January, 1939

Hibernation of tlie Corn Ear Worm In Southern Connecticut

U N ~ T I ~ D STATES DEPARTMENT OF ~ \CR~CULTURR, WASNINGTOW, D. C . IN C O ~ ~ P E I ~ A T I O N \\'IT11 THE CONP~ECTICUT ~ G ~ I C U L T U R A L EXPERTAIENT STATION.

Hibernation of the Corn Ear Worm In Southern Connecticut

CONTENTS PAGE

HIBERNATING HABITS OF THE INSECT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

COMPARISON OF THE WINTERS 1935-36, 1936-37 and 1937-38 . . . . . . . . . . . . . . . . . . . . . 5

RESULTS OF EXAMINATION OF CAGES PLACED OUTDOORS.. . . . . . . . . . . . . . . . . . . . . . . .12

Survival of pupae in outdoor cages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I6

Survival in fall. . ,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Survival in spring.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . l G

Survival in summer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Depth of pupae in the soil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1'6

Durability of the emergence burrows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Factors leading to destruction of burrows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Heavingofthesoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Earthworms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -21

Roots of growing plants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Emergence of moths.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

OR several consecutive years prior to 1935 the corn ear worm (Heliolhis Fobsolela F.) caused serious injury to corn in southern Connecticut dur- ing the growing season, particularly in the fall. Outbreaks had occurred occasionally for many years, but not annually. I t was apparent that there had been a definite change in status of the insect in this area.

Previous to 1935 a number of experiments had been performed to deter- mine whether or not the corn ear worm hibernated successfully in southern Connecticut. All the results proved negative, and a t that time there was no recorded instance of the insect having overwintered there.

In view of the recent annual occurrence of the pest the question arose whether this was due to annual migration of moths, successful hibernation of pupae under conditions not yet known, or to the arrival of the insect through interstate shipment of its green vegetable hosts, particularly corn, tomatoes, and beans. The study described herein was therefore initiated in the summer of 1935 and continued until the spring of 1938 to determine whether the insect did hibernate successfully in the area.

HIBERNATING HABITS OF THE INSECT

In the Northeastern States the corn ear worm spends the pel.iod from September to June in the soil as a pupa. The periods during which pupae go into hibernation in the fall and moths emerge early in the summer cover, in each case, from one to two months.

During the fall, larvae which mature in rorn ears burrow into the soil. They dig to depths of from 1 to 8 inches below the surface, and then each larva opens a tunnel extending from the deepest point attained to within about half an inch of the surface. The walls of this tunnel are compacted and

I possibly lined with silk or cementing material; consequently, unless inter- fered with by an external agency such as earthworms, roots of plants, or heaving during freezing and thawing, the tunnel remains.open until the following summer. The purpose of this tunnel is to enable the moth to es- cape from the soil. When the tunnel is finished, the larva returns to the deepest point, which is slightly enlarged, and within a few days becomes a pupa. The pupa rests in this position for a t least nine months, or until the warmth of early summer stimulates its emergence. Then, as a moth, it crawls up to the top of the burrow, forces its way through the narrow bar- rier of earth, attains the surface, spreads its wings, feeds on the nectar of flowers, mates, and seeks out corn or other suitahle plants on which to deposit its eggs.

* G . W. Barhcr Division of Cereal and Fora e Illsect Investi ntions. Burenu of E~ttomology and Plnnt Quarantine, U. S. bepartment of Agriculture. &ashington, D. E.

4 Connecticut Experiment Stalion Bulletin 419 Methods of Study 5

SEASONAL ABUNDANCE OF THE INSECT

The seasonal occurrences of the ear worm in southern Connecticut were similar during the period 1935 to 1937, inclusive.

In 1935 full-grown larvae were found during the last week of July in two fields on elevated ground, one a t Woodbridge and one a t Mount Car- mel. Eggs probably had been laid during the first week of July. Elsewhere the insect was scarce or u7as not found. Infestation continued a t a very low rate until mid-August, when populations began to increase. During the first two weeks of September heavy infestations occurred, usually amount- ing to 100 percent in ears of late sweet corn.

In 1936 larvae were first found on July 13 as partly grown individuals in a field in Hamden. Infestation was a t a rate of 4.7 percent of ears in this field, but the insect was much less abundant elsewhere. Eggs probably had been laid during the first week of July. Infestation until mid-August varied from none to 2 percent, but by that time it began to increase, and by Sept- ember 15 as many as 50 percent of sweet-corn ears, and as many as 25 per- cent of ensilage corn ears, were attacked.

In 1937 larvae were first found on June 22 in the same field where the earliest infestation was found the previous year. Fifth-instar larvae were recovered a t this time, but the infestation was a t a rate of less than 1 per- cent. Eggs apparent,ly had been deposited about June 10. During July and August the insect was very scarce in most cornfields or was not present a t all. It began to appear and increase during the last week of August, and by mid-September from 75 t,o 100 percent of ears of late sweet corn were attacked.

The insect caused no appreciable injury t,o corn before mid-August in any of the three years. Until that time it.s populat,ion per field was uniform- ly low.

Infestation each season apparently began with very few individuals. Throughout July and August large acreages of corn in stages of growth attractive to the insect were available, but the individuals of the first brood were scarce and their rate of reproduction was not high enough to cause noticeable infestation of this large acreage. By the third or fourth week of August, however, ripening of the corn and ensilage cutting began, and from that time on the supply of corn in suitable stages of growth for attack by the ear worm rapidly decreased, resulting in a concentration of the expand- ing population of the insect in the limited acreage of late corn. I t was only a t this time, during any of the years observed, that economic damage was caused by ear worms. When the relation betweeq the abundance of the ear worm and the acreage of corn in a condition attractive to i t is considered, i t is believed that all the infestations found throughout each of the three yearsof observation can he accounted for as natural increase, during two or more generations, from the few individuals that occurred in June or July in the first brood of a year1.

1 This explanation of the seasonal abundance was first developed by F. F. Dicke who has presented supporting data in a manuscript en!itled "Seasonal Abundance of the Corn Earworm." to hr euhmitted for in the Journal of Agr~culfr~rnl Resmrch.

COMPARISON OF THE WINTERS 1935-36, 1936-37 and 1937-38

The three winters during which these experiments were conducted were quite variable in severity. That of 1935-36 might be classed as very severe, that of 1936-37 as very mild, while that of 1937-38 was intermediate. Dur- ing January, February, and March of 1936, snow covered the ground much of the time, often to depths of a foot or more; but in the two succeeding winters little snow cover occurred, especially during the winter of 1936-37, which was nearly snow-free. Thus, during the course of these experiments, a wide range in winter weather occurred. This is indicated in the data given in Table 1, applicable to New Haven.

Of the several environments in which experiments were placed, the cli- mate of Milford was similar to that of New Haven. That of Mount Carmel, a t an elevation of 200 feet, and that of Windsor, 45 miles north, were more severe, with minimum temperatures sometimes falling from 10 to 15 degrees lower than a t New Haven. The climate of West Dennis, Mass., on the other hand, was somewhat milder than that in New Haven.

METHODS OF STUDY

To study the survival of pupae and the emergence of moths from hiber- nation, i t was necessary to protect pupae against attack by predators, such as moles and skunks, and to trap the emerging moths. For this purpose cages were used such as had been employed in previous studies elsewhere.

Twenty cages 30 inches wide, 30 inches long, and 10 inches deep were constructed of redwood. Lids two inches deep and covered with wire screen- ing were attached with hinges and hooks. The wood was first painted with linseed oil and later with green paint to ensure durability. During summer, cages were sunk in soil in the field to a depth of about nine inches so that the contained soil would become naturally packed by fall. The soil was also packed by treading.

In September of each year, 100 ear worm larvae were allowed to dig into the soil of each cage. Larvae were collected in September of 1935 and 1937 from sweet corn, and in 1936 from ensilage corn. Because of the cannibal- istic habits of this insect i t was necessary to handle each larva separately. Individuals as nearly full-grown as possible, mostly in the sixth instar, were collected, isolated in two-ounce tin salve boxes, and given fresh, soft kernels of corn daily. When the larvae were nearly mature, the covers were removed and the boxes containing the worms were inverted over the soil in the cages. Here again food was renewed daily as long as they continued to feed. By this method larvae could finish feeding and burrow into the soil a t will, without disturbance, during this critical period of their lives. As soon as 100 had burrowed in a given cage, the tin boxes and food refuse were removed.

Cages were usually set out in series, so that one could be examined in the fall and one in the spring, and a t least one was allowed to remain for pos- sible moth emergence. The lids were removed after the soil had frozen, were left off during the winter, and replaced in spring before the ground thawed.

Conneciicul Experimenl Station Bulletin 419 Melhods of Study 7

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Larvae entered the soil of cages during the second and third weeks of September each year. The number digging on each date is given in Table 2. The weather in 1935 and 1936 was favorable for their development and digging activities and the required number entered the soil within about 10 days. Cool, cloudy, or rainy weather during this period in 1937 was un- favorable for their development, arid not only did they enter the soil more slowly but many died. In the three-year period a total of 60 cages, each containing 100 individuais, were studied. In 1936 and 1937, these were augmented by 10 cages, each containing 25 individuals. It was not possible to obtain ear worms in suflicient numbers to stock hibernation cages until the second week of September in any of the three years because, as ex- plained in a previous section, i t was not until this time that concentrated populations of the insect occurred.

TABLE 2. DATES WHEN CORN EAR Wonni LARVAE DUG INTO THE SOIL OF HIBERNATION (

New Haven, Milford, Mount Carmel, and Windsor, Conn.

Datc Number of Larvae t ha t Entered the Soil September

Total 1 1,800 1 1,800 1 2,000 I

* Cages at West Dennis, Mass.

While the larvae were digging into the soil of cages, i t was found neces- sary to protect them from carnivorous ants, that were attracted to and killed larvae entering the soil or feeding under salve boxes. One species in particular, the cornfield ant, Lasi~zs niger var. americanus Emery (de- termined by W. M. Mann), was a nuisance in this respect and killed larvae in the first cages set out a t Mount Carmel and Milford in July, 1933. In all subsequent experiments, when larvae were presenl, they were protected by a narrow barrier of kerosene poured on the earth about the cages. When renewed daily, this barrier prevented ants from approaching the cages.

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the surface, or an average depth of 1.7 inches. Of the burrows, 25, or 44..6 percent, were open; 21, or 37.5 percent, were partly open; and 10, or 17.9 percent, were closed. This cage contained 29 earthworms.

numbers by the last week of that month. I t is probable that the percent- age gradually increases from the first of August onwards. Hibernation of individuals that entered the soil in the middle of September was complete, for in none of the three years i d moths emerge from these cages i n the fall. These results seemed to show that no larvae entering the soil during the

last week of July hibernated, and that their emergence burrows were partly obliterated during the summer, principally through the activity of earth- worms. Larvae entering the soil for pupation a t this time did not dig nearly so deeply as t.hose that went into the soil in September; the depths of the burrows of the latter are given in Table 8.

Information was also obtained on the possible hibernation of individuals entering the soil during July. The results from two such cages N ere render- ed inconclusive in 1935, because, when an examination was made the follow- ing September, i t was found that ants had gained access to them. A cage a t Mount Carmel, in which larvae entered the soil July 27 to 30 inclusive, was examined on September 16. No live pupae were recovered. A total of 21 empty pupal skins were found a t depths of from 0.9 to 3.7 inches below the surface, or an average depth of 1.6 inches. Of the burrows, 13, or 61.9 percent, were open; 7 , or 33.3 percent, were partly open; while 1, or 4.8 percent, was closed. Six earthworms were present in this cage. A cage a t Milford, in which larvae had entered the soil July 27 to 30 inclusive, was examined on September 17. No live pupae were recovered. A total of 56 empty pupal skins were found a t depths from 0.9 to 4.3 inches below

EFFECT OF SNOW COVERAGE ON PUPAL SURVIVAL

In September, 1935, two cages a t New Haven (cages 17 and 18, Table 4) were stocked with pupae in the usual way and set up for study of the effect of snow coverage on survival of pupae. The following winter was the only one having sufficient snowfall to make such an experiment possible. One cape was kept free of snow throughout the winter; the second was kept

Depth of pupae in soil c~nches) 1 I - I'upae encumbered by

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CONNECTI- CUT

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Survival of Pupae in Outdoor Cages

Survival in fall

Over the three-year period, four cages were examined during November a t h'Iount Carmel and a like number a t Milford. Of an expected population of 400 in the soil of the Mount Carmel cages, 387 were recovered, or 96.8 percent. Of these, 326, or 84.2 percent, were live pupae; 31, or 8 percent, were dead pupae; and 30, or 7.8 percent, were dead larvae. Of an expected population of 400 in the soil of the R'Iilford cages, 391, or 97.8 percent, ivere recovered. Of these, 287, or 73.4 percent, were live pupae; 51, or 13 percent, were dead pupae; and 53, or 13.6 percent, were dead larvae. Thus i t ap- peared that the Milford environment was less favorable for pupation arid survival than the Mount Carmel environment, probably owing to the better soil drainage of the latter location. Whatever the environment, however, some larvae failed to pupate, and some pupae died during the fall.

Survival in spring

Over the three-year period 25 cages were examined during April and RiIay. Live pupae were found in only five. In a cage a t West Dennis, Mass., 41 live pupae were found on May 4, 1937 (cage 19, Table 5), a survival a t the rate of 39.4 percent of recovered individuals. In two cages in this location examined on April 27 and 28, 1938, a total of six live pupae were recovered (cages 19 and 20, Table 6), or a survival a t the rate of 3.3 percent of re- covered individuals. Again, six live pupae were found in a cage of washed sand a t Milford, examined on May 7 , 1937, (cage 8, Table 5), or a survival a t the rate of 12 percent of recovered individuals. Thus, during the three- year period, no living pupae were recovered in spring from outdoor cages in any soil environment other than sand.

In soil of an open shed a t Mount Carmel, examined on May 1, 193'7 (cage 15, Table 5), 36 pupae, or 38.3 percent, of recovered individuals were alive.

Survival in summer

Of 18 outdoor cages examined during summer, live pupae were recovered in but one, a cage a t West Dennis, Mass., examined on July 23, 1937 (cage 20, Table 5). Only a single live pupa was recovered, but by this time moths had emerged from most hibernating pupae which had survived the winter. Three live pupae were recovered from a cage placed in the soil of an open shed a t Mount Carmel when examined on July 28,1937 (cage 16, Table 5).

Depth of Pupae in the Sod

The depth to which larvae dug before forming the emergence burrows, and a t which they transformed to pupae, varied in the several localities and years. A few pupae were found only a little below the surface of the soil and some occurred a t depths greater than 6 inches, but most (60.64 percent) of them were between 2 and 4 inches below the surface. The greatest, least,, and average depths of pupae in each cage examined are given in Tables 4, 5, and 6, the numbers of pupae found a t various depths during each year

Exnmination of Outdoor Cages 17

are given in Table 7, and the average depths of pupae in the several en- vironments and localities during the three years are given in Table 8. Lar- vae had dug deepest, on an average, in September, 1936, which was followed by the mildest winter and the highest survival. while they dug the shallowest burrows in September 1935, which was followed by the most severe winter of the period and the lowest survival. ,

Sand was the only soil in which pupae survived the winter outdoors dur- ing the three-year period. I t is notable that the larvae dug deeper in sand (soil of West Dennis, Mass., and washed sand a t Rlount Carmel and Mil- ford, Conn.) than in other soils each year. The deeper location of pupae in the sand, with correspondingly greater protection against the lowest winter temperatures, together with the much better drainage, may have been important factors in the survival of pupae.

TABLE 7 . DEPTH OF CORN E n n WORM PUPAE BELOW THE SURFACE OF THE SOIL

Depth of I Pupae recovered at dieerent depths pupae I

(illchcs) 1 1935-36 1 1936-37 1 1937-38 1 Total I %

Less than 1 1.1 to 2 2.1 to 3

6.1 to 7 7.1 to 8 8.1 to 9

Total

Durability of the Emergence Burrows

In order that moths may emerge during spring or summer i t is necessary that the emergence burrows prepared by the larvae remain open. Emer- gence burrows in cages that were protected from precipitation and that contained no earthworms or other fauna, or growing plants, remained open perfectly. Such was the condition of burrows in cages placed in an open shed a t Mount Carmel, in a tobacco shed at Iliindsor, and in an insectary a t New Ilaven. But in outdoor environments the number of partly or wholly closed burrows increased p~ogressively from the time wrhen dug by the larvae until the following summer. Table 9 summarizes the records for the three years of burrow conditions found on examination of cages located outdoors. Although, on examination during fall, 88.06 percent of burrows were open, only 24.76 percent were found in this condilion in spring, and by summer only an average of 7.54 percent were open.

Burrows remain open in some soils more perfectly than in others. Thus, a t Mount Carmel, in well-drained loam containing some gravel, burrows remained open longer than a t Milfcrd, where the loam was less well drained and contained no gravel. In sand a t West Dennis lhey remained open until

TABLE 8. ~ V E R A G E DEPTHS BELOW THE SOIL SURFACE AT WHICH CORN EAR WORM PWAE WERE RECOVERED IN DIFFERENT ENVIRONMENTS

I I

Environment Average depth of pupae (inches) 1 1 1935-36 1 1936-37 \ 1931-38 1 All seasons

Outdoors : In cultivated soil Mt. Carmel

Milford New Haven West Dennis'

Average for the above four soils I In washed sand

In alfalfa field Protected from rainfall:

In soil of an open shed In soil of a tobacco shed

Mt. Carmel 1 Milford 1 Mt. Carmel

Mt. Carmel Windsor

1 Town in Massachusetts; others are in Connecticut.

Fall

Spring

Summer

Burrows Found in the SpeciGed Conditions

Number of cages

examined

Total number of pupae

recovered

PARTLY OPEN

Number OPEN 1 l'ercent Number Percent

CLOSED

Number 1 Percent

Connecticut Experiment Station Bulletin 419

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spring, more perfectly than in loam either a t Mount Carmel or Milford, but were destroyed more rapidly during the period between spring and summer examinations. A comparison of the condition of emergence burrows in these three soil types during three seasons over a three-year period is given in Table 10.

Factors Leading to Destruction of Burrows

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Many factors influence the d'urability of the emergence burrows, but none seems to be more important generally than aiternate freezing and thawing of the soil, the activities of earthworms, and the penetration into them of roots of growing plants.

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The soil of southern New England usually has a high moisture content due to abundant and frequent rainfall. When it freezes expansion occurs, which results in some parts being thrust upward. On thawing the soil con- tracts, and if i t has been frozen deeply the contraction results in the open- ing of fissures several inches deep similar to those occurring in the drying of clay.

In our experiments this was more noticeable in soils of high clay content, such as occur in the Milford environment, less noticeable in soil with some sand content, such as that a t Mount Carmel and a t New Haven, and least noticeable in soil of high sand content, such as that of West Dennis, Mass. No such effect was observed in soil protected against winter precipitation, such as that under the shed a t Mount Carmel, in the insectary a t New Haven, or in' the tobacco shed a t Windsor. The emergence burrows re- mained in perfect condition after repeated and heavy rainfall, but the move- ment of soil during freezing and thawing, or in drying after saturation, caused cracks lo open which interrupted the walls of the burrows. Obser- vations seemed to show that soil may then be washed into them and the pupae buried. The degree to which the burrows are filled in this way and the number affected depend not only on the soil type and its susceptibility to cracking but on the depth to which the soil has been frozen and the num- ber of times i t was frozen and thawed during winter.

Earthworms

-Earthworm populations vary from many to none, depending on the type of soil and its moisture and humus content. Such differences were observed in the soils in which hibernation cages were set, and the average recovery of earthworms per cage is given in Table 11. The most abundant earth- worm population was found in Milford loam, the least in sandy soils. Un- disturbed soil protected from rainfall contained none, probably because i t was too dry.

The earthworms observed in these cages varied'from minute, recently hatched individuals feeding in dead pupae to full-grown specimens. De- pending on the moisture content of the soil, they wel'e found a t depths from near the surface to below the bottoms of cages. They were usually active during fall and spring but were frequently found resting in cells a t various depths during dry periods of summer.

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