The White Butterfly and the Diamond-backed Moth

New Zealand Journal of Agriculture, Volume 63, Issue 5, 15 November 1941, Page 371

The White Butterfly and the Diamond-backed Moth

By

J. MUGGERIDGE and B. B. GIVEN,

Entomology Division, Department of Scientific and Industrial Research, Cawthron Institute, Nelson. t mmiiimiimmmmimmimmmmmmimmmmmmmmmmmmmimmmmmimmim

T'WO of the most common pests of cabbages, rape, turnips, chou moellier, etc., are the white butterfly and the diamond-backed moth. No doubt a good deal of confusion exists among the farming community in regard to the damage caused by the white butterfly and the diamond-backed moth, and injury caused by the latter insect is all too commonly blamed on the white butterfly. The fact that butterflies are seen over a crop is not sufficient evidence for attributing any injury to their presence, and the only way to determine the cause of the injury is to examine the crop. In the following account differences between the two insects and the type of injury they cause are described.

THE white butterfly, which has been in this country for about eleven years, is probably far better known than the moth, which has been here for more than sixty years. The white butterfly is a conspicuous object, and its size and creamy whitish colour . make it readily seen from a distance. It .may be seen flying by the roadside” over grass paddocks, over some crop in the field, or visiting flowers of different kinds for the nectar they contain.

The moth, on the other hand, is inconspicuous. It is a small, greyish, night-flying moth which rests during the daytime amid the crop foliage, and unless searched for is hardly likely to be seen. Its presence may be readily detected, k. however, if by strolling

through a crop, the foliage is sufficiently disturbed, when numbers of the moths will be seen to rise, fly ,a short distance, and then settle again. In Fig. 1 the two insects are compared in regard to size and form. Both the butterfly and the moth lay their eggs on the crop foliage. The butterfly eggs (Fig. 2) are small, sculptured, bullet-shaped objects. They are deposited singly, in an upright position, generally (but not necessarily) on the underside of the leaf near its edge, whereas those of the moth are very much smaller, oval in outline, and are usually found on the undersurface of the leaves, where they are deposited either singly or in batches beside the ribs or veins. Either insect may

avpmff o ° r e ggS ’ slthough the average number laid would be considS and the moth hatch in about . five days, although the time taken may be longer or shorter, depending on temperature conditions. Under cold conditions, for example, the eggs may take up to forty days to hatch, The tiny butterfly and moth grubs both look the same to: the untrained eye, and shortly after hatching they . begin to feed on the plant foliage.

How the Damage Is Caused

The butterfly larva begins feeding on the surface tissue, but as it grows older and stronger it eats right through the leaf, continuing to feed where the hole is made. By eating around the edges the larva considerably enlarges the original hole, and if there are sufficient caterpillars to a leaf, in time the whole of the tissue between the main ribs will be eaten out, and the

plant will appear as shown in Fig. 3a. In . Fig 4 the results of the butterfly °” & P ' The young moth larva, on the other hand, first burrows into the leaf, and once inside, begins to eat out all the soft green matter between the upper and lower surface of the leaf, with the result that a whitish irregular patch appears. As the larva grows it emerges from its burrow and begins to feed on

the lower leaf surface, in time eating out all . the green matter and leaving only the upper leaf surface and the veins. When eaten in this fashion, the leaves have a white, tissue-paper-like appearance (Fig. 3c) and, seen from a distance, an entire crop affected in this manner has a white, blasted appearance. Quite frequently, when the foliage is very tender, the larvae will eat right through the leaf, leaving holes similar to those caused by the butterfly larvae. In this case it is almost impossible to tell which of the two insects was responsible for the damage, although usually the holes made by the moth are relatively small.

Which Is Which?

As has already been pointed out, it is difficult to distinguish between the larvae of the butterfly and the moth in their younger. stages. The writers have found that the simplest method to determine which is which is to touch the grubs at the posterior extremity. The result is that the butterfly larva will shift but little, whereas the moth larva will begin to wriggle, and if the leaf is held at a suitable angle the larva will drop from it towards the ground on a silken thread attached to the leaf surface (Fig. 6).

As the larvae grow older the differences between them become quite marked. That of the butterfly, at first light green in colour, becomes intense leaf-green as it grows to full size. A thin orange stripe appears down its

back, and the whole body is covered with a short pile of hairs, which give it a soft velvety appearance. It measures approximately one inch when full grown, and varies but little in width from head to tail. The moth larvae

vary a good deal in colour, probably the largest proportion of them being light green. When full grown the larva measures 3-Bths of an inch in length, and the body is widest at its

middle, tapering towards the head and tail ends. In Fig. 5 the two larvae are compared in regard .to size and shape.

Danger in the Larval Stage

. It is in the larval stage only that the two insects are destructive to crops, and it is at the conclusion of this stage that both, well fed at the farmers’ expense, repair to a suitable spot to transform to the passive pupa or chrysalid. The butterfly larvae usually leave the plant on which they have been feeding, and seek some surrounding object on which to pupate. They will travel quite a distance to find a suitable place, although what the exact requirements are it is difficult to say, as they may be found on dried leaves near the host plant, on Scotch thistles, on the foliage of grass or other vegetation bordering a crop, on bulrushes 80 or 90 feet away from, the nearest host plant, on fencing posts, on gates, on trees, on the weather boarding of houses, under ■ the eaves or even in-

side the house itself. In fact, it would be difficult to say on which objects it would not pupate.

The chrysalid measures approximately -inch in length (Fig. 7b). It may be green or greyish in colour, and very frequently changes to a colour in harmony with its background. ■ln contrast to the butterfly, the moth larva generally remains on the host plant to pupate, although it occasionally pupates on nearby objects. When about to pupate it first spins an open network cocoon (Fig. 7a), inside of which it transforms to the pupa or chrysalid form. The backs of leaves infested with the moth , frequently have large numbers of these cocoons attached to them.

The times occupied in completing the egg, larval and pupal stages of both the moth and the butterfly are all very indefinite, as all are governed to a very large extent by prevailing climatic conditions, and in this respect, temperature in particular is important. For,example, the egg, larval, or pupal stage of either* the butterfly or the moth may be lengthened or shortened by prevailing temperature conditions. Low temperatures lengthen the cycle, while high temperatures shorten it. In the accompanying table the. approximate minimum and maximum period for the development of each stage is given, and beyond these limits this range cannot be lengthened or shortened by any further temperature adjustment.

During the winter months the butterfly remains dormant in the pupal stage, and as daily temperatures rise in the spring, development again proceeds and butterflies begin to emerge. As far as the moth is concerned, all stages continue to develop at a very slow rate throughout the winter months.

White Butterfly Parasites

The parasite liberated against the butterfly is now well established throughout New Zealand. It has proved very successful as a controlling factor, as in the field 80 to 90 per cent, parasitism of the host is common. Because of climatic conditions, however, in a wet locality or in any place where the rainfall is persistent to the extent that crops. and their surroundings are kept

in a fairly wet condition, the parasite is less effective than under dry conditions. Again, a crop infested with flowering weeds, Scotch thistles, etc., always appears to be a source of attraction to butterflies, even though the butterfly population as a whole in the vicinity is relatively small. When the butterfly increases to epidemic proportions under the conditions referred to, there is not much that can be done to check it by parasite control. The idea that, the liberation of more parasites would'’ rectify the position is hardly supported by facts, as the conditions which bring about an increase in the butterfly population are also responsible for reducing the effectiveness of the parasite. The position might be improved by introducing species of parasites which would prove

effective under a wider range of climate, and which would thrive under wet conditions, and work on this aspect of the problem is now being carried on.

The parasite (technically known as Pteromalus puparum') , which is already doing, such good work throughout New Zealand, is a small inspect approximately one-sixteenth to one-eighth of an inch long (Fig. 9c). . It is black or green in colour, with four clear wings. Its eggs, laid in the chrysalid stage of the butterfly (Fig. 7b), hatch into small grubs, which devour the contents of the chrysalid. A chrysalid which has been attacked usually becomes dry and brittle, and if it is broken open . it will be seen to be packed with the small parasite grubs "as shown in Fig. Ba. These grubs, eventually turn into the small winged parasites already referred to, which escape from the chrysalid by cutting a small hole in it (Fig. 8b). They then take to the wing in search of other chrysalids to parasitise.

Diamond-backed Moth Parasites

Two kinds of parasite have been introduced and liberated against the moth, one to attack the larva and the other to attack the pupa. The larval parasite, Angitia cerophaga (Fig. 9a), attacks the larval stages (Fig. sa) from the youngest to the half-grown caterpillar. The egg, which is laid inside the moth larva, hatches into a small grub, which feeds and grows at the expense of its host, which it eventually destroys. ; The parasite then spins a silken cocoon and passes into a pupal stage. After a short time an adult winged parasite emerges, and so the cycle goes on. The parasite which attacks the pupal* stage lays its eggs inside the pupa (Fig. 7a). A small grub hatches out which. eventually destroys the moth pupa, after f which the grub itself changes into a pupa and eventually into a winged parasite (Fig. 9b). Both these parasites have been liberated and are now well established in New Zealand. They are as yet not widespread throughout the country, but they are being spread as fast as circumstances permit. There is no doubt that they will play some part in reducing the moth pest, but whether they will give control in an economic sense remains to be seen.

Protection. Methods

In the event of parasites being unable to control the moth and the butterfly to the extent required (and this may sometimes be the case where the home garden or. the market garden is concerned), spraying, dusting, or some other means of protection must

be resorted to. As the methods of treating the home garden and the market garden crop differ somewhat, the treatment of each is dealt with under two separate headings. ,

HOME GARDEN. The average home garden contains relatively few plants for treatment, and white butterfly eggs or grubs when the infestation is light can be dealt with by hand picking the caterpillars or by crushing the eggs (while on the leaves) between the fingers. If this method is not found suitable, sprays or dusts can be used. These are best applied by means of small, hand spray or dust guns obtainable at florist or hardware stores. Various proprietary sprays and dusts are on the market, but those most suitable for the home garden are the non-poisonous variety, such as derris or pyrethrum, both of Which are sold , under various proprietary names. For control of a mixed population of moth and butterfly caterpillars some writers consider

derris dusts as superior to pyrethrum. The derris dust should.have a rotenone content from 0.5 to 1.0 per cent., and should be freshly packed. Dusts and sprays of this kind which have been packed for a long time tend to deteriorate. It is important to keep the heart of the plant free from injury, and if dusts are not available one of the proprietary pyrethrum . sprays (as used in the house against flies, etc.) applied with the household spray gun will prove quite suitable for the task. MARKET GARDEN. Crops grown on a commercial scale for human consumption require to be kept in good condition if they are to command a good market price and show a good return to the grower. Gui* found that the best control of moth and butterfly was 'obtained by spraying or dusting with Paris green or derris powder at intervals of ten days. Paris green dust is mixed with a carrier, such as talc or lime, in the proportions of one of. the former to ten of the latter. Flour may be substituted as a carrier, as it improves the adhesive qualities of the dust. Paris green, if used as a spray, can be mixed in the proportions of 21b. of Paris green to 40 gals, of water, plus a spreader. Calcium arsenate dust applied at the rate of 15 to 201 b. per acre of crop is also said to be effective.. It is pointed out that poisonous compounds such as Paris green or arsenicals should be used only when the crops are young or up to 40 days before harvesting; derris compounds should be used after this period. Derris compounds are. marketed under

various proprietary names, and such products generally have all necessary instructions on the container or package, and these instructions should be adhered to. Whether home-mixed or commercial, the rotenone content should not be less than 0.5 to 1 per cent. Thorough application of the sprays or dusts is important if good results are to be expected.

* Rev. App. Ent., vol. 26, p. 651.

' | Butterfly , Moth. Min. | Max. Min. | Max. Number of days to complete egg stage ( 3 to 35 3 to 40 Number of days to complete larval stage 7 „ 55 6 „ 50 Number of days to complete pupal stage 6. „ 49 3 „ 36 Full cycle, in days, which ranges , from 16 „ 139 12 „ 126

Minimum and maximum number of days occupied in completion of the life cycle of the moth and butterfly.