Insects Injurious to Beans and Peas

INSECTS INJURIOUS TO BEANS AND PEAS. By F. H. CHITTENDEN, Assistant Entomologist, INTRODUCTION. Beans, peas, cowpeas, and other edible legumes are subject to injury by certain species of beetles, commonly known as weevils, which deposit their eggs upon or within the pods on the growing plants in the field or garden and develop Ivithin the seed. Four forms of these weevils, members of the genus Bruchus of the family Bru- chidae, which inhabit the United States, are very serious drawbacks to the culture of these crops in many portions of the country. The spe- cific enemy of the pea is the pea weevil, and of the bean, the common bean weevil, both of sufficiently wide distribution and abundance to hold the highest rank among injurious insects.- Cowpeas are attacked by two species of these beetles, known, respectively, as the four- spotted bean weevil and the cowpea weevil. These latter are of considerable importance economically in the Southern States and in tropical climates, as well as in northern localities in which cowpeas are grown or to which they are from time to time shipped in seed from the South and from abroad. As with the insects that live upon stored cereals, the inroads of the larvse of these weevils in leguminous seeds cause great waste, and particularly is this true of beans that are kept hi store for any considerable time. In former times popular opinion held that the germination of leguminous food seed was not impaired by the action of the larval beetle in eating out its interior, but this belief was erroneous, as will be shown in the discussion of the nature of the damage by the pea weevil. It is not probable that any serious trouble follows the consumption by human beings of the immature weevils in green peas or beans, but the use for food of badly infested dry seed filled with the dead bodies and excrement of the beetles would very naturally be attended with unpleasant consequences. Kirby and Spence narrate that *4n the year 1780 an alarm was spread in some parts of France that people had been poisoned by eating worm-eaten peas, and they were forbid- den by authority to be exposed in the market." When the crop is made and harjv^ested, and even before, the insects 234 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. which have undergone their successive changes from larva and pupa within and at the expense of the seed, gnaw their way out and make their appearance, often in great numbers, in the garden or in the bags, barrels, and other receptacles in which the seed has been stored. Then for the first time the farmer, merchant, or housekeeper becomes aware of their presence. All of the above-mentioned species of insects live within the infested seed throughout the winter, and all but one of them (the pea weevil) continue to breed for successive generations in the stored material. In addition there are other weevils which injure seed in a similar manner in foreign countries and whose introduction into the United States in seed material is to be guarded against. In the fields the growing pods are invaded by the boll worm and the larva of the pea moth, and the foliage is preyed upon by numerous insects. None, however, save in exceptional years and in limited dis- tricts, is of as much importance as the species which develop within the matured seed. Of foliage feeders, the bean leaf-beetle and bean ladybird, blister beetles, cutworms, and other caterpillars are deserv- ing of special mention. Several forms of plant-bugs, leaf-hoppers, and plant-lice also exhaust the plants by sapping their juices. All of the species considered in the present paper that inhabit the Eastern United States have been under observation by the writer in the vicinity of Washington, D. C. THE PEA WEEVIL. (Bruchus piso rum Linn.) GENERAL APPEARANCE AND METHOD OF WORK. Seed peas are often found with a single round hole in them due to the attack of the pea weevil, or, as it is sometimes incorrectly called, fche "pea bug." This is the largest of the pea- and bean-feeding weevils found in this country, measuring about a fifth of an inch (5mm.) in length and half that in width. Its ground color is black, but it is thickly covered with brown pubescence, variegated with black and white markings arranged as shown in fig. 6G, a. The sides of the thorax are notched or toothed, and the abdomen, which projects beyond the elytra or wing-covers, is coated with whitish pubescence and marked by two black spots. The hind femora or thighs are thick- ened and each bears two prominent teeth. HISTORY AND ORIGIN. This weevil has long been known as an enemy to the pea. About the middle of the last century the celebrated Swedish traveler and naturalist Pehr Kalm gave an account of it and its ravages in America, stating, in 1748, that at that time the culture of the pea had been abandoned in Pennsylvania, New Jersey, and southern New York on account of this insect. INSECTS INJURIOUS TO BEANS AND PEAS. 235 Most writers on economic entomology have agreed that this species, presumably because it was first found in peas in North America, is indigenous to this country, from whence it has been introduced into southern and middle Europe. There are reasons for believing, however, that it came originally, with so many other injurious insects which live upon cultivated seeds, from the Orient. ^ This species now occurs over nearly the entire globe, wher- ever peas are cultivated. It is scarcely known, however, in the colder countries of north- b CL C ern Europe, and does com- FIG. 66.-J5rucÄu. l>i.ori*m; a, adult beetle; Marva; paratively little damage in c, pupa—all greatly enlarged (original). the colder parts of Canada; hence, it happens that seed peas for planting in the United States are largely imported from Canada or are bought from seed dealers who obtain them from our more Northern States. What was true in Kalm's time in New Jersey, when peas could not be sown on account of the insects which consumed them, may almost be said at present of many localities in the United States. NATURE OF INJURY. It is no uncommon sight to see every pea in a pod infested with this weevil; and although nearly every one is familiar with the appearance of "buggy" peas, it is not generally known that in eating green peas we often eat also a larva or ''worm" with nearly every pea. The only external evidence of infestation in a green pea is a minute dot on its surface, but in dry seed the cell inhabited by the insect is visible under the skin, while later the beetles may be seen with their heads poking through holes which they have made in the skin. Until recently the belief was more or less prevalent that the larva working within the seed, by some wonderful instinct, avoided the germ or embryo, and that weevil-infested peas were therefore of equal value for seed to uninfested ones, but* this is incorrect. Many ''weeviled" seed will germinate, but as they are deficient in plant food the resulting plant is apt to be weakly and nonproductive. Of 500 peas tested by Prof. E. A. Popenoe in Kansas in 1890 only one- fourth germinated, and the partial destruction of the cotyledons ren- dered the future growth of these doubtful, while an examination of 275 injured peas showed only 69 in which the germ was not wholly Í Two reasons for this belief may be mentioned: It belongs to a group of the genus Bruchus, having the sides of the thorax toothed, and not represented by any native species; furthermore, peas are not native to North America, and no other food plant is known for this weevil. 236 YEARBOOK OF THE DEPARTMENT OF AGRICULTURE. or partially destroyed. Of a lot of seed tested in Canada by Dr. James Fletcher, Dominion entomologist, 17 per cent grew, but only 2 per cent made strong looking plants producing seed, all the others being stunted and weak. Another lot tested in which the radicle had been injured by the weevil in escaping from the seed did not grow at all. LIFE HISTORY AND DEVELOPMENT. The beetles appear on the vines when the peas are in blossom and the eggs^ are deposited singly upon the surface of the pods, being attached by a peculiar viscid secretion which turns white in drying. The minute, newly hatched, or post-embryonic larva'^ bores through the pod opposite a pea, which it enters and casts its skin, after which it appears as a larva of the ordinary Bruchid type. Within the developing pea the larva feeds and grows apace, and when fully matured presents the appearance indicated at 6 of fig. 06. It is somewhat maggot-like in general aspect, nearly cylindrical, fleshy, strongly wrinkled and perfectly white throughout except in the region of its minute mouth-parts, FIO. Q7.—Bruchus pisorum: a, egg on pod; ö, where it is brown. The larva in cross section of opening of larval mine; c, this stage has' been described as young larva and opening on inside of pod by which it had entered, enlarged; d, d, d, egge apodous, or footless, but this is on pod, sliglitly enlarged; e, embryonic larva, an error, as it has three pairs of greatly enlarged; /, leg of same; gr, protho- racic spinous processes, more enlarged (reen- very apparent legs.

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