Popular Science Monthly/Volume 7/October 1875/The Colorado Potato-Beetle
By Prof. C. V. RILEY.
FEW insects have done more serious injury, or attracted greater attention, than this, even in America, where insect depredations attain a magnitude scarcely dreamed of in this country. Feeding originally on the wild Solanumr ostratum in the Rocky Mountain regions of Colorado and other Territories, it fell upon the cultivated potato as soon as civilized man began to grow this esculent within its reach. With large fields of palatable food, instead of scattered plants of the wild Solanum to work upon, it multiplied at a marvelous rate, and began to spread from its native home toward the East. Reaching a point 100 miles west of Omaha, Nebraska, in 1859, its progress has been carefully recorded each year since, until last year it reached the Atlantic coast at a number of different points in Connecticut, New Jersey, New York, Pennsylvania, Delaware, Maryland, and Virginia. The present year we hear of it being still more numerous on the Atlantic coast, and of its swarming around New York City, and covering the nets of fishermen. It has thus, in sixteen years, spread over 360 geographical miles, in a direct line; and, if we consider the territory
Fig. 1.—Colorado Potato-Beetle
a, a, Eggs; b, b, b, larvae of different sizes; c, pupa; d, d, beetle; e, left-wing: cover magnified to show lines and punctures; f, leg enlarged. Colors: of egg, orange; of larvæ, Venetian-red; of beetle, black and yellow.
actually invaded, which includes the States of Kansas, Nebraska, Missouri, Iowa, Minnesota, Wisconsin, Illinois, Indiana, Kentucky, Michigan, Ohio, Ontario (Canada), New York, Vermont, Massachusetts, Pennsylvania, Maryland, and Virginia, it has overrun an area of 800,000 square miles. The natural history of the species was first made known by me in 1863, The beetle hibernates either beneath the ground or beneath any other shelter that it can obtain. Early in spring it issues from its winter quarters, and may be seen flying about, on sunny days, long before there are any potato-tops for it to devour. In flight it presents a very pretty appearance, its gauzy, rose-colored under-wings contrasting agreeably with the striped yellow and black elytra or wing-covers. The sexes pair, and, as soon as the potato haulms push out of the ground, these beetles break their long winter fast, sometimes even working their way down toward the sprout before it is fairly out of the earth. The eggs, which are orange-yellow, are laid in small clusters on the under sides of the leaves, and the same female continues to thus lay at short intervals for a period of over forty days, until the number laid by a single specimen may aggregate from 500 to over 1,000. There are, in the latitude of St. Louis, three broods each year; but, from the fact that a single female continues to deposit as above described, and from the irregularity of larval development, the insect may be found in all stages throughout the summer months. In from thirty to forty days from the time the egg is deposited, the insect hatching from it goes through all its transformations and become a beetle, the pupa state being assumed under-ground. The prolificacy of the species may be imagined when it is remembered that the progeny of a single female may exceed a hundred millions in the course of a single season! The beetle feeds as well as the larva, though not so voraciously. Its attacks are principally confined to plants of the family Solanaceæ and it is particularly fond of those belonging to the genus Solanum. Yet I have recorded many
instances of its acquiring new habits in its march to the Atlantic, and of its feeding, when hard pushed, on plants of other families. There are various means of destroying the insect, and in the earlier invaded territory of the States, though it continues its ravages, thereby making the cultivation of potatoes more laborious, and increasing their market price, yet it is no longer dreaded as it at first was, for the reason that it is controlled with comparative ease. The natural enemies of the species are encouraged by the intelligent cultivator, and poultry may be taught to feed upon it. Of over twoscore predaceous and parasitic species of its own class which I have enumerated, those herewith figured may be considered the most important. The only true parasite is a species of Tachina-fly (Lydella doryphoræ, Riley), somewhat resembling a house-fly, which fastens its eggs to the doryphora larva. From these eggs hatch maggots, which feed upon the fatty portions of the said larva, which, after entering the ground, succumbs to its enemy, and, instead of eventually giving forth a beetle, as it naturally should do, gives forth, instead, the Tachina-flies. A number of different lady-birds (Coccinellidæ), of which the convergent lady-bird is the most common, devour the eggs of the doryphora. Of true bugs the spined soldier-bug (Arma spinosa, Dallas) is the most effective, though several other rapacious species assist it, all of them piercing and sucking out the juices of their prey. Of artificial remedies there are various mechanical contrivances for knocking the insects off the haulm and catching them—some such even being worked by horsepower. The sun is, also, so hot in some of the Mississippi Valley States that the larvæ are roasted to death if shaken from the haulm on to the hot soil at mid-day. The remedy of all others, however, and the one universally employed, is Paris-green, which is used either in the form of a powder, or in that of a liquid, being combined in the former case with from twenty-five to thirty parts of some dilutent, as flour-middlings, plaster, etc., and in the latter with one tablespoonful of pure green stirred into an ordinary bucketful or about three gallons of water. Enormous quantities of the poison have thus been used in America, especially since it has proved a perfect remedy for the cotton-worm in the Southern States as well as for the potato-beetle in question. Cautiously and judiciously used it proves cheap and effective, and a large experience goes to show that no ill effects follow such use of it. There is a very closely-allied species, the Doryphora juncta of Germar, called the bogus Colorado potato-beetle, which, very naturally, has often been confounded with, and mistaken for, the genuine depredator. It differs, however, in the eggs being paler; in the larva being paler, and in having but one row of black dots on each side instead of two; and in the beetle having the second and third black lines of the elytra (counting from the outside) joined, instead of the third and fourth; in the punctures of said elytra being more regular and distinct, and in the legs haying pale instead of dark tarsi, and a spot on the thighs. Singularly enough, this species, though it feeds and thrives on , will not touch the cultivated potato, and is, therefore, perfectly harmless to man.
The English reader is more particularly interested in this insect, because of its possible introduction into Europe; and on the subject of its introduction I cannot do better than quote some passages from my seventh report: "Those who have watched the gradual spread of this potato-beetle during the past seventeen or eighteen years from its native Rocky Mountain home to the Atlantic, and who have seen how lakes, instead of hindering its march into Canada, really accelerated that march, can have no doubt that there is. danger of its being carried to Europe. Yet I must repeat the opinion expressed a year ago—and which has been very generally coincided in by all who have any familiarity with the insect's economy—that if it ever gets to Europe it will most likely be carried there in the perfect beetle state on some vessel plying between the two continents. While the beetle, especially in the non-growing season, will live for months without food, the larva would perish in a few days without fresh potato-tops, and would, I believe, starve to death in the midst of a barrel of potatoes, even if it could get there without being crushed; for, while it so voraciously devours the leaves, it will not touch the tubers. The eggs, which are quite soft and easily crushed, could, of course, only be carried over on the haulm or on the living plant; and while there is a bare possibility of the insect's transmission in this way, there is little probability of it, since the plants are not objects of commercial exchange, and the haulm, on account of its liability to rot, is not, so far as I can learn, used to any extent in packing. Besides, potatoes are mostly exported during that part of the year when there are neither eggs, larvæ, nor potato haulm in existence in the United States. There is only one other possible way of transmission, and that is in sufficiently large lumps of earth, either as larva, pupa, or beetle. Now, if American dealers be required to carefully avoid the use of the haulm, and to ship none but clean potatoes, as free from
Fig. 4. Bogus Colorado Potato-Beetle.
a, a, eggs: b, b, larva; c, beetle: d, left-wing cover, enlarged, showing marks and punctures; e, leg enlarged. Colors: of egg, pale-yellow; of larva, cream-yellow; of beetle, black, yellow, and brown.
earth as possible, the insect's transmission among the tubers will be rendered impossible; and when such precautions are so easily taken, there can be no advantage in the absolute prohibition of the traffic in American potatoes. As well prohibit traffic in a dozen other commodities, in many of which the insect is as likely to be imported as in potatoes. The course recently adopted by the German Government, in accordance with the suggestion made in my last report, is more rational, and will prove a better safeguard: It is to furnish vessels, plying between the two countries, with cards giving illustrated descriptions of the insect in all stages, with the request that passengers and crew destroy any stray specimens that may be found. Let England and Ireland, together with the other European governments, co-operate with Germany in this plan, and have such a card posted in the warehouses of seaport towns, and the meeting-rooms of agricultural societies, and a possible evil will be much more likely avoided." Some English journals are discussing the question as to whether, with the more moist and cool climate of this country, the ten-line potato-beetle would thrive here even if imported. "There cannot be much doubt that it will rather enjoy the more temperate clime; for while it thrives best during comparatively dry seasons, both excessive heat and drought, as well as excessive wet, are prejudicial to it. It is argued by others that on the Continent of Europe our doryphora would not thrive if introduced; and, in a recent letter received from M. Oswald de Kerchove, of Gand, Belgium, author of an interesting pamphlet on the insect, that gentleman says, 'I do not think that the doryphora awakened by our early warm weather, could resist the effects of the late cold which we are apt to have in these European countries.' The idea that the climate of North America is less extreme than that of Europe is rather novel to us of the cisatlantic; and, from a sufficiently long residence in England, France, and Germany, I am decidedly
of opinion that they delude themselves who suppose that doryphora could not thrive in the greater part of Europe; and that to abandon all precautionary measures against its introduction on such grounds would be foolish. An insect which has spread from the high table-lands of the Rocky Mountains across the Mississippi Valley to the Atlantic, and that flourishes alike in the Stales of Minnesota, Wisconsin, and Connecticut, and in Maryland, Virginia, and Texas—in fact, wherever the potato succeeds—will not be likely to be discomfited in the potato-growing districts of Europe. Some few, again, have ridiculed the idea of the insect's passage to Europe in any state, arguing that it is an impossibility for any coleopterous insect to be thus transferred from one country to another. Considering that half the weeds of America, and a large proportion of her worst insect pests, including two beetles, viz., the asparagus-beetle (Crioceris asparagi) and the elm-leaf beetle (Galeruca calmariensis), in the very same family as our doriphora, have been imported from Europe, there would seem poor foundation for such an argument. Moreover, a number of other insects—among them some beetles—of less importance, may be included in the number of importations; and the rape-butterfly (Pieris rapæ), whose progress westward has been simultaneous with that of the doryphora eastward, and whose importation dates back but a few years, bears witness to the fact that insects more delicate, and with fewer chances of safe transport than doryphora may succeed in getting alive from one country to the other, and in gaining a foothold in a new home. The ravages of the insect, bad as they are, very naturally get exaggerated at such a distance from its native home, and the following from an English gardening periodical gives altogether a too gloomy picture: 'When once a field of potatoes has been attacked, all hopes of a harvest must be given up; in a few days it is changed into an arid waste, a mere mass of dried stalks.' It should not be forgotten that the American cultivator, by means of intelligence and a little Paris-green, is pretty much master of the doryphora. It is to be hoped that this exposition of the facts and probabilities of the case will put people on their guard, and cause intelligent action to be taken to prevent the importation of so dangerous a pest as this potato-beetle.—The Garden.