Insects injurious to vegetables

Preface

Among the hordes of insect foes with which the American farmer has to deal, those affecting vegetable crops are in manyrespects most troublesome. Vegetable plants are exceptionally perishable, and the control of their insect enemies entails a very considerable expenditure of money and time. The annual losses due to insect attack on vegetable crops is estimated at 20 per cent., or double that of the average farm crop. The injurious vegetable-feeding forms outnumber in species the insect enemies of any other single class of crops, excepting possibly deciduous fruits, and this nearly endless variety of pests necessitates information in regard to each. Many are intermittent in attack, hence the grower should be forewarned in order to guard against injury or to check it before irreparable damage has been accomplished. The progressive vegetable grower should be as amply equipped with knowledge as the fruit grower, and if he would be entirely successful in avoiding losses from insect ravages he should be provided with a complete outfit for spraying operations and should keep on hand or know where to obtain at short notice a good supply of necessary insecticides. The more general observance of certain farming methods with a view to the prevention of insect injury will greatly lessen the losses from this source. Until within recent years few farmers in planning the management of the farm for the season considered the effect which any given method of tillage would have upon injurious insects. Too frequently they fail to look far ahead, and as a rule rotation of crops where practiced is more for the sake of soil improvement than for the avoidance of insect injury, and yet crop rotation is the best and sometimes the only remedy for

VI PREFACE

certain species of insects. Among other general methods of farming strict cleanliness, including the destruction of weeds and burning over fields after harvest, fall plowing, crop rotation, the use of fertilizers, and the selection of the proper place and time for planting, must be considered. A knowledge of the classification of insects sufficient to enable the farmer to distinguish friends from foes is valuable, and finally comes a knowledge of what insecticides and repellents to use and the best means of preparing and applying them.

Accounts of most of our noxious species of insects have been published. These accounts, however, are distributed through government and state publications, reports of agricultural societies, magazines, and periodical publications of entomological societies, and even the daily press. As an example of the number of such publications on American economic entomology, the Bureau of Entomology, United States Department of Agriculture, has cited no less than 12,645 titles that had appeared to January 1, 1905, and the number of references to noxious insects is about 72,000. The average farmer has neither time nor opportunity to consult a tithe of these 12,- 000 odd works, and it is therefore the object of the following pages to collate concise accounts of the principal insects which affect one class of crops — vegetables. The order which will be followed is, as far as practicable; alphabetical, beginning with the insect enemies of asparagus, and ending with those which affect sweet potato, and finally miscellaneous or unclassified crops.

The insect enemies of vegetables have not hitherto been considered as a special topic in comprehensive form. Separate accounts, however, on the economic entomology of certain vegetables have been published, for example, of beets and of sweet potato.

In presenting this work to the public its author does not claim originality for its contents. It is, however, largely com-

PREFACE Vil

piled from his own writings, although it has been found necessary to draw also from the works of others, and is based on an experience of about ten years with the subject with which it deals. The illustrations are in large part the same, or adaptations of, figures previously used in the Bureau of Entomology, United States Department of Agriculture, and are so credited.

F. H. Chittenden.

United States Department of Agriculture, September, ipo/.

Contents

PAGE

Preface . . . v-vii

Introduction . . . x-xiv

Chapter II

Selection of place and time for planting — The maintenance of vigorous growth — Burning over fields and waste lands — Crop rotation — Diversified agriculture — Fall plowing and cultivating — Clean farming methods — Farm inspection — Cooperation in the control of insects.

Introduction

Vegetables are subject to attack by insects from the time the seed is planted until the edible portion is ready for cooking. Insect injury manifests itself in different ways according to the plant attacked, the insect concerned, the stage of the insect, and the age and condition of the plant.

Manifestations of Insect Injury. — If plants soon after the seed has been sown fail to appear in due time, such failure is apt to be attributed to unfavorable atmospheric conditions or to imperfect seed, but examination will frequently show that some insect is at work. Among insects destructive to planted seed are wireworms and root-maggots, and in some cases insects that have fed on the seed stock while in store are planted with the seed, and this they destroy by eating out the germ. Familiar examples are the bean and pea weevils.

Asparagus tips when ready for cutting are

ruined for market by the asparagus beetles (see

Fig. l— Aspara- **£• I ) * ^ plants like tomato that are reset are

gus tip, showing cut off abruptly near the ground, cutworms are

by asparagus near ty always at work. If the minute leaves of

beetles. (Au- plants like cucumber, soon after beginning to

thor's illustra- . , • i i

tion, u.s.Dept. sprout, are found eaten away, causing the plants A § r) to die, cucumber beetles are present. Corn is

similarly affected by flea-beetles, as are also potato and other vegetables.

When stems or stalks of various plants are found with one or more holes of varying size, from that of a straw to considerably larger, this is evidence of a borer within, particularly

INTRODUCTION

XI if excrement is exuding. Common species which do this injury are the common and corn stalk-borers and the grass- worm. When plants like corn make unequal starts, a hill here and there showing greater thrift than elsewhere, injury is apt to be due to root-aphides. As a rule, these insects are accompanied by ants, which in most cases foster the "lice," and sometimes feed on the seeds.

When the leaves of plants are seen to be withering, and aphides or other sucking insects cannot be detected above ground, search will usually reveal the presence of white grubs, wireworms or other insects working below the surface, and the same is true of corn plants that fall after windstorms, root-worms also being present at such times. Another manifestation of the presence of root-worms is in the plants requiring too long a time for maturing, and producing sterile stalks, and, in the case of corn, yielding nubbins instead of

complete ears.

When young leaves are found with small round holes of about the size of a pea or a little smaller, leaf-

Fig. 2.— Bollworm entering bean pod, Somewhat enlarged (Authors illustration, U. S. Dept. Agr.)

beetles are usually present, while much smaller holes occurring in great profusion over leaves usually betoken the presence of flea-beetles. Extremely large and irregular holes in leaves of older growth are made by grasshoppers, crickets, and the larger caterpillars such as "woolly bears."

Fruits such as melons are attacked by the melon worm and pickle worm; tomato, beans and corn by the bollworm, tomato

Xll INTRODUCTION

fruit worm or ear worm (fig. 2), and the edible roots of such plants as beet and carrot, are subject to injury by the carrot beetle, while potato tubers are damaged by the potato tuber worm. Seed pods and similar coverings of seeds are attacked by numerous insects, such as the corn ear worm, cucumber beetles, and others. Growing seeds are liable to be injured by some of the last mentioned insects as well as by some others which begin development when the seed approaches maturity. Familiar forms are the bean and pea weevils, and the Angoumois grain moth.

Many other forms of injury might be cited, but it may suffice to briefly mention the curling and dying down of leaves like melon, due to the presence of the melon aphis; the wilting and dying of squash caused by the severance of the stalks by the vine borer feeding within; the discoloration of leaves such as radish and their subsequent drying, owing to leaf-miner attack; and the destruction of whole plants of various kinds by army worms and migratory cutworms.

Every year that passes brings with it some new entomological problem to be solved, and this is especially true of insect injury to vegetables. The cause is usually a general or local outbreak of one or more serious pests, and the species concerned may be an old and well-known injurious form; it may be a comparatively unknown species or one that has not hitherto been identified with injury to useful plants; again the habits of the species may never have been studied owing to previous scarcity. It has perhaps lived in obscurity since time immemorial before any considerable outbreak attracts attention. It may be new to our country or even new to science.

Determination of the Injurious Insect. — The first problem that confronts the grower whose crops suffer from insect injury is the identification of the insect. Most insects have popular names, more or less local, which may be apt or may be rank misnomers, conveying no definite meaning.

INTRODUCTION

Xlll

Thus if a southern farmer complain of "the budworm," failing to state what plant is being injured, it is an impossibility to identify the insect concerned. The budworm of corn is the twelve-spotted cucumber beetle of the northern states, while the budworm of tobacco is the same as the tomato fruit worm and the corn ear worm or bollworm or a related species.

The first pest is Diabrotica 12-punctata; the second is

Fig. 3.— Diabrotica 12- punctata. (Riley, U. S.

Dept. Agr.)

Fig. 4.— Bollworm moth in natural position. About twice natural size. (Quaintance, U.S. Dept. Agr.)

Heliothis obsoleta. 1 The parent of the first is a beetle (fig. 3) and of the latter a moth (fig. 4)-. It should be added that the first named species is also known as drill worm, while the latter is also called the shatter worm, this last name being shared also by the larger corn stalk-borer and perhaps by other species having the same habits, such as the fall army worm. The last is the grass-worm of the South and the fall army worm of the

1 It is to be regretted that the scientific names are not more stable, but the bollworm was generally recognized under the name of Heliothis, until it was recently changed to H. obsoleta.

XIV INTRODUCTION

North. It is the "alfalfa worm" of Kansas and Nebraska and elsewhere it is simply called the army worm. Its technical name is Laphygma frugiperda.

It will thus be seen from the examples cited that the popular name of an insect has frequently little bearing on its identity. The scientific name must be determined.

A frequent source of injury to plants is due to contiguous growers who raise the same, or similar crops, for different purposes, e. g., some may raise cucumbers for pickling, and in the immediate vicinity others grow melons for fruit, so that pickle-growers pick their crops while quite young, and the fruit-growers when older. This means that after the picklegrower has stopped pickling, numerous vines remain, and the insects scatter from them to melon fields.

Chapter I General Considerations

For an intelligent understanding of the subject of insect control by agricultural practice one must know not alone that certain conditions produce an increase or decrease of certain forms of insects, but how this is accomplished, why the alternation of one crop with another is apt to result in insect injury, and why a system of crop rotation that would be of value in the control of one class of insects might be ineffective against another; how fall plowing, though destructive to one species, would not affect a different insect, and so on. In short, a knowledge of economic entomology beyond the fact that arsenicals are the proper remedies for mandibulate or chewing insects, and that kerosene will kill aphides or plant-lice, scale insects, and other soft-bodied insects, is a prerequisite to intelligent effort in the control of noxious insects. Before we can hope to avert losses we must know what our insect enemies are, what species are destroying each crop, which ones are responsible for primary injury, which are secondary or merely auxiliary, how injury is accomplished, when injury begins each year, when it ends, as well as other facts.

Similarly desirable is it to be able to recognize useful insects, such as ladybirds, syrphus flies, tachina and ichneumon flies and other parasites, that these may not be unnecessarily destroyed, but, if possible, encouraged in their useful work.

i

The different stages of some insects are so diverse that they can be identified only by specialists, and many entomologists are unable to recognize them without reference to technical descriptions and illustrations. Some knowledge of the distribution and origin of a species is of value, as well as some acquaintance with its history and literature.

A knowledge of the life history of an insect consists in knowing : when, where and how its eggs are deposited ; how the larva feeds, and how many stages there are in this period; the habits of the larva, whether diurnal or nocturnal, whether omnivorous or a dainty feeder; how and where it transforms to pupa; how and where and in what stage it passes the winter; the number of generations produced each year; the first appearance of the insect and its disappearance, and the same of each generation; its food plants, natural and cultivated, and above all, its favorite foods, both as larva and adult. If to this we add a knowledge of the effect of farm practice and of insecticidal and mechanical methods on the insect we have, in a general manner, the main facts desired.

We must determine in what stage and at what time the insect is most vulnerable, and by practice and experiment learn the best remedy. A knowledge of the appearance and place of deposition of the eggs will, in the case of some species, furnish means for their control, for many insects can be combatted successfully merely by destroying the eggs. Others may be killed in their cocoons.

The most valuable weapon that can be used in combatting an insect consists in an intimate knowledge of the insect itself and its life economy, its natural enemies, its susceptibility to natural influences, heat and cold, dryness and moisture, and their effect upon its increase or decrease directly, or indirectly by destroying or favoring the growth of its enemies. A knowledge of the weeds and wild plants that furnish food for these insects, in addition to cultivated plants, and the soils in which they attain their greatest development is also desirable. To this we must add a knowledge of the effects of different farm practices upon the insects, as well as of insecticides, There are many insects with which we cannot cope by the use of poisons, and it is only by a thorough knowledge of their life economy from the time the eggs are deposited until the perfect insect emerges, that we are able to mitigate losses from their ravages. The knowledge of some one or more facts apparently trivial in themselves may frequently enable us to prevent by farming methods injury which we cannot cure by means of poisons, mechanical or other direct methods

After seed has been selected with reference to its adaptability to the soil and climatic and other conditions one of the next problems that confront the grower is how to protect the crop from noxious insects and diseases. Fortunately we know approximately the life history and habits of a large proportion of the injurious insect inhabitants of this country, with the exception of some few species which have only been recently associated with injury, or which have lately been introduced from abroad.

As a necessary preliminary to the discussion of the insect enemies of the various vegetable crops, some idea of the structure and classification of insects must be given. It is pertinent to follow as a matter of course with general methods of control, which include (1) mechanical methods, (2) farm practice as preventives and (3) the preparation and means of applying poisons for the destruction of insects. Following this the different groups of insects which affect various crops and are not attached to single crops will be considered.

CLASSIFICATION OF INSECTS

If we would have an intelligent understanding of the causes that have led to the destruction of our crops, we must know something of the classification of insects and their nearest relatives in order that we may be able to distinguish friends from foes and true insects from related forms. It is, of course, not essential that all of the Latin names which insects bear should be memorized, nor that anything approaching a complete classification be studied. The object of what is here presented on this topic is to assist in the ready identification by orders of such creatures as may come under observation as enemies or suspected enemies to plants under cultivation.

It is first desirable to eliminate animals which are not true insects but are related to them. These are all included (with

true insects) under the branch Arthropoda of the Animal Kingdom and are distinguished by having their bodies composed of rings or segments more or less similar, joined together, most of them bearing jointed legs, the last character separating them from earthworms, eel-worms, or hairworms, which have many segments but no legs. This branch is divided into four classes:

Crustacea (Crabs, lobsters, shrimps, crawfish, and sow-bugs [Oniscidce]). — Of this class only the sow-bugs or pill-bugs are apt to be confused with insects, and are of some economic importance, though not so injurious as many suppose. A common injurious species is shown in figure 5.

Arachnida (Scorpions, daddy long-legs, spiders, mites, etc.) . — The scorpions are well known in the South and need no description. The same is true of the daddy long-legs or harvestmen, and spiders are everywhere. Among the mites, however, we have one species, the so-called red spider, which is quite injurious at times and which as it is commonly supposed to be an insect we will treat in one of the following chapters.

Fig. 5.— Water-cress sowbug (Mancasellus brachyurus). Enlarged. (After Richardson.)

Myriopoda (Thousand-legged worms). — These creatures are known to most persons, and are divided into two orders: The Centipedes constitute a group in which each segment bears only a single pair of legs, while the body is generally flattened, and the antennae are long with many joints. They live mostly by preying upon other insects. The Millipedes (fig. 6) have two

Fig. 6.— Myriopod. Enlarged

pairs of legs to each segment except the first three; the body is more or less cylindrical, and the antennae are shorter with few joints. Most species feed upon decomposing vegetable matter, but some attack growing plants, more particularly those of the garden and greenhouse. Injury by these creatures, however, is frequently exaggerated, as in the case of the sow-bugs, previously mentioned.

Hexapoda (Insects). — This brings us to the true insects which are distinguished from the other three classes that have been mentioned by having the body divided into three distinct portions, — head, thorax (chest), and abdomen (belly) (fig. 7).

They have a single pair of antennae or feelers, normally three pairs of legs, and in the mature stage, one or two pairs of wings (save in exceptional cases). In

, , ,. Fig. l.—Polistes bellicosus. Somewhat

our present advanced state of enlarged . (Marx del, u. S. De P t. Agr.) knowledge of the classification

of true insects they have been divided into no less than nineteen 1 orders, but for present purposes what is known as the

1 For a list of these orders the reader is referred to pp. 77-81 of Comstock's Manual for the Study of Insects, Comstock Publishing Co., Ithaca, N. Y.

old Linnsean classification will suffice. This embraces seven orders, — Coleoptera, Orthoptera, Lepidoptera, Hymenoptera, Neuroptera, Diptera, and Hemiptera. The first six of these orders are mandibulate or chewing insects, at least in the most active stage of the insect, while the last are haustellate or sucking insects. This is important to know as on this classification depends the question as to whether stomach poisons, such as the arsenicals, or contact poisons, such as kerosene and its different preparations, are best suited for their destruction.

ORDERS OF INSECTS

Coleoptera or Beetles. — Beetles are distinguished by having a pair of more or less horny elytra or wing-covers which nor-

Fig. 8.— Convergent ladybird (Hippodamia convergens) . a Adult; b, pupa, c, larva. All much enlarged. (Author s illustration, U. S. Dept Agr )

mally meet in a straight line down the back. Beneath them are the true membranous wings, usually folded. The mouth-parts of beetles and their larvae (the latter usually called grubs) are formed for biting. A common beetle, a beneficial ladybird, is shown in figure 8, in different stages. Many species of beetles are injurious both in the adult and larval stages.. Among the best known forms of this order that injuriously affect vegetables are wireworms, white grubs and their parents the May and June beetles, leaf-beetles, flea-beetles, pea and bean weevils, blister beetles, bill-bugs and other snout-beetles. Lepidoptera (Butterflies and moths). — This order consists of insects having four membranous wings covered with more or less minute overlapping scales. The mouth-parts of the adults are formed for sucking, but the larvae (called eater-

Fig. 9.— Cabbage looper (Autographa brassicse). a, Male moth: b, egg; c, caterpillar; d, pupa in cocoon, a, c, d. One-third larger than natural; b, more enlarged, (a, c, d, after Howard; b, Chittenden, U. S. Dept. Agr.)

pillars, "worms," etc.) have well-developed chewing mouths. The Lepidoptera are of about equal importance with the Coleoptera or beetles as pests.

Examples of noxious forms that are destructive to vegetable crops are found in the cutworms, army worms, webworms, cabbage and tomato worms and various caterpillars. The cabbage looper is shown in figure 9 in its four principal stages.

Hymenoptera {Saw flies, ants, wasps, bees, etc.). — In this order there are two pairs of membranous wings with comparatively few veins, the hind-wings being the smaller pair. The mouth-parts of the adults are formed for both biting and sucking, and those of the larvae, which are injurious, for biting. The females are furnished with stings, piercers or saws. In this order there are comparatively few noxious forms, and most of these are confined to the sawflies whose larvae, known as slugs and false-worms, consume vegetation, doing injury similar

Fig. 1 0.— Screw-worm (Compsomyia macellaria). a. Maggot; b, head of same; c, anal segment from rear; d, puparium; e, adult fly;/, head from side. All enlarged.

to that accomplished by caterpillars. Some species of ants are troublesome, both directly and indirectly, in the latter case by acting as carriers of aphides or plant-lice, scales, and some other insects. This order, however, contains many beneficial forms, such as ichneumon and chalcis flies — parasites of noxious insects, and wasps which also destroy insect pests. One of these is shown in figure 7.

Diptera, or Flies. — Insects of this order have a single pair of wings which are borne on the mesothorax or middle portion of the thorax. The metathorax or hind portion bears a pair of knobbed thread-like processes called halteres, poisers, or balancers. The mouth-parts are formed for sucking in the adult condition, but in the larvae (called maggots) the mouth-parts are for biting. To the Diptera belong such pernicious insects as mosquitoes, house and horse flies, and root-maggots. For illustrations of the Diptera, see figures io«and 20. Orthoptera (Grasshoppers, crickets, roaches, katydids, etc.). — In this order the insects have two pairs of wings, — the first somewhat horny and overlapping when at rest, the second pair thin and folded when at rest in plaits like a fan. The metamorphosis is incomplete, and all forms of the insect (except the egg) are active, with biting mouth-parts. The Rocky Mountain locust is an example of this order (figs, n and 12). Hemiptera. — This order divides into three suborders, — the Heteroptera or true bugs, the Homoptera containing aphides, leafhoppers, etc., and the Physopoda or thrips. These groups

have in common four wings, the mouth-parts in all stages ss?4ifeL JFBC — formed for sucking, with in-

complete metamorphoses.

Fig. 11. — Rocky Mountain locust (Melanoplus spretus). a, a, Newly-hatched nymph; b, full-grown nymph; c, pupa, natural size. (After Riley.)

Fig. 1 2.— Rocky Mountain locust (Melanoplus spretus). Adult. Natural size. (After Riley.)

Suborder Heteroptera. — In the true bugs the anterior wings are thickened at the base and thinner at the extremities and overlap on the back, and the beak arises from the anterior portion of the head. It includes various forms of noxious insects, such as the chinch bug, numerous plant-bugs, (fig. 13), squash bug, and certain beneficial species, such as soldier-bugs.

Suborder Homoptera. — In this group the wings are of uniform thickness and usually slope at the sides of the body, the beak arising from the hinder portion of the lower side of the head. In this suborder most injurious pests are found in the aphides (fig. 14), leaf-hoppers and the like.

Suborder Physopoda. — The thrips have two pairs of wings of similar form — long, narrow, membranous, not folded, and

Fig- 1 3.— Brown plant-bug (Euschfstus variolarius) . Adult at right; last riymph stage at left. Enlarged. (Howard, U. S. Dept. Agr.)

with few or no veins. These are fringed with long hairs and do not fold, but are laid horizontally along the back when at

rest 1 (fig. 15).

Neuroptera. — This order has been subdivided by recent writers into numerous other orders, but as they are of comparatively little if any economic importance, these

Fig. 15.— Enthrips tritici. a, Adult thrips; b, antenna; c, leg. All highly magnified. Fig. 14 -An aphis. Much enlarged (Aftgr Hubbard) u# S . Dept . Agr .)

subdivisions need not be discussed here. The Neuroptera, in fact, have served as a "catch-all" for the groups that could not

1 According to recent classification the Physopoda constitute a, distinct order, but the Parasitica, which includes the parasites of man and other mammals, is a suborder of equal rank with the Heteroptera and Homoptera.

II be classified with other orders. Among neuropteroid insects of interest to the farmer are the aphis lions or young of the lacewinged flies (fig. 16) which are beneficial by feeding on noxious insects and the dragon flies, which also do some good in destroying injurious forms.

The Coleoptera, Lepidoptera, Hymenoptera, Neuroptera and Diptera have what is termed a complete metamorphosis, which means that they undergo four totally different stages, of egg, larva, pupa and adult or imago. In the remaining

Fig. 1 6.— A lace-wing with eggs at right

Fig. 1 7- —Tarnished plant-bug; four stages of nymphs. Enlarged (After Forbes)

two orders, Orthoptera and Hemiptera, the metamorphosis is incomplete, which means that in the stages between the egg and the imago the insect undergoes only a gradual change, each successive substage (nymph) after the first being very like the one that precedes or follows it (fig. 17).

NATURAL ELEMENTS IN THE CONTROL OF INSECTS

The benefits which the agriculturist reaps from the friendly assistance of various forms of insects which prey upon noxious forms is very considerable. Every tiller of the soil should recognize their usefulness, but some are prone to expect too much from them in the subjugation of farm pests, and opinions are so diverse that the grower is sometimes in doubt as to whether the insects which are indicated as his allies are not, in reality, pests. A little study is necessary in many cases to discriminate between noxious and innoxious species and those which are truly and exclusively beneficial.

Organisms beneficial to agriculture may be variously classified, but fall naturally into four groups. The most important

Fig. 18. — Fiery ground-beetle, a, Larva; b, beetle. (From Riley)

a

Fig. 19. —A soldier-bug {Milyas cinctus). (Riley, U. S. Dept. Agr.)

forms are: (i) predaceous insects, consisting of those which feed externally upon their prey; (2) predatory animals other than insects, such as birds and mammals; (3) parasitic insects which live in the bodies of their hosts; (4) fungi and diseases of bacterial origin.

The weather has quite as great effect in the control of insects as in the yield of the crops themselves. Extremes of heat or cold, excess of moisture or dryness have the same effect on insect as on plant life. The results of severe rainstorms, sudden cold snaps and prolonged drought on many insect pests are well known.

Of predaceous insects the most useful are undoubtedly the ladybirds 1 (fig. 9), from their destruction of aphides alone, although some forms also do as good if not even better work in limiting the numbers of scale insects. Ladybirds also devour the eggs and larvae of various other insects, and especially of soft-bodied forms. Several ground-beetles (fig. 18) live at the expense of cutworms and other vegetable-feeding caterpillars and the larvae of beetles. Of this number the great Lebia 2 follows the Colorado beetle wherever it goes, and appears to

Fig. 20.— Syrphus-fly (Syrphus ribesii). a. Fly; b, lateral view of head; c. larva or active immature form. All much enlarged. (Author's illustration, U. S. Dept Agr.)