This section is from the book "The Gardener's Monthly And Horticulturist V21", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The rose is undoubtedly beset by a greater variety of insect enemies than any other garden shrub. Among these pests the well known and wide-spread " slug " has a "bad eminence." This insect was first described and named by Dr. Harris, in his Insects Injurious to Vegetation, in which account the statements concerning it are mainly correct, but the Doctor omits certain interesting particulars in its history and makes one serious mistake.
With us, in the latitude of central Missouri, the parent flies appear as early as the first of May, when they may be observed in considerable numbers on the rose bushes, where their sluggish habits admit of their easy capture. They are about one-fifth of an inch in length, of a glossy black color, and have the wings closely folded when at rest. The females soon begin the process of oviposition. With their saw-like ovipositors they pierce the edges of the leaves and force their eggs toward the tips of the serrations under the cuticle on the under side. The egg is circular, about one-twentieth of an inch in diameter, and so flat at first as to be imperceptible except upon the closest scrutiny. It hatches in eight or nine days, and before the larva escapes, swells considerably, appearing like a minute blister on the under side of the leaf, within which the coiled embryo is distinctly visible. The young slug upon emerging is one-tenth of an inch in length, and about the diameter of No. 30 spool cotton, the round, tawny-yellow head being the broadest part. The color is greenish white with a dark-green vesicular line as soon as it begins to feed.
When full grown it is rather more than one-third of an inch long, broadest at the thoracie joints, the color being a translucent dull yellow, shading to green on the back. It is not in the least slimy, as are some closely allied species, but on the contrary has a velvety surface. It feeds at night upon the green tissue of the upper surface of the leaf, and rests during the day hidden upon the under side. It attains its growth in fourteen or fifteen <lays, molting meanwhile four times. After the last molt it ceases to feed, acquires a more opaque color, and soon drops or crawls to the ground into which it burrows to the depth of an inch or two, and encloses itself in a frail", oval cell formed from particles of earth cemented with a viscid excretion. "The slugs having finished their transformations," Harris proceeds to say, " and changed to flies within their cells, they come out of the ground early in August, and lay their eggs for a second brood of young. These in turn perform their appointed work of destruction in the Autumn; they then go into the ground, make their earthen cells, remain therein throughout the Winter, and appear in the winged form in the following Spring and Summer".
This is the point where Dr. Harris is in error, unless the rose slug of Massachusetts is a different species from the one from which we suffer, which, as the two insects seem to agree in all other particulars is scarcely supposable. Having watched the insect through its transformations for several successive years, I am convinced that it is not double-brooded with us, and as our season is much longer than Summer in Massachusetts, it stands to reason that it is not double-brooded in more northern latitudes.
Dr. Harris, in making this statement probably reasoned from analogy, as the cherry slug and several other closely allied species are double-brooded; but it is not always safe to reason thus in the case of insects, as there is often great diversity of habit among species nearly related.
As Dr. Harris's mistake has been followed by all the subsequent writers on the subject, it occurred to me that it would only be the part of kindness to rose culturists to undeceive them, or rather to reassure them on this point. There is an adage to the effect that " it is not necessary to paint a certain personage blacker than he is," which holds good in the present case. It is certainly bad enough to have to contend with one brood of this destructive pest, without the discouraging information that almost as soon as the Spring brood disappears the Autumn brood will hatch. Therefore, let all who entertain such fears take heart. If the slugs can be kept from blighting the foliage during the months of May and June, no further trouble need be apprehended from them until the following year, as they remain unchanged within their cells for more than ten months. Although the individual larva feeds only for about two weeks, yet as the flies live and continue to lay their eggs for some time, the slug season lasts for nearly a month, and if the insects are neglected at the end of that time the foliage of the infested plants, with the green tissue eaten in large irregular patches from the upper surface, will appear as though scorched with fire.
Bourbon, Tea, and other perpetual roses naturally suffer most in this ordeal.
The rose slug has a number of natural enemies, such as ichneumon parasites, lady-bird larva?, cannibal bugs, spiders, and the like, but none of these are, as yet, equal to the task of keeping it sufficiently in check.
The most thoroughly effectual remedy is whale-oil soap, in the proportions of one pound of soap to eight gallons of water. This should be applied at night, the plants being thoroughly drenched. Three applications, at intervals of two or three days will almost, if not quite exterminate the pest. Powdered White Hellebore and the Persian Insect Powder dusted on the plants while the dew is on them are also excellent remedies.