THE COFFEE BUG. Lecanium Coffeæ.
Until she has nearly reached her full size, she still possesses the power of locomotion, and her six legs are easily distinguishable in the under surface of her corpulent body; but at no period of her existence has she wings. It is about the time of her obtaining full size that impregnation takes place[4382]; after which the scale becomes somewhat more conical, assumes a darker colour, and at length is permanently fixed to the surface of the plant, by means of a cottony substance interposed between it and the vegetable cuticle to which it adheres. The scale, when full grown, exactly resembles in miniature the hat of a Cornish miner[4391], there being a narrow rim at the base, which gives increased surface of attachment. It is about 1/8 inch in diameter, by about 1/12 deep, and it appears perfectly smooth to the naked eye; but it is in reality studded over with a multitude of very minute warts, giving it a dotted appearance. Except the margin, which is ciliated, it is entirely destitute of hairs. The number of eggs contained in one of the scales is enormous, amounting in a single one to 691. The eggs are of an oblong shape, of a pale flesh colour, and perfectly smooth.[4392] In some of the scales, the eggs when laid on the field of the microscope resemble those masses of life sometimes seen in decayed cheese.[4393] A few small yellowish maggots are sometimes found with them, and these are the larvæ[4394] of insects, the eggs of which have been deposited in the female while the scale was soft. They escape when mature by cutting a small round hole in the dorsum of the scale.
It is not till after this pest has been on an estate for two or three years that it shows itself to an alarming extent. During the first year a few only of the ripe scales are seen scattered over the bushes, generally on the younger shoots; but that year's crop does not suffer much, and the appearance of the tree is little altered.
The second year, however, brings a change for the worse; if the young shoots and the underside of the leaves he now examined, the scales will be found to have become much more numerous, and with them appear a multitude of white specks, which are the young scales in a more or less forward state. The clusters of berries now assume a black sooty look, and a great number of them fall off before coming to maturity; the general health of the tree also begins to fail, and it acquires a blighted appearance. A loss of crop is this year sustained, but to no great extent.
The third year brings about a more serious change, the whole plant acquires a black hue, appearing as if soot had been thrown over it in great quantities; this is caused by the growth of a parasitic fungus[4401] over the shoots and the upper surface of the leaves, forming a fibrous coating, somewhat resembling velvet or felt. This never makes its appearance till the insect has been a considerable time on the bush, and probably owes its existence there to an unhealthy condition of the juices of the leaf, consequent on the irritation produced by the coccus, since it never visits the upper surface of the leaf until the latter has fully established itself on the lower. At this period the young shoots have an exceedingly disgusting look from the dense mass of yellow pustular bodies forming on them, the leaves get shrivelled, and the infected trees become conspicuous in the row. The black ants are assiduous in their visits to them. Two-thirds of the crop is lost, and on many trees not a single berry forms.
This Lecanium, or a very closely allied species, has been observed in the Botanic Garden at Peradenia, on the Citrus acida, Psidium pomiferum, Myrtus Zeylanica, Rosa Indica, Careya arborea, Vitex Negundo, and other plants. The coffee coccus has generally been first observed in moist, hollow places sheltered from the wind; and thence it has spread itself even over the driest and most exposed parts of the island. On some estates, after attaining a maximum, it has generally declined, but has shown a liability to reappear, especially in low sheltered situations, and it is believed to prevail most extensively in wet seasons. While in its earlier stages, it is easily transmitted from one estate to another, on the clothes of human beings, and in various other ways, which will readily suggest themselves. Dr. Gardner, after a careful consideration and minute examination of estates, arrived at the conclusion, that all remedies suggested up to that time had utterly failed, and that none at once cheap and effectual was likely to be discovered. He seems also to have been of opinion that the insect was not under human control; and that even if it should disappear, it would only be when it should have worn itself out as other blighte have been known to do in some mysterious way. Whether this may prove to be the case or not, is still very uncertain, but every thing observed by Dr. Gardner tends to indicate the permanency of the pest.