CHAPTER V.

REMEDIES AGAINST COCCIDIDÆ.

Many people are under the impression that scale-insects out-of-doors are not of much consequence. They are aware that in greenhouses and hothouses these insects are a trouble to gardeners, and that they probably injure flowering or fruit-bearing plants in such situations. But they imagine that in the open air, and on large well-grown trees, Coccids do no very great harm; or, if the trees are for a time injured, that recovery and health will come before long, and the pest will disappear. This is not the place in which to controvert this or any other opinion. A work professedly dealing with facts should be as free as possible from controversial discussion. Whatever, therefore, may be the grounds of the opinion just stated, or the reasons for rejecting it, it will be sufficient here to say that there seems to be nothing to lead to the belief that New Zealand is likely to be different from other countries in this respect. To institute a comparison, it would be manifestly absurd to include such countries as England, or Germany, or, on the other hand, India, or Central America, or North Australia--Firstly, because in the greater part, or at least in the northern parts, of Europe the winters are much more severe than in New Zealand, and almost certainly the great cold is injurious to such insects as Coccids. Secondly, because in tropical countries it seems that the too great heat is equally obnoxious to them; and, with the exception of a few species, tropical Coccids are comparatively harmless. But it is to the warmer temperate or the subtropical regions that we must look for comparison--regions where there is neither too scorching a summer nor too ice-bound a winter. And, for this purpose, we have only to take such lands as California, Florida, the South of France or Northern Italy, the Cape of Good Hope, the southern regions of Australia, &c. The experience of these is, that some species of Coccids do injure, in every way, whether as regards ornamental or commercial value, a number of trees and plants on which the people of the country depend largely for subsistence or profit. In the South of France the olive industry has been in some years greatly cut down. In Florida, California, Australia, the Cape of Good Hope, oranges and apples have been so damaged that the value of an orchard or a grove has been reduced sometimes by 80 per cent. It may be said, moreover, that even in tropical countries the attacks of scale-insects are often most damaging: in Mauritius the sugar-cane and in Ceylon the coffee plantations have suffered from their ravages. The experience of American fruit-growers is certainly not to be despised, and the fact that both in California and Florida the people strain every nerve to get rid of the insect pests on open-air trees would seem to be distinctly against the notion that these little enemies can be neglected with impunity.

Nor, indeed, can it be said that in New Zealand itself the attacks of scale-insects out-of-doors are harmless. Apple orchards throughout the country bear evidence to the contrary: lemon-groves can be seen about Auckland where, instead of the thousands of fruit formerly grown, a few stunted lemons are all that the withered trees afford; and nobody can glance round the plantations at Nelson or Napier without recognizing the devastating powers of a scale-insect (_Icerya_).

The opinion that Coccids are not likely to do much harm in the open air is therefore scarcely tenable, and it will be of use to inquire what remedies can be provided against them.

There is a point, however, to be noted at the outset, and it is, that in reality there is not, as far as is yet known, any _certain_ remedy against scale-insects. Not that ingenuity and experiment have not succeeded in inventing plans and substances quite efficient in killing the insects when applied to them. It is easy enough to kill an insect when you can get at it, in most cases; but the problem in this instance is not only to kill individual insects, but to do more. What is wanted is to get rid of whole communities of them, and, at the same time, to prevent their eggs from hatching and a new brood coming forth. Many of those who profess to know all about destroying "scale"--especially if they belong to that class which prides itself upon being "practical men"--being generally quite ignorant of the habits and life-history of the insects, are satisfied when they have tried some rule-of-thumb plan which seems to kill most of the adult insects, not dreaming that they have left the eggs unharmed and ready to send forth a fresh swarm at hatching-time. There is another obstacle which often prevents success in eradicating "scale." This is the difficulty of making sure of the effects of any remedy. A plan which has answered well in one place will fail in another, and this, not only as regards different countries, but even in the same district for neighbouring gardens, or even for neighbouring trees in the same garden. Tree-growers must be prepared to find the very same remedy which has cleared their neighbours' trees fail for their own; and in this country the author has seen, in one and the same orchard, some trees quite cleared, while on others, treated in exactly a similar manner, the "scale" was scarcely injured.

It is from want of knowledge of this and the like points that persons who have tried various remedies recommended to them have complained of failure, and condemned both the remedy and their adviser, whilst really neither was in fault.

Nothing need be said here of carelessness or unskilfulness in applying a remedy, beyond the following instance: A person whose apple-trees were being very much damaged by _Mytilaspis pomorum_ was advised to apply, by way of painting the trunks and branches, a mixture of kerosene and some other ingredient. In two or three months he found violent fault with his adviser, for he said every tree was dead or dying. On inquiry it was found that, from over-zeal or want of knowledge, he had applied the mixture as if painting a house, had used it much too strong, and, to make assurance doubly sure, had given his trees two good coats of the oil.

An intelligent appreciation of the life-history and habits of scale-insects is necessary to enable any one to select and apply, with a probability of success, a remedy, and the details given in Chapter I. of this work will be found useful for this purpose. It will be apparent from them that, without regarding the generic or specific characters of these insects, we may lay down a few general principles to start with, thus:--

I. _Whatever damage is done is effected by the sucking of the juices of the plant through the rostrum of the insect._ It follows from this that applications of any fluid to the tree externally, with the object of poisoning the insects in their feeding, would be useless, as their food is drawn from beneath the surface.

II. Neglecting entomological distinctions, we may divide the Coccididæ, roughly, into--

(_a._) Insects attacking deciduous plants; (_b._) Insects attacking evergreen plants; or, again, (_c._) Insects living usually on the bark; (_d._) Insects living usually on the leaves; (_e._) Insects living on both bark and leaves; or, lastly, (_f._) Insects covered with hard shields or "scales;" (_g._) Insects covered with cotton; (_h._) Insects naked.

It will be clear that a different method will be required for destroying these different classes; but any one insect will belong to more than one class. Thus _Mytilaspis pomorum_, the apple-scale, belongs to (_a_), (_c_), and (_f_), and indeed may be placed also in (_b_), as such plants as hawthorns, which it attacks, are as bad as evergreens in the difficulty of reaching the insect on them; or, _Lecanium hesperidum_ is in (_a_), (_d_), and (_h_); Lecanium oleæ in (_a_), (_b_), (_e_), and (_h_).

As far as regards the injurious species of Coccids it may usually be taken for granted that those infesting deciduous plants (class _a_) live chiefly on the bark (class _c_), and are either naked (class _h_) or covered with a hard scale (class _f_). If naked they are chiefly _Lecanium_; if covered, either _Mytilaspis_, _Aspidiotus_ or _Diaspis_.

_Icerya_ is exceptional, being omnivorous, feeding equally on bark or leaves, deciduous or evergreen plants; it belongs to every class except (_f_). Every method of destruction has therefore to be resorted to against it.

The treatment of a deciduous plant infested by Coccids is simple as to its method. For two reasons the dead winter-time must be chosen for it--first, because, the leaves being off, the whole plant can be easily got at; secondly, because the eggs of the insect have not yet been hatched, and the whole brood can be destroyed at once. The first operation should be the pruning of the tree, so as to reduce the labour required to a minimum. A brushing over all the trunk and branches with a good hard stiff brush and one of the liquid remedies given below is then often successful. Brushing with a _dry_ brush is adopted by some persons; but, although this doubtless clears away a good many insects and scales, and may do the tree itself some good by cleaning off fungus-growths and incrustations, yet it necessarily fails to destroy all the eggs, and in consequence the work is only half done. _Any one who wishes to extirpate Coccids must make certain that he has destroyed the eggs_--a fact which is quite ignored by numbers of those who glibly talk of their own success, and blame the advice of others. The object being, therefore, twofold, the operation should be performed with a hard, stiff brush dipped in one of the fluids recommended below; and care should be taken that there is no part of the trunk or branches escaping untouched. In fact, what should be aimed at is a kind of painting of the tree, but with a thin coating of the fluid, so as to close the pores as little as may be; while at the same time the brush clears away as many as possible of the "scales" and their enclosed broods of insects and eggs.

Bearing in mind what has been said just now of the want of _certainty_ in any remedy whatsoever, the tree-grower who follows these directions will most likely find his work successful and his deciduous plants cleaned of "scale" on the bark.

A second method may be adopted--namely, the painting-over of the trunk and branches, without attempting to forcibly detach any "scales" with the brush. This, _properly performed and with proper fluids_, is likely to be just as efficacious as the other, for the fluid should "run in" under the scales, surround the eggs, and prevent them from hatching. It gives less trouble than the hard brushing, and is equally destructive to the Coccids. It has, however, of course, not the same cleaning effect upon fungoid growths or incrustations impeding the free "respiration" of the plant.

For deciduous trees, then, such as apple- or pear-trees in an orchard, the simple remedy is severe pruning at the dead of winter, and the coating of the trees with a destructive fluid, laid on with a brush _on every part_, preferably with a hard brush vigorously used, but leaving a thin coat of the fluid on the bark.

_It must be thoroughly understood that, a week or two after the first application, the "scales" left on the tree should be examined, and, if the eggs are not killed, a second coating of the fluid should be applied._

The treatment of evergreen plants, or of plants which are attacked both on the bark and leaves, is really the same as the above as regards its object, but it necessarily differs in its method. Here, again, it is desired not only to kill the insects themselves, but also to devitalize the eggs; but in this case the work is much harder, for the eggs are especially difficult to reach. Still, there is this advantage: that in dealing with evergreens the season of the year need not be specially studied, and, in default of touching the eggs, one may get at the young larvæ. The remedy is again a fluid, but it must be applied in the form of spray. Coccids are sometimes found on the upper surfaces of leaves, but as a rule they affect the lower sides. This, of course, renders it much more difficult to get at them; and the ordinary rose of a garden syringe would not, as a general thing, distribute the fluid in a sufficiently-fine form. The finer the spray and the more it is forced into every corner and nook of the plant the better. Various force-pumps and spray-throwers have been invented for this purpose in the United States; but probably tree-growers in this country need only procure the finest possible rose for their syringes, and use them in the ordinary way. The fluid should be thrown as thoroughly as possible on all parts of the plant, every care being taken to direct it most fully against the under sides of the leaves.

The work, then, to be done is in itself simple enough. A destructive fluid must be selected and applied according to the character of the insect and its position on the tree. For covered or naked insects on the bark, apply it with a hard, stiff brush; for covered or naked insects on the leaves, apply it in the form of the _finest spray_ thoroughly forced as much as possible into every nook and cranny, and especially against the under side of the leaves.

The question, "What is the best fluid to use?" is more complicated. Many answers have been given to it: many fluids have been strongly recommended by different people. It must be well remembered that, as stated above, a sure and sovereign remedy has yet to be discovered, and failure may attend even the best suggested at present. Bearing this in mind, tree-growers will find in the following list the result of the experience of a number of observers, which may serve as a useful guide. It does not profess to be more than a summary, compiled from the researches of entomologists such as Mr. Comstock, Professor Riley, Mr. Hubbard; from suggestions by gardeners and others, embodied in various parliamentary and private documents; and from actual observation and experiment in this country: but it is believed that the information here given may be accepted and relied on.

Some of the substances here given are manifestly unsuitable for general use on account of their expense, at any rate in the open air. Yet it is well to include them, as they are all suggested in some work or other, or in the replies of gardeners and fruit-growers to parliamentary inquiries; and the objections to them ought also to be known:--

1. _Alcohol._ Will certainly kill any individual insect; but "sprayed over scale-insects produced no apparent effect" (Comstock).

2. _Ammonia._ Whether used pure (diluted) or in urine, damages the plants much more than it does the insects (Hubbard; Comstock).

3. _Ashes._ Powdered, or mixed with lime, salt, soot, &c. Of no value whatever (Hubbard; Personal experiment).

4. _Carbolic acid._ Of no avail, either as spray or brushed on, unless used in such strength as to seriously damage the tree (Hubbard; Riley; Comstock).

5. _Castor-oil._ Has been found efficacious in cleaning hawthorn-trees at the Agricultural College, Lincoln (T. Kirk). It was mixed with soot for some unexplained reason. The time of the year when it was applied is not stated; but the author's experiments seem to show that castor-oil _does not effectually kill the eggs_. Still, it is doubtless a valuable remedy if applied repeatedly, so as to kill larvæ and adults, supposing it to be sufficiently cheap.

6. _Cole's Insect-exterminator._ Apparently a mixture of about 2 parts of "green soap" with 100 parts of strong alcohol. It is "effectual as an insecticide, and harmless to growing plants;" but "the cost is too great, except on a small scale, as in conservatories" (Comstock).

7. _Gasoline._ Seems to have been used in California on pear-trees: result, doubtful (New Zealand Parliamentary Papers: Codlin Moth Committee Report, 1885, page 8).

8. _Gishurst compound._ Very favourably spoken of in many quarters. In New South Wales it has been found efficacious on orange-trees against _Aspidiotus coccineus_ (Alderton); in Nelson it is said to be used beneficially against _Icerya purchasi_. _It does not, however, kill the eggs with certainty_ (Personal experiment). Applied warm, and properly diluted, it may be recommended as a good remedy; but applications of it should be repeated.

9. _Kerosene._ Seemingly the most valuable of all remedies, when properly applied. "Almost the only substance which will certainly kill the eggs without at the same time destroying the plant" (Hubbard).

But the application of this remedy must be carefully performed. Some trees may endure it without injury, even undiluted or unmixed; but this is scarcely to be expected, and the oil should therefore be applied in some mixed form. Also, it is important to remember _that a hot sun increases the injurious effect of kerosene_; consequently winter, or cloudy weather, should be chosen for its employment.

(_a._) _Pure kerosene._ As just stated, it is probably not advisable to use this. Still, "a young shoot of orange, not more than fourteen days old, was uninjured by an application of pure kerosene which thoroughly wet every leaf;" (Comstock); and _Lecanium hesperidum_ on ivy, similarly treated, was destroyed, without injury to the plant (ibid.).

(_b._) _Kerosene and milk._ An excellent mixture, if milk can be obtained cheap (Riley; Hubbard; Comstock). It must be applied in the form of an "emulsion," sprayed over the tree or brushed on the bark. Hubbard gives the following directions for use: Heat the milk nearly to boiling-point and mix with double the quantity of kerosene; churn violently from ten minutes to half an hour, according to temperature, until a creamy thick fluid is obtained; dilute this with nine or ten times the quantity of water. The mixture is of course purely a _mechanical_ one, as far at least as the water is concerned, and it must be kept constantly stirred, to prevent the substances from separating from the water. For evergreen trees impel the mixture on leaves and branches _in the finest possible spray_. Sour milk is as useful as fresh.

The object of the milk is not only to lessen the injurious qualities of the kerosene, but also to induce it to mix more freely with the water; but it is the oil alone which destroys the insects _and their eggs_.

(_c._) _Kerosene and soap._ When milk is not obtainable, or too dear, nothing is so excellent as this mixture. Soap itself (see below) is a useful insecticide, and in combination with kerosene includes the good qualities of both substances. The cheapest possible qualities of soap will do. The mixture, which is, even more than the last, purely _mechanical_, must be made first of all an "emulsion." The American experiments result in the following recipe and method of using:--

Formula:

Common soap 1/2lb. Kerosene 2 galls. _Soft_ water 1 gall.

Dissolve the soap in the water heated to boiling, then add the kerosene, and churn the mixture until a creamy fluid results which thickens on cooling. Dilute with nine or ten times the quantity of water: the quantities given above will make about thirty gallons of liquid. Whale-oil soap, soft-soap, or any other kind will do. As with the milk emulsion, apply in the form of the finest spray for evergreens (Riley; Hubbard; Personal experiment).

(_d._) _Kerosene and oil._ Castor-oil, linseed-oil, whale-oil, may be used. A mixture of this kind, in the proportion of 1 part kerosene to 3 or 4 of oil, has been found very efficacious for apple- and other fruit-trees attacked by the common apple-scale (_Myt. pomorum_). But, as observed above, the mixture must not be laid on too thick. Thinly brushed all over trunk and branches, at dead of winter, it has been found quite successful in destroying both insects _and eggs_, without injury to the trees (Personal experiment). It would probably not answer for evergreens, on account of expense.

On the whole, it may be said that, as far as certainty can be attained in the matter, there is no substance better for destroying Coccids _and their eggs_ than kerosene in the form of milk or soap emulsion, diluted with water for evergreens or for trees with insects on the leaves as well as on the bark. Probably, for deciduous fruit-trees the kerosene-and-oil mixture is the best.

The great point in favour of this substance is that _it destroys the eggs_; this, few if any of the others will accomplish.

10. _Lime._ Of no avail whatever.

11. _Lye._ Concentrated lye is very frequently recommended. In the New Zealand Parliamentary Papers (Codlin Moth Committee Report, 1885, page 7) several statements will be found apparently most favourable to it; yet in places we find admissions that "it cannot reach all the eggs." In America generally, it has not been found satisfactory: "inferior to kerosene in killing-power, and far more injurious to trees when used in solutions strong enough to be effective as insecticides."

It is quite possible that the action of lye on the fungus accompanying the scale-insects (see Chap. III.) may have led "practical" gardeners to imagine that it cleaned their trees of scale. Comstock says, "I saw most excellent results from the following mixture: 1lb. concentrated lye, one pint gasoline or benzine, half pint oil, five gallons water." Probably the good results here were due, not to the lye, but to the gasoline and oil.

12. _Pyrethrum._ Useless against Coccids (Comstock).

13. _Salt._ Useless (Comstock).

14. _Soap._ Undoubtedly a valuable remedy, and perhaps, in some cases, as efficient as kerosene; _but it does not destroy the eggs_. A solution of 3/4lb. of soap to a gallon of water, _applied hot_, was entirely successful in California: three months after its application no living scale-insect could be found (Comstock). The time of the year is not stated. In another case the solution was applied cold: "four days after no living insect could be found;" but, again, the time of the year is not stated, and no mention is made of the eggs. Still, a strong solution of soap may be said to be one of the best remedies against the larvæ and adult insects--proportions from 1/4lb. to 1/3lb. soap to one gallon of water (Comstock; Hubbard; Personal experiment).

15. _Soda, caustic._ Strongly recommended by many persons. It injures the tree, _and does not kill the eggs_--two things which are decidedly against its use. Gardeners may have been led to employ it from finding that in some instances it clears away the black fungus-growths (Chap. III.), and imagining this to be a clearance also of the scales.

16. _Soda, silicate._ Kills some insects, _but no eggs_, and injures the tree (Hubbard).

17. _Sulphate of iron._ "A common ingredient in patent remedies;" most injurious to vegetation. It does not affect scale-insects (Hubbard).

18. _Sulphur._ Another substance, the object of a kind of superstitious veneration amongst gardeners. It is excellent against fungoid growth, but of little value against scale-insects. Here, again, the clearing of the black fungus has probably been taken to mean also the destruction of the insects (Hubbard; Comstock; Personal experiment). Comstock says that in America people often bore holes in their trees and stuff them with sulphur, under the notion that the substance will be taken up by the sap, and poison the insects: quite a futile idea.

19. _Sulphur and lime._ A dangerous compound, and useless unless applied in such strength as to kill the tree. Its fumes are poisonous, and it may seriously injure the face and hands (Hubbard).

20. _Sulphur and snuff._ Equal parts mixed and dusted over _Lecanium hesperidum_ on a wet day were quite successful (Comstock). But the mixture would be too expensive except for conservatory plants, and doubtless the snuff alone would be quite as efficacious.

21. _Sulphuric acid._ "Killed nearly all the scale-insects, and very nearly killed the tree" (Hubbard). No mention is made of its action on the eggs.

22. _Soot._ Useless (Hubbard; Comstock; Personal experiment).

23. _Tobacco._ A good remedy against larvæ and adults; _doubtful against the eggs_. Fumigation has no effect on scale-insects, except sometimes on Dactylopidæ, or "mealy bugs" (Hubbard; Comstock; Personal experiment). The tobacco should be applied in a pretty strong solution; but the expense in this country would be probably too great for general use.

24. _Whale-oil and whale-oil soap_ have been already alluded to under the head "Kerosene." They are both useful ingredients in mixture with that substance, if procurable cheaply.

From the foregoing list it will be gathered that, if experiment, combined with knowledge of the habits and life-history of scale-insects, can be relied on, there is no substance better adapted for their destruction than kerosene, mixed with oil, or milk, or soap solution, and _carefully applied_. It has been already observed that the killing of the eggs is absolutely necessary for thorough clearing-away of the insects; and, to quote again the words of Mr. Hubbard, kerosene is "almost the only substance which will with certainty kill the eggs without at the same time destroying the plant."

But precautions must not be neglected. Persons who recklessly use any remedy, or who apply it too thickly or in too strong proportions, must expect their trees to suffer. Nor must the weather and the time of the year be overlooked. _Winter is the best season for all remedies_; and, preferably, cool and cloudy days. Again, if substances soluble in water, such as potash or soda lye, soap solutions, &c., be employed, it must be expected that a day's rain will wash a good deal of them off, and greatly reduce their efficacy. These are things which many people forget; they fancy that because somebody has cleared his trees with, say, castor-oil in winter they can do the same thing in full heat of summer; or, because a lye solution has done well in the dry climate of California, that it will be equally good in the rains of New Zealand. Still more is it a fallacy to imagine that rule-of-thumb methods, not founded upon any knowledge of the nature, habits, and life-history of the insects, are likely to be really efficacious.

Little need be said here of a remedy which has had, to some extent, the authority of Professor Riley, and which is recommended by Mr. Howard (Report U.S. Dep. of Agric. 1880-81, p. 351): viz., the transportation or acclimatization of parasites on scale-insects. Doubtless the thing could be done, as experiments in America have shown. But there are plenty of parasitic insects in New Zealand already, and, although they seem to have hitherto confined their work to the native and mostly to the innoxious Coccids, they may at any time begin to attack the others, and it is only a question of time when they will act usefully as efficient checks (see Chap. IV.).

There is one Coccid of which it must be said that, whilst kerosene mixtures will undoubtedly destroy it, by far the best remedy of all is to destroy and burn at once the infested trees. _Icerya purchasi_ is so voracious and universal a feeder, so repulsive in its aspect, and so destructive in its effects that the most drastic remedy is the best. Any one, therefore, having a tree, especially an ornamental or a fruit tree, attacked by _Icerya purchasi_, is strongly recommended to make no delay, but to cut down and burn every stick of the tree as soon as possible.

It was observed at the beginning of this chapter that some people hold the opinion that the damage done by scale-insects is not of importance. The foregoing remarks upon remedies are not directed to those who hold this view, which is contradicted by the experience not only of other countries but of New Zealand itself.

_Authorities referred to in this Chapter._

U.S. Department of Agriculture--

Reports by Professor Riley, Professor Comstock, Mr. Hubbard, Mr. L. O. Howard.

N.Z. Parliamentary Papers, 1885--

Report of the Select Joint Committee of both Houses on the Codlin-moth, and "various blights to which fruits are subject."

Personal experiment by the author and friends.

Replies of farmers, gardeners, and tree-growers to inquiries, official or private.