THE SOUTHWESTERN CORN BORER IN ARKANSAS

DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By

LAWRENCE HUBERT ROLSTON, A.B., M.S.

The Ohio State University 1955

Approved by:

Department of Zoology and Entomology ACKNOWLEDGMENTS

The author is particularly indebted to; Dr. Ralph H. Davidson, of The Ohio State University, and the staff of the Department of Entomology at the University of Arkansas for suggestions during the course of this investigation and critical reading of the manuscript; to student assist­ ants Messrs. Philip Callahan, James Hawkins and Ralph Mayes; to Mr. Henry Vose, Manager of the substation at Van Buren for his cooperation; and to Dr. Lloyd 0. Warren for the photography. A special expression of gratitude is due the many farmers who tolerated trespassing, mutilation of their corn and other nuisances.

ii TABLE OP CONTENTS

Introduction ...... 1 Review of Literature Dlatraea species In the United States ...... 2 History and distribution ...... 3 Life and Seasonal History Life history ...... 8 Seasonal history ...... 8 Description of Injury ...... 13 Adult Description ...... 17 B e h a v i o r ...... l8 Emergence ...... 20 Longevity ...... 20 Proportion of sexes ...... 22 Mating ...... 23 Oviposltion...... 24 Fecundity and fertility...... 23 Oviposltion preference ...... 26 Flight range ...... 28 Light ...... 31 Egg Description...... 32 Location of eggs ...... 33 Development and Incubation ...... 33 Eclosure ...... 34 Larva Description...... 33 Pall activity ...... 36 Survival of overwintering brood ...... 4l Spring activity ...... 42 Number and duration of Instars ...... 42 Larval position on corn ...... 46 Prepupa ...... 32 Pupa Description...... 32 Duration of pupal stage ...... 34 Hosts Other Than Corn ...... 34 Natural Enemies ...... 60

111 Control Methods Cultural control ...... 62 Insecticidal control ...... 63 S u m m a r y ...... 67 Literature Cited ...... 69

LIST OP ILLUSTRATIONS

I, Distribution of the southwestern c o m borer In Arkansas ...... 7 II, Emergence of the principal broods at Fayetteville and Van Buren, 1952-54 ...... 13 III, Female of the southwestern c om borer ...... I9 IV, Girdled stalk ...... 37 V. Overwintering larva In base of s t a l k ...... 38 VI. Distribution of 1,365 field collected larvae according to head capsule width ...... 47

Iv Introduction

In the brief interval following the entrance into Arkansas of the southwestern c o m borer, Diatraea grandio- aella Dyar, it has become a c o m pest of great importance. A thorough investigation of the biology and possible control measures was necessitated by the rapidity with which the borer has spread and the resulting crop damage. Reduction of yield arises in several ways. Young plants may be killed or dwarfed by borer activity in the stem; the yield may be reduced by extensive tunnelling in the stalk; and mature grain may be lost as a result of fall girdling activity. Heavy infestations in sweet corn ears, although infrequent, are disastrous to the grower when they occur. Relatively little has been published about the south­ western c o m borer in spite of its economic importance and of its having been in the United States for many years. The first comprehensive investigation was that of Davis and his co-workers in New Mexico and Arizona (2). More recent bul­ letins have been issued by Kansas (13) and Oklahoma where particular attention has been paid to the development of cultural control practices (14). Since Arkansas differs considerably in climate and to­ pography from the locales of previous investigations, there was no assurance that the seasonal history of the borer would parallel that in neighboring states or that cultural 2 practices recommended there would be effective under Arkansas conditions. The biology of the borer has been given particular at­ tention during this investigation. The habits of both the larva and adult have been studied to ascertain the possibili­ ties they offer for control as well as the limitations they impose. Previously suggested cultural control measures have been evaluated under local climatic conditions.

Review of Literature Diatraea species in the United States The genus Diatraea contains a large number of species but nearly all are confined to tropical and subtropical re­ gions. Several species are important pests as borers in cul­ tivated grasses, particularly sugar cane and corn. Of the six Diatraea species known from the United States, three are apparently rare and of no economic importance: D. evanescens Dyar, D. lisetta (Dyar) and D. venosalis (Dyar). The sugar cane borer, D. saccharalis (Pabr.), is well known as a pest of both sugar cane and com. It is found along the Gulf Coast of Texas, Louisiana and Mississippi and in the southern half of Florida. The southern corn stalk borer, D. crambidoides (Grote), is capable of damaging c o m severely but is not consistently a major pest. Its range extends from Maryland to Florida and apparently westward into Kansas but heavy populations 3 are confined to the southeast. This species is widely known under the synonym D. zeacolella Dyar. The southwestern c o m borer, D. grandioaella Dyar, con­ cludes the list of known Diatraea species from the United States. The range of this insect and the damage caused are discussed in detail under appropriate sections. yet another species, D. lineolata (Walker) has been er­ roneously reported from the southwest. Until 1911 D. saccharalis (Pabr.) was believed to be the sole representative of the genus in the United States and all papers and records on the southern c o m stalk borer and south- westem corn borer, as well as the sugar cane borer, were pub­ lished under this name. In that year Dyar published a revi­ sion of the Diatraea based on the color and extemal morphology of the adult but the characters used have proved Insufficient to consistently separate all the species (3). The main im­ pact of this paper on economic literature was the establish­ ment of the southern c o m stalk borer, under D, zeacolella Dyar, as a species distinct from the sugar cane borer of southem Florida and the Gulf Coast. Holloway later gave a method for distinguishing the larval forms of these two spe­ cies by means of the setal pattem (7). In Dyar's revision, the southwestem c o m borer was described as a new species from Mexico and D. lineolata (Walker) was redescribed from specimens from Mexico and a single specimen from Arizona. This latter specimen was undoubtedly not D, lineolata (Walk.) 4 but the southwestern c o m borer, for the northern limit for D. lineolata (Walk.) Is now known to be central Mexico (1). The revision of Dyar and Heinrich In I927 has done much to clear up the confusion concerning the identity of Diatraea species (6). As a result of these authors' detailed study of the genitalia for diagnostic purposes there was a complete change In species concepts, many names being reduced to syn­ onymy and others recognized as valid. References to Diatraea species prior to 1927, and par­ ticularly those before 19II, must be regarded critically. Sometimes the Identity of the species involved can be deduced from the locality. Adequate descriptions of habits, damage

and the egg are of great help, of the three common Diatraea the sugar cane borer Is the only species not habitually over­ wintering In the stalk base below soil level. The eggs of the southwestern corn borer are distinctive, having three red bars that the eggs of the other two species lack, and the larva's habit of girdling the stalk near ground level In the fall Is unique. For a technical description of American Diatraea, along with excellent drawings of the genitalia, the reader Is re­ ferred to Dyar and Heinrich (6), In this paper D, lisetta (Dyar) Is described and figured under the synonym lesta li­ setta Dyar and D, crambidoides (Grote) under D, zeacolella Dyar, 5 The synonymy of American species and new species, as well as photomicrographs of genitalia, are given by Box (1).

History and distribution What may be the first reference to the southwestem c o m borer occurs In a note appended to a paper by L. 0. Howard In 1891> stating that larvae of the sugar cane borer had been found Infesting c o m In two localities In New Mexico (8), These larvae were probably the southwestem c o m borer, since It Is the only Diatraea species normally found In this area. The distribution In 1931, as given by Davis et al.. Included southeastem Arizona, the southeastern two-thirds of New Mexico, most of the Panhandle and Big Bend of Texas, most of the Oklahoma Panhandle and a few counties of southeastem Colorado and southwestem Kansas (2). No appreciable exten­ sion of this range occurred until 1941, when the borer resumed Its easterly advance across Oklahoma and northerly advance across Kansas. The borer spread across Kansas quite rapidly, reaching a few counties In south central Nebraska In 1945 (15). A survey In the fall of 1950 showed the northern limit of the borer's range to have receded from Nebraska and the northem third of Kansas but the eastern limit to have advanced across Oklahoma Into west central Arkansas and the southwestem cor­ ner of Missouri (13). In 1954 the borer was again present In all but five north­ western and two northeastem counties of Kansas (9). In Missouri> ten southwestern counties were infested in 1953 and twenty-five counties in 195% (12). There are apparently two principal areas of infestation in Texas; one in the northeast comer involving at least fifteen counties and probably as many as twenty-five and the other in the Pan­ handle covering at least thirteen counties and probably twenty-eight (4), It is apparent that the distribution of the borer has been fluid but with a considerable east­ erly extension.

The first counties infested in Arkansas were Sebastian and Franklin along the Arkansas River Valley in 1950. No survey was made in 1951 but borers were found in two addi­ tional counties, Washington and Crawford. The following year the borer was present in twelve western counties and in 1953 four additional counties had been entered. The area known to be infested by the borer in 1954, which in­ cludes twenty-two counties, is shown in Figure I, For the past two years the heaviest infestations were in the Arkansas

River Valley and Springfield Plateau region of Benton and

Washington Counties. These are the major c o m producing areas in westem Arkansas. Pig. 1. Distribution of the southwestern corn borer in Arkansas. Dates in­ dicate year each county was first found infested. 8

Life and Seasonal History Life history A brief and generalized outline of the borer's life his­ tory is necessary for the ready comprehension of the subse­ quent discussion. This account will be qualified and en­ larged In following sections. The borer overwinters as full grown larvae In the ex­ treme stalk base below soil level. Most larvae girdle the stalk internally near ground level as part of the preparation for overwintering. In the spring, pupation takes place In the stubble, the moths emerging via a previously prepared tunnel. Eggs are laid on the leaves and stalk of the new host and the larvae develop on and In the same plant, the later Instars tunneling extensively throughout the stalk. The summer generations, unlike the overwintering brood, pu­ pate mostly In the stalk. In northwest Arkansas, approxi­ mately one-fourth of the second generation larvae overwinter, there being two complete and a partial third generation a year. In the Arkansas River Valley there may be three com­ plete generations and a trace of a fourth which does not, apparently, overwinter.

Seasonal history The seasonal history of the borer was followed over a three-year period near Fayetteville, In Washington County, and for two years and part of a third near Van Buren, 9 Crawford County, In the Arkansas River Valley. These loca­ tions are representative of the two large regions in which infestations have generally been serious. Since Fayetteville is approximately forty-five miles north of Van Buren and has about one thousand feet greater elevation, the same biologi­ cal event can be expected at a later date in the Fayetteville area. The seasonal history of the borer was followed by dis­ secting stubble or c o m plants, usually at weekly intervals, and counting larvae, pupae and pupal cases. Since moth emer­ gence was of primary interest, the results obtained by dis­ sections were frequently checked against emergence from caged stubble or against oviposition records. Over the three-year period at Fayetteville, there were consistently two complete generations and a partial third. All of the first generation pupates in mid-summer and about three fourths of the second generation pupate in late summer. The remaining larvae of this generation overwinter and pupate the following spring. The percentages of second generation larvae pupating in late summer were 68, 72 and 76 for the years 1952-54, respectively. All third generation larvae overwinter and pupate the following spring. The seasonal history at Van Buren has been variable. In 1952, there were three complete generations and a trace of a fourth. Less than one per cent of the third generation pupated in the fall, the remaining larvae overwintering. 10 The fourth generation was consequently very small and the larvae did not develop sufficiently to survive the winter. The following year there were two complete generations and a partial third. About three-fourths of the second generation pupated in late summer, the remaining second generation lar­ vae and all third generation larvae overwintering. In 195^ there were two complete generations, a partial third and a trace of a fourth. Eighty per cent or more of the second generation pupated in late summer. The exact percentage of the second generation pupating could not be determined, for reasons to be explained, but some larvae molted to the over­ wintering form and the generation was therefore not complete. Again, less than one per cent of the third generation pupated in the fall. To follow the development of the overwintering brood, several hundred stubble showing girdling activity were col­ lected from fields in the fall or spring and replaced in the soil to their original depth* Samples were taken from this collection at intervals until emergence of the overwintering brood was complete. Following the seasonal history of this brood in the field generally proved quite frustrating, since fields selected for this purpose were nearly always plowed before emergence was complete. The seasonal history of the first generation could be followed without difficulty. However, the picture of second generation moth emergence was confused if the field had been 11 previously infested by the first generation. Many first generation pupal cases remained intact in the stalks and these were indistinguishable from those of the second gen­ eration. To overcome this difficulty, special plantings were made soon after eggs from overwintering brood moths were no longer found. This c o m was not, of course, infested by the first generation but was tall enough to be attractive for oviposition by the time the first generation moth flight be­ gan. The resulting second generation could be followed with­ out complications from first generation pupal cases. Second generation moths also oviposited on these plantings but the third generation larvae could usually be separated by size from second generation larvae until pupation and emergence of the second generation had ceased. Such was not the case, in 1954, at Van Buren, for pupation of second generation lar­ vae was prolonged until third generation larvae were full grown. Quantitative data could no longer be taken inasmuch as larvae of the two generations were inseparable. plantings were also made after the first generation moth flight so that they would be infested only by the third gen­ eration. Pupation and emergence of the overwintering brood was found to vary considerably from field to field within a rela­ tively small area. Differences in soil types, in exposure to the sun and in air drainage are probable factors in these variations. As an illustration, data on seasonal history 12

In three fields, lying within a five mile radius, are given

In Table 1. One of these fields was plowed before pupation

was completed and before any emergence had occurred. Never­

theless, It Is evident that development In the three fields

was not synchronized. Less variation was found among fields

Infested with the first or second generation.

Table 1. Variations In seasonal history of the over­ wintering brood In the three fields In Benton County In 195^.

Date of Per cent pupae per cent emerged exami­ Field Field Field Field Field Field nation 1 2 3 1 2 3 5/19 68 24 0 0 0 0 5/25 92 76 44 0 0 0 6/1 92 100 88 , , 8 0 0 40 64 _ 1/ 60 36 — 6/14 0 0 - 100 100 - 1/Pleld plowed, terminating records.

Moth flights at Van Buren were consistently earlier than those at Fayetteville (Fig. II), Disregarding the first and last ten per cent to emerge, most moths of the overwintering brood and first generation emerged at nearly the same date In a given area during the years observations were made. The second generation moth flights were quite variable, however. 13

. U ^ S t ,'5 3 :^4 >3 Overwintering Brood Overwintering Brood. Fayetterilfe Van Buren

c I «- t — r = ‘ ■ I ' I I, I I I ♦ * ' I I ^ ‘ ' ‘ ' ^ / o ^ c / t 3 ^4 S/19 V24 S/Z4 iyJ •/#' fc/J D ate Date

FrTit Generation Fifit Generation / 75. Fauetteville , \fan Buren / . '

I- ^ I I I f ^ •'I ' 1 ' I ) I Tfi 9* r/t* ^ ^ ^fs 46/ ^ r/n T/* 7/ H •y* Date Date

Second Generation Second Gen. ». Fagettevilk Van Buren Î Ï

1 ♦ ■ * • • • 1 ^ ' * I * I I , 1/ r f I I, I I I I 9^ 6^ ÿv ^9 1^ a/s iÿs'' syh ^a> tfrs Dote Date

Pig. II. Emergence of the principal broods at Fayetteville and Van Buren, 1952-54. 14

The main moth flights of the overwintering brood and first generation cover a period of approximately three weeks, while the second generation moth flight Is likely to be more protracted. At Fayetteville the main moth flight of the overwintering brood may be expected during the first three weeks of June and that of the first generation during the three weeks following the Fourth of July. The second genera­ tion moth flight should be underway by mid-August and strag­ gle Into September. At Van Buren the main moth flight of the overwintering brood may be expected during the last week of May and first two weeks of June and that of the first generation during the last week of June and first two weeks of July. The second generation moth flight may begin by the first of August and continue Into September.

This resumé of seasonal history should be useful as a guide In the examination of fields for eggs. As previously pointed out, emergence of overwintering brood moths may vary

In a rather small area In the same season. Variations In moth flights of all broods are to be expected, and have oc­ curred, from year to year. Examination of fields for eggs

Is necessary, therefore, to fix the oviposltion period ac­ curately In a particular season and area. 15

Description of Injury In c o m that has not tasseled, most early Instar larvae feed between the leaves of the whorl. Typical feeding areas are elongated, parallel to the leaf veins, with either the upper or lower epidermis remaining Intact, A small amount of dry frass Is deposited on and about the feeding site. By the time plant growth has exposed this Injury, the membranous leaf epidermis has often dried up and fallen out and the frass has been dislodged. The young larvae may also bore through the tightly furled leaves of the whorl, producing a series of small holes across the leaves when they unfurl. The amount of leaf consumed by the feeding Is usually quite small and the Injury of little consequence. Similar Injury Is caused by the European c o m borer, c o m earworm, fall armyworm and other "bud worms”. The most spectacular Injury caused by the southwestern corn borer Is "dead heart", resulting from destruction of the merlstematlc tissues of the young plant. Splitting the young plant through Its longitudinal axis reveals the stem, which comprises most of the merlstematlc tissues, to extend only a few Inches above ground level and to be enveloped by tightly furled leaves. The stalk of young plants Is com­ posed largely of furled leaves but the stalk of mature plants Is composed largely of stem. The site of most early borer activity Is In the whorl and separated from the merlstematlc 16 tissues by considerable amounts of leaf tissues. However, larvae entering the stalk from beneath the leaf sheaths of the lower leaves do so In the vicinity of the stem and fre­ quently destroy a large portion of the merlstematlc tissues. The central leaves are severed and soon blanch, contrasting strongly with the remaining green leaves. Hence the name "dead heart". Plants thus affected become deformed, stunted and unproductive. Occasionally fields are almost totally destroyed by "dead heart". Injury caused by larvae tunneling throughout the stalks Is often Insidious, This activity does not result In exten­ sive lodging but may cause dwarfing and heavy loss of grain production. Severe Injury Is quite obvious but that caused by moderate and light Infestations Is difficult to determine and seldom appreciated. Walton and Bleberdorf found yield reductions In Oklahoma to range from 8.7 per cent to 100.0 per cent In fields with more than half the stalks Infested (14). The ears also may be attacked by larvae. Entrance Is made through the silk channel, shank or husk, particularly where ear and stalk touch. Typically, the larvae bore be­ tween two rows of kernels and next to the cob but Injury may be confined to a small area at the point of entrance or to the cob. The damage to field c o m resulting from borer ac­ tivity In the ears Is of little Importance but In sweet c o m as much as one fourth the crop has been lost In this manner. 17 The high loss in sweet c o m is due to grading standards rather than actual grain consumed. Larvae are particularly obnoxious in sweet c o m since they are likely to be over­ looked in preparations for processing. If the borer has gone unnoticed during the growing sea­ son, its girdling activity will certainly attract attention in the fall. Overwintering larvae weaken the stalks by cut­ ting one or more v-shaped grooves around the inside at, or near, ground level. Some infested stalks escape entirely, and others are not girdled sufficiently to cause felling, but many girdled stalks fall of their own weight or are blown over by wind. The loss to rodents, mold and rot oc­ casioned by this activity may be great, depending on weather conditions, prevalence of rodents and other factors. Where mechanical harvesters are used the down c o m necessitates gleaning, an operation that is both expensive and distaste­ ful, or turning livestock into the field.

Adult The adult was originally described by Dyar (5) and re­ described by Dyar and Heinrich (6), Box (1) and Heinrich (2). The last three papers contain illustrations of the genitalia. The following description is intended principally for field identification in this area.

Description The moths are usually 15-20 mm, long with a wing expanse 18 of 30-40 nun. The males are generally smaller than the fe­ males and dwarf specimens of both sexes are sometimes en­ countered. The palpi are prominent and snout-like. The veins of the forewlngs and the Intervenular streaks In cells

R]|. to 1st A are buff on a light background. These Intervenu­ lar streaks, seven on each wing, terminate In minute, brown or black dots at the wing edge. A discal dot Is usually present on newly emerged adults but Is frequently lacking on old specimens. The veins of the hind wings, as well as the wing edge, are buff on a nearly white background. There Is considerable variation In the color Intensity of the veins and Intervenular streaks. The general appearance of the moth Is shown In Figure III.

Considering both appearance and behavior, there Is no other common moth likely to be confused with the southwestern c o m borer In Arkansas. However, final diagnosis should rest upon examination of the genitalia.

Behavior During the day moths are found resting quietly In the corn or on foliage bordering the field. When disturbed they may fly a short distance or merely run along the plant a few Inches. At night they move about actively but their flight is comparatively slow and erratic. 19

Pig. III. Female of the southwestern corn borer. Several of the terminal dots on the Intravenular streaks are just visible. About twice natural size. 20 Emergence In rearing moths from the pupal stage it was necessary to maintain high humidity and to cover the pupae with some material that would hold the pupal cases during the process of emerging, yet be sufficiently loose to allow the moths easy passage through it. The technique finally evolved was to place the pupae in a battery jar on plaster of Paris and cover them with a layer of excelsior. A glass tube imbedded vertically in the plaster of Paris and closed at the top with a one hole stopper permitted wetting the plaster of Paris without pouring water directly on the pupae. The top of the battery Jar was covered with muslin. It was observed that of 90 moths, 72 per cent emerged between 5 P.M. and 10 P.M., I8 per cent between 10 P.M. and 8 A.M., 3 per cent between 8 A.M. and noon and 7 per cent between noon and 5 P.M.

Longevity Field observations suggested a brief life span, since oviposition terminated shortly after adult emergence was com­ plete. To obtain more precise information on longevity, moths were collected daily from emergence cages, marked on the wings with model airplane dope and released in a large cage in the insectary. Dead moths were taken from the cage daily, sexed and their longevity determined from the code markings. Un­ marked moths were used as often as possible to afford a check 21 against possible adverse effects of the dope. No differences in longevity between marked and unmarked moths were observed. Although the females tended to live longer than the males, there was no great difference between the sexes in this regard. Individuals of both sexes lived as long as ten days but the average life span was near three days for the adults of the overwintering brood and between five and seven days for those of the first and second generations. The dif­ ferences in longevity among the three groups of moths were probably due to temperature. The average temperatures during the period the moths of the overwintering brood and those of the first and second generations were under observation were 82.1° P., 74,5^ P. and 75.7° P., respectively. The data on longevity are presented in Table 2,

Table 2. Longevity of adults. Overwintering First Second Days brood generation generation lived Males Females Males Females Males Females Number 1 0 2 0 0 0 0 2 12 7 0 0 0 0 3 14 19 1 3 0 0 ? 4 4 5 4 1 2 5 2 2 5 1 3 2 6 0 0 3 3 2 4 7 0 0 3 4 0 1 a 0 0 2 6 0 2 9 0 0 3 3 0 0 10 0 0 1 2 0 0 No. moths 32 34 22 26 6 11 Days Average longevity 2,9 2,9 6,1 6,6 5.2 5.9 22

Davis observed a limited number of moths, the males of which lived two to four days and the females five or six days (2). Failure to mate apparently did not affect longevity of the females. Eighteen virgin females were compared with twenty females caged with males and with fifteen females observed mating. The results are given In Table 3.

Table 3, Longevity of mated and virgin females.

Days No. virgin No. mated No. females caged lived females females with males 2 0 1 0 3 3 1 7 4 7 7 6 5 7 5 6 6 0 1 0 7 1 0 0 8 0 0 0 9 0 0 1 No. moths 18 15 20

Days Average longevity 4.4 4.3 4.2

Proportion of sexes Examination of field collected pupae and pupal cases showed a rather constant ratio between the sexes with the females predominating. Collections of pupae and pupal cases of each brood or generation were made on three different dates, the first when more than half the borers had pupated 23 and the last when pupation was complete. The results are summarized in Table 4.

Table 4. Proportion of sexes.

pupae and Proportion of sexes Per cent pupal cases Brood pupatedexamined Males Females per cent Overwintering 68-100 153 38.6 61.4 First 87-100, / 99 44.4 55.6 Second 58-100±/ 90 46.7 53.3 All 342 42.4 57.6 ^ T h i s is a partial generation. No pupae, only pupal cases, were found when the last examination was made.

Mating The courting male takes a position slightly behind and to one side of the female and facing in the same general di­ rection. A foot race sometimes develops at this point until the male is able to grasp the female with his genitalia or until she flies away. Immediately after grasping the female, the male moves in a short arc until the two moths face in nearly opposite directions with the wings of the female over­ lapping those of the male. If undisturbed, the moths remain in this position throughout copulation, which usually lasts from one to three hours. Of twenty-four females, sixteen mated the same night they emerged, seven the first night and one the second night fol­ lowing emergence. All of the seventeen males observed mating 24

did BO initially on the night of emergence. Eight of these mated a second time on the following night. Females appar­ ently mate only once. Fertile eggs were not obtained when freshly emerged moths were caged in pairs regardless of cage size or loca­ tion, even though one such pair appeared to mate,

Oviposition caged moths will oviposit on any smooth surface such as glass, metal, wood, or wax paper. Sheets of wax paper pinned to the side of the cage are ideal for collecting eggs. In addition, it is necessary to cover the floor with wax paper, because moths continue to oviposit after they can no longer fly, and to wrap the wooden frame of the cage with muslin to prevent oviposition on the exposed wooden surfaces. In large cages few eggs are deposited on screen wire. When first extruded the eggs are pliable and globular, being tamped into their characteristic form by the female's ovipositor. Heavily ladened females deposit an egg every three or four seconds, placing as many as thirty or forty in a mass before moving to a new site. In the field, however, large masses are rare. Most moths did not deposit eggs until the night follow­ ing mating, although two of fifteen individuals deposited a few eggs and one moth deposited a large number of eggs on the night of emergence and mating. 25

Fecundity and fertility Virgin females deposited considerably fewer eggs than did mated females. Eighteen virgin females, confined to­ gether, deposited an average of 94 eggs each while 15 mated females, held separately, deposited an average of 313 eggs each. Records of the mated Individuals are given In Table 5. In some cases part of the eggs were deposited on the wire cage and could not be Included In the fertility record. With two exceptions the percentage of fertile eggs was quite high. Both females depositing few or no fertile eggs were the sec­ ond mate of a male. The fecundity of moths of the three flights was esti­ mated by confining both sexes from emergence until death In a large cage In the Insectary. Twenty overwintering brood females deposited an average of 147 eggs each, twenty-six first generation females an average of 26? eggs each and eleven second generation females an average of 194 eggs each. The low average of the overwintering brood and second genera­ tion females may be due to failure of some Individuals to mate, since an adequate number of males was not available throughout the test. On the other hand. It Is doubtful that many females realize their potential egg production In the field, for moths that cannot fly become easy victims of pred­ ators and, as previously mentioned, caged moths often con­ tinue to oviposit after they can no longer fly. 26

Table 5. Fecundity and fertility of individual moths.

Number of eggs deposited No. eggs Per cent Days after mating observed for fertile 0 1 2 3 4 Total fertility eggs 239 277 79 0 605 560 88.4 0 316 133 0 449 449 98.0 0 288 116 0 — 404 404 100.0 0 200 177 4 0 381 381 98.8 0 276 42 0 — 318 318 96.2 0 158 108 43 0 309 216 97.7 0 163 129 0 - 292 292 97.6 0 228 27 0 - 255 255 94.5 0 158 66 0 0 224 224 98.7 0 112 0 103 0 215 172 96.5 6 166 0 -- 172 172 20 283 3 0 - 306 306 0 216 49 —- 265 265 0 229 33 0 — 262 245 0 241 0 241 241 313.2 aver. if 1/Moth dead. ^Second mate of male.

Oviposition preference It has been repeatedly observed that few eggs are de­ posited on very young or very old c o m when succulent, large plants are available. Since these observations were made on plantings that were, at best, adjacent, factors other than plant development might have been operative. However, egg counts made on replicated, dates-of-pianting trials bear out these observations as shown in Table 6. 27

Table 6. Relationship between plant height and oviposition.

Average No. Planting Plant height Plant eggs on date in inches stage August 28 July 1 72 Dough 58.2 July 15 63 Silk 180.2 August 1 27 Whorl 144.8 August 15 8 Whorl 36.0 LSD at 5^...

Twenty plants in each replication were examined for eggs on August 28, 1952. The earliest planting was in the dough stage and the leaves had begun to dry out and turn brown. The second planting was silking and the last two plantings were in the whorl stage* There was no significant differ­ ence at the 5 per cent level between the number of eggs found on the youngest and the oldest com. The difference between the two intermediate plantings bordered on statistical sig­ nificance and the difference between either of the intermedi­ ate plantings and the first or last plantings was highly sig­ nificant. It appears, therefore, that moths are attracted to c o m according to the stage or rate of development which may, within limits, be correlated with plant height * That plant height alone does not determine oviposition preference was demonstrated in dates-of-planting trials in 1954. The mid-May planting was severely affected by drought but had reached nearly full height. The mid-June planting 28 was In the early whorl stage and, although somewhat wilted, still succulent. An egg count on July 19 gave 24 eggs per hundred plants on the mid-May planting and 134 eggs per hun­ dred plants on the mid-June planting or roughly six times as many eggs on the younger plants. Besides corn, eggs have been found in fields of sorghum and on Sudan grass, millet and Johnson grass growing in corn fields. To determine the relative attractiveness for ovipo­ sition, egg counts were made on replicated plantings of corn, grain sorghum, millet and Sudan grass. The plantings were drilled in late June and egg counts made August 11-13 over ten feet of each of four replications. The average counts were 29.2, 5.2, 2.5 and 0,5 for corn, Sudan grass, sorghum and millet, respectively, indicating that only c o m is highly attractive. It should be pointed out that this planting was adjacent to c o m fields on both sides, Oviposition on crops other than c o m may have been increased by their proximity to com, for no eggs have been found in pure stands of Sudan grass.

Plight range It has not been feasible to determine adult flight range by the release and recovery of marked moths because no effi­ cient recovery method is known and no method has been devised for mass rearing or collecting. 29 In the late winter of 1952-53 a survey was made in the sparsely populated Boston Mountain Addition of the Ozark National Forest to find isolated, infested fields and to de­ termine the closest point from which the infestation could have been derived. One heavily infested field was one and one-half miles from any known source of infestation, there being no other cultivated fields within that distance. Other infested fields were found to be more than a mile from known sources of infestation. The habits of the borer are such that it is unlikely to be spread passively in any numbers, so it may be assumed that the moths are capable of flying at least one and one-half miles. Periodic surveys showing the borers' progress across Oklahoma (14), Kansas (15), and Arkan­ sas suggest that the moths may travel considerable distances, perhaps with the aid of winds. It is important to know whether moths of the overwinter­ ing brood fly to the closest corn field upon emerging in the spring or whether they fly, or are blown, about randomly. If moths are strongly attracted to the closest corn, an indi­ vidual or small group would benefit from the use of good cul­ tural control measures directed against overwintering larvae regardless of practices in the general area. If the moths are not strongly attracted to the closest corn, destruction of overwintering larvae would have to be undertaken over a rather large area to be of much value. 30 To clarify this point, three large areas were chosen, prior to emergence of the overwintering brood, that con­ tained undisturbed fields of heavily infested stubble and fields of young corn of about the same size and age. There were also several fields of plowed stubble in these areas. However, none of the plowed fields were close to fields of young c o m and their contribution to the moth population is believed to have been small. The percentage of infested plants in each field of young corn was estimated from five, fifty-plant samples per field taken June 29-30. At that time larvae derived from the overwintering brood moths were well established in the young corn. The fields of young corn were divided into three groups according to their dis­ tance from the nearest field of stubble. One group included those fields less than a quarter of a mile from the closest field of stubble, a second group those fields between a quar­ ter of a mile and one mile and a third group those fields more than a mile. Each of the three areas contained fields falling into two or more of these gmups. The results are summarized in Table 7. Although there were several uncontrollable variables, such as unequal sources of infestation, uneven growth of the young corn and varying relationships between young corn and stubble with respect to prevailing winds, any strong tend­ ency of the moths to fly to the closest field of young corn should have been evident. Prom these observations, a strong 31 attraction to the closest corn is not apparent. No conclu­ sions could be drawn as to the role played by prevailing winds In moth dispersal.

Table 7. Relationship between degree of Infesta­ tion and distance from closest source of Infestation,

Miles from closest source of Infestation Less than 1/4 1/4 - 1 Over 1 Per cent of plants Infested Minimum Infestation 17.2 4.4 14.4 Maximum Infestation 28.4 30.0 20.8 Average Infestation 21.8 18.3 16.8 No. of fields 3 6 4

Light The moths are not strongly attracted to light from Incandescent bulbs. New Jersey type light traps with the screen removed were operated along the borders of corn fields at times when moths were abundant In the field and oviposi­ tion was high. Males predominated In the catch by a three to one ratio. Usually only two or three moths were taken per trap In a night, A trap using four 15-watt fluorescent bulbs was operated at one station for three hours nightly over a twelve-day per­ iod while second generation moths were flying. Black light and white light bulbs were used on alternate nights and at­ tracted moths collected by hand. No moths were collected at 32 the white light but sixteen males and fourteen females were taken at the black light. Although black light cannot be considered very attractive, it might be useful in determin­ ing when moths are flying.

Egg Description The eggs are approximately 1,0 mm, wide, elliptical and decidedly flattened. They are deposited singly or in chains or masses with the individual eggs overlapping like shingles. About three eggs per mass was the average for 426 masses, in­ cluding single eggs, examined over the season. When first laid the eggs are entirely white, dull and opaque but older, fertile eggs are distinctly marked with three red, transverse bars. Shortly before hatching, the head capsule of the embryo appears as a black spot in the egg and the bars are no longer apparent, the red color being more or less diffused. Infertile eggs, and those with dead embryos, discolor in varying degrees but do not develop the characteristic markings. Eggs parasitized by Triehogramma minutum Riley are readily distinguished by the intense black color. During extremely hot and dry weather some deviation from normal egg development was noticed. The eggs became wrinkled, possibly because of desiccation, and were light yellow rather than white. The red bars were frequently 33

Irregular or reduced to discontinuous bands, flecks or splotches. Many embryos, apparently fully developed, died before or during eclosion. The eggs of the European corn borer are not uncommon in some areas of the state and might be mistaken for freshly deposited eggs of the southwestern corn borer. However, the egg masses of the European corn borer usually contain many eggs and have a noticeable sheen. The Individual eggs are somewhat smaller than those of the southwestern corn borer.

Location of eggs Most eggs are found on the upper surface of leaves, par­ ticularly on the basal portion. The lower surface of the leaves is the position of second choice and a few eggs are placed on the stalk. Second and third generation eggs were tabulated weekly according to their position on corn in the whorl stage. Of 1,263 eggs examined, 60.2 per cent were on the upper surface of the leaves, 26.7 per cent were on the lower surface of the leaves and 13 .I per cent were on the stalk.

Development and incubation Observations on development and Incubation were made on eggs deposited on wax paper by caged adults. These eggs were removed from the cage daily and held in jelly glasses. The observable rate of development varied according to temperature. The red bars appeared on the first day after 34 oviposition, although they were faint and Incomplete In cool weather, and deepened In color for one or more days after their appearance. Head capsules were apparent on the day before hatching began. Incubation required from four to seven days with most eggs of the same age hatching over a two-day period. The data given In Table 8 summarize observations made on all three generations. The temperature given Is an average of readings made twelve times dally from the time the first eggs were collected until hatching was complete.

Table 8. Incubation period. 1953.

Aver. No. Per cent hatch Deposition P. of dates temp. eggs 4th day 5th day 6th day 7th day

July 8-10 71 494 0.0 0.0 51.4 48.6 August 10-12 80 995 28.1 69.4 2.5 0.0 June 10-16 83 1055 75.6 21.7 1.9 0.5

Eclosure The majority of eggs hatch during the early morning, the larva emerging through a silt cut In the exposed end of the egg. The larva, as soon as It Is free of the egg, raises the head and thorax above the leaf surface, swaying from side to side for several seconds. Following this ritual. It Immedi­ ately leaves the vicinity of the egg mass and crawls randomly over the plant surface until reaching the whorl or some other secluded place, positive thlgmotroplsm Is evident In larval 35 behavior but no phototroplsm or geotropism was observed. The larvae are not Inclined to drop on a spun thread from one leaf to another.

Larva Description The first instar and fully grown larvae have been de­ scribed by Heinrich (2) and methods for separating the three common species, Diatraea saccharallis (Fabr.), D, crambio- doides (Grote) and D. grandiosella Dyar, are given by Peterson (10). In Arkansas, the southwestern corn borer is the only Diatraea species known to be present so that field identification is simplified. The first instar larvae have a definite reddish cast that is seen, under magnification, to be more or less con­ fined to the prothorax and to a broad, dorsal band across most abdominal segments. The second instar larvae retain this color in varying degrees, particularly in the prothorax. The head capsule and cervical shield are dark brown in the first instar and become increasingly lighter in following instars. The reddish color of the first and second instars is sufficiently reliable for field identification. The third and succeeding instars are easily distinguished from other larvae commonly found in corn and related crops by the regular pattern of black spots, the pinacula, on a yellowish- white background. The overwintering brood of larvae molt in 36

the fall, assuming the immaculate, yellowish-white winter form with this molt. The black pinacula of the summer form gradually fades to a light brown in some, but not all, lar­ vae prior to this molt,

Pall activity Grown larvae of the overwintering brood converge on the stalk base below ground level, sometimes descending part of the distance externally and re-entering the stalk at a lower point. Seldom more than one overwintering larva survives in a stubble, although in the fall larvae may be numerous in the stalk. This reduction is presumably due to cannibalism and to the death of larvae forced to take less protected posi­ tions in the stubble. After tunneling to the extreme stalk base the larvae usually ascend to, or near, ground level and girdles the stalk internally (Fig. IV). An occasional stalk is girdled in more than one place but it is not certain that this is the work of a single larva. Both summer and winter form larvae are found in girdled stalks, so it is assured that summer form larvae are responsible for some girdling. Overwintering larvae normally assume a vertical posi­ tion in the stalk base, oriented toward the soil surface (Pig. V). The tunnel into the stubble is closed by a few silk strands and varying amounts of frass. Where the stalk is girdled at soil level, dirt and debris frequently cover 37

Pig. IV, Girdled stalk. Note even break and sharp edge. 38

Fig. V. Overwintering larva in base of stalk. 39 the entrance. Larvae In poorly sealed stubble often drown following the first fall rains, for the concavity in the girdled stubble serves to funnel water Into the tunnel. Girdling activity has been noted as early as August 20 in the Arkansas River valley but does not generally start until the last few days in August or early September. In the vicinity of Fayetteville a few stalks may be girdled about the first of September and girdled stalks are present in most fields during the second week of September. Appar­ ently early girdling activity is due entirely to the activ­ ity of second generation larvae, in one field at Van Buren in which both second and third generation larvae were pres­ ent girdling began August 27 and continued until September 24. In an adjacent field of late corn infested with third generation larvae only, girdling did not begin until after September 11 and continued until October 7. The data are presented in Table 9.

Table 9. Girdling activity in fields Infested with both second and third generation and with third generation larvae only. Van Buren, 1953.

Second and third generation Third generation Per cent stalks Per cent stalks Date girdled Date girdled August 27 Less than 1 September 11 0 September 4 31 September 24 26 September 10 49 October 2 44 September 24 72 October 7 68 40

The amount and thoroughness of girdling vary consider­ ably from field to field. Some stalks are girdled completely around their circumference while others, although infested, escape entirely. All gradation between these extremes are found. In November 1953# ten fields in Benton and Washing­ ton Counties were examined for the frequency and degree of girdling. Ten consecutive stubble in five places were dug up, split and examined in each field, for a total of fifty stubble per field. Infested stubble were classified accord­ ing to the degree of girdling. Those stubble girdled more than half their circumference were classed as "girdled", those girdled less than half their circumference as "par­ tially girdled" and those showing no girdling whatsoever as "not girdled". The observations are summarized in Table 10.

Table 10. Degree of girdling in ten c o m fields in Washington and Benton Counties. November, 1953.

Per cent infested stubble that were: Partially Not Per cent stubble Girdled girdled girdled infested 94.1 5.9 0 34 93.1 0 6.9 58 83.3 3.3 13.3 60 82.9 5.7 11.4 70 70.6 11.8 17.6 60 69.4 13.9 16.7 72 64.0 24.0 12.0 50 48.7 17.9 33.3 78 48.5 42.4 9.1 66 44.8 27.6 27.6 58 Average 69.9 15.2 14.8 60.6 41

There were bo many variables among the fields examined that no explanation of the observed differences in the de­ gree of girdling can be advanced. Wilbur et observed that the per cent of infested stalks girdled varied from field to field and correlated the difference with time of planting (15). A smaller percentage of stalks were infested in the earlier plantings and fewer infested stalks were gir­ dled. In Arkansas, however, no consistent relationship be­ tween planting date and the degree of girdling has been observed.

Survival of overwintering brood The overwintering brood is considerably decimated by the time the first moth flight begins. Physical factors, rain and cold, are responsible for most of the reduction in population. That this mortality is a very important natural check is evident from the relatively small number of first generation eggs deposited. Survival of overwintering brood larvae in 1952-53 was determined by sampling the population in seven fields during August and again in the same fields in May. This method was uneconomical of labor, for of thirty odd fields examined in the fall only seven remained undisturbed by May. Survival in undisturbed stubble during 1953-54 was estimated by ex­ amining, in May, samples of girdled stubble from ten fields. Girdling, however slight, was taken as evidence that a 42

stubble was once infested by an overwintering larva. Inasmuch as not all overwintering larvae girdle, estimates of survival obtained in this manner would be slightly biased in the event survival of non-girdling larvae should differ materially from that of girdling larvae. During the winter of 1952-53, there was a minimum of survival of l6 per cent, a maximum of 54 per cent and an average of 37 per cent in the seven fields examined. The following winter the minimum survival found among ten fields was 10 per cent, the maximum 72 per cent and the average 30 per cent.

Spring activity The overwintering larvae resume activity in early spring, cleaning an escape tunnel of debris and closing the entrance with a thin web. Most larvae then assume their former posi­ tion in the crude pupal cell in the stalk base although a few pupate in that part of the stubble above ground, A few larvae held in soil cages molted once in the spring before pupating.

Number and duration of instars The number and duration of instars are variable, appar­ ently being affected by temperature and by the kind and con­ dition of food. All data are from larvae confined individ­ ually in vials partially filled with moist plaster of Paris 43

and closed with a one-hole stopper, the hole being loosely plugged with cotton. Each vial was exsunlned dally for cast head capsules and the food changed every other day or dally If wilted or spoiled. The data In Table 11 are from larvae reared on corn In the Insectary, Host plants from the same planting were used to minimize possible differences due to plant age. The first and second Instars were fed tender leaves taken from the whorl and third and succeeding Instars were fed pieces of the stalk. The time required for one complete generation In the field, coincident with the generation being reared, was estimated from the date of fifty per cent emergence of the overwintering brood to the same stage of development of the first generation and found to be forty-one days. The reared Insects required an average of 40,7 days if one day Is allowed for preovlposltlon. Reared pupae were consider­ ably smaller than those obtained In the field. The number of Instars varied from five to eight. Of the twenty-four larvae successfully pupating, one passed through eight Instars, nine through seven Instars, thirteen through six Instars and a single larva through five Instars. The minimum, maximum and average duration of the Instars are given In Table 11, These data agree substantially with those given by Davis £t al, (2), These authors found the larvae to have as many as nine Instars but most of them pupated following the sixth Instar. 44

Table 11. Duration of Instars on corn in days.

Instar number All 1 2 3 4 5 6 7 8 instars Minimum 2 2 2 2 2 2 5 9 20 Maximum 5 6 5 6 12 9 9 35 Average 3.1 2.9 3.0 4.2 7.0 6.9 9.0 27.7 No. of larvae 24 24 24 24 24 23 10 1 24

As a preliminary study of the effects of different hosts on development, larvae were reared on corn, sorghum, millet and Johnson grass. The rearing was done in an incubator at 80° P. in order to eliminate temperature from consideration in comparing instars and stage durations. Host age was not considered, except that mature and very young plants were avoided. The larvae were given a choice of leaves and stalk or stem throughout the larval period but were otherwise han­ dled as before. At least some of the larvae started on each host completed their development, although there was a marked difference in survival and in the time required to reach the various stages of development. Corn was apparently the most satisfactory host, as far as rate of development is concerned, followed by sorghum. Millet and Johnson grass were least suitable. Survival on both corn and sorghum was much greater than on either millet or Johnson grass. The results are summarized in Table 12. 45

Table 12. Average duration of Instars on various hosts.

Johnson Corn Sorghum Millet grass Number Number Number Number Instar larvae Days larvae Days larvae Days larvae Days

1st 22 2.6 21 3.3 15 5.7 16 3.9 2nd 21 2.9 21 2.5 11 6.1 14 2.7 3rd 19 2.8 21 2.9 9 6.3 14 4.1 4th 19 3.2 20 4.0 9 6.3 14 4.6 5th 17 5.B 17 4.6 9 5.2 10 5.2 6th 8 8.1 15 7.1 9 7.3 8 7.8 7th 1 6.0 5 10.0 7 9.0 6 6.7 8th 1 7.0 1 9.0 4 6.0 2 11.5 9th - - 0 - 1 3.0 - - Entire larval 15 22.0 13 29.0 4 48.7 1 45.0 stage

Of the larvae reared on corn In the Incubator, fifteen pupated successfully: eight following the fifth Instar, six following the sixth Instar and one following the eighth In­ star. The first three Instars required an average of 8.3 days or 38 per cent of the larval period as compared to an average of 9.0 days or 32 per cent of the larval period for the larvae reared In the Insectary, Thirteen larvae reared on sorghum pupated successfully, nine of these following the sixth Instar and four following the seventh. One of the lar­ vae reared on millet pupated following the seventh Instar, two following the eighth and one following the ninth. The single larva successfully reared on Johnson grass pupated following the seventh Instar. 46

Larval position on corn It Is fundamental to the successful use of contact or residual Insecticides to establish the time that larvae enter the stalk and become Inaccessible. Reconstruction of larval movement on the plant and the manner of entrance Into the stalk may offer a clue to plant characteristics essential to effective resistance. For these reasons an Investigation was undertaken to establish when, and In what manner, the larvae enter the stalk. As a prelude to this Investigation, measurements were taken of the head capsule width of reared larvae of known Instars and of 1,365 field collected larvae to establish the variation occurring within each Instar, The larvae taken from various parts of the plant could then be placed both developmentally and chronologically. Head capsules were measured with a calibrated ocular micrometer mounted In a binocular. Under the magnification used, the space between adjacent lines on the micrometer represented O .05 mm. Measurements were taken to the closest line. I.e. to ± 0.025 mm. of the actual head capsule width. The frequency chart of head capsule measurements of field collected larvae does not show the limits of the In­ stars In a definite fashion, although the Instar to which most larvae belong can be recognized (Pig. VI). The size variation In reared larvae of known Instars (Table I3 ) 47

• 0 _

4 4

Head capsule width in

Pig. VI, Distribution of 1,365 field collected larvae according to head capsule width. 48

serves to assign most of the field collected larvae to their respective Instars. Larvae with head capsule widths of O.65 and 0.70 mm. cannot be placed, from these data. In the sec­ ond or third Instars exclusively. None of the reared larvae fall In this range but It Is probable that Individuals of both Instars are Included. The same situation applies to larvae In the 1,05 mm, category. This group probably con­ tains some third Instar larvae and some fourth Instar larvae. An overlapping occurred In reared larvae of the fourth and fifth Instars In the 1,45 and 1.50 mm. range. No attempt was made to distinguish between fifth and any succeeding In­ star, since a distinction was not essential to the problem.

Table 13. Head capsule width of reared larvae of known Instars.

Head capsule , Instar width In mm,j/ 1st 2nd 3rd 4th 5th Minimum 0.35 0.45 0.75 1,10 1.45 Maximum 0,40 0.60 1,00 1.50 2.05 Average 0.36 0.53 0.86 1.34 1.77

No, larvae 41 56 41 42 13

\/ ± 0.025 mm.

Dissections of whorl stage corn were made during and after ovlposltlon to determine the location of larvae of the various Instars. For this purpose the plant was divided along lines of physical similarities rather than by organs. 49 The stalk refers to the supporting structure above ground and below the uppermost flagged leaf. This includes the tightly furled leaves enveloping the stem, as well as the stem proper, but excludes the more loosely applied sheaths of flagged leaves. The whorl refers to the leaves above the stalk. All but a few of the first and second instar larvae were feeding in the whorl, either between the loosely furled leaves or boring in those that were tightly furled. The larvae of these two instars found in the stalk were usually located in the superficial leaf layers and beneath the leaf sheaths. Occasionally a small larva had bored into the leaf midrib. Most of the third instar larvae were still in the whorl, although about ten per cent were boring in the stalk. More than half of the fourth instar larvae and nearly all of the larvae beyond the fourth instar were located in the stalk. The data, summarized in Table 14, exclude the few larvae that could not be placed definitely as to instar.

Table 14. Location of larvae on whorl stage corn.

Percentage in; Instar No. larvae Whorl Sheath Stalk

1st 154 93.5 4.5 2.0 2nd 151 91.3 6.0 2.7 3rd 117 80.3 9.4 10.3 4th 82 33.0 8.5 58.5 5th and 6th 86 8.1 1.2 90.7 50 Using the average stadia of reared larvae, the position of the various instars on whorl stage corn can be fixed chronologically. On the third day from hatching approxi­ mately two per cent of the larvae will have entered the stalk and on the sixth day about three per cent. As pre­ viously pointed out, the penetration of larvae of the first and second instars is usually not deep enough to afford much protection to the larva. By the ninth day about ten per cent of the larvae will have bored into the stalk, most of them penetrating beyond the superficial leaf layers to well shel­ tered positions. Three or four days later nearly sixty per cent of the larvae will have become borers and in another four to six days about ninety per cent. In dissections, the percentages of larvae in the stalk are always smaller than those given above because larvae of other than the initial hatch are included. Prom examination of young corn it appears that the ma­ jority of larvae, threatened with exposure by the flagging of leaves, move beneath the leaf sheaths and enter the stalk from this position. This is not obvious from examination of mature corn because lengthening of the internodes and loss of the lower leaves and their sheaths expose the entrance holes. Then, too, additional holes are often created by the larger larvae. In c o m past the whorl stage nearly all larvae were found in the stalk, beneath the leaf sheaths and in or on 51 the carpellate Inflorescences. The carpellate inflorescences are commonly called lower ear shoots, nubbins and ears, de­ pending upon the extent of their development, but there is a continuous gradation from the smallest lower ear shoot to the largest ear. In dissections, those inflorescences protrud­ ing from the leaf sheath were arbitrarily classed as "ears", while those concealed beneath the leaf sheath were classed as ear shoots, of larvae in the first four instars, 4.6 per cent were found in the stalk, 39.S per cent in the "ears" and 55.6 per cent beneath the leaf sheath. More than half of the full grown larvae were in the stalk and the remainder equally divided between the leaf sheath and ears. Larvae beneath the leaf sheath were feeding on the internal surface of the sheath and on the succulent lower ear shoot. It is anticipated that the relative number of third and fourth in­ star larvae in the stalk may increase on less mature corn with softer stalks. Occasional larvae were found between brace roots, in the leaf midrib and at the base of the ligules. Second and third instar larvae are able to penetrate the stalk of whorl stage corn but first instar larvae have some difficulty in doing so. To test the ability of the first three instars to effect an entrance, short sections of stalk of month-old field corn were closed on both ends with sealing wax and one larva confined with each section. Twenty-five larvae in each of the first three instars were 52 used. Thirteen first instar, twenty-two second instar and twenty-four third instar larvae had successfully entered the stalk by the following day. That most larvae in these in­ stars are leaf feeders in the whorl under field conditions seems to be a matter of chance or choice rather than ne­ cessity. A similar test was conducted with sections of stalk from the same field when the corn was three and a half months old and bearing immature ears. The stalk was extremely hard and brittle at this age. Only one second and one third in­ star larva managed an entrance in two days. No first instar larvae were tested. Apparently the boring habit on grown corn is limited by the inability of early instars to pene­ trate the stalk, if for no other reason.

Prepupa The prepupal stage is not very distinct. During this stage the insect is less active, responds slowly to stimuli and assumes a more or less rigid, linear position. The pre­ pupal stage apparently lasts a day or less.

Pupa Description Only two species of Lepidoptera, the southwestern corn borer and the European corn borer, commonly pupate in corn stalks in Arkansas. The pupae of these insects are readily 53

separated by differences In the posterior abdominal segments.

The pupa of the southwestern corn borer Is comparatively blunt posteriorly and the 8th, 9th, and 10th abdominal segments bear numerous sharp, stout, black tubercles. These are ap­ parent to the naked eye. There Is no cremaster. Abdominal segments 5, 6, 7 and 8 have a median band of fine, rugose sculpturing encircling each segment. The thorax and first four abdominal segments are similarly sculptured dorsally. The European corn borer pupa tapers more sharply posteriorly than does the southwestern corn borer pupa. It has neither the tubercles nor sculpturing but does have a cremaster.

In the southwestern corn borer pupa the anal opening Is the shape of an Inverted Y and located on the 10th abdominal segment. The sll",-llke genital opening lies entirely within the 9th abdominal segment of the male and between a pair of tubercles. In the female the genital opening appears to lie

In both the 8th and 9th abdominal segments. The anterior margin of the 9th segment Is sharply convex medially. The anterior margin of the 10th segment Is likewise convex and touches the genital opening. There are no tubercles near the genital opening. Males are usually smaller than females from the same host. The pupa of the southwestern corn borer has been de­ scribed and figured in detail by Heinrich (2). 54

Duration of pupal stage Individuals of the overwintering generation held in soil cages for pupation and emergence records had a minimum pupal period of eleven days, a maximum of twenty days and anaver­ age, for thirty-two pupae, of 14.8 days. In 1952, pupae of the first generation, when held under insectary conditions, required a minimum of nine days, a maximum of eleven days and an average, for twenty-one pupae, of 10.0 days for the pupal stage. Thirty-five pupae of the first generation in 1953, handled in the same manner, spent a minimum of eight days, a maximum of thirteen days and an average of 11.4 days in the pupal stage.

Hosts Other Than Corn Although corn is the only crop seriously damaged by the southwestern corn borer at present, sugar cane, sorghums, broom corn, Sudan grass and Johnson grass have been reported as hosts (15). Since secondary hosts might provide an im­ portant source of infestation in com, produce additional broods by altering the period of larval development or be­ come severely infested in the absence of corn, some know­ ledge of borer survival and development on common local, secondary hosts is desirable.

The initial work on this problem has been presented in part in the discussion of the number and duration of instars. 55

In general, the secondary hosts investigated increased both the number and duration of instars.

Additional rearing was carried out in the insectary, simulating field conditions as closely as possible. Larvae reared on corn, sorghum and millet were fed on leaves taken from the whorl until half grown and then on stalks, since these crops were in the whorl stage and only leaves would be available to most young larvae in the field. Larvae reared on Sudan grass and Johnson grass were offered both leaves and stems. For survival records, larvae living less than one day and not feeding were replaced until at least forty- nine larvae were established on each host. One larva reared on sorghum, six larvae on Sudan grass and two on Johnson grass molted to the overwintering form after apparently at­ taining full growth. These larvae were not included in the average duration of the larval stage to avoid undue distor­ tion.

The results obtained from insectary and incubator rear- ings are summarized in Table 15. In^ji#th instances corn was the superior host Judging from survival and rate of develop­ ment. 56

Table 15, Effect of host on larval development and survival.

Larval period In days Survival Host Min. Max. Aver. No. per cent Reared In Incubator at 80® p. Corn 17 36 22.0 15 63.6 Sorghum 22 49 29.0 13 61.9 Millet 41 57 48.7 4 19.0 Johnson grass 45 45 45.0 1 4.3 Reared In Insectary Corn 18 38 77.5 Sorghum 32 3 ^ : 6 ^ 27 54.0 Sudan 27 40.4^7 28 50.0 Johnson grass 34 1 43.1^ 16 32.0 ^ Excludes overwintering larvae.

Field survival and development on corn, sorghum, Sudan grass and millet were checked by artlflcally Infesting these hosts with eggs deposited on wax paper by caged overwintering brood moths. The egg masses were randomly divided Into four nearly equal lots and one mass was stapled to a leaf of each host plant. Natural ovlposltlon was very light. Plants were dissected for survivors Just before pupation was expected to begin. No larvae were found on millet and comparatively few survived on Sudan, but survival on sorghum was nearly equal to that on corn. Larval growth, as Indicated by size, was slower on sorghum than on corn while on Sudan it compared favorably with that on com. The data are given In Table 16. 57

Table 16. Effect of host on larval survival and development of the first generation in the field.

Head capsule y Number of larvae on width in mm.*/ C o m Sorghum Sudan Millet

- 0.70 2 1 0 0.75 - 0.95 5 11 1 - 1.00 - 1.20 15 24 1 — 1.25 - 1.45 15 53 6 — 1.50 - 1.70 47 33 8 - 1.75 - 1.95 29 2 5 - 2.00 - 2.20 13 0 6 - 2.25 - 2.45 7 0 1 - 2.50 - 1 0 0 - Total No. larvae 134 124 28 0

No. egg masses 57 57 57 46 y ± 0.025 mm.

The same planting was also artificially infested with the second generation and dissections were made when pupa­ tion on adjacent corn was near fifty per cent. As in the previous experiment, egg masses were divided randomly into four lots and a single mass attached to each plant. The most striking result was the relatively low survival on all hosts. C o m was the superior host with respect to survival, sorghum and Sudan grass producing less than half as many borers. Again the rate of development on Sudan grass com­ pared favorably with that on c o m (Table 17). 58

Table 17. Effect of host on larval survival and development of the second generation In the field.

Number egg Number Number Number Host masses larvae pupae emerged Total

Corn 72 14 10 4 28 Sudan 90 6 6 0 12 Sorghum 90 11 0 0 11 Millet 77 1 0 0 1

Pupae from secondary hosts were smaller than those from corn. Even though survival on sorghum and rate of develop­ ment on Sudan grass may compare favorably with that on corn,

the former hosts apparently are deficient In promoting lar­ val growth.

Growth of reared larvae was noticeably retarded from

late fourth Instar on, resulting In abnormally small pupae on all hosts. The pupal weights of field collected larvae and reared larvae are given In Table l8. 59

Table l8. Average weight in milligrams of collected and reared pupae on various hosts.

Males Females Source and host Weight Number weight Number Collected in field Corn 125 10 227 10 Sorghum 116 18 173 18 Sudan 88 3 174 5 Reared, Lot 1 Corn 88 6 127 8 Sorghum 102 6 104 6 Millet 50 4 - 0 Johnson grass 43 1 - 0 Reared, Lot 2 C o m 98 19 I4l 16 Sorghum 75 13 102 7 Sudan 64 6 93 11 Johnson grass 65 5 70 1

The longevity and fecundity of moths emerging from pu­ pae collected on sorghum and corn were comparable. Since pupae were not available from both hosts on the same date and in the same area, the pupae on sorghum were collected at Van Buren and those on corn at Fayetteville, Moths from both lots emerged at about the same dates. The observed differences, shown in Table 19, between moths from the two hosts cannot be considered significant. 60

Table 19. Longevity and fecundity of moths from c o m and from sorghum.

C o m Sorghum Criterion Males Females Males Females Days Minimum longevity 4 4 4 2 Maximum longevity 6 8 8 7 Average longevity 5.2 6.0 5.2 5.0 Number Average number eggs 194.2 179.6 Number moths 6 11 19 28

The possibility of the borer maintaining itself on sor­ ghum, at least, cannot be entirely dismissed. Sorghum is not usually infested to any extent because moths prefer to ovi­ posit on corn. However, in the absence of succulent corn, moths will oviposit freely on sorghum. At the time of the second generation moth flight in 1954, nearly all c o m in the vicinity of the Main Experiment Station was drought striken, dry and unattractive for ovlposltlon but a field of sorghum and another field of late. Irrigated corn were heavily infested with third generation larvae.

Natural Enemies With the exception of a few overwintering larvae de­ stroyed by nematodes, no parasitized larvae have been found in the field. A few reared larvae were killed by an hyme- nopterous parasite, identified by B. D. Burks as a braconfd. 61

A single parasite emerged from each fourth or fifth Instar host but did not complete Its development« suggesting that the borer was not a normal host. Since the larvae were not accessible to adult parasites. It Is assumed that the eggs were parasitized In the field, A braconld, Apanteles dla- traeae Mues., has been reported as a southwestern c o m borer parasite In Arizona but the eggs of this species are ovi­ posited In the larvae (3). A small number of overwintering larvae are destroyed, apparently Indirectly, by subterranean termites working In the corn stubble during the fall and winter. The stubble are often completely hollowed out, exposing the borer lar­ vae to rain and cold. Rodents, presumably mice, sometimes cut a hole Into the stubble at ground level and feed on the overwintering larvae. Where c o m Is shocked In the field, mice are often numerous and a large proportion of overwintering larvae may be de­ stroyed. Excepting special conditions, rodents are a minor control factor. Egg parasites of the genera Trlchogramma, Prospatella and Teleonomus have been reported from southwestem c o m borer eggs (2). Only Trlchogramma mlnutum Riley, determined by B. D. Burks, has been found parasitizing borer eggs In Arkansas. The degree of parasitism has varied greatly but has usually been less than twenty per cent. One notable exception occurred In the fall of 1952 when three-fourths 62 of the third generation eggs were parasitized. Parasitized eggs are an intense black color and contain, usually, two or three parasites. The adult also destroys, by stinging, some eggs in which it does not oviposit. No predator or disease of consequence has been observed.

Control Methods The entire borer problem can be avoided where it is fea­ sible to substitute small grain or other non-hosts for com. Sorghum, though sometimes infested, is not seriously damaged and may be a suitable replacement in many instances. There is, however, a substantial acreage on which corn is a reasonably dependable and highly profitable crop. Here the borer problem must be squarely met rather than avoided. The potential overwintering population is greatly re­ duced by the low survival of the third, or partial third, generation on early planted corn and by the competition among the larvae for the favored overwintering position in the stalk base. Further larval mortality occurs during the winter. Nevertheless, the larval population in undisturbed stubble may still be quite large in the spring.

Cultural control To determine the effect of fall plowing on larval sur­ vival, a hundred stubble were randomly placed in a pit and covered with earth to depths of one to six inches in November. 63

Samples taken from stubble collected for this experiment In­ dicated a larval population of 1?6 per hundred stubble at the time of burial. Even though the stubble was badly decomposed when dug up In late April, 120 living larvae were recovered. The survival of burled larvae was 68 per cent, compared to 16 per cent survival In the undisturbed stubble. Plainly, burying the larvae Is not detrimental to their welfare. In pursuance of this study, two hundred stubble thought to be infested were removed from the field In May and ran­ domly divided Into four lots. Two pits were dug and fifty stubble burled In each at depths of one to six Inches. The other two lots were set In the ground to simulate the un­ disturbed condition, A cage was then placed over each of the four lots and examined dally for moths. No moths emerged from one lot of burled stubble and three emerged from the other lot; twenty moths emerged from one check lot and twenty-three from the second, A week after the last moth emerged all the stubble was dug up and examined. Most of the burled borers could not be accounted for but some were found dead In all stages. Ex­ amination of the surface debris on fields plowed In the fall or spring gave no Indication that burled larvae had worked to the surface and entered surface debris. Infested stubble was collected In March the following year, 1954, and the burial experiment repeated. One moth was recovered from one lot of burled stubble and three from the 64

other. Twenty-one and seventeen moths were recovered from the check cages. Burial thus reduced the number of emerging moths 93.0 and 89,5 per cent, respectively, in the two ex­ periments. On a larger scale, three 10 by 12 feet plots were staked out and 18O stubble placed in each plot. On May 4 one plot was double disked as though for seeding small grain, the second plot double disked once and turned, with a moldboard plow, and the third plot left undisturbed. Each was then covered with a muslin tent and emerging moths counted and removed. One moth was recovered from the disked plot, four from the disked and turned plot, and nineteen from the check. Although recovery of moths from the check was unusually low, both treatments showed considerable reduction in the number of emerging moths. Overwintering larvae are susceptible to winter tempera­ tures when not protected by soil around the stubble. Walton and Bieberdorf investigated this phase of control extensively in Oklahoma (14). Work along this line in Arkansas was limited to ascertaining that similar results could be ob­ tained under local conditions. Listing stubble to the sur­ face in late November, to take advantage of the year's lowest temperatures in December, January and February, gave 73.7 and 88,2 per cent control by mid-February in two fields in 1953- 54. Hand pulling the stubble resulted in 97.1 per cent con­ trol. 65 Combining fall listing with spring plowing and disking will produce maximum control. Stubble treatment must be practiced over a large area to be beneficial, for It has been observed that fields a mile or more from the closest stubble have nearly as many plants Infested as do fields much closer to stubble. Early planting Is Indicated by available data Inasmuch as this practice reduces loss from "dead heart" and harvest­ ing may be completed before girdling begins. In a dates-of- plantlng test at Fayetteville, corn planted In mld-Aprll, mid-May and raid-June had 1, 8 and 40 per cent, respectively, of the plants affected by "dead heart". Girdling of In­ fested c o m can be largely avoided If harvesting Is com­ pleted by September first In the Arkansas River Valley and a week or ten days later In the vicinity of Fayetteville.

Insecticidal control Insecticide treatment may be desirable on highly produc­ tive land or where the crop Is of special value, such as c o m grown for seed production. At present, effective treatments have been developed only for corn In the early whorl stage and these are Intended primarily to reduce damage from "dead heart" and stunting. The results of Insecticide Investiga­ tions have been published separately but may be briefly sum­ marized here (11). The treatment consists of two spray applications of suf­ ficient volume to obtain run-off Into the whorl where the 66 small larvae are feeding. The exact volume of spray per acre depends upon the size of the corn and must be adjusted to the particular situation. Timing of the applications Is critical. The first application should be applied on the ninth day after eggs In the black-head or hatched stage are found. The second application should be applied on the seventeenth day. The reason for this timing Is evident from examination of the location of larvae on whorl stage corn. About ninety per cent of the larvae from the Initial hatch are still feeding In the whorl, where they can be reached and killed by Insecticides, on the ninth day. A larger percentage of larvae hatching between the first and ninth day are also In the whorl. However, a few days later many larvae of the Initial hatch have bored Into the stalk where they cannot be reached by Insecticides. Since the ovlposltlon period of the overwintering brood and of the first generation covers a period of about three weeks, two applications are necessary. EPN, Isodrln and endrln have given effective controls at rates of one, one-half and one-eighth pounds, respec­ tively, of active material per hundred gallons of spray.

In two tests EPN reduced the number of Infested plants by

90,1 and 87.3 per cent. In single tests, Isodrln reduced the number of Infested plants by 87.3 per cent and endrln by 72.8 per cent. 67

While this method of control is practical on young corn, particularly on late plantings where loss by "dead heart" is commonly high, it is not suited to older, larger corn because of the high volume of spray required and the need for special machinery.

SUMMARY The southwestern corn borer, Diatraea grandiosella Dyar, is an important, though recent, pest of corn in Arkansas. The insect has been reported present in nine southwestem and midwestern states : Arkansas, Arizona, Colorado, Kansas, Missouri, Nebraska, New Mexico, Oklahoma and Texas, Only small areas of Colorado and Nebraska have been infested. Yield is reduced by destruction of meristematlc tissues in young plants, resulting in stunting and "dead heart", and by tunneling in growing c o m and girdling of stalks in the fall. The insect overwinters in the larval stage below soil level in the stubble. In the spring pupation takes place in the stubble and moths fly to young corn where they de­ posit most of their eggs on the leaves. The larvae feed be­ tween the leaves of the whorl during the first three instars but bore into the stalk in the later instars. The insect pu­ pates above ground excepting the overwintering brood. There are from two and a partial third generations to three and a trace of a fourth depending upon the season and locality. 68

Moths live but a short time, averaging less than a week, and do not feed. Usually mating takes place the night of emergence and oviposition begins the following night. Fe­ males deposited an average of about two hundred eggs, al­ though over six hundred have been obtained from an indi­ vidual. Fertility is normally quite high.

Moths have a strong preference for succulent, large corn but will oviposit on small corn, maturing c o m and other crops. Larvae can develop on sorghum, Sudan grass,

Johnson grass and millet but these hosts are not extensively attacked under field conditions. Usually fewer larvae sur­ vive on secondary hosts and the larval stage is often pro­ longed.

No parasites, predators or diseases of importance have been observed.

The overwintering larvae are vulnerable to freezing when the stubble are brought to the surface. Burial of the stubble is also effective in reducing moth emergence. Loss from "dead heart" and stunting can be reduced by early plant­ ing and loss from girdling by early harvesting. Under some conditions insecticide treatments may be desirable. 69

LITERATURE CITED 1. Box, Harold E. 1931. The Cramblne genera Diatraea and Xanthopeme (Lep, Pyral.). Bui. Ent. Res. 22:1-50. 2. Davis, E. G., J. R. Horton, 0. H. Gable, E. V. Walter and R. A. Blanchard. (With technical descriptions by Carl Heinrich). 1933. The southwestern corn borer. USDA Tech. Bui. 388. 3 . Davis, E. G. 1944. Apanteles diatraeae, a braconid parasite of the southwestern corn borer. USDA Tech. Bui. 871. 4. Davis, Geo. W. Jr. Personal communication. 5 . Dyar, Harrison G. I9II. The American species of Di­ atraea (Lep. Pyralidae). Ent. News 2 2 :199-207. 6. Dyar, Harrison G. and Carl Heinrich. 1927. The Ameri­ can moths of the genus Diatraea and allies. Proc. U.S. Nat. Mus. Vol. 71, Art. 19 (No. 269I), pp. 1-48. 7 . Holloway, T. E. I916. Larval characters and distribu­ tion of two species of Diatraea (D. saccharalis cramb- doldes Pab. and D. zeacolella Dyar). Jour. Agr. Res. 6:621» 8. Howard, L. 0. I891. The larger corn stalk borer (D. saccharalis P.). Insect Life 4:95-103. 9 . Matthews, David L. Jr. 1954. Kan. Ent. Comm.— USDA Cooperative Insect Detection Service Rpt. 4l (Nov. 28- Dec. 3). 10. Peterson, Alvah. Larvae of insects. Part 1. Edward Bros, Inc., Ann Arbor, Mich. 1948. 11. Rolston, L. H. 1955. Insecticide tests against the southwestern c o m borer. Jour. Kan. Ent. Soc. 12. Thomas, Geo. W. 1954. Cooperative Economic Insect Rpt., USDA Vol. 4, No. 42 (Oct. 22). 1 3 . Walkden, H. H. and G. D. White. 1951. Status of the southwestern corn borer in Kansas, Oklahoma, Missouri and Arkansas in the fall of 1950. USDA Bur. Ent. and Plant Quar., Insect Pest Survey. Special Supplement (NO. 2, 1951). 70

14. Walton, R. R. and G. A. Bieberdorf. 1948. Seasonal history of the southwestem c o m borer, Diatraea grandiosella Dyar, in Oklahoma; and experiments on methods of control. Okla. Agr. Expt. Sta. Tech. Bui. T-32. 15. Wilbur, D. A., H. R. Bryson and R. H. Painter. 1950, Southwestem c o m borer in Kansas. Kan. Agr. Expt. Sta. Bui. 339. 71

AUTOBIOGRAPHY

I, Lawrence Hubert Rolston, was b o m April 14, 1922, In Parkersburg, Wood County, West Virginia. I received my secondary schooling In the Wood County public schools. In 1949> I completed my undergraduate work at Marietta College The following year I received the Master of Science degree from The Ohio State University and continued in residence for an additional year. I held the position of instructor and Junior entomologist at the University of Arkansas and the Arkansas Agricultural Experiment Station from 1952 to 1955. In April 1955, I was appointed to the position of instructor at the Ohio Agricultural Experiment Station.