Let us begin with the produce of our fields.—Bread is called "the staff of life:" yet should divine Providence in anger be pleased to give the rein to the various insects which, in the different stages of its growth, attack the plant producing it, how quickly would this staff be broken! From the moment that wheat begins to emerge from the soil, to the time when it is carried into the barn, it is exposed to their ravages. One of its earliest assailants in this country is that of which Mr. Walford has given an account in the Linnean Transactions, taking it for the wire-worm; but, as Mr. Marsham observed, not correctly; it being probably the larva of some coleopterous insect, perhaps of one of the numerous tribe of Brachyptera or rove-beetles which are not universally carnivorous. This animal was discovered to infest the wheat in its earliest stage of growth after vegetation had commenced; and there was reason to believe that it began even with the grain itself. It eats into the young plant about an inch below the surface, devouring the central part; and thus, vegetation being stopped, it dies. Out of fifty acres sown with this grain in 1802, ten had been destroyed by the grub in question so early as October[270].—Other predaceous Coleoptera will also attack young corn. This is done by the larva of Zabrus gibbus, particularly with respect to wheat. In the spring of 1813 not less than twelve German hides (Hufen), equal to two hundred and thirty English acres, were destroyed by it in the canton of Seeburg, near Halle in Germany; and Germar (who with other members of the Society of Natural History, at that place, ascertained the fact,) suspects that it was the same insect, described by Cooti, an Italian author, which caused great destruction in Upper Italy in 1776.—Not only is the larva, which probably lives in that state three years, thus injurious; but, what one would not have expected, the perfect beetle itself attacks the grain when in the ear, clambering up the stems at night in vast numbers to get at it.—Along with the larvæ of this insect were found, in the proportion of about one fourth, those of another beetle (Melolontha ruficornis), which seemed to contribute to the mischief[271].
Mr. Markwick has given us the history of a fly that attacks wheat in a later period of its growth, which, if it be not indeed the same, appears to be nearly related to the Musca Pumilionis of Bierkander[272], (Oscinis F.) accused by him of being extremely injurious to rye in the spring. Our insect was discovered on the first sown wheats early in that season, making its lodgement in the very heart of the principal stem just above the root, which stem it invariably destroyed, giving the crop at first a most unpromising appearance, so that there seemed scarcely a hope of any produce. But it proved in this and other instances that year (1791) that the plant, instead of being injured, derived great benefit from this circumstance; for, the main stem perishing, the root (which was not hurt) threw out fresh shoots on every side, so as to yield a more abundant crop than in other fields where the insect had not been busy. These flies therefore seem to belong to our insect benefactors; and I should not have introduced them here, had it not been probable that in some instances later in the spring they may attack the lateral shoots of the wheat, and so be injurious. It is also not unlikely that the new progeny, which is disclosed in May, may oviposit in barley or some other spring corn, which would bring the next generation out in time for the wheat sown in the autumn.—These flies are amongst the last, and, in some seasons, the most numerous, that take shelter in the windows of our apartments when the first frosts indicate the approach of winter, previous to their becoming torpid during that season. When this little animal was first observed in England, it created no small alarm amongst agriculturists lest it should prove to be the Hessian fly, so notorious for its depredations in North America; but Mr. Marsham, by tracing out the species, proved the alarm to be unfounded[273]. That there was sufficient cause for apprehension should it have so turned out, what I have formerly stated concerning the latter insect, and the additional facts which I shall now adduce, will amply show.
The ravages of the animal just alluded to, which was first noticed in 1776, and received its name from an erroneous idea that it was carried by the Hessian troops in their straw from Germany, were at one time so universal as to threaten, where it appeared, the total abolition of the culture of wheat; though, by recent accounts, the injury which it now occasions is much less than at first. It commences its depredations in autumn, as soon as the plant begins to appear above ground, when it devours the leaf and stem with equal voracity until stopped by the frost. When the return of spring brings a milder temperature the fly appears again, and deposits its eggs in the heart of the main stems, which it perforates and so weakens, that when the ear begins to grow heavy, and is about to go into the milky state, they break down and perish. All the crops, as far as it extended its flight, fell before this ravager. It first showed itself in Long Island, from whence it proceeded inland at about the rate of fifteen or twenty miles annually, and by the year 1789 had reached 200 miles from its original station. I must observe, however, that some accounts state its progress at first to have been very slow, at the rate only of seven miles per annum, and the damage inconsiderable; and that the wheat crops were not materially injured by it before the year 1788. Though these insect hordes traverse such a tract of country in the course of the year, their flights are not more than five or six feet at a time. Nothing intercepts them in their destructive career, neither mountains nor the broadest rivers. They were seen to cross the Delaware like a cloud. The numbers of this fly were so great, that in wheat-harvest the houses swarmed with them to the extreme annoyance of the inhabitants. They filled every plate or vessel that was in use; and five hundred were counted in a single glass tumbler exposed to them a few minutes with a little beer in it[274].
America suffers also in its wheat and maize from the attack of an insect of a different order; which, for what reason I know not, is called the chintz-bug-fly. It appears to be apterous, and is said in scent and colour to resemble the bed-bug. They travel in immense columns from field to field, like locusts destroying every thing as they proceed: but their injuries are confined to the states south of the 40th degree of north latitude[275]. From this account the depredator here noticed should belong to the tribe of Geocorisæ, Latr.; but it seems very difficult to conceive how an insect that lives by suction, and has no mandibles, could destroy these plants so totally.
When the wheat blossoms, another marauder, to which Mr. Marsham first called the attention of the public, takes its turn to make an attack upon it, under the form of an orange-coloured gnat, which introducing its long retractile ovipositor into the centre of the corolla, there deposits its eggs. These being hatched, the larvæ, perhaps by eating the pollen, prevent the impregnation of the grain, and so in some seasons destroy the twentieth part of the crop[276].
One would think, when laid up in the barn or in the granary, that wheat would be secure from injury; but even there the weevil (Calandra granaria), in its imago as well as in its larva state, devours it; and sometimes this pest becomes so infinitely numerous, that a sensible man, engaged in the brewing trade, once told me, speaking perhaps rather hyperbolically, that they collected and destroyed them by bushels; and no wonder, for a single pair of these destroyers may produce in one year above 6000 descendants.—There are three other insects that attack the stored wheat, which are more injurious to it than even the weevil. One is a minute species of moth (Tinea granella), happily not much if at all known in this country; of which Leeuwenhoek has given us a full history under the name of the wolf. Another is a species of the same genus, at present not named, which, as we are informed by Du Hamel, at one time committed dreadful ravages in the province of Angoumois in France. The third is Trogosita caraboides, a kind of beetle, the grub of which called Cadelle, Olivier tells us, did more damage to the housed grain in the southern provinces of France than either the weevil or the wolf[277].—Here I may just mention a few other insects which devour grains that are the food of man, concerning which I have collected no other facts. The rice-weevil (Calandra Oryzæ) is very injurious to the useful grain after which it is named, as is likewise another small beetle, Lyctus dentatus, F. (Sylvanus, Latr.): and an Indian grain called in the country Joharré, which appears to be a species of Holcus or Milium, is the appropriate food of another species of Calandra[278], which I found abundant in it.
Rye, in this island, is an article of less importance than wheat; but in some parts of the continent it forms a principal portion of the bread-corn. Providence has also appointed the insect means of causing a scarcity of this species of food. The fly before noticed (Oscinis Pumilionis) introduces its eggs into the heart of the shoots of rye, and occasions so many to perish, that from eight to fourteen are lost in a square of two feet.—A small moth also (Margaritia Secalis) which eats the culm of this plant within the vagina, thus destroys many ears[279]. In common with wheat and barley it also suffers from Leeuwenhoek's wolf and the weevil.
Barley likewise, another of our most valuable grains, has several insect foes. The gelatinous larva of a saw-fly (Tenthredo, L.) preys upon the upper surface of the leaves, and so occasions them to wither. Musca Hordei of Bierkander also assails the plant. A tenth part of the produce of this grain, Linné affirms, is annually destroyed in Sweden by another fly, not yet discovered in Britain, (Oscinis Frit,) which does the mischief by getting into the ear, as does likewise O. lineata, F.—A small species of moth described by Reaumur, though not named by Linné, which may be called Tinea Hordei, (Ypsolophus granellus?) devours the grain when laid up in the granary. This fly deposits several eggs, perhaps twenty or thirty, on a single grain; but as one grain only is to be the portion of one larva, they disperse when hatched, each selecting one for itself, which it enters from without at a place more tender than the rest;—and this single grain furnishes a sufficient supply of food to support the caterpillar till it is ready to assume the pupa. Concealed within this contracted habitation, the little animal does nothing that may betray it to the watchful eye of man, not even ejecting its excrements from its habitation; so that there may be millions within a heap of corn, where you would not suspect there was one[280].
I have not observed that oats suffer from insects, except from the universal subterranean destroyer of the grasses, the wire-worm, of which I shall give you a more full account hereafter; and occasionally from an Aphis. The only important grain that now remains unnoticed is the maize or Indian corn. Besides the chintz-bug-fly, a little beetle[281] (Phaleria cornuta) appears to devour it; and it has probably other unrecorded enemies. The Guinea corn of America (Holcus bicolor), as well as other kinds of grain, is, according to Abbott, often much injured by the larva of a moth (Noctua frugiperda, Smith), which feeds upon the main shoot[282].