The Lepidoptera, which before were rare, became more and more common in
1879, 1880, and 1881, and so much so that during the last named year they
abounded; and species that had formerly been detected only at certain
favored points spread over the entire coast and into the interior of the
country. The geometers appeared in numbers that were unheard of. But this
change was especially striking as regards the Noctuellae, in view of the
previous rarity of the individuals belonging to this family.
We have here an example of the direct relation of cause to effect,
although I am not in a position to assert that the effect is always
produced in the same way. To me there is no question as to the fact that
the constitution of those insects which nature has accorded the faculty
of liberating is strengthened, and that their chances of life are
increased, if the cold of winter is intense enough to plunge them into an
absolute rest, and is not unseasonably affected by warm, spring-like
days. It is certain that such cold is capable of contributing largely to
the multiplication of the individuals of such species as hibernate in the
egg state, and it also has a beneficent influence upon those species
which, like the small social larvae, pass this season upon the earth
enveloped in a silken envelope, or, like the larvae of the Noctuellae,
between dead leaves or upon the ground itself.
On another hand, it cannot be doubted that mild winters greatly
contribute to the bringing about of a destruction of larvae and chrysalids
in two ways: First, they favor the development of mould, which, as well
known, attacks the larvae of insects when these have been enfeebled by an
excess of rain or dampness; and second, they permit beasts of prey to
continue to exercise their activity. Now, these latter are numerous.
Moles, instead of burying themselves deeply, then continue to excavate
near the surface, and shrew mice are constantly in search of food. These
small mammals, which abound in this district, destroy a large number of
chrysalids of Lepidoptera.
It is the same with birds. As soon as severe cold begins to prevail in
the north and east, they come in troops to the open fields and the
sheltered slope of the hills of our district. But it is scarcely worth
while to stop to tell of the skill and perseverance of these destroyer of
larvae. We may mention, the woodpecker, however, as a skillful searcher
for insects that lie hidden in places where the sun has melted the snow.
The carnivorous Coleoptera and the Forficulae are likewise generally in
motion during mild winters. Doubtless these last-named do not make very
large inroads in the ranks of larvae and chrysalids every day; yet, having
no other food, they destroy a goodly number of them. But I believe that
the devastations made in the army of insects by all these enemies united
do not equal those made by certain crustaceans--the wood lice.
During mild winters these pests multiply, eat, and prosper out of bounds,
and to such a point that, in a climate like ours, they become a true
scourge that prevails everywhere, out of doors and within. Once in a
place, they begin to look for larvae and chrysalids, which they devour.
The severe cold seems to have destroyed a certain number of them, since
they are now not so numerous by far; and it has at least certainly put a
stop to their devastations at an epoch when the larvae are more
particularly exposed to the attacks of their enemies. It is to this
cause, as well as to the preceding, that I am led to attribute the
extraordinary multiplication of so many species during the three last
summers, which were separated by severe winters. Last winter was mild,
and there is therefore no reason to expect that there will be another
multiplication; but I hope that the harm done by such a season will be
slight. It is the progressive multiplication of the destroyers, joined to
the correlative disappearance of the victims caused by a series of
temperate seasons, that is to be feared.
In support of the proposition that I maintain, I may mention still
another fact. While this district (Pembroke, Wales) is relatively poor in
species whose larvae feed and hibernate in the open air a few species of
Noctuellae, whose larvae live buried in the earth, are always abundant. The
country is relatively rich in spices of _Tortrix_, which develop and
hibernate in the stalks or roots of plants. It is also worthy of remark
that very few of our species seem to be incapable of enduring a severe
winter.--_C.G. Barret, in Science et Nature_.
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