D., M.Sc., F.E.S., Chief Entomologist and Director of Weeds

Control Research, Cawthron Institute, Nelson

The object of this article is to place before those interested a general account of the present position of the researches into the control of ragwort through the agency of insects.

In 1927 definite investigations into the possibility of controlling noxious weeds in New Zealand by means of insects were commenced at the Cawthron Institute.* At the outset it was decided to concentrate upon the four weeds of major —blackberry, ragwort, gorse, and piripiri. . In the case of blackberry, gorse, and piripiri the investigations are still in the preliminary stages, but the work on ragwort has advanced to such an extent that field liberations of the cinnabar moth (Tyria jacobaeae), imported for the control of the weed, were made last autumn.

Although good results have been secured in other countries e.g., in the control of prickly pear in Australiathe utilization of plantfeeding insects in the control of weeds, especially of weeds closely related to economic plants, is fraught with many dangers, in that there is always a possibility of the insects in question extending their food range beyond weeds to include agricultural crops. Under the noxious-weeds control scheme, and in order to guard as . far as possible against this possibility, only insects that are known to be restricted to the weeds in question, or to plants of no great economic value in their natural haunts, are selected for importation to New Zealand. The insects are then kept under strict quarantine in special insectaries until the possibility of economic plants being attacked by them under New Zealand conditions has been thoroughly tested. If this proves negative the insects may be liberated in the field, as has been done in the case of the cinnabar moth.

It must be fully realized by the farming community that the liberations of the cinnabar moth already made were merely preliminary, and only a phase of an experiment to determine the behaviour of the insect in the field under its new environment. Farmers at the present stage must not be too optimistic concerning the control of ragwort by this insect, and must still actively continue to check the weed by cutting or by other means at their disposal, until it can be definitely shown, that the insect will act as an efficient controlling factor. Unfortunately, too much publicity has been given to the possibilities of weed-control by means of insects ; and any one believeirig that ragwort, or any other weed, will suddenly disappear as soon as .an insect is liberated will stand

* During the first year (1927) the noxious-weeds control research was under the direction of Dr. R. J. Tillyard who initiated the scheme, and he was followed by the present writer in 1928. Mr. A. L. Tonnoir, Field Entomologist, has been responsible for carrying on the researches since the inception of the scheme. Our thanks are due to Mr. A. H. Cockayne, Director of the Fields Division of the Department of Agriculture, for placing his field staff at our disposal in the distribution of .the cinnabar moth'. '

to be deeply disappointed. The most that can be expected of any practicable means of control is to sufficiently check, but not exterminate.

HISTORICAL

Of the several insects found attacking ragwort in Europe and Great Britain, the caterpillar of the cinnabar moth was selected as giving most promise as a means, of controlling the weed in New Zealand. Its normal food plants are groundsel, and ragwort, and in its natural haunts it sometimes occurs in epidemic form, when it clears large areas of the host plants. Furthermore, the insect is normally attacked by some nine or ten parasites, which tend to keep it from developing abnormally ; but before it is liberated in this country these parasites are eliminated, so that the insect should have every opportunity of increasing to a far greater extent in its new environment than it would in its country of origin.

As the caterpillars of moths pass through a dormant chrysalis or pupal stage before developing to the perfect insect, advantage of this was taken in transporting, the cinnabar moth, the pupal period of which is a lengthy one. During the years 1927 and 1928 several consignments of the pupae were secured by Dr. A. D. Imms, of the Rothamsted Experiment Station, and sent to New Zealand in cool store, and this method of transport has proved a success. An unsuccessful attempt was also made to send the eggs of the insect in cool store. From the pupae, moths later developed in the quarantine insectaries at Nelson, ’and in due course a supply of caterpillars, used for carrying on the research, was secured from the eggs laid by these moths.

Although the cinnabar moth caterpillars are not known to attack plants of economic value, in Europe or Britain, it was nevertheless necessary to submit the insect to a series of starvation tests in New Zealand before considering the question of liberation. Except in the case of ragwort, groundsel, and cineraria, these tests proved negative, and an official permit from the Government was given authorizing liberation. However, owing to later reports received from England, some doubt existed as to whether the larvae would attack potatoes,

although the tests already made in Nelson had proved negative. In view of this, liberation in New Zealand was postponed until further and more detailed tests with potatoes had been carried out both at Nelson and at Rothamsted in England. These tests again showed that the insect could not survive on potatoes, and liberation was proceeded with.

DESCRIPTION OF THE CINNABAR MOTH

The moth (Fig. la), which measures about fin. long when the wings are closed, is black in colour with crimson wing-markings ; the forewings are black with a crimson stripe along the full length near the fore margin, and two crimson spots close to the outer margin, while the. hindwings are completely crimson with a narrow black margin.

The eggs are globular, and laid in clusters on the underside of the leaves of the food plant (Fig. 3) ; they are yellowish in colour at first, but turn to brownish^in a few days. . The caterpillar (Figs. 4 and 5 a) is distinctly marked with alternating broad blackish and narrow orange bands, and is sparsely clothed with long pale hairs ; when fully fed it measures about 1 in. long. The chrysalis or pupa (Fig. se) is rather plump and about -J- in. long; it is more or less blunt at both ends, reddish to blackish-brown in colour, and is sometimes enclosed in a loosely spun silken cocoon. LIFE-HISTORY. Under conditions the eggs (each female moth laying about 150) hatch in about eight days, just prior to which they change in colour from yellowish to brownish. The caterpillars grow . rapidly as they

devour the plants ; during growth they pass through five stages, each stage being separated by a short quiescent period during which the protecting “ skin ” is moulted and replaced by a more roomy one to accommodate the growing insect. These moultings are accompanied by certain changes in colour : the minute freshly emerged caterpillar is yellowish except for the black head ; in the second stage of growth (i.e., between

the "first and second moults) the colour is almost black, later turning to orange with black dots over the body ; in the third stage (i.e., after the second moult) the characteristic alternating orange and black bands make their appearance and persist through the fourth and fifth stages. The caterpillar takes about four weeks to complete its growth under summer conditions. At first the young caterpillars are gregarious, feeding in colonies ; but as growth proceeds they commence to wander and spread in all directions over the area covered by the food plants. Both leaves and flowers are devoured, the insect showing a preference for the latter When fully fed the caterpillars do not burrow into: the ground, but wander to some sheltered spot among the debris < about the base- of the

plants, or into crevices, or under'lumps of earth or stones, where each may spin a very imperfect cocoon of silk, and there, passing through a final moult, transform to the pupa. The pupal stage is a prolonged one, and the insect passes the winter as such, the moths not emerging from the pupae until the following spring.

In Europe and Britain the cinnabar moth develops through only one generation each year, the moths appearing in the spring and during summer, when the caterpillars become abundant ; in the autumn and winter pupation is at its height. Under New Zealand conditions it is hoped that two generations may develop, when the insect would certainly be a much greater factor in the control of ragwort.

LIBERATION OF THE INSECT IN NEW ZEALAND.

On nth February, 1929, field liberations of the cinnabar moth in New Zealand were commenced, it being found most convenient to distribute the insect in the egg stage. In order to secure the supplies of eggs, fertilized moths were placed in muslin cages enclosing ragwort growing' in the Cawthron Institute• grounds (Fig. 6), . and later the leaves upon which the eggs were deposited (Fig. 3) were removed and distributed in tin boxes. Altogether some 500,000 eggs have been

distributed at the following points : Cawthron Institute grounds, and in the Te Puke, Hamilton, Stratford, and Invercargill districts.

It was not intended that these liberations should be made to all ragwort districts of the Dominion, but only to selected areas, in order to ascertain the effect, of the insect upon ragwort, and its manner of

behaviour under field conditions where it would be exposed to the influence of its new environment. However, it is intended to make more widespread distribution of the insect next spring,* and with this in view a supply of several thousand cinnabar moth pupae is expected to reach the Dominion in time for the liberations to be made. This supply is being secured by Dr. W. R. Thompson, of the Imperial Bureau of Entomology, London, who has now taken charge of the European end of the noxious-weed control work.

OTHER INSECTS ATTACKING RAGWORT.

In New Zealand there are at least five species of insects that, as larvae, normally attack ragwort, but none of which actually serves as a sufficiently permanent means of control, though at times the weed may be temporarily checked by one or more of them. ■ These insects

are the magpie moth. (Nyclemeva annulata'), two species of moth stemborers (Homoeosoma faranaria and H. vagelld), a fly pith-borer (Agromyza sp.), and a leaf-miner (Agromyza affinis).

The magpie moth and its black hairy caterpillars are common features of the countryside. As this insect has already been thought by many farmers to be the cinnabar moth, a description is given herewith. The moth (Fig. ib) is somewhat larger than the cinnabar moth, and is black except for a pair of white or cream-coloured spots on the forewings, a single one on each of the hindwings, and orangeyellow bands on the abdomen. The globular eggs are placed on the underside of leaves, but, unlike those of the cinnabar moth, are palegreen at first, turning to yellow and finally to a leaden colour. The full-grown caterpillar (Fig. sb, c) measures about in, long; it is black with narrow brick-red lines - along the body which is clothed with tufts of long blue-black hair, each tuft arising from a blue-black tubercle. When freshly emerged from the eggs the caterpillars are pale yellow in colour with a black head, and the body clothed with black hairs, while the tubercles supporting the hairs turn black after a few hours, giving a spotted appearance to the body. A few days later the yellow markings appear, and the characteristic features of the fully grown caterpillars become apparent. Winter is passed in the caterpillar stage. Before pupation a conspicuous cocoon of silk is spun, the caterpillars sheltering for this purpose in locations similar to those taken up by the cinnabar moth. The pupa (Fig. sd) is about -4- in. long, and more pointed at the posterior end than that of the cinnabar moth it is yellowish at first, but later becomes blackish and mottled, the abdomen being conspicuously ringed with orangeyellow, though in older pupae these markings disappear. The magpie moth may undergo three or four generations during the year.

The magpie moth is common every year, and periodically becomes epidemic, when it acts as a severe check to ragwort. However, these epidemics are not of sufficiently frequent occurrence to be of permanent economic value. One of the factors controlling this insect is that of parasites, there being two common species, Ecthromorpha intricatoria and Phorocera casta.

The two species of moths breeding in the stems of ragwort are well established throughout New Zealand they occur also in Australia, and .possibly in the Pacific islands. Some seasons they occur in localized epidemic form, when they act as temporary check to their host plant. lake the magpie moth, they are controlled by parasites. The damage they do to ragwort is shown in Fig. 5/).

The larvae of the fly -borer and of the leaf-miner are usual associates of ragwort, but do but little appreciable damage, if any, to the plant.

PROSPECTS OF RAGWORT-CONTROL BY THE CINNABAR MOTH

In the insectaries the devastating effect of the cinnabar moth as extremely apparent, and if this condition could be maintained each year in the field one could safely say that ragwort would no longer rank as a major weed. But under field conditions the insect enters a complex of factors any one or all of which might be so detrimentally

influential as to prevent it from maintaining a standard sufficiently epidemic for effective control. Among such possible- detrimental factors are climate, natural enemies, and disease. Regarding climate, one should expect that the meteorological conditions of New Zealand would be rather favourable than otherwise, especially in the North Island ; in Southland, on the other hand, the greater severity of the climate will probably have a staying influence upon the insect.

Of possible natural enemies predaceous and parasitic insects will certainly influence the problem. Even at this early stage of establishing the insect it has been found that the two parasites attacking the early stages of the magpie moth are both parasitic upon the cinnabar moth. To what extent these parasites will affect the general issue is not yet known.

The disease factor is not to be overlooked, since in the insectaries a polyhedral disease destroyed a very great proportion of the caterpillars. In the insectary, however, conditions were favourable to the development of this disease, owing to overcrowding and high temperature and humidity. It is hoped that similar epidemics will not destroy the cinnabar-moth caterpillars under open field conditions, although the writer has noted a somewhat similar disease destroying large numbers of the army-worm caterpillar (Cirphis unipunctata) during an outbreak of the latter.

Note. —The foregoing article will also appear in the New Zealand Journal of Science and Technology.

* Distributions will be made through Fields Division officers of the Department of Agriculture located in the different ragwort areas, and not direct from the Cawthron Institute to farmers.