This section is from the book "The Florist And Garden Miscellany". Also see: All New Square Foot Gardening: Grow More in Less Space!.
We are indebted to the Editor of the Gardeners' Chronicle for the following article, extracted from that popular journal, and also for the use of the accompanying woodcut. The subject is one of no common interest, and opens a wide field for observation.
"If our readers will take the trouble to refer back to our volume for 1848, p. 2.54, they will observe an article entitled the ' Florist and the Rose-maggot,' requesting our particular attention to the history of this destructive insect, with reference to the observations then recently published in the Florist, and extracted then into our columns of the same year, p. 191, announcing the discovery, in little cylindrical burrows in the dead snags of Rose-bushes, of the winter-quarters of a small black-headed maggot, which was therein asserted to be the young state of the Rose-maggot (a term applied, by the way, to the larvee of any of the numerous small species of moths which burrow into the Rose-bud and eat out its heart). In a subsequent page (299) a woodcut is given of a Rose-snag cut open, shewing a number of cells, each of which is said to have been the birthplace of these destructive larvae.
Since the publication of these several notices, our attention has been directed to this subject; and the result of our examinations renders it necessary for us to advise the gardener not to cut off the dead ends of these snags; since, although in a very few instances we have met with the little black-headed maggot in question, yet in the majority of cases we have found that the burrows are inhabited, and, in fact, formed by a different insect, whose economy is in the highest degree singular, and which deserves the protection of the gardener, from the benefit which it confers upon him by destroying great numbers of his enemies, the plant-lice. But this is not the whole of the result of our examinations, for we have thereby picked up some facts in regard to the natural history of four or five other species of insects inhabiting the same situation, the economy of which has hitherto remained unknown, or has baffled the researches of entomologists; so that we may safely affirm that these Rose-snags offer a mine of interesting observation to any person disposed to regard such investigations in their proper light.
Cemonus unicolor, magnified and of the natural size, carrying an aphis in its mouth; together with a Rose snag cut open, shewing a provisioned cell, a cell with the full-grown larva, and one with the insect in its cocoon.
In the present article we shall confine ourselves to the species which, from it:-carious economy, may be aptly termed the Aphis Sexton of the Rose-snags. On the 20th of June, 1848, whilst looking over some standard Roses, to examine the state of the dead ends, which had been cut off obliquely, we observed a little glossy black fly, full of activity, alight on the top of one of the twigs, and as suddenly creep into one of the little circular burrows which we had observed formed in the soft pithy part of the twig. Our attention was immediately roused, not only by the action of the insect itself, but also from the fact that it carried some small green object in its mouth. It soon flew out of the hole, and after patiently Matching the stem, we saw it again approach, but more carefully, in consequence of our being bo close to its burrow. We had, therefore, an opportunity of observing that its load was a green aphis, which it at length took into its hole, as it had done with the former.
This observation let us into the whole secret of the manufacture of the burrows, the insect by which, and the object for which, they were made. Our little black fly was one of the small burrowing wood-wasps; and it made these holes for the purpose of fitting up a cell for the abode of its progeny still unborn; the food of which, when hatched, was to consist of the store of aphides which it was then engaged in burying. Such is the course of proceeding of many species of sand and wood-wasps, whose economy has been often observed, and is well known; but this species presented several circumstances worthy of notice. The aphis was carried by the wasp in its jaws; whereas many of the allied species make use either of their fore, middle, or hind legs to clasp their prey, but here it is so weak and so light in weight, that the wasp is able to carry it without the assistance of its legs. Several aphides are thus deposited in the little oval cell previously formed at the bottom of the burrow, together with an egg, and then a covering is fixed over the whole; another cell is then filled, which is, of course, nearer to the top of the twig than the first, and so on, several cells are thus successively formed and filled.
It is important to notice that the imprisoned aphides are not killed by the parent wasp (in which case they would become putrid), but that the young larvae, when hatched, have a store of fresh food; they do not, however, keep their victims long in suspense, as they are quickly hatched, and are very voracious; indeed, by the end of July or August, they have attained their full size, and entirely consumed their stock of food, which the parent fly has the instinct to apportion to their entire wants; but what is sufficiently curious, although they are full-grown at this early period (when the heat of the weather is sufficient to cause them quickly to undergo their transformations), yet they remain unchanged as larvae all through the winter in the shape of small yellow footless grubs, enclosed in a shining silken kind of case, becoming pupae in the spring, and assuming the perfect winged state about the end of May. We have reared the same insect in burrows formed in the pith of elder-sticks stuck in our garden; and Mr. F. Smith states that it also burrows into bramble-sticks, depositing small green caterpillars in its cells.
This wasp, which belongs to the Hymenopterous family of Crabronids, is named, from its uniform black colour, Cemonus unicolor. It varies from rather less than a quarter to more than a third of an inch in length; it is slightly covered with fine silvery down on the head and thorax, and has the abdomen glossy, finely punctured, and affixed to the thorax by a curved footstalk scarcely longer than the remainder of the first segment. The male differs from the female in having more of the silvery down on the face. It is one of our commonest of insects. - J. O. W".
In availing ourselves of the kind permission of the editor of the Gardeners' Chronicle to introduce the interesting article at page 43, it must not be understood that we subscribe to the advice tendered to the gardener, "not to cut off the dead end of these snags." We repeat the advice originally given by us, to remove them all; for we engage that, for every Aphis Sexton found in these snags, there shall be found fifty caterpillars like the accompanying figure, reprinted from our Vol. 1848, and which are the very worst enemies the Rose-bud has. Nevertheless, J. O. W.'s article is a most interesting one, giving additional evidence of the truth of the observation of Dean Swift:
"So fleas have lesser fleas to bite 'em, And so go on ad infinitum.""
But there is one plan which we strongly recommend all our readers to adopt in this and similar cases, and that is to examine for themselves. Take nothing for granted; go to your Rose-bushes with our sketch in hand, and you will soon find what is indicated; and if you will only take a snag or two, and allow the maggots to emerge in a wafer-box covered with a piece of open muslin, and feed them afterwards on fresh Rose-shoots, you will be gratified with the result.
1. Shewing a snag on the shorter shoot to the right.
2. A similar one cut off and magnified.
3. The same split down, and the maggot exposed from its gallery in the pith.