considerations, and to have carried out some interesting demonstrations
of the more frequent significance of colour-markings as means of
concealment and illusion than had been recognised before his work. At
the same time, it is not possible to consider the yellow and black
livery of wasps, of certain evil-tasting grubs, and of poisonous
salamanders as anything but a “danger-flag,” a warning to other animals
that the yellow and black animal had better not be bitten and tasted. So
the previous experience of animals who have bitten yellow and black
creatures is appealed to, and ensures the safety of the yellow and black
gentry from tentative bites which would kill them. Other recognition
marks by which ill-tasting, nauseous butterflies are distinguished, and
in consequence of which they escape attack, and, not only that, but are
“mimicked” (as the yellow and black poisonous wasp is mimicked by some
innocuous flies which thus escape attack) by other pleasant-tasting
butterflies which fly with them, are considered by Mr. Thayer to be
wrongly interpreted as recognition or “warning” marks. He shows, with
more or less success, that the markings of the butterflies known as
Heliconiæ are effective as concealment, and is therefore inclined to
deny their value as “warning” marks, serving to indicate a noxious
quarry best left untasted.
It is, of course, quite possible that what are “concealment markings”
when viewed by an aggressive bird or lizard at a distance, may be
recognised as “warning marks” when seen by the same observers at close
quarters, and it is also possible that the latter may have become the
more important or only important result of the colour marks of a given
butterfly which were once useful as “concealment.” The possible change
of significance of colour spots and markings in wild animals may be
illustrated by the effect on human beings of the burglar’s crêpe mask.
At the present moment probably the most prominent result of the
appearance in a house full of people in the dead of night of a man with
a crêpe mask over his face would be terror to those who saw him. The
mask would be interpreted as a “mark” or “sign” of evil, not to say
violent intentions on the part of the masked man. It would be a “warning
colour,” and most unathletic individuals would severely avoid it; in
fact, retire from it in alarm. But actually, the burglar’s mask—as
possibly some noxious insects’ distinctive markings—was not invented
for the purpose of causing alarm. Far from it! The burglar, or nocturnal
malefactor, dons his crêpe mask in order to cover the white glitter of
his face, and so to escape observation. In origin it is a protective
coloration leading to invisibility, and only secondarily has it become a
“warning colour” or “mark” at close quarters. There will be much more
ascertained, and much instructive discussion as to the colours and
markings of not only animals, but also of flowers and foliage, before
this wonderful subject is thrashed out. I have only been able here to
indicate its outlines.
XXXIV
HOP-BLIGHT
Hops have for many years now been a very uncertain investment for those
who, in England, devote capital to the growing, drying, and marketing of
this crop. In some years a fortune may be made, in some years a dead
loss, in many a bare return of expenditure. Hence, it is not surprising
that English hop-growers should wish for legislation which shall make
their business more secure by taxing the hops produced in other
countries, and imported by our brewers. The whole subject of “hops” is a
very complicated one. It is the fact that every plant and animal
cultivated by civilised man has led to the accumulation of an
astonishing amount of detailed knowledge and experience in each case,
and that there are increasing difficulties and surprises in regard to
varieties, and the competition of new supplies brought from all quarters
of the globe. New areas of cultivation, new methods of transport, new
fashion and taste continually disturb, and even destroy, old-established
industries. It is for statesmen to consider how far the remorseless
current of unforeseen changes should be checked and manipulated, so as
to prevent disaster in the old-established and flourishing industries of
the countryside.
The hop (called _Humulus lupulus_ by botanists) is a native of this
country, and of the more temperate parts of Europe. The Greeks and
Romans never made “beer,” and were unacquainted with the use of the hop.
More than a thousand years ago the German and Scandinavian peoples made
use of various fragrant herbs (sweet gale, bark of tamarisk and oak) to
flavour the sweet beer which they brewed from malted grain, just as
borage, cucumber, and other plants are still used to flavour “cups.”
Wild hops were used, amongst other herbs, for this purpose, and
gradually—but only gradually—became the favourite source of flavour.
The hop owes its selection not merely to its bitter tonic quality, but
also to its wonderful and most delicate perfume. Not only that, but the
hop is found to be effective in checking continued fermentation and
souring—and also to have a narcotic sleep-producing quality, for which
it is still used medicinally. Distinct chemical compounds are found in
hops to which these several properties are due. A warm “hop-pillow”—a
pillow stuffed with dried hop-flowers—has given, and still gives, sleep
to many a wakeful countryman. The older use of other fragrant plants in
the making of beer survives in some foreign beers, such as the Norwegian
ale, the beer of Louvain, and the “green” spruce-beer of Jena.
Hops were first cultivated with a view to obtaining varieties which
would furnish abundant and large, well-flavoured flower-heads. The
flower-heads are “cones,” consisting of numerous minute flowers,
protected by overlapping green-coloured scales or bracts. The cultivated
hop was brought to this country in the time of Henry VIII, and the
cultivation of hops in hop-gardens and the skilful drying of the
flower-heads in large bulk was commenced, and regulated by law. The male
or pollen-producing hop-vine is distinct from the female seed-bearing
hop-vine; it is the female flower-cone which carries the valuable
fragrant and resinous products which the brewer desires. Hops are
artificially propagated by root-cuttings, and it is interesting to note
that the hop-grower finds that it is not desirable to allow the female
flowers to be fertilised, since, although the hops weigh more after the
setting of the seed, the valuable extractive substances contained in the
flower are diminished, used up in the growth of the seed. Hence, often
only one male hop-vine to every 200 female hop-vines is allowed in a
hop-garden.
[Illustration: FIG. 55.—Early winged female hop-louse, produced
viviparously by the first generation of daughters of the “Foundress,”
Fig. 58. These winged females migrate from the plum tree, where they
were born, to the hop-vines by aid of their wings, and produce
viviparously the form drawn in Fig. 57.]
[Illustration: FIG. 56.—Male hop-louse, not appearing until late
autumn.]
It does not follow because a plant is a native of a given country that
it can be easily cultivated anywhere in that country, or that its finest
cultivated varieties will be hardy. Only a few limited territories
(owing to the nature of the soil, climate, and exposure) in Germany
(chiefly in Bavaria), and in Kent, Sussex, Worcestershire, and
Herefordshire, seem to be really favourable to hop-growing in Europe.
Certain parts of the Pacific coast of the United States have of late
years proved a very successful ground, although hops were introduced
from Europe and first cultivated with considerable success in the State
of New York. The same dangers and troubles attend the hop-crop in all
these regions. These are blight, red-spider, mildew and mould, besides
several less important insect pests. The hop-blight, or “black-blight,”
is a plant-louse or aphis (Fig. 55) like the rose-aphis, and does great
and increasing damage to the hop-crop in England, destroying the young
and tender shoots in the months of June and July. In 1882 the hop-crop
was reduced from 459,000 cwt. (of the preceding year) to 115,000 cwt. by
this insect, and the wages paid for hop-picking from £350,000 to
£150,000. These figures give an idea both of the damage done by blight
and of the amount and value of the annual crop, for the mere picking of
which so large a payment is made. Red-spider is a small mite or acarid
which has done a good deal of damage in Kent. But mildew and mould are
more serious. These are due to a delicate, thread-like kind of fungus,
which spreads on the leaf. Many kinds are known in various parts of the
world and on various plants. They may grow on one kind of plant without
doing injury to it, but if they get on to another, cause deadly
destruction of the foliage. It was an otherwise harmless mould, or
leaf-fungus, which destroyed the coffee plantations of Ceylon. It had
lived in the Ceylon forests on other plants without attracting notice;
but when the coffee tree was introduced and cultivated in large areas,
this little fungus seized on it, grew with terrible activity, and
received the name “vastatrix” from the botanists who traced its history,
and showed that it was the destroyer of the coffee plantations.
[Illustration: FIG. 57.—Ordinary wingless female hop-louse, multiplying
parthenogenetically throughout the summer.]
Hop-growers are constantly contending with these pests in the same way
as other growers of crops have to contend with similar pests, but the
hop-growers have the more difficult and delicate “patient” to steer
through its diseases. The finest kinds of hops are not robust; it is a
chance whether or no they will suffer from a wet and cold season, or
other irregularity of climate, to such a degree as to fall ready victims
to blight and mildew. Yet they pay better, provided the season is
favourable, and so the grower risks planting the fine, delicate variety
instead of being content with the more certain but smaller profits
yielded by a more robust variety of hop. The hop-lice, or blight
insects, are destroyed by washing with soft soap and quassia—a process
requiring, even when a machine is used, a good deal of care and labour.
Mildew and mould are destroyed and also prevented by dusting the
hop-vines in hot summer weather with finely powdered sulphur. But both
diseases can be combated by keeping the source of infection away from
the hop-garden. The mould-fungus can be checked by burning all leaves
and plants attacked by it within the hop-garden. If the infected leaves
are left to rot they carry on the parasitic fungus to a new season.
An interesting fact has been discovered about the hop-blight aphis
(called by zoologists _Phorodon humuli_). It appears that the winter
brood of this little insect (when the hop-vine has died down) deposit
their eggs on the bark of the sloe (the wild plum), and also that any
cultivated plum trees serve them for the same purpose. When the hop is
dead they must of necessity get nourishment and shelter from the plum
tree. Clearly, then, if you can keep all plum trees at a distance of
half a mile from your hop-garden you will render it very difficult, if
not impossible, for the blight aphis to carry on from season to season.
It will rarely, if ever, travel half a mile, and not in any number. But
hop-growers have not always the control of the cultivation for half a
mile around their hop-fields, though large growers should be able to
acquire it. The skilful grower even finds it useful to leave one or two
plum trees in the hop-field, so as to attract the winter brood of the
blight aphis to them, and then he falls upon the devastating but minute
rascals with quassia and other poisons, and ensures their destruction.
The increase of plum orchards in the neighbourhood of hop-gardens is
probably a chief cause of the increased loss by hop-blight of late years
in Kent.
The hop-louse has other enemies besides the grower. These are the
lady-birds (less prettily called “lady-bugs”), which feed greedily on
the parasites, so that when the hop-grower sees plenty of them on a
hop-vine he does not trouble to wash it. And there are other predaceous
insects which tend to keep the hop-lice down. Cultivation and excessive
production have resulted in putting, as it were, too heavy a task upon
the natural enemies of the pest, whilst the more delicate but valuable
varieties of hop cannot withstand the attacks of blight, which less
valuable varieties would tolerate without fatal injury.
Another complicated and difficult problem for the hop-grower is the
“curing” of the hops when gathered. He has to arrange to grow a number
of varieties which will not be all ready for picking at the same moment,
so that the hop-pickers may be employed for some six weeks, and gather
each kind at the exact time of ripeness. Then the gathered hops have to
be “dried” and “cured.” In Germany (where the highest-priced hops are
produced) small cultivators dry them in the sun, and they are “cured” by
the purchaser, but in England they are dried in kilns (called “oasts” in
Kent) near the hop-grounds. They are cured with sulphur fumes on the
spot as soon as dried. The object of the drying and curing is quickly to
get rid of the water, which forms 75 per cent. of the weight of the
green flower-heads, but is reduced by drying to 10 per cent., and to
destroy the “mould” (fungus) which may be present, and to keep the hops
free from new access of mould by the slight deposit of sulphur fumes on
their surface. The drying and fumigating require a great deal of skill,
and a fine crop may be injured or even rendered worthless by want of
care, rapidity, and judgment in treating the freshly gathered
flower-cones. It is said that it takes years to acquire the art, and
that skilled hop-curers are more difficult to obtain than formerly.
The natural difficulties and fluctuations with which the English
hop-grower has to contend are made far more serious by the fact that he
does not know what will be the yield of the American and German
hop-plantations, and so cannot prepare beforehand for the demands of the
market. It appears that ice-storage is now being made use of in some
districts to hold over any excess of produce of particular kinds of hop
beyond the special demand for those kinds. But a formidable source of
trouble exists (and, it appears, must always exist) in the enormous
changes and expansion of the brewing industry in all parts of the globe.
It is actually the case that there has been a greatly increased and
unforeseen demand for hops of less highly developed aroma, for the
purpose of brewing light ales with little of the perfume given by the
finest and hitherto most highly priced hops. So that, having expended
skill and money to produce the finest hops, and having been favoured by
the weather, a grower may find that his pains have been thrown away, and
that there is a sudden falling-off in the demand for the beautiful
high-priced crop which he has gathered in. There is no remedy for these
world-wide fluctuations in the market, and the only way in which the
grower can protect himself is by combining with others to procure
information from every part of the world as to the probable production
and the probable demand of the various qualities of hops a year or more
in advance of his planting. More has been done in America and in Germany
in this way than in England, and it is probable that the future success
or failure of hop-growing in this country depends more on the
possibility of obtaining correct information in regard to the tendencies
of production in all hop-growing countries, and in regard to the demand
in all the brewing industries of the world, than on anything else.
This brief sketch of the hop-growing industry is sufficient to show what
a very difficult problem is before those who desire to take legislative
measures for the preservation of the old industry of the hop-garden in
this country. But it must not be at once assumed, because the case is a
difficult and complicated one, that nothing can be done, and that the
beautiful hop-vines and the finest hops are necessarily to be banished
from the English soil.
XXXV
GREEN-FLIES, PLANT-LICE, AND PARTHENOGENESIS
The minute “green-flies” which attack all kinds of plants, and among
which are ranked the hop-louse or hop-blight, the rose aphis or
green-fly of rose trees, the woolly blight or aphis of apple trees and
pear trees, and the terrible vine-killer—the _Phylloxera
vastatrix_—form a special group of bug-like insects known as the
Aphides. They have soft cylindrical bodies, six legs, sometimes two
pairs of transparent wings, sometimes none, and a sharp beak (in some
kinds this is one and a half times as long as the body), with which they
prick the soft parts of plants, when they suck up the juices which issue
from the wound (Fig. 59). There is in the temperate regions of the world
a special kind of aphis or plant-louse peculiar to each of many kinds of
flowering plants, including most trees. A very complete, illustrated
account of the kinds or species of British aphides, amounting to some
two hundred, was produced by the late Mr. Buckton, F.R.S., and published
by the Ray Society.
There are many facts of extraordinary interest about these tiny swarming
insects. In the first place, they are closely related to the minute
scale-insects or _Coccidæ_, several species of which produce the
celebrated lac of lacquer-work and the dyes known as lake, cochineal,
and kermes, the latter a dye manufactured in South Europe and used to
colour wool and cloth crimson before cochineal reached us from Mexico.
The _Coccidæ_ include also the “mussel-scale” and other destructive
diseases of fruit trees. A beautiful purple colour can be extracted from
crushed masses of some kinds of aphides (as well as from _Coccidæ_), and
has been used as a dye. The aphides have very generally a green colour,
like many insects (caterpillars and leaf insects) which pass their lives
upon green leaves and feed on them. It is often supposed that this green
colour is merely the green colouring matter (so-called chlorophyll) of
the leaf, taken up by the insects in feeding on the leaf. But this is
not so; it is a peculiar substance derived in a crude state from the
plant-juice, but digested in the stomach and completed in the insects’
blood and tissues. Then, again, the aphides produce curious secretions,
often in great abundance, which surround them as the lac surrounds the
lac-insect. The threads which are produced in such abundance, by the
woolly aphis of apple trees, as to look like masses of cotton wool
adhering to the twigs of the tree, are of this nature.
[Illustration: FIG. 58.—Foundress or stock-mother of the hop-louse: the
individual hatched from a winter egg, laid on the bark of a plum tree,
who produces viviparously a wingless virgin brood. That brood produces
wing-bearing young, which fly off to the hop-plants.]
[Illustration: FIG. 59.—Side view of winged viviparous female of the
hop-louse, _b_, the stabbing beak.]
[Illustration: FIG. 60.—An ant “milking” a “plant-louse” or “green-fly”
for honey-dew. The drop of honey-dew is seen exuding from one of the two
long tubes or spouts (called “cornicles”) on the back of the plant-louse
at _a_. These spouts are seen at the hinder part of the body in the
drawings of the hop-louse (Figs. 55 to 59). The ant is causing the aphis
to pour out its honey-dew (in fact “milking” it) by “drumming” on the
body of the plant-louse with its clubbed antennæ, and has taken the drop
of honey-dew between its jaws. This drawing was made from life by the
late Mr. Buckton, F.R.S., a great student of these creatures. The ant is
that kind known as _Myrinica rubra_. The plant-louse is the _Aphis
sambuci_ or blight of the elder-tree.]
Another curious production of the aphides—common on the leaves of elms
and other trees infested by them—is known as “honey-dew.” It is sticky
and sweet, and was supposed by old writers to have distilled from the
stars, or otherwise to have dropped from heaven. It is this sweet
secretion which has led to the establishment of a most curious
friendship between ants and aphides, or plant-lice. It has long been
known that an ant will approach an aphis, and tickle it, when at once
the aphis exudes from its cornicles (see Fig. 60) a drop of sweet
honey-dew, which the ant swallows—just as a man may milk a cow and
drink the milk. And the resemblance goes further, for the ants take
possession of certain aphides, and keep them either underground or in
specially constructed chambers, where they can gain ready access to them
and “milk” them for honey-dew. There has been a certain amount of
exaggeration in the description of these facts by some of the older
writers; but it is undoubtedly true that some species of ants keep
special flocks or herds of aphides, and feed on their sweet secretion.
Other small insects nourish themselves on the enormous swarms of
plant-lice in a less gentle way, but a way which man is very glad to see
in active operation, namely, by biting them and sucking out their soft
entrails—thus destroying them in great numbers. The lady-bird beetle is
especially active in this matter, both when it is a grub and on
attaining its adult form. A trustworthy observer saw as many as forty
aphides consumed by a lady-bird in an hour. Where the plant-lice or
aphides abound, there come also in countless swarms the beetles known as
lady-birds. In the year 1869, such a cloud of these beetles passed over
and settled on the fields and gardens of Kent, Sussex, and Surrey, as to
cause something like terror; it was impossible to walk in the lanes
without crushing hundreds under foot. But the little lady-birds are not
like the terrible locust, which appears in millions and devours all
vegetation before it; on the contrary, they are what are called
“beneficials,” and come solely to feed on and destroy the plant-lice of
the hops, plum trees, and apple trees. A first-rate hop crop in the year
1870 was the consequence of the abundance of lady-birds in 1869. It is
this beneficent activity of the lady-birds which has given them their
name. In Italy they are called _Bestioline del Signore_, also
_Madonnine_, and _Marioline_, and in France _Bête à Dieu_. In English
they are “our lady’s blessed bugs,” which save the crops from
destruction.
The exertions of the aphides in pricking the plants they infest so as to
get at their juices lead to the growth of galls on the leaves, and also
on the rootlets of many plants, and often the leaves become rolled up
into bag-like bodies filled with aphides. Many trees and smaller plants
are killed by these attacks, but it is probable that where the plants
have not been rendered delicate by nursing and cultivation, and where
the aphides are not a strange foreign kind, introduced by man’s
carelessness or by some rarely (if ever) occurring wind or flood, the
aphides do not actually destroy any plants by their visitation,
excepting the weaklings, and that their numbers are kept within bounds
by their natural enemies the lady-birds and other such carnivorous
insects.