THE SOUTHWESTERN CORN BORER IN ARKANSAS

By L. H. Rolston

AGRICULTURAL EXPERIMENT STATION College of Agriculture and Home Economics University of Arkansas, Fayetteville

June, 1955 Bulletin 553 CONTENTS

Page

In tro d u c tio n ______3

Review of Literature______3

L ife and Seasonal H is to ry ______—... 6

Description of Injury______... 10

A d ut l ______:______12

Egg------20

L a rva ______21

P re pu p a ______31

P upa______31

H osts Other Than Corn______, ______32

N a tural Enemies______35

C o nrol t Methods____ .'.______36

S u m ma ry ______39

L ite ra tu re C ite d ------40

COVER PICTURE

The two plants in the foreground are affected by “dead heart”. Notice the dead, blanched central leaves, excessive tillering, and stunting. The plant in the right background is of normal height.

This bulletin covers the m aterial used in a thesis presented as partial fulfillm ent of the requirements for the degree of Doctor of Philosophy at Ohio State University.

Agricultural Experiment Station, University of Arkansas College of Agriculture and Home Eco­ nomics. Lippert S. Ellis, director; John W. White, associate director. Main Station, University; with Cotton Branch Station, Lee County; Rice Branch Station, Arkansas County; Fruit and Truck Branch Station, Hempstead County; and Livestock and Forestry Branch Station, Independence County. PPC3M655 The Southwestern Corn Borer in Arkansas By L. H. Rolston Department of Entomology

In the brief interval following the entrance into Arkansas of the southwestern cornDiatraea borer, grandiosella Dyar, it has become a corn pest of great importance. A thorough investigation of its biology and possible control measures was necessitated by the rapidity with which the borer has spread and the resulting crop damage. R eduction 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 tunneling 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 southwestern corn borer in spite of its economic importance and of its having been in the United States for many years. The first comprehen­ sive investigation was that of Davis and his co-workers in New Mexico and Arizona (2). More recent bulletins have been issued by Kansas (15) and Oklahoma where particular attention has been paid to the development of cultural control practices (14). Since Arkansas differs considerably in climate and topography from the locales of previous investigations, there was no assurance that the seasonal history of the borer would parallel that in neigh­ boring states nor that cultural practices recommended there would be effective under Arkansas conditions. The biology of the borer has been given particular attention during this investigation. The habits of both the larva and adult have been studied to ascertain the possibilities they offer for control as well as the limitations they impose. Previously sug­ gested 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 regions. Sev­ eral species are im portant pests as borers in cultivated grasses, particularly sugar cane and corn. Diatraea Of the species six known from the United States, three are apparently rare and of no economic importance:D. evanescens Dyar, D. lisetta (D yar), andD. venosalis (D yar). 4 A rkansas E xperiment Station, B ulletin 553

The sugar cane borer,saccharalis D. (Fabr.), is well known as a pest of both sugar cane and corn. It is found along the Gulf Coast of Texas, Louisiana, and Mississippi and in the southern half of Florida. Thesouthern corn stalk borer,D. crambidoides (Grote), is capable of damaging corn severely but is not consistently a major pest. Its range extends from Maryland to Florida and apparently westward into Kansas, but heavy populations are confined to the southeast. This species is widely known under D. the synonym zeacolella Dyar. Thesouthwestern corn D. borer grandiosella Dyar concludes the list of knownDiatraea species from the United States. The damage caused by this insect and its range are discussed in detail under appropriate sections. Y e t another species,D. lineolata (Walker) has been errone­ ously reported from the southwest. U n til 1911 D. saccharalis (Fabr.) was believed to be the sole representative of the genus in the United States and all papers and records on the southern corn stalk borer and southwestern corn borer, as well as the sugar cane borer, were published under this name. In that year Dyar published a revisionDiatraea of the based on the color and external morphology of the adult (5), but the characters used have proved insufficient to consistently sep­ arate all the species. The main impact of this paper on economic literature was the establishment of the southern corn stalk borer, underD. zeacolella D yar, as a species d istin ct fro m the sugar cane borer of southern Florida and the Gulf coast. Holloway later gave a method for separating the larval forms of these two species by means of the setal pattern (7). In Dyar’s revision, the southwest­ ern corn borer was described as a new species fromD. Mexico and lineolata (Walker) was redescribed from specimens from Mexico and a single specimen from Arizona. This latter specimen was un­ doubtedly D. not lineolata (Walk.) but the southwestern corn borer, for the northern D.lim lineolata it for (Walk.) is now known to be central Mexico (1). Therevision of Dyar and Heinrich in 1927 has done much to clear up the confusion concerning theDiatraea identity species of (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 synonymy and others rec­ ognized as valid. References toDiatraea species prior to 1927, and particularly those before 1911, must be regarded critically. Sometimes the T he Southwestern Corn B orer in Arkansas 5 identity of the species involved can be deduced from the locality. Adequate descriptions of habits, damage, and eggs are of great help. Of the three commonDiatraea the sugar cane borer is the only species not habitually overwintering 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. F or a technical description of AmericanDiatraea, along with excellent drawings of the genitalia, the reader is referred to Dyar and Heinrich (6). In this paperD. lisetta (Dyar) is described and figured under the synonymlesta lisetta D yar andD. crambidoides (Grote) underD. zeacolella D yar. Thesynonymy of American species and new species, as well as photomicrographs of genitalia, are given by Box (1). History of Distribution What may be the first reference to the southwestern corn borer occurs in a note appended to a paper by L. O. Howard in 1891, stating that larvae of the sugar cane borer had been found infesting corn in two localities in New Mexico (8). These larvae were probably the southwestern corn borer, since it is the only Diatraea species normally found in this area. The distribution in 1931, as given by Davis et al, included southeastern 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 southeastern Colorado and southwestern Kansas (2). No appreciable extension of this range occurred until 1941, when the borer resumed its easterly advance across Oklahoma and north­ erly 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 lim it of the borer’s range to have receded from Nebraska and the northern third of Kansas but the eastern lim it to have advanced across Okla- homa into west central Arkansas and the southwestern corner of Missouri (13). In 1954 the borer was again present in all but five northwest­ ern and two northeastern counties of Kansas (9). In Missouri, 10 southwestern counties were infested in 1953 and 25 counties in 1954 (12). There are apparently two principal areas of infestation in Texas; one in the northeast corner involving 15 counties and probably as many as 25, and the other in the Panhandle covering 13 counties and probably 28 (4). It is apparent that the distribu­ tion of the borer has been fluid, with a considerable easterly ex­ tension. 6 A rkansas E xperiment S tation, B ulletin 553

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 additional counties, Washington and Crawford. The following year the borer was present in 12 western counties, and in 1953 four additional counties

Figure 1. Distribution of the Southwestern Corn Borer in Arkansas Dates indicate year each county was first found infested.

had been entered. The area known to be infested by the borer in 1954, which includes 22 counties, is shown in Figure 1. For the past two years the heaviest infestations were in the Arkansas River Valley and in the Springfield Plateau region of Benton and Wash­ ington counties. These are the major corn producing areas in western Arkansas. LIFE AND SEASONAL HISTORY Life History A brief and generalized outline of the borer’s life history is necessary for ready comprehension of the subsequent discussion. This account w ill be qualified and enlarged in following sections. T h e S outhwestern C orn B orer in A rkansas 7

The borers overwinter as full grown larvae in the extreme stalk base below soil level. Most larvae girdle the stalk internally near the ground level as part of the preparation for overwintering. In the spring, pupation takes place in the stubble, the moths emerg­ ing 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, pupate mostly in the stalk. In northwest Arkansas approximately one-fourth of the second generation larvae overwinter, there be­ ing two complete and a partial third generation a year. In the Arkansas River Valley there may be three complete generations and a trace of a fourth which does not, apparently, overwinter.

Seasonal History The seasonal history of the borer was followed for a three-, year period near Fayetteville, in Washington County, and for two years and part of a third near Van Buren, Crawford County, in the Arkansas River Valley. These locations are representative of the two large regions in which infestations have generally been seri­ ous. Since Fayetteville is approximately 45 miles north of Van Buren and has about 1,000 feet greater elevation, the same biolog­ ical event can be expected at a later date in the Fayetteville area. The seasonal history of the borer was followed by dissecting stubble or corn plants, usually at weekly intervals, and counting larvae, pupae, and pupal cases. Since moth emergence was of pri­ mary interest, the results obtained by dissections were frequently checked against emergence from caged stubble or against oviposi- tion records. Over the three-year period at Fayetteville, there were consis­ tently two complete generations and a partial third. A ll of the first generation pupate 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 1 per cent of the third generation pupated in the fall, the remaining larvae overwintering. The fourth generation was consequently very small and the larvae did not develop sufficiently to survive the winter. The following year there were two com- 8 A rkansas E x p e r im e n t S t a t io n, B u l l e t in 553 plete generations and a partial third. About three-fourths of the second generation pupated in late summer, the remaining second generation larvae and all third generation larvae overwintering. In 1954 there were two complete generations, a partial third, and a trace of a fourth. Eighty per cent or more of the second genera­ tion 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 overwintering form and the generation was therefore not complete. Again, less than 1 per cent of the third generation pupated in the fall. To follow the development of the overwintering brood, sev­ eral hundred stubble showing girdling activity were collected from fields in the fall or spring and replaced in 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 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 generation. To overcome this difficulty, special plantings were made soon after eggs from overwintering brood moths were no longer found. This corn was not, of course, in­ fested by the first generation but was tall enough to be attractive for oviposition by the time the first generation moth flight began. The resulting second generation could be followed without compli­ cations 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 sec­ ond generation larvae 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 generation. Pupation and emergence of the overwintering brood was found to vary considerably from field to field w ithin a relatively small area. Differences in soil types, exposure to the sun, and air drainage are probable factors in these variations. As an illus­ tration, data on seasonal history in three fields lying within a T h e Southwestern C orn B orer in A rkansas 9 five-mile radius are given in Table 1. One of these fields was plowed before pupation was completed and before any emergence had occurred. Nevertheless, 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 Overwintering Brood in Three Fields in Benton County in 1954

Date Pupae Pupal cases of exami­ Field 1 Field 2 Field 3 Field 1 Field 2 Field 3 nation

Per cent of total borers found May 9 1______68 24 0 0 0 0 May 5 2______92 76 44 0 0 0 June 1______92 100 88 8 0 0 June 8 ______40 64 __1 60 36 June 14______0 0 .... 100 100

1 Field plowed, terminating records.

Moth flights at Van Buren were consistently earlier than those at Fayetteville (Fig. 2). Disregarding the first and last 10 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.

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 generation moth flight should be underway by mid-August and straggle into September. At Van Buren the main moth flight of the overwintering brood may be expected dur­ ing 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 resume' of seasonal history should be useful as a guide when examining fields for eggs. As previously pointed out, emerg­ ence 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 occurred, from year to year. Examina­ tion of fields for eggs w ill be necessary, therefore, to fix the ovipo- sition period accurately in a p a rticu la r season and area. 10 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

Figure 2. Emergence of the Principal Broods at Fayetteville and at Van Buren, 1 9 5 2 -5 4

DESCRIPTION OF INJURY In corn that has not tasseled, most early instar larvae feed between the leaves of the whorl. Typical feeding areas are elon­ gated parallel to the leaf veins, with either the upper or lower epidermis of the leaf 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. T he S outhwestern Corn B orer in A rkansas 11

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 is of little consequence. Sim­ ilar injury is caused by the European corn borer, corn earworm, fall army worm, and other “bud worms”. The most spectacular injury caused by the southwestern corn borer is “dead heart”, resulting from destruction of the meristem- atic tissues of the young plant. Splitting the young plant through its longitudinal axis reveals that the stem, which comprises most of the meristematic tissues, extends only a few inches above ground level and is enveloped by tightly furled leaves. The stalk of young plants is composed 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 meri­ stematic 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 frequently de­ stroy a large portion of the meristematic tissues. The central leaves are severed and soon blanch, contrasting strongly w ith the remaining green leaves. Hence the name “dead heart”. Plants thus affected become deformed, stunted, and unproductive. Occa­ sionally 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 extensive lodging but may cause dwarfing and heavy loss of grain production. Severe injury is quite obvious but that caused by moderate and light in­ festations is difficult to determine and seldom appreciated. Walton and Bieberdorf in Oklahoma found yields were reduced from 8.7 per cent to 100.0 per cent in fields in which more than half the stalks were infested (13). 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 between 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 corn resulting from borer activity in the ears is of little importance but in sweet corn as much as one-fourth the crop has been lost in this manner. The high loss in sweet corn is due to grading stand­ ards rather than actual grain consumed. Larvae are particularly obnoxious in sweet corn since they are likely to be overlooked in preparations for processing.

I f the borer has gone unnoticed during the grow ing season, its girdling activity will certainly attract attention in the fall. 12 A rkansas E xperiment Statio n, B u lle tin 553

Overwintering larvae weaken the stalks by cutting one or more v-shaped grooves around the inside at, or near, ground level. Some infested stalks escape entirely, and others are not girdled suffi­ ciently 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 occasioned by this activity may be great, depending on weather conditions, prevalence of rodents, and other factors. Where mechanical harvesters are used the down corn necessitates glean­ ing, an operation that is both expensive and distasteful, or turning livestock into the field. ADULT The adult was originally described by Dyar (5) and rede­ scribed 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 w ith a wing expanse of 30-40 mm. The males are generally smaller than the females, and dwarf specimens of both sexes are sometimes encountered. The palpi are prominent and snoutlike. The veins of the forewings and the intervenular streaks in cells R4 to 1st A are buff on a light background. These intervenular 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 fre­ quently 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 3. Considering both appearance and behavior, there is no other common moth likely to be confused with the southwestern corn 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. T h e S outhwestern C o r n B o r e r i n A r k a n s a s 13

Figure 3. Female of the Southwestern Corn Borer Several of the terminal dots on the intravenular streaks are just visible. About twice natural size.

Emergence In rearing moths from the pupal stage it was necessary to maintain high humidity and to cover the pupae w ith 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., 18 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 oviposi- tion terminated shortly after adult emergence was complete. To obtain more precise information on longevity, moths were collected daily fro m emergence cages, m arked on the wings w ith model a ir­ plane 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 marking. Unmarked moths were used 14 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

as often as possible to afford a check 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 10 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 differences 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° F., 74.5° F., and 75.7° F., respectively. The data on longevity are presented in Table 2.

Table 2. Longevity of Adults

T o ta l Overwintering brood First generation Second generation days liv e d M ales F em ales M a le s Fem ales M a le s Fem ales

Number 1 ______0 2 0 0 0 0 2 ______12 7 0 0 0 0 3 ______14 19 1 3 0 0 4 ______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 8 ______0 0 2 6 0 2 9 ______0 0 3 3 0 0 10 ______0 0 1 2 0 0 N o .o f m oth s ______32 34 23 26 6 11 Days A ve r. lo n g e v ity ______. .. 2.9 2.9 6.1 6.6 5.2 5.9

Davis observed a lim ited number of moths, the males of which lived two to four days and the females five to six days (2).

Table 3. Longevity of Mated and Virgin Females

T o ta l V ir g in M a te d days Females caged liv e d fe m a le s fe m a le s w ith males

Number 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 N o . o f m o th___ s .______18 15 20 Days A v e r. lo n g e v ity______4 .4 4.3 4.2 T h e S outhwestern C o r n B o r e r i n A r k a n s a s 15

Failure to mate apparently did not affect longevity of the females. Eighteen virgin females were compared w ith 20 females caged with males and with 15 females observed mating. The results are given in Table 3. Proportion o f Sexes E xam ination of field-collected pupae and pupal cases showed a rather constant ratio between the sexes with the females pre­ dominating. 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 and the last when pupation was complete. The results are summarized in Table 4.

Table 4. Proportion of Sexes

P e r cent Pupae and pupal Proportion of sexes B rood pup ated cases examined M a le s Fem ales

Number P e rcent O vew r in te rin g . 68-100 .. 153 38 .6 61.4 F irs t ______87-100 99 44 .4 55.6 Second ...... 5 8 -1 0 0 1 90 46 .7 53.3 A ll ______342 42 .4 57.6

1 This is a partial generation. No pupae, only pupal cases, were found when the last examination was made.

M a tin g The courting male takes a position slightly behind and to one side of the female and facing in the same general direction. 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. Imme­ diately 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 overlapping those of the male. If undisturbed, the moths remain in this position throughout copulation, which usually lasts from one to three hours. Of 24 females, 16 mated the same night they emerged, 7 the first night, and 1 the second night following emergence. All of the 17 males observed mating did so initially on the night of emerg­ ence. Eight of these mated a second time on the following night. Females apparently mate only once. Fertile eggs were not obtained when freshly emerged moths were caged in pairs regardless of cage size or location, even though one such p a ir appeared to mate.

Oviposition Caged moths w ill 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 16 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

necessary to cover the floor with wax paper, because moths con­ tinue to oviposit after they can not 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 depos­ ited on screen wire. When first extruded the eggs are pliable and globular, being tamped into their characteristic form by the female’s ovipositor. H e a vily laden females deposit an egg every three or fo u r seconds, placing as many as 30 or 40 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 following mating, although two of 15 individuals deposited a few eggs, and one moth deposited a large number of eggs, on the night of emerg­ ence and mating. Fecundity and Fertility Virgin females deposited considerably fewer eggs than did mated females. Eighteen virgin females, confined together, de­ posited 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

Table 5. Fecundity and Fertility of Individual Moths

Number of eggs deposited N o . eggs P e r cent observed for fe rtile Days after mating f e r t ilit y eggs 0 1 2 3 4 T o ta l 239 277 79 0 10 605 560 88.4 0 316 133 0 1 449 449 98.0 0 288 116 0 4 0 4 4 0 4 100.0 0 200 177 4 0 381 381 98.8 0 276 42 0 31 8 31 8 96.2

0 158 108 43 0 30 9 21 6 97.7 0 163 129 0 292 292 97.6 0 228 27 0 255 255 94.5 0 158 66 0 0 22 4 2 24 98.7 0 112 0 103 0 215 172 96.5

6 166 0 172 172 99.5 20 283 3 0 30 6 30 6 9 4 .8 2 0 216 49 265 265 54.7 2 0 229 33 0 262 245 93.4 2 0 241 0 241 241 . 0.0 2

1 M oth dead. 2 Second mate of male. of the eggs were deposited on the wire cage and could not be in­ cluded in the fertility record. W ith two exceptions the percentage of fertile eggs was quite high. Both females that deposited few or no fe rtile eggs were the second mate of a male. The fecundity of moths of the three flights was estimated by confining both sexes fro m emergence u n til death in a large cage in the insectary. Twenty overwintering brood females deposited T h e Southwestern C orn B orer in A rkansas 17

an average of 147 eggs each; 26 first generation females, an average of 267 eggs each; and 11 second generation females, an average of 194 eggs each. The low average of the overwintering brood and second generation females may be due to failure of some individ­ uals 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 predators and, as previously mentioned, caged moths often continue to oviposit after they can no longer fly. Oviposition Preference It has been repeatedly observed that few eggs are deposited on very young or very old corn when succulent, large plants are avail­ able. 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, date-of- planting trials bear out these observations, as shown in Table 6.

Table 6. Relation Between Height of Corn and Oviposition

D ate corn Plant height Plant stage Average eggs was planted in inches on Aug. 28th1

J u ly 1 72 D ough 58.2 J u ly 15 63 S ilk 180.2 Aug. 1 27 W h o rl 144.8 Aug. 15 8 W h o rl 36.0

1 Difference required for significance at 5 per cent level, 51.7.

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 difference at the 5 per cent level between the number of eggs found on the youngest and those found on the oldest corn. The difference between the two inter­ mediate plantings bordered on statistical significance, and the dif­ ference between either of the intermediate plantings and the first or last planting was highly significant. It appears, therefore, that moths are attracted to corn according to the stage or rate of devel­ opment which may, within limits, be correlated with plant height. That plant height alone does not determine oviposition prefer­ ence was demonstrated in date-of-planting trials in 1954. The mid-May planting was severely affected by drought but had reached nearly full height. The mid-June planting was in the early whorl stage and although somewhat wilted was still succu­ lent. An egg count on July 19 gave 24 eggs per hundred plants on the mid-May planting and 134 eggs per hundred plants on the mid- 18 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

June planting, or roughly six times as many eggs on the younger plants. Besides being found on corn, eggs have been found in fields of sorghum and on Sudan grass, millet, and Johnson grass growing in corn fields. To determine their relative attractiveness for ovi- position, eggs counts were made on replicated plantings of corn, grain sorghum, millet, and Sudan grass. The plantings were drilled in late June and egg counts were made August 11-13 over 10 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 m illet, respectively, indicating that only corn is highly attractive. It should be pointed out that this planting was adjacent to corn fields on both sides. Oviposition on the crops other than corn may have been increased by their proximity to corn, for no eggs have been found in pure stands of Sudan grass.

Flight Range It has not been feasible to determine adult flight range by the release and recovery of marked moths because no efficient recovery method is known and no method has been devised for mass rearing or collecting. In the late winter of 1952-53 a survey was made in the sparsely populated Boston Mountains Addition of the Ozark National Forest to find isolated, infested corn fields and to determine the closest point from which the infestation could have 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 the borer is spread passively in any num­ bers, 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 Arkansas sug­ gest that the moths may travel considerable distances, perhaps with the aid of winds.

It is important to know whether moths of the overwintering 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 individual or small group would benefit from the use of good cultural control measures di­ rected 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 under­ taken over a rather large area to be of much value. T h e S outhwestern C o r n B o r e r i n A r k a n s a s 19

To clarify this point, three large areas were chosen, prior to emergence of the overwintering brood, that contained 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 corn 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, 50-plant samples per field taken June 29-30. At that time larvae derived from overwintering brood moths were well established in the young corn. The fields of young corn were divided into three groups according to their distance 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 quarter of a mile and one mile, and a third group those fields more than a mile away. Each of the three areas contained fields falling into two or more of these groups. The results are summarized in Table 7.

Table 7. Relationship Between Degree of Infestation and Distance from Closest Source

Miles from closest source of infestation______Infestation Less than 1/4 1/4 to 1 More than 1

Per cent of plants infested M inimum infestation______17.2 4 .4 14.4 M aximum infestation______2 8 .4 3 0 .0 20.8 A verage infestation______2 1 .8______18.3______16.8 Number N umber of fields______3______6 4

There were several uncontrollable variables, such as unequal sources of infestation, uneven growth of the young corn, and vary­ ing relationships between young corn and stubble w ith respect to prevailing winds. However, any strong tendency of the moths to fly to the closest field of young corn should have been evident. From these observations, a strong attraction to the closest corn was not apparent. No conclusions could be drawn as to the role played by prevailing winds in moth dispersal.

Light The moths are not strongly attracted to light from incandes­ cent 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 Oviposition was high. Males pre­ dominated in the catch by a three to one ratio. Usually only two or three moths were taken per trap in a night. 20 A r k a n s a s E x p e r i m e n t S t a t i o n, B u l l e t i n 553

A trap using four 15-watt fluorescent bulbs was operated at one station for three hours nightly over a 12-day period while second generation moths were flying. Black light and white light bulbs were used on alternate nights and attracted moths were col­ lected by hand. No moths were collected at the white light, but 16 males and 14 females were taken at the black light. Although black light cannot be considered very attractive, it might be useful in determining when moths are flying.

EGG Description The eggs are approximately 1.0 mm. wide, elliptical, and decid­ edly 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, including single eggs, examined over the season. When first laid the eggs are entirely white, dull, and opaque but older, fertile eggs are distinctly marked w ith three red, trans­ verse bars. Shortly before hatching, the head capsule of the em­ bryo 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 Tparasitized ri- by chogamma minutum Riley are readily distinguished by the intense black color. D uring 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 irregular or reduced to discontinuous bands, flecks, or splotches. Many embryos, appar­ ently fully developed, died before or during hatching. The eggs of the European corn borer are not uncommon in some areas of the state and might be mistaken for freshly depos­ ited eggs of the southwestern corn borer. H ow ever, the egg masses of the European corn borer usually contain many eggs and have a noticeable sheen. The in d iv id u a l eggs are som ewhat sm aller than those of the southwestern corn borer. Location o f Eggs Most eggs are found on the upper surface of leaves, particu­ larly 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 th ird generation eggs were tabulated w eekly ac­ cording to their position on corn in the whorl stage. Of 1,263 eggs T h e S outhwestern C o r n B o r e r i n A r k a n s a s 21 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 13.1 per cent were on the stalk. Development and Incubation Observations on development and incubation were made on eggs deposited on w ax 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 tem­ perature. The red bars appeared on the first day after 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 w ith 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.

Table 8. Relationship of Temperature to Incubation Period, 1953

Deposition Average N um ber Per cent hatch dates Fahrenheit o f temp. eggs 4th day 5th day 6th day 7th day J u ly 8 -1 0______71 494 0.0 0.0 51.4 48.6 Aug. 1 0-1______2 ______80 995 28.1 69.4 2.5 0.0 June 6-16 ... ______83 1,055 75.8 21.7 1.9 0.5

The temperature given is an average of readings made 12 times daily from the time the first eggs were collected until hatching was complete.

Eclosure The majority of eggs hatch during the early morning, the larva emerging through a slit cut in the exposed end of the egg. The larva, as soon as it is free from the egg, raises its head and thorax above the leaf surface, swaying from side to side for several seconds. Following this ritual, it immediately leaves the vicinity of the egg mass and crawls randomly over the plant surface until reaching the whorl or some other secluded place. Positive thig- motropism is evident in larval behavior but no phototropism 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 described by Heinrich (2). Methods for separating the three common spe­ cies, Diatraea saccharallis (F abr.),D. crambiodoides (Grote), and 22 A r k a n s a s E x p e r i m e n t S t a t i o n, B u l l e t i n 553

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. Thefirst instar larvae have a definite reddish cast that is seen, under magnification, to be more or less confined to the prothorax and to a broad, dorsal band across most abdominal segments. The second instar larvae retain this color in varying degrees, particu­ larly 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 suc­ ceeding instars are easily distinguished from other larvae com­ monly 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 the fall, assuming the im­ maculate, 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, larvae prior to this molt. Fall Activity Grown larvae of the overwintering brood converge on the stalk base below ground level, sometimes descending part of the

Figure 4. Girdled Corn Stalk Note even break and sharp edge. T h e S outhwestern C o r n B o r e r in A r k a n s a s 23 distance externally and re-entering the stalk at a lower point. Sel­ dom 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 positions in the stubble. After tunneling to the extreme stalk base the larva usually ascends to, or near, ground level and girdles the stalk internally (Fig. 4). 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 position in the stalk base, oriented toward the soil surface (Fig. 5). The

Figure 5. Overwintering Larva in Base of Stalk 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 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 24 A r ka n sa s E x p e r im e n t St a t io n, B u l l e t in 553 few days in August or early September. In the vicinity of Fay­ etteville 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. Apparently early girdling activity is due entirely to the activity of second generation larvae. In one field at Van Buren in which both second and third generation larvae were present girdling began August 27 and continued until Sep­ tember 24. In an adjacent field of late corn infested with third generation larvae only, girdling did not begin until after Septem­ ber 11 and continued until October 7. The data are presented in Table 9.

Table 9. Girdling Activity in Fields Infested with Second and Third Generation Larvae, Van Buren, 1953

Second and third generation larvae Third generation larvae only Per cent stalks Per cent stalks D a te g ird le d D a te g ird le d

A u g . 2 7 .Less than 1 S e p t. 11 0 S e p t. 4 _ 3 1 S e p t. 2 4 2 6 S e p t. 10 4 9 Oct. 2 ... 4 4 S e p t. 2 4 72 O c t. 7 ... 6 8

The amount and thoroughness of girdling vary considerably from field to field. Some stalks are girdled completely around their circumference while others, although infested, escape en­ tirely. All gradations betwen these extremes are found. In No­ vember, 1953, 10 fields in Benton and Washington counties were examined for the frequency and degree of girdling. Ten consecu­ tive stubble in five places were dug up, split, and examined in each field, for a total of 50 stubble per field. Those stubble girdled around more than half their circumference were classed as “gir­ dled” ; those girdled around less than half their circumference, as “partially girdled”, and those showing no girdling whatsoever, as “not girdled”. The observations are summarized in Table 10.

Table 10. Degree of Girdling in Ten Corn Fields in Washington and Benton Counties, November, 1953

P e r c e n t o f Per cent of infested stubble that were s tu b b le in fe s te d G ir d le d Partially girdled N ot girdled

3 4 9 4 .1 5 .9 0 5 8 9 3 .1 0 6 .9 6 0 8 3 .3 3 .3 1 3 .3 7 0 8 2 .9 5 .7 1 1 .4 6 0 7 0 .6 1 1 .8 1 7 .6

72 6 9 .4 1 3 .9 1 6 .7 5 0 6 4 .0 2 4 .0 1 2 .0 7 8 4 8 .7 1 7 .9 3 3 .3 6 6 4 8 .5 4 2 .4 9 .1 5 8 4 4 .8 2 7 .6 2 7 .6

A v e r . 6 0 .6 6 9 .9 1 5 .2 1 4 .8 T he Southwestern Corn B orer in A rkansas 25

There were so many variables among the fields examined that no explanation of the observed differences in the degree of girdling can be advanced. W ilbur et. al. observed that the per cent of in­ fested 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 girdled. In Arkansas, however, no consistent relation­ ship between 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 deter­ mined by sampling the population in certain fields during August and again in May. This method was uneconomical of labor, for of 30 odd fields examined in the fall only seven remained undis­ turbed by May. Survival in undisturbed stubble during 1953-54 was estimated by examining, in May, samples of girdled stubble from 10 fields. Girdling, however slight, was taken as evidence that the stubble was once infested by an overwintering larva. Inasmuch as all overwintering larvae do not girdle, estimates of survival obtained in this manner would be slightly biased in the event that survival of non-girdling larvae should differ materi­ ally from that of girdling larvae. D u rin g the w in te r of 1952-53, the m inim um su rviva l was 16 per cent. The maximum was 54 per cent, and the average was 37 per cent in the seven fields examined. The following winter the minimum survival found in 10 fields was 10 per cent, the maxi­ mum was 72 per cent, and the average was 30 per cent.

Spring Activity Overwintering larvae resume activity in early spring, clean­ ing an escape tunnel of debris and closing the entrance with a thin web. Most larvae then assume their former position 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 vary, apparently being affected by temperature and by the kind and condition of food. All data are from larvae confined individually in vials partially 26 A r k a n s a s E x p e r i m e n t S t a t i o n , B u l l e t i n 553

filled with moist plaster of Paris. The vials were closed with a one-hole stopper, and the hole was loosely plugged with cotton. Each vial was examined daily for cast head capsules. The food was changed every other day or daily if wilted or spoiled.

Table 11. Duration of Instars on Corn

Instar number A ll D u ra tio n 1 2 3 4 5 6 7 8 instars

Duration in days M in im u m______...... 2 2 2 2 2 2 5 9 20 M a xim u m ______5 6 5 6 9 12 9 0 55 A v e ra g e___ .______3.1 2.9 3.0 4.2 4.8 7.0 6.9 9.0 27.7 Number L a rv a e______24 24 24 24 24 23 10 1 24

The data in Table 11 are from larvae reared on corn in the insectary. Host plants from the same planting were used to mini­ mize 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 re­ quired for one complete generation in the field, coincident with the generation being reared, was estimated from the date of 50 per cent emergence of the overwintering brood to the same stage of development of the first generation. It was found to be 41 days. The reared insects required an average of 40.7 days if one day is allowed for preoviposition. Reared pupae were considerably smaller than those obtained in the field. The number of instars varied from five to eight. Of the 24 larvae successfully pupating, 1 passed through eight instars, 9 through seven instars, 13 through six instars, and a single larvae through five instars. The minimum, maximum, and average dura­ tion of the instars are given in Table 11. These data agree sub­ stantially with those given by Davis et al. (2). These authors found the larvae to have as many as nine instars, but most of them pupated following the sixth instar. As a preliminary study of the effects of different hosts on de­ velopment, larvae were reared on corn, sorghum, m illet, and John­ son grass. The rearing was done in an incubator at 80° F. in order to eliminate temperature as a 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 handled as before. At least some of the larvae that were started on each host completed their devel­ opment, although there was a marked difference in survival and in the time required to reach the various stages of development. T h e S outhwestern C o r n B o r e r i n A r k a n s a s 27

Corn was apparently the most satisfactory host, as far as rate of development is concerned, followed by sorghum. M illet and John­ son 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.

Table 12. Average Duration of Instars on Various Hosts

C o rn S o rg h u m M ille t Johnson grass In s ta r N u m b e r N u m b e r N u m b e r N u m b e r larvae Days larvae Days larvae D a ys la rvae D ays

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 4 th ______19 3.2 20 4 .0 9 6.3 14 4.6 5 th ______17 5.8 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 0 _ Duration of larval period A ll ______15 22.0 13 2 9 .0 4 48 .7 1 45.0

Of the larvae reared on corn, 15 pupated successfully, 8 follow­ ing the fifth instar, 6 following the sixth instar, and 1 following the eighth instar. 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, 9 of these following the sixth instar, and 4 following the seventh. One of the larvae reared on m illet pupated following the seventh instar, 2 following the eighth, and 1 follow­ ing the ninth. The single larva successfully reared on Johnson grass pupated following the seventh instar.

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 chronolog­ ically. 28 A r k a n s a s E x p e r i m e n t St a t i o n, B u l l e t i n 553

The frequency chart of head capsule measurements of field- collected larvae does not show the limits of the instars in definite fashion, although the instar to which most larvae belong can be recognized (Fig. 6). The size variation in reared larvae of known Number o f larvae

Head capsule width in mm.

Figure 6. Distribution of 1,365 Field-Collected Larvae According to Head Capsule Width T h e S outhwestern C o r n B o r e r i n A r k a n s a s 29

instars (Table 13) serves to assign most of the field-collected larvae to their respective instars. Larvae with head capsule widths of

Table 13. Head Capsule Widths of Reared Larvae of Known Instars

Head In s ta r capsule w id th 1st 2nd 3 rd 4 th 5 th

Width in millimeters1 M in im u m . . ______0.35 0.45 0.75 1.10 1.45 M a x im u m______0.40 0 .6 0 1.00 1.50 2.05 A verage______0.36 0.53 0.86 1.34 1.77 Number Larvae ______41 56 41 42 13

1 Plus or minus 0.025 mm.

0,65 and 0.70 mm. cannot be placed, from these data, in the second or third instars exclusively. None of the reared larvae fell 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 contains 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 to 1.50 mm. range. No attempt was made to distinguish between fifth and any suc­ ceeding instar, since a distinction was not essential to the problem. Dissections of whorl-stage corn were made during and after Oviposition 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. 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. A ll 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 that were 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 lar­ vae were still in the whorl, although about 10 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 in either of two instars, the second or third and the fourth or fifth. Using the average stadia of reared larvae, the position of the various instars on whorl-stage corn can be fixed chronologically. 30 A r ka n s a s E x p e r im e n t St a t io n, B u l l e t in 553

Table 14. Location of Larvae on Whorl-State Corn

Percentage in In s ta r N o . la rv a e W h o r l S h e a th S ta lk

1 s t ______... 1 5 4 9 3 .5 4 .5 2 .0 2 n d...... - ... 1 51 9 1 .3 6 .0 2 .7 3 r d ______... 1 1 7 8 0 .3 9 .4 1 0 .3 4 t h ______... 8 2 3 3 .0 8 .5 5 8 .5 5 th and 6th ... 8 6 8.1 1.2 9 0 .7

On the third day from hatching approximately 2 per cent of the larvae w ill have entered the stalk and on the sixth day, about 3 per cent. As previously 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 10 per cent of the larvae w ill have bored into the stalk, most of them penetrat­ ing beyond the superficial leaf layers to well sheltered positions. Three or four days later nearly 60 per cent of the larvae w ill have become borers and in another four to six days, about 90 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. From examination of young corn it appears that the majority of larvae, threatened with exposure by the flagging of leaves, move beneath the leaf sheaths and enter the stalk from this posi­ tion. 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 corn past the whorl stage nearly all larvae were found in the stalk, beneath the leaf sheaths and in or on the carpellate inflorescences. The carpellate inflorescences are commonly called lower ear shoots, nubbins, and ears, depending 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 protruding 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, 39.8 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 with 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 suc­ culent lower ear shoot. It is anticipated that the relative number of third and fourth instar larvae in the stalk may increase on less mature corn with softer stalks. Occasional larvae were found be­ tween brace roots, in the leaf midrib, and at the base of the ligules. T h e S outhwestern C orn B orer in A rkansas 31

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 used. Thirteen first instar, 22 second instar, and 24 third instar larvae had successfully entered the stalk by the following day. That most larvae in these instars are leaf feeders in the whorl under field conditions seems to be a matter of chance or choice rather than necessity. 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 instar larvae managed an entrance in two days. No first instar larvae were tested. Ap­ parently the boring habit on grown corn is limited by the inability of early instars to penetrate 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 prepupal stage apparently lasts a day or less. PUPA Description of Pupa 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 separated by dif­ ferences 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 apparent 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 sculp­ tured 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. The anal opening is in the shape of an inverted Y and located on the 10th abdominal segment. The slit-like genital opening lies entirely within the 9th abdominal segment of the male and be­ tween a pair of tubercles. In the female the genital opening ap- 32 A rkansas E xperiment S tation, B ulletin 553

pears 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 gen­ ital opening. Males are usually smaller than females from the same host. The pupa of the southwestern corn borer has been described and figured in detail by Heinrich (2).

Duration of Pupal Stage Individuals of the overwintering generation that were held in soil cages for pupation and emergence records had a minimum pupal period of 11 days, a maximum of 20 days, and an average, for 32 pupae, of 14.8 days. In 1952 pupae of the first generation, when held under insectary conditions, required a minimum of 9 days, a maximum of 11 days, and an average, for 21 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 8 days, a maximum of 13 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 important source of infestation in corn, produce additional broods by altering the period of larval development, or become severely infested in the absence of corn, some knowledge of borer survival and develop­ ment 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. In gen­ eral, 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, sor­ ghum, 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 49 larvae were established on each host. One larva reared on sor­ ghum, 6 larvae on Sudan grass, and 2 on Johnson grass molted to the overwintering form after apparently attaining full growth. T h e S outhwestern C o r n B o r er in A r k a n s a s 33

These larvae were not included in the average duration of the lar­ val stage to avoid undue distortion. The results obtained from insectary and incubator rearing are summarized in Table 15. In both instances corn was the superior host, judging from survival and rate of development.

Table 15. Effect of Host on Larval Development and Survival

Larval period in days S u rv iv a l H o s t M a x . M in . A ve r. N o . P e r cent

Reared in incubator at 80°F. C o r n______. 36 17 2 2.0 15 63.6 S orgh um...... 49 22 29.0 13 61.9 M i l l e t______. 57 41 4 8.7 4 19.0 Johnson g ra s s ______45 45 4 5.0 1 4.3

Reared in insectary C o rn ...______35 18 23.9 38 77.5 S org h u______m 5 0 1 32 3 9 .6 1 2 7 54.0 S ud a__ n ______5 0 1 27 4 0 .4 1 28 50.0 Johnson grass______5 7 1 3 4 4 3 .1 1 16 3 2.0

1 Excludes overwintering larvae.

Field survival and development on corn, sorghum, Sudan grass, and m illet were checked by artificially 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 oviposition was very light. Plants were dissected for sur­ vivors 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 corn. The data are given in Table 16.

Table 16. Effect of Host on Larval Survival and Development of the First Generation in the Field

Head capsule Number of larvae on width of larvae C o rn S orghum S u d a n M ille t

Millimeters 1 Up to - 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 and more...... 1 0 0

Total no. larvae...... 134 124 28 0 No. egg masses______. 57 57 57 46

1 Plus or minus 0.025 mm. 34 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

The same planting was also artificially infested with the sec­ ond generation and dissections were made when pupation on ad­ jacent corn was near 50 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. Corn was the superior host w ith 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 corn (Table 17).

T a ble17. Effect of Host on Larval Survival and on Development of the Second Generation in the Field

H o s t N o . egg N u m b e r N u m b e r N u m b e r masses la rvae pup ae em erged T o ta l

C o r n____ ..... 72 14 10 4 28 S orgh um .... 90 11 0 0 11 S u dan__ .... 9 0 6 6 0 12 M i l l e t ...... 77 1 0 0 1

Pupae from secondary hosts were smaller than those from corn. Even though survival on sorghum and rate of development on Sudan grass may compare favorably with those on corn, the former hosts apparently are deficient in promoting larval growth. Growth of reared larvae was noticeably retarded from the 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 18.

Table 18. Average W eight of Collected and Reared Pupae on Various Hosts

S ource M a le s Fem ales and host W e ig h t N o . W e ig h t N o .

Collected in field Milligrams Milligrams C o r n______.... 125 10 2 2 7 10 S orgh um______. ------.... 116 18 173 18 S ud a----- n ------.... 88 3 174 5

Reared, Lot 1 C o rn______.... 88 6 127 8 S org h u------m ------.... 102 6 104 6 M i l l e t______— — ------.... 50 4 0 Johnson grass------.... 43 1 0

Reared, Lot 2 C o r n______.... 98 19 141 16 S org h u m------... 75 13 102 7 S ud a----- n ------.... 64 6 93 11 Johnson grass------.... 65 5 70 1

The longevity and fecundity of moths emerging from pupae 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 T he Southwestern Corn Borer in A rkansas 35 on about the same dates. The observed differences, shown in Table 19, between moths from the two hosts cannot be considered significant.

Table 19. Longevity and Fecundity of Moths from Corn and from Sorghum

Corn Sorghum Longevity and fecundity Males Females Males Females

Longevity in days Minimum longevity______4 4 4 2 Maximum longevity...... __ .______6 8 8 7 Average longevity ______.. ____ 5.2 6.0 5.2 5.0

N u m b e r Aver. no. of ______eggs 194.2 179.6 No. of moths______6 11 19 28

The possibility of the borer’s maintaining itself on sorghum, at least, cannot be entirely dismissed. Sorghum is not usually in­ fested to any extent because moths prefer to oviposit on corn. How­ ever, in the absence of succulent corn, moths w ill oviposit freely on sorghum. At the time of the second generation flight in 1954, nearly all corn in the vicinity of the Main Experiment Station was drought-stricken, dry, and unattractive for Oviposition, but a field of sorghum and another field of late, irrigated corn were heavily infested with third generation larvae.

NATURAL ENEMIES W ith the exception of a few overwintering larvae destroyed by nematodes, no parasitized larvae have been found in the field. A few reared larvae were killed by a hymenopterous parasite, identified by B. D. Burks as a braconid. A single parasite emerged from each fourth or fifth instar host but did not complete its de­ velopment, suggesting that the borer was not a normal host. Since the larvae were not accessible to adult parasites, it is as­ sumed that the eggs were parasitized in the field. A braconid, Apanteles cliatraeae Mues., has been reported as a southwestern corn 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, appar­ ently indirectly, by subterranean termites working in the corn stub­ ble during the fall and winter. The stubble are often completely hollowed out, exposing the borer larvae to rain and cold. Rodents, presumably mice, sometimes cut a hole into the stub­ ble at ground level, and feed on the overwintering larvae. Where corn is shocked in the field, mice are often numerous and a large proportion of overwintering larvae may be destroyed. Except under special conditions, rodents are a minor control factor. 36 A rkansas E x p e r im e n t St a t io n, B u l l e t in 553

Egg parasites of the generaTrichogramma, Prospatella, and Teleonomus have been reported from southwestern corn borer eggs (2). OnlyTrichogramma minutum 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 20 per cent. One notable exception occurred in the fall of 1952 when three-fourths of the third generation eggs were para­ sitized. Parasitized eggs are an intense black color and contain, usually, two or three parasites. The adult also destroys by sting­ ing some eggs in w hich 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 feasible to substitute small grains or other non-hosts for corn. 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 reduced 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 popu­ lation in undisturbed stubble may still be quite large in the spring.

Cultural Control To determine the effect of fall plowing on larval survival, a hundred stubble were randomly placed in a pit and covered with earth to depths of one to six inches in November. Samples taken from stubble collected for this experiment indicated a larval pop­ ulation of 176 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 buried larvae was 68 per cent, compared to 16 per cent survival in the undisturbed stubble. Plainly, burying the larvae is not detrimen­ tal to their welfare.

In pursuance of this study, 200 stubble thought to be infested were removed from the field in May and randomly divided into four lots. Two pits were dug and 50 stubble buried in each at depths of one to six inches. The other two lots were set in the ground to simulate the undisturbed condition. A cage was then T he Southwestern Corn B orer in A rkansas 37 placed over each of the four lots and examined daily for moths. No moths emerged from one lot of buried stubble and three emerged from the other lot; 20 moths emerged from one check lot and 23 from the second. A week after the last moth emerged all the stubble was dug up and examined. Most of the buried borers could not be accounted for but some were found dead in all stages. Examination of the surface debris on fields plowed in the fall or spring gave no indica­ tion that buried larvae had worked to the surface and entered surface debris. Infested stubble was collected in March of the following year, 1954, and the burial experiment was repeated. One moth was re­ covered from one lot of buried stubble and three from the other. Twenty-one and 17 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 experiments. On a larger scale, three 10 - by - 12 feet plots were staked out and 180 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 w ith 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, 4 from the disked and turned plot, and 19 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 temperatures when not protected by soil around the stubble. Walton and Bie- berdorf investigated this phase of control extensively in Oklahoma (14). Work along this line in Arkansas was limited to ascertain­ ing that similar results could be obtained under local conditions. Listing stubble to the surface in late November, to take advan­ tage of the year’s lowest temperatures in December, January, and February, gave 73.7 and 88.2 per cent control by mid-February in tw o fields in 1953-54. H and p u llin g the stubble resulted in 97.1 per cent control. 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 m ile 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 harvesting may be 38 Arkansas E xperiment Station, B u lletin 553 completed before girdling begins. In a dates-of-planting test at Fayetteville, corn planted in mid-April, mid-May, and mid-June had 1, 8, and 40 per cent, respectively, of the plants affected by “dead heart”. Girdling of infested corn can be largely avoided if harvesting is completed 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 productive land or where the crop is of special value, such as corn grown for seed production. At present, effective treatments have been de­ veloped only for corn in the early whorl stage and these are in­ tended primarily to reduce damage from “dead heart” and stunt­ ing. The results of insecticide investigations in Arkansas have been published separately (11), but may be briefly summarized here.

The treatment consisted of two spray applications of suffi­ cient volume to obtain run-off into the whorl where the small lar­ vae are feeding. The exact volume of spray per acre depends on 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 stage or hatched eggs are found. The second application should be applied on the 17th day. The reason for this timing is evident from an examination of the location of larvae on whorl-stage corn. About 90 per cent of the larvae from the initial hatch are still feeding in the whorl, where they can be reached and killed by the insecti­ cides, on the ninth day. A large 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 Oviposition period of the overwintering brood and of the first gen­ eration covers a period of about three weeks, two applications are necessary.

EPN, isodrin, and endrin have given effective controls at rates of one, one-half, and one-eighth pounds, respectively, 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, isodrin reduced the number of infested plants by 87.3 per cent, and endrin by 72.8 per cent.

While this method of control is practical on young corn, par­ ticularly on late plantings where loss by “dead heart” is commonly high, it is not suited for older, larger corn because of the large volume of spray required and the need for special machinery. T he Southwestern Corn B orer in A rkansas 39 SUMMARY The southwestern cornDiatraea borer, grandiosella Dyar, is an important pest of corn over a large area of the southwestern and midwestern United States. Although the range of this insect has fluctuated over the years, there has been a considerable exten­ sion to the north and east during the period reliable records have been kept. The eastern boundary of the infested area now lies in Arkansas and Missouri. Young corn may be killed or dwarfed by larval activity in the meristematic tissue. Yields may be further reduced by tunneling in growing corn, and harvesting is complicated by girdling activity in the fall. The insect overwinters in the larval stage below soil level in the stubble. In the spring it pupates in the stubble and moths fly to young corn where they deposit their eggs. In Arkansas the number of generations varies from two and a partial third to three and a trace of a fo u rth , depending on the season and locality. The eggs are distinctively marked with three red bars and are u sually deposited in sm all, fla t masses. The incubation period normally requires four to seven days. The first three instars are predominately surface feeders in the whorl or other secluded positions. Fourth and later instars are predominately borers. There are five or more instars and the entire larval period is passed in three to four weeks. The summer- form larvae are white with conspicuous black spots but the over­ wintering form is an immaculate yellowish white. Pupation occurs in the stalk above ground, excepting the over­ wintering brood. The pupal period requires about ten days to two weeks. The nocturnal 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. Females deposit an average of about tw o hundred eggs, although more than six hundred have been obtained from an individual. Fertility is normally quite high. Moths show a strong preference for succulent, large corn but w ill oviposit on small or maturing corn 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 survive on secondary hosts than on corn, the larval stage is often prolonged, and the insect is smaller. 40 A rkansas Ex p e r im e n t S t a t io n , B u l l e t in 553

No parasites, predators, or diseases of importance have been observed. At present, relief from severe damage depends prim arily on cultural practices. The overwintering larvae are vulnerable to freezing when the stubble are brought to the surface in the fall, and moth emergence is reduced by burial of the stubble in the spring. Loss from “dead heart” and stunting can be reduced by early planting and loss from girdling by early harvesting. Under some conditions insecticide treatments may be desirable.

LITERATURE CITED

1. Box, Harold E. “The Crambine generaDiatraea and Xanthoperne (L e p . P y ra l.) ” Bul. Ent. Res. 22:1-50, 1931.

2. D a vis, E. G., J. R. Horton, C. H. Gable, E. V. Walter, and R. A. Blanchard. “The southwestern corn borer.”U.S. Dept. Agr. Tech. Bul. 388, 1933.

3. Davis, E. G. " Apanteles diatraeae, a braconid parasite of the southwestern corn bo re r.” U.S. Dept. Agr. Tech. Bul. 871, 1944.

4. Davis, Geo. W. Jr. Personal communication in author’s file, 1955.

5. Dyar, Harrison G. “The American species ofDiatraea (Lep. Pyralidae).” Ent. News. 22:199-207, 1911.

6. D yar, Harrison G. and Carl Heinrich. “The American moths of the genus Diatraea and allies.” Proc. U.S. Nat. Mus. Vol. 71, Art. 19 (No. 2691), pp. 1-48, 1927.

7. H oloway, l T. E. “Larval characters and distribution of two species Diatraeaof (D. saccharalis cramhdoides Fab. and D. zeacolella D y a r ) .” Jour. Agr. Res. 6:621, 1916.

8. H oward, L. O. “The larger corn stalk borer(D. saccharalis F . ) ” Insect Life 4:95-103, 1891.

9. M atthews, David L. Jr. Kan. Ent. Comm.— USDA Cooperative Insect Detec­ tion Service Rpt. 41 (Nov. 28-Dec. 3), 1954.

10. Peterson, Alvah. Larvae of Insects, Part I, Edward Bros., Inc., Ann Arbor, Mich., 1948.

11. Rolston, L. H. “Insecticide tests against the southwestern corn borer.” Jour. Kans. Ent. Soc. (in press).

12. Thomas, Geo. W. Cooperative Economic Insect Rept.,U.S. Dept. Agr. V o l. 4. No. 42 (Oct. 22), 1954.

13. Walkden, H. H. and G. D. White. “Status of the southwestern com borer in Kansas, Oklahoma, Missouri and Arkansas in the fall of 1950.”U.S. Dept. Agr. Bur. Ent. and Plant Quar., Insect Pest Survey, Special Supplement No. 2, 1951.

14. Walton, R. R. and G. A. Bieberdorf. “ Seasonal history of the southwestern corn bo rer,Diatraea grandiosella Dyar, in Oklahoma; and experiments on methods of control.”Okla. Agr. Expt. Sta. Tech. Bul. T-32, 1948.

15. W ilbur, D. A., H. R. Bryson, and R. H. Painter. “Southwestern corn borer in Kansas.” Kans. Agr. Expt. Sta. Bul. 339, 1950.