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.