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THE PLUM PLANT-LOUSE.
(Myzus mahaleb Fonsc.)
By THEODORE PERGANDE. Several species of plant-lice occur upon the plum, and one of these is so much like Phorodon humuli, the hop plant-louse, in certain stages of its development that without great care the observer will be led astray. Its habits are different, and it never migrates to the hop. This species is Myzus mahaleb Fonscolombe. It was formerly considered as a true Phorodon, and is quite generally mentioned in literature under this genus. On account of the confusion liable to arise, we may devote some little space to a consideration of this species. The generic characters separating Myzus from Phorodon are as follows: Frontal tubercles prolonged at inner angle into a prominent, slender, por
rected tooth; first antennal joint bluntly but distinctly gibbous......Phorodon. Frontal tubercles gibbous at inner angle; first antennal joint without a tooth.....
Myzus. In P. humuli the inner apical angle of the frontal tubercles is prolonged into a rather long and slender branch, reaching in the apterous females, larvae, and pupæ, except those of the first generation, to about the apex of the first antennal joint, being somewhat shorter, though still slender, in the winged form, and having a distinct, stout, and blunt tubercle near the inner apical angle of the first antennal joint. In M. mahaleb, however, the inner prolongation of the frontal tubercles is rather insignificant, stout, and rounded, and the tooth of the first joint is wanting in all generations. The only generation in which both spe. cies are difficult to be separated is the first, in which the frontal tubercles in both species are scarcely indicated, whereas in the last or sexual generation the females of both may be separated without difficulty.
The synonymy of the species is as follows: Myzus mahaleb Fonsc. Aphis pruni-mahaleb Fonscolombe, Ann. Soc. Ent. de France, X, 1841, p. 175. Aphis mahaleb Koch, l’tlanzenlänse, 1857, p. 113. Myzus mahaleb Passerini, Aphidida Italicæ, 1863, p. 26. Phorodon humuli var, malaheb Buckton, British Aphides, 1876, p. 168. Phorodon humuli var, mahaleb, Thomas, Vox, and Benef. Ins. Ill., 1878, p. 72. Siphonophora achyrantes Monell (?), Nox, and Benef. Ins. Ill., 1878, p. 187. Regarding the synonomy of this species much uncertainty has previ. ously existed. Notwithstanding that Fonscolombe and Koch have indicated its specific characters and Passeriui its generic position, it is still considered by some authors as but a variety of Phorodon humuli, simply, as it appears, on account of its occurrence on plum in company with humuli and on account of the general similarity in color and markings.
During 1886 and 1887, its life history was carefully studied by the writer, and its identity with the species described under this name by Fonscolombe, Koch, and Passerini affirmed. That considerable difficulty has been experienced in this task goes without saying.
FOOD PLANTS OF THE SPECIES COMPARED.
Notwithstanding the great superficial similarity of the two species, the life habits of both during the course of a season are quite dissimilar. Whereas Phorodon humuli subsists, as far as known, exclusively upon the different varieties of plum and hop and produces but one return migrating brood in fall, mahaleb, from the time that its migratory form leaves the plum, is never found on the hop, but is able to accommodate itself through succeeding generations to the peculiarities of a considerable number of quite dissimilar plants, which are in no way
а related to each other nor to the plum or the hop. The diversity of habit of the species after leaving the plum or peach is well illustrated by the fact that its progeny will thrive equally well upon the pear, sunflower, Rumex, kohlrabi, chrysanthemum, shepherd's purse, Portulaca, and a number of other plants in the conservatory of the Department of Agriculture, among which the following ones appear to be most subject to its attacks: Rhamnus, Mallotus, Rhus, Alfredia, and a species of tobacco, on most of which plants it multiplies with wonderful rapidity. Considering the great variety of plants which it inhabits, it will not be strange if in the future it shall be found, after more careful observations, both in this country and Europe, that quite a number of described species are identical with mahaleb, and we believe that we are justitied in referring to this species the form described by Mr. Monell under the name of Siphonophora achyrantes (8th Rep. Nox. and Benef. Ins. Ill., 1879, p. 187).
Koch's description of mahaleb is quite comprehensive, and can not be mistaken for humuli. If Koch had not seen and examined both species at the same time as found upon the plum, he would certainly not have expressed himself in the words he used in his description, that, notwithstanding its close resemblance to humuli, it differs markedly from that species in the absence of the tooth at the inner apical angle of the first antennal joint, as well as in other minor characters.
No one who has studied plant-lice carefully, or who has had occasion to study these two species simultaneously, will entertain a doubt as to the correctness of Koch's observation. The characters of the frontal tubercles and of the first antennal joint, which separate the two species, are so obvious and of such constancy through all generations, except perhaps in the first, that it would at least be hasty to consider them as varieties. There can be no doubt whatever that the two species are in no way specifically related, and that the true position of mahaleb is where Passerini correctly placed it, in the genus Myzus, whereas humuli, on account of its frontal and antennal characters, belongs to Phorodon.
LIFE HISTORY OF MYZUS MAHALEB.
The following account is based mainly upon observations made at Washington, D. C.:
The hatching period of the eggs in this latitude ranges from about the 20th of March to the first week in April, being either accelerated or retarded according as the weather is favorable or otherwise, but always at the time when the sap commences to rise and the buds to swell. This period is the most critical in the life history of the species, as numerous individuals will perish from cold and wet if weather changes occur suddenly and frequently, for their ranks may be depleted to such an extent that perhaps not more than 5 or 10 out of 1,000 may succeed in reaching maturity.
The hatching commences generally about the 15th or 20th of March, so that in a few days, if the weather be favorable, there appear hundreds of young larvae on all the twigs and buds. Cold showers and frost, however, during the following days reduce their numbers to such an extent that few can be found. Those which survive attain maturity about the 10th of April-about twenty days or a little more after hatching--and commence at once to deposit their larvæ, increasing in number quite rapidly as it becomes warmer and as the leaves expand. Before acquiring full growth the stem-mother casts five skins, changing but little in general appearance, except in size, in the number and length of the antennal joints, and in the length and shape of the nectaries. The antenna of the mature stem-mother are six-jointed.
The second generation grows more rapidly than the first, so that many are full-grown before the 25th of April, when they have assumed the general characters of the species, which they retain with but slight alterations through all following generations. The antennæ are now seven-jointed and the nectaries long and slender. This generation, as a rule, is apterous, like the preceding generation, though occasionally there develop in one or the other colony one or a few winged individuals. These winged specimens are very shy and drop and fly off at the slightest jar. This early appearance of a few winged specimens is an evident provision of nature to preserve the species from destruction in case a third or genuine migratory generation should fail to reach maturity. The lice now increase quite rapidly in numbers, and station themselves along the midrib of the young leaves, causing them to twist and curl into different shapes.
The third generation grows still more rapidly, so that many of the oldest individuals are already winged by or before the 5th of May, while the great bulk reached the winged stage between the 9th and 20th of the month. In this generation, as in the preceding, a few individ. uals remain apterous and produce a fourth generation, most of which become winged toward the end of the month or early in June. Some of the winged fourth generation deposit a few larvæ on the tree on which they were born before leaving in search of other suitable plants.
On warm days during the swarming period of the third generation the air is often filled toward evening in the neighborhood of the infested trees with the winged individuals searching for suitable plants upon which to settle.
The few remaining apterous females of the third generation have now a hard struggle to escape their enemies, which by this time have become extremely numerous, so much so that very few escape to establish colonies of the fourth generation. Still fewer escape to form the fifth. Some of both the fourth and fifth become winged. Natural enemies or a prevalent fungous disease destroy so many that the last few stragglers may be seen on the trees early in June, and this appears to be the extreme limit of the spring generations on the peach or plum, at least in the vicinity of Washington.
In the meantime, however, the migrating forms have been able, as stated before, to establish and distribute their progeny on a number of different plants over a wide area of country, often miles away from any peach or plum trees, raising immense colonies, many individuals of which become again winged from time to time, till the raids of their enemies reduce them more and more, so that from the 10th to the 20th of July hardly an individual is to be found on any of the infested plants. They apparently disappear completely, though there can be no doubt that they still exist in one form or another either above or below the surface of the ground on different kinds of plants. No trace of their whereabouts has been found so far.
After an intermission of four or five months, after leaving the peach and plum, the winged female return migrants suddenly make their appearance again on the trees, from about the first of October till toward the middle of November, so that at times all the leaves and twigs are literally covered with them and with their larvæ. All the larvæ which are now produced belong to the true sexual generation, and are composed exclusively of apterous females, which, however, differ considerably in appearance from all other apterous generations, the oldest of which reach full maturity by the end of the month or the first of November, by and after which time, for about two or three weeks, a large number of migrants appear. These are almost entirely males, which at once commence to pair with the females, after which, in about a day, the latter deposit their eggs, to the number of from 5 to 8, on buds, twigs, and trunk. The plants from which these return migrants come are in part those on which they settled in spring after deserting the plum and peach. There seems to be, however, some influence at work which prevents many from acquiring wings and forces them to remain where they were born. These last, one would
naturally suppose, would gradually succumb and die during winter, but this does not appear to be the case. Repeated examinations of certain plants from the time the last migrants appeared on the trees till some time in February the following year, both outdoors and in hothouses, even after several severe frosts and snowstorms, showed the lice on all the plants in large numbers in every stage of development, and among them often many winged ones as lively and active as though the weather had been most agreeable to them.
It may be well to state, however, that the only season in which this observation has been made was that of 1888–89, in which the winter was a remarkably open one. The lice have not been searched for during other and more severe winters.
DESCRIPTIONS OF THE DIFFERENT STAGES.
FIRST GENERATION (STEM-MOTHER).
First stage.-Length of recently hatched larva, about 0.6 mm.; color almost black, with a slight greenish tinge; antennæ five-jointed, the fifth somewhat longer than the third; joints 1, 2, and 4 shortest and subequal in length; frontal tubercles wanting; legs very stout, nectaries about as long as thick, and apparently somewhat shorter than the second antennal joint.
Second stage.—Length, about 0.8 mm.; color darker or paler greenish-gray, with a slightly darker medio-dorsal line; head and all members darkest; the head somewhat polished, with a distinct and quite deep fovea on the vertex; antenna still five-jointed, the third joint as long or somewhat longer than the two last together, with an indication of a division into two joints; frontal tubercles still wanting; rostrum shorter than the body; nectaries slightly longer than thick, with a constriction or annulus at extreme tip.
Third stage.—Length, about 1 mm.; color either greenish-gray, greenish, or greenish-yellow, medio-dorsal line darkost, while the lateral margin and end of body are often slightly ferruginous; the dorsum is also frequently marked with faint but darker irregular green mottlings; head darker or lighter dusky, polished, and with a paler median line; antennæ six-jointed, dusky, joint 3 palest at base; the third and sixth longest, about equal, or the sixth is slightly longest; the fourth and fifth subequal, each rather less than half the length of the sixth; eyes almost black;
egs still stout, though more slender than in previous stages; femora and tibiæ somewhat dusky toward the end; tarsi blackish; body almost oval, broadest at the middle of the abdomen; nectaries at least twice as long as thick, reaching to tip of abdomen, slightly curved, and somewhat thinnest a short distance above base; apex dusky, the constriction as before.
Fourth stage.—Length, 1.4 mm.; general color, greenish-yellow, marked more or less distinctly on the head, dorsum, and lateral margin with crimson or pinkish spots; head faintly concave in front, with a slight median swelling; antennä very similar in the proportions of their joints to those of the previous stage, though longer and piore slender, greenish, with the tip of joints 3 and 4 and the two last joints blackish; eyes brown; rostrum rather short, reaching to about the median legs; legs of color of body, with about their terminal third dusky; tarsi blackish; body oval, tapering to a point posteriorly, and distinctly filled with almost fully developed pseudova; nectaries rather slender, slightly curved, almost of equal thickness, slightly stoutest at base, four to five times longer than thick, faintly dusky, darkest at tip; tail not yet present.
Fifth stage.—Length, 1.6 to 1.8 min.; general, color pale greenish-yellow, with a more or less distinct darker green medio-dorsal line, marked along the lateral border