However useful insects may be in the general economy of nature, it is
but too true that farmers and gardeners often find them a pest, and
with each returning summer the pages of agricultural journals abound
with remedies, offensive and defensive, against the obnoxious
invaders. In such cases, it becomes desirable to know what remedial
means are the most efficacious, and we are glad to find that the
question has been taken up by persons competent to discuss it. Among
these, Dr J. Davy has given the results of his inquiry in a paper, 'On
the Effects of certain Agents on Insects,' which has just been
published in the Transactions of the Entomological Society, and is
well worth reproduction in a condensed form. The experiments were
begun in the winter of 1850, the season, as will be remembered, being
so mild that insects were readily met with. Their objects were
threefold--to test the effects of temperature, of gases, and of
vapours. In the former, recourse was had to extremes of heat and cold.
A bee placed in a temperature of 32° became at first more active, but
the next morning was found torpid, as if dead; a register-thermometer
shewing that 25° had been the lowest temperature during the night.
Transferred to a temperature of 52°, the bee revived in half an hour,
and on the following day exhibited the same results under the same
conditions. A fly which, on December 8, was lively on the wing, in a
temperature of 52° indoors, was disinclined to move at 40°; and still
more so, stirring only when touched, at 33°, but did not become
torpid, as in the case of the bee, even at 23°, signs of life being
distinctly visible. Several trials made with different species of
flies all gave the same result--a remarkable power of sustaining life.
The method adopted was to enclose the insects in a glass tube, and
place them out of doors all night; and though the tube was frequently
covered with frost, they soon revived in the warm temperature of a
room. It is perhaps scarcely possible to estimate the degree of cold
which insect life will bear without destruction, since many of these
creatures survive the terrible winters of the arctic regions. Still, a
knowledge of the effects of reduction of temperature will be valuable,
as affording data by which to judge of the effects and probable
duration of visitations of insects, and of the nature of the
precautionary measures to be adopted. In an experiment of alternate
temperature from 40° to 65° tried for five days on a bee, the creature
at last 'ceased to give any sign of vitality.'
The influence of heat appears to be much more rapid than that of cold:
a fly exposed to a temperature of 120°, died in two or three minutes;
and 113° proved fatal to another; while a third, placed in a
temperature increased gradually to 96°, remained alive for more than
an hour. Others bore from 80° to 90° for two hours; and in one
instance, a fly survived from 86° to 100° for several hours, but
became uneasy with a slight rise, and died at 105°. A bee, taken on
March 15, from a temperature of 45°, was exposed to 80° without any
apparent diminution of activity; at 90° it ceased to buzz; and at 96°,
ceased altogether to move, and did not revive. Although these results
are too few to enable us to determine the laws with respect to the
influence of temperature on insects, they may serve a purpose, in
shewing that the effect is not that gradual one of hybernation, where
activity and torpor succeed each other but slowly.
In the series of experiments with gas, it was found that flies placed
in carbonic acid gas became instantly motionless, and died if left for
any length of time. Some revived after an hour's immersion; others,
after two or three hours--the revival being slow in proportion to the
time of exposure to the gas. Somewhat similar results were obtained
with flies and bees in hydrogen and azote. To try the effect of
deprivation, a fly was shut up in a tube with but a small quantity of
common air, on the 5th February, in a temperature varying from 52° to
60° during the whole time of the experiment. The insect manifested no
uneasiness until the 25th day, and was found dead on the 28th. Another
fly, enclosed in a similar tube, with a quantity of air not more than
a few times its own volume, became languid on the second day, and
motionless on the twelfth, but revived on being taken out.
Flies immersed in oxygen were found dead the second day, with a
diminution of the quantity of the gas. Coal-gas produced almost
immediate insensibility, with a few feeble attempts at revival, but in
no case effectual. Sulphuretted hydrogen also proved especially
fatal--an instant's immersion was sufficient to destroy life; though
withdrawn at once, not one of the flies recovered. It was the same
when the portion of gas diffused in the air of the tube was so minute
as to be scarcely appreciable. On bees, too, the effect was similar;
the deadly nature of the gas on their delicate organisation being
invariably destructive. Like results were obtained with chlorine.
In the class of vapours, ammonia proved fatal in one case, and
harmless in another; muriatic acid stupified in two, and killed in
twenty-four hours. The vapour of nitric acid was equally fatal with
sulphuretted hydrogen; and, in alcoholic vapour, at a temperature of
74°, 'for a few minutes the fly shewed increased activity; in a few
more, it became nearly motionless; after about a quarter of an hour,
it appeared to be torpid. Now, exposed to the air of the room, in a
few minutes a slight motion of its feet was seen; after a couple of
hours, it was nearly as active as before the experiment; two hours
later, it was found dead.' The same effects, with slight variations,
were produced on other flies. With ether, cessation of motion was
almost instantaneous, followed, however, by revivification, except in
one instance: brief immersion in chloroform did not prevent revival,
but an exposure of eight minutes killed: camphor and turpentine were
both fatal: with attar of roses, musk, or iodine, no ill effect was
perceptible.
The experiments with prussic acid are worthy the attention of
entomologists, with whom it is often a matter of importance to kill an
insect with the least possible amount of injury. In these instances,
the plan pursued was to charge a small tube with the acid, and place
it inside that containing the insects. The vapour of 1-16th of a grain
was sufficient to destroy bees and flies; and that of seven grains
proved fatal to large beetles, and the largest kind of bees. Although
as yet the investigation has taken but a limited range, it will be
seen that it opens a wide field of research: the next step will be to
group or class those agents which appear to have produced similar
effects. It is remarkable, as Dr Davy observes, 'that most of the
substances which, even in minute portions mixed with common air,
prevent the slow combustion of phosphorus, as indicated by its shining
in the dark, have the effect, on the insects on which they were tried,
of suspending animation.'
He says further: 'Some of the results may not be undeserving notice
for practical purposes--as those in the instances of sulphuretted
hydrogen, oil of turpentine, and camphor, in relation to the
destruction of parasitical insects, whether infesting plants or
minerals, or to the preservation of substances from the attacks of
insects. To be applicable to the preservation of plants, of course it
is necessary that the agents to be used should not exercise on them
any materially injurious effects. This must be determined by
experiments made expressly for the purpose. The few trials I have yet
made on seeds seem to shew, that the steeping them in a solution in
water of sulphuretted hydrogen has not prevented their germination.
The seeds tried were mignonette, cress-seed, and that of a Nemophila:
analogy--namely, that of steeping the seed of the cerealia in a
solution of the white oxide of arsenic, is in favour of the same
conclusion. Further, for the preservation of articles, whether of
clothing or furniture, it is hardly less necessary that the substances
to be employed should have no offensive odour. Judging from the
effects of attar of roses, and from what we know of scented woods not
being liable to be attacked by insects, the probability is, that any
volatile oil of agreeable perfume will answer the purpose required,
and prove a true instance of the _utile et dulce_ combined.
'As carbonic acid gas, and some of the other agents mentioned, produce
merely a temporary torpor, it may be a question whether this gas, or
simple immersion in water, may not be advantageously substituted for
the fumes of burning sulphur, destructive of life, at the yearly
gathering of honey; the former, indeed, may be said to be in use in
the Levant, where the smoke of the fire of leaves, in which the
carbonic acid generated may be considered as chiefly operative, is
employed to stupify the bees preparatory to the spoiling of their
hives.'