LEPTOMONAS
PYRAUSTAE nov. sp.
Examining
some Corn Borer larvae, collected in a field some four kilometers west of
Bletterans in August 1927, 1 found in the malpighian vessels and in the
alimentary canal of some of them a new flagellate. The new species had the
morphological and cytological characters of the genus Leptomonas and was named Leptomonas
pyraustae.
It
is impossible to distinguish the infected and the healthy larvae by simple
observation. By dissection it is easy under binocular microscope to recognize
the Corn Borers infected by Leptomonas
pyraustae, and also to distinguish them from those parasitized by Perezia
pyraustae.
The
malpighian vessels are slightly hypertrophied and are in their full length
grayish, very different in color from the white vessels of the larvae infected
by Perezia. Examined under the
microscope, the malpighian tubes appear filled with vibrating or rapidly moving
parasites. By staining pieces of the malpighian vessels or of the mid‑intestine
with Giemsa, the parasites are plainly seen. It is striking, that the specimen
from the malpighian tubes and those from the intestines differ morphologically.
It looks as if they belonged to two different species.
The
specimens from the intestines are more or less oblong with a compact nucleus in
the front part of the cells. In front of the nucleus, a transversally elongated
blepharoblast is found in the bottom of a protoplasmatic cavity opening on the
surface of the cell. Eosinophile grains or short filaments forming a flagellum
are found at the outside border of the protoplasmatic cavity (Fig. 14). Some of
the parasites are motile. In that case, they carry a normal flagellum. 11 is
difficult, however, to demonstrate its presence. The cytoplasm behind the
nucleus is rich in rather large chromatophile bodies. They are probably reserve
substances. The division takes place in the regular fashion. The blepharoblast
moves back to the nucleus and both divide longitudinally. The two individuals
produced by the division remain for a shorter or longer time connected at the
posterior end. If they divide once more before separating they form rosettes.
These are, however, much more rare than in other flagellates. Besides the
typical specimen with swollen anterior part there are needle‑shaped forms
(No. 7) in all Leptomonas species.
By
fixing an infected Corn Borer in Duboscq‑Brasil fluid and staining
microtome sections, the almost continuous layer formed by parasites along the
epithelium' of the mid‑intestine can be shown (Fig. 15). They are attached
with their anterior end. to the epithelium. Their presence does not seem to
cause deeper injuries. By fixing the larvae in accordance with the methods
applied in the study of mitochondriae, and staining the sections after Kull's
method, it can be shown that 'the mitochondriae, the most fragile elements of
the cells, are unchanged in the epithelium. No parasites can be found in the
foreand hind‑intestines. Leptomonas
pyraustae lives as a commensal rather than as a parasite in the alimentary
canal of the Corn Borer.
The
specimens from the malpighian vessels are all of the type with
Other
types of the parasite were found in a hibernating Corn Borer collected in
February, 1928 in the same region
where the previous material was obtained. Typical specimens from the malpigbian
vessels of this larvae are pictured in
Fig. 17. Some specimens (no.
8) correspond to the typical form with the exception that a flagellum is clearly
visible, and directly connected with the blepharoblast. Other specimens (no. 7)
have a relatively short flagellum but the position of the blepharoblast is
modified, the protoplasmatic cavity is absent, and some chroatophile bodies are
visible in front of the nucleus. Some of the parasites (nos. 1, 2, and 5) are
round and look like cysts without a wall. The blepharoblasts, when present, are
round and not located in a cavity. The chromatophile bodies are distributed
throughout the protoplasm.
The
blood of the larva, examined in February 1928, proved to be infected with motile
parasites. In blood preparations, stained with Giemsa, I have found specimens of
peculiar form (Fig. 18). Most of them possessed a flaggellum attached to the
blepharoblast which is located at the nucleus or behind it in the bottom of an
invagination opening at the anterior end of the cell. The length of this
invagination, and its central position in the cell, makes it look like [lie
axostyle described by certain authors, notably CHATTON. I have observed conjugation
characterized by the fusion of nuclei an(] blepharoblasts. The gametes are
approximately of the same size. They place themselves side by side with the
front end in opposite directions. The conjugation seems to produce an individual
without flagellum and cavity and with chromatophile bodies distributed in all
parts of the cell (no. 9).
Leplomonas pyraustae have
been found only in a relatively small number of cases. In the field where the
parasite was discovered in September, 1927, only 4 out of 620 Corn Borers were
infected. During the winter, 500 larvae from the same locality were examined and
the flagellates found only in two.
My
experiments with artificial contamination have all been unsuccessful. The
methods of transmission of the parasite from individual to individual
and from generation to generation remain to be investigated.
The
present importance of the parasite is insignificant. Because the degree of
Parasitism may increase in the future, I have considered it worth while to study
this flagellate rather extensively.
IV.
PREDATORS
Most
authors claim that birds destroy a considerable number of Coro Borers. It is
difficult to estimate the importance of birds in this connection. I have not
made a single observation that would indicate that birds destroy the Borers in
eastern France.
I
have, however, observed that ants of the genus Lasius
destroyed a very
large part of the Corn Borers in a pile of corn stalks in the garden of the
Entomological Station. A large number of these small .ants were found in the
Corn Borer tunnels in the dry stalks. I have observed how they preyed upon [lie
hibernating Borer larvae. It is probable that the same thing occurs in the
field. It is known that in certain cases ants are of great importance in the
natural destruction of injurious insects. The instance, discussed hi my Paper on
Neurotoma
nemoralis, is
suggestive in this direction. The effect of ants on the ,Corn Borer is surpassed
by that of two not hitherto observed predators. 'The most important is a very
common lace‑wingfly, Chrysopa vulgaris; the other is a mite of the genus Allothrombium.
CHRYSOPA
VULGARIS Schneider
The
adult insect (Fig. 20) is more or less abundant everywhere. It deposits its eggs
on an,,, object even in houses. The eggs are very easily recogniz
ed (Fig. 19). They look like
tiny grains of greenish colour, and are attached to the supporting object by a
silken filament. The larvae (Fig. 21) are very motile. They possess two strong
mandibles through which passes a fine canal. Through this canal they absorb
their food directly from the body cavity of various larvae and plant‑lice.
In
1927, 1 had occasion to capture Chrysopa larvae
attached to Corn Borers in their tunnels within the corn stalks. The same
observation
‑was
made in 1928. The attraction which the Corn Borer exerts on the Chrysopa
larvae can readily be demonstrated by placing them together in a glass
container. The predacious larvae instantaneously attack the Corn Borers, stab
them with their mandibles and imbibe the contents ,Of their body cavity.
Sometimes the Chrysopa larvae have
difficulty in piercing the skin of the biggest Corn Borers, and these, by
violent ,contortion, may succeed in throwing off the attacker. The action of the
larvae of Chrysopa vulgaris is most
efficient on the eggs and young larvae of the Corn Borer. During my examination
of the corn fields in the Jura region, July 19‑25, 1928, 1 discovered that
the Chrysopa larvae very frequently
attack the egg masses of the Corn Borer. The larvae pierced the eggs with their
mandibles and rapidly absorbed their contents. Every egg in the attacked masses
was destroyed in that way. The sucked eggs look like eggs from which the larvae
have hatched, but by examining them tinder binocular microscope the holes
pierced by the mandibles may be seen. I have found many egg masses destroyed by Chrysopa
vulgaris. It is difficult to estimate the number of Corn Borers, destroyed
by this predacious insect, but it is certainly large, to Judge from the number
of adult Chrysopa in the corn fields. In some fields, Chrysopa eggs may be found on every corn plant, so that many larvae
hatch at the very place where they find the eggs and young larvae of the Corn
Borer on which they feed.
ALLOTHROMBIUM
FULIGONOSUM Herm.
The
larvae of this mite are extremely common (Fig. 22); they are ,easily recognized
by their red color. On July 25, 1928, 1 observed them in a corn field near
Bletterans on. Corn Borer eggs the content of which they were about to absorb
through a small hole bored by their mandibles. Because the Allothrombium larvae are much smaller than the Chrysopa larvae, they consume fewer eggs than the latter species.
Their effect is, however, far from negligible. Other insect eggs are also
attacked. My collaborator R. PUSSARD have found them on Chaitophorus
aceris.
It
is hard to tell whether these predators are responsible for the high mortality
of the Corn Borer in its first stages. Many authors have called attention to the
fact that the adult Corn Borer moths only represent a very small percentage of
the eggs. ZWOLFER estimates
that ,only 10‑15 per cent. of the Corn Borer eggs complete their
development.
CAFFREY
cites similar figures. CAESAR
has recorded from Canada that
the mortality of the young larvae reached 77.7 per cent. in 1924, and 93.6 per
cent. in 1925. Most authors explain this mortality by the action of physical
factors (temperature, humidity, etc.). THOMPSON
& PARKER See the reason in the low vitality of the larvae. All
these explanations are, unfortunately, only hypotheses. The influence of
physical factors has never been demonstrated. The meteorological conditions of
the months July and August have been entirely different in 1927 and 1928, the
weather being cold and humid in 1927 and abnormallv, warm and dry in 1928.
Nevertheless I have not been able to. find any difference in the mortality of
the young Corn Borer larvae in these two years.
OTHER
FACTORS DESTROYING THE CORN BORER
The
destruction of the old corn stalks is the most important factor limiting the
spread of the Corn Borer. This factor is more important than the action of all
parasites and predators taken together. The Corn Borers in the few corn stalks
which are not used as live stock feed,. or destroyed, are those which propagate
the pest. The utilization of the corn stalks very definitely influences the
equilibrium of the insect.
ROUBAUD
has called attention to the fact that the common mugwort (Artemisia
vulgaris) in the environs of Paris is preferred by the Corn Borers as host
plant, and that this wild plant detracts the Borer from the corn fields. Mugwort
rarely occurs in the corn region of the Jura and the Saone valley, and in the
region of Lyon, where the plant is abundant along the country roads, I have not
observed any infestation by Pyrausta nubilalis.