G. W. Barber.
The Importance of Winter Mortality in the Natural Control of the European Corn Borer in New England.
Psyche 31:279-292, 1924.

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19~1 Winter Mortality oj European Corn Borer 279 THE IMPORTANCE OF WINTER MORTALITY IN THE NATURALCONTROLOFTHEEUROPEAN CORN
BORER IN NEW ENGLAND.1
Cereal and Forage Insect Investigations, Bureau of Entomology, United States Department of Agriculture. Among the factors that limit the spread and occurrence of an injurious insect in numbers sufficient to cause injury to crops, the ability of a species to withstand the rigors of winter occupies an important place, particularly in colder climates. If an insect is unable to pass successfully through this troublesome period of the year, it probably will rarely become a pest of first im- portance) although by means of annual migration it may occur frequently in some numbers. Such a condition is found, for example) in the case of the corn earworm (Heliothis obsoleta Fab.) which as far as our knowledge goes cannot successfully winter in New England or other northern sections of the United States. However, this insect appears nearly every summer in numbers ranging from a mere trace to serious abundance in such areas) and as the writer has shown, (Jour. Agri. Res. 1924, XXVII p. 65) it became a pest of importance in New England in 1921. The accepted explanation of this phenomenon is that the moths of this insect migrate northward each spring and summer, arriving in larger numbers some years than in others, and in the case of favorable growing seasons) possibly pass through a partial second generation, in which case injury is most noticeable. It may happen in the case of another species that a large number of individuals may perish in the overwintering stage) more dying during severe winters than during mild winters. This too may result in the species becoming injurious during some seasons and unimportant during others following a high or low rate of mortality during the preceding winter, Investigations of the European corn borer (Pyramta nu- 1Contribution from the Bureau of Entomolo U. S. Department of Agriculture in cooperation with the ~ntomologicaf?aboratory of the Bussey Institution, Harvard University, (Busse~ Institution No. 242.)



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280 Psyche [~ecem ber
bilalis Hubn.) in the New England area have been conducted for several years to determine the importance of winter mortality as a limiting factlor in the spread and injurious occurrence of this insect. It has been found that the spread of the insect has been greatest along the coast of New England and least inland from Fig. 1.
Map showing the southern extent of the area of high winter mortality of the European corn borer in New England during the winters of 1922-1923 (AA) 1923-1925 (BB). the sea. This condition may be due in part to an ability of the overwintering larvz to survive more successfully along the coastal region where the winter temperatures due to the moderating effect of the ocean are less severe than farther inland where such a moderating effect is less noticeable. On the other hand no such condition may exist, and the more rapid spread along the coast may be the result of other causes.



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19241 Winter Mortality of European Corn Borer 28 1 In the following account of winter mortality of the European corn borer, information has been obtained in two ways. Field
counts of larvz in the spring have usually been made in localities where the insect occurs abundantly and in host plants that have remained undisturbed. Over much of the area now infested in New England) however) the insect occurs in insufficient num- bers to permit such counts in numbers large enough for accuracy. In this case experiments were placed in the field in representative localities in the fall after activity of the insect had ceased and collected in the spring before activity was resumed. In the studies of the results obtained in this work) the conditions found to exist in the spring of 1922) 1923 and 1924 may be considered separately.
Observations in the spring of 192%'.
During the spring of 1922 observations of winter mort,ality were confined to host plants that. had passed the winter undis- TABLE No. 1.
No. of
Host Plant localitie:
PP
..... Sweet Corn.. 9
Cocklebur
...
(Xanthium sp.). 5
Barnyard Grass
(Echhzochloa crus-
galli (L). Beauv). 4
Smartweed
(Polygonum sp.). .. 2
Beggar-ticks
...... (Bidens sp.). 2
P~gweed (Amaran-
thus retroflexus
L.) ............. 2
Lamb's quarters
(Cheno odium al-
bum L$... . .'. ....
Total
No.
larv~e
txamin-
ed
:I Least.
No.
larvae
dead
Aver. %
winter
mor-
tality
Greatesi
% winte~
mor-
tality
for any
locality
% winter
mor-
tality
for any
locality
6.2%
2%
10 %
0%
0%
15%
....




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Psyche
[December
turbed. These observations were in corn and certain other host plants in which the insect occurs abundantly. The observations
were also confined to that section of New England where the insect occurred in large numbers) an area bounded by Marble- head, Mass., on the north, Arlington) Mass, on the west and Quincy) Mass. on the south. The results of these observations are shown in Table I.
For all these localities and in all host plants the average winter mortality was 9.4 per cent, being highest in pigweed (Amaranthus retroflexus L.) and least in Cocklebur, (Xanthium sp.) and Smartweed (Polygonurn sp.) .
TAELE No. 2.
I INTER &~ORTALITY FOU~D IN THE SPRING OF 1923 IK ~ ~ ~ 1 0 ~ s -
Vum-
er oj
ocali-
ties
Host Plant
Sweet Corn.. ......
Beggar-ticks (Bidens
sp.) .............
Pigweed (Amaran-
thus retroflexus L.).
Horseweed (Erigeron
.... canadensis L.)
Cocklebur (Xanthi-
1 um sp.). .........
Barnyard grass (E-
chinochZoa crusgd-
.. Zi (L.) Beauv.)..
Smartweed (Polygo-
num sp.). ........
Dahlia (Dahlia sp.). .
Prince's feather (Pe-
Zygonum orien,tale
..............
L.)
Hemp (Cannabis sa-
tzva L.) ..........
Flase ragweed (Iva
xanthzfohz Nutt.).
Tot a1
num-
ber
0 f
lanae
Num-
ber of
Col-
lec-
tions
50
9
9
I
10
8
7
4
I
2
I
Num-
ber
arvae
dead
hl ean
Per
cent
arvae
dead
Highest
)er cent
lead-in
any col-
lection
Lcwest
)er cent
dead, in
any col-
lection




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19241
Winter Mortality of European Corn Borer
Obsefvations in the spring of 1993.
Investigations of winter mortality during the spring of 1923 were of two distinct sorts; first, observations of the winter mortality in host plants that had remained undisturbed through the winter and second, observations of the winter mortality in corn stalks placed in representative localities the previous fall for the purpose of obtaining information on this subject. In Table 2 the results of the observations of winter mortality in host plants that had remained undisturbed during the winter are shown. The highest rate of winter mortality was found in pigweed (Amaranthus retro$exus L.) and false ragweed (Iva xanth<folia Nutt.) ; the least mortality in prince's feather (Poly- jonurn orientale L.) and hemp (Cannabis sativa L.). These exa- minations were confined to localities within the heavily infested area of Massachusetts as were the corresponding observations of t,he preceding spring. The average winter mortality for all localities and host plants was found to be 8.3 per cent. In table 3 winter mortality is compared in corn stalks that had passed the winter in various conditions. Here mortality appears to have been highest in corn stubble and lowest in corn that was biled up in the fall and passed the winter in this condi- tion.
TABLE No. 3.
WINTER MORTALITY IN CORN STALKS THAT PASSED THE WINTER IN SEVERAL CONDITIONS.
Mean Greatest
Num- per winter
ber of cent mor-
larvae of tality,in
dead larvae any col-
dead lection
Condition of
corn stalks
Least
winter
mor-
tality, in
any col-
lection
Num-
ber of
collec-
tions
Standing stalks.. . . .
Stalks lying on sur-
face of soil. . . . . . .
Stalks placed in piles
in the fall.. . . . . , .
Corn stubble. . . . . . .




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Psyche [December
None of the localities summarized in tables 2 and 3 were far enough inland to show any marked difference in winter mortality that might be associated with any climatic factor. In fact all these localities are in a very small area, somewhat larger than the area covered by the examinations of the previous year but confined to the heavily infested regions for the reason previously stated. -
The experimental work, on the other hand, shows very clear differences in winter mortality. An experiment was placed in the late fall of 1922 in each of twenty localities representing all sections infested by this insect at that time. These experiments were placed in the field in November, 1922, after activity of the insect had ceased and were recovered in April, 1923, before spring activity began. Migration of larvee from the corn stalks was for this reason limited to a few individuals. Each experiment consisted of ten stakes to each of which were fastened six in- fested stalks. The findings in the spring of 1923 may be best shown in the following table.
TABLE No. 4.
WINTER MORTALITY FOCND IN THE EXPERIMENTAL MATERIAL IN THE I
Date Date
Locality placed in re-
field covered
Bristol, N. H.
Farmington, N. H.
Wells, Me.
Tyngsboro, Mass.
Concord, Mass.
Methuen, Mass.
Worcester, Mass.
Walpole, Mass.
Quincy, Mass.
Manomet, Mass.
Wareham, Mass.
Falmouth, Mass.
Harwich, Mass.
Wellfleet, Mass.
Total
num- Num-
ber of ber of
larvae larvae
re-cov- dead
ered
Per
:ent of
winter
mor-
tality
)er cent
)f winter
mortal-
ity in
group
35.5
22.6
3.9
.88
,
.
.
.
Average
r
c
-
-
-
-




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19241 Winter Mortality of the European Corn Borer . 285 Several of the experiments placed in the field and recovered are not shown in this table because birds removed such a large number of the larvae from the stalks that too few remained from which to draw conclusions. However, several areas stand out rather distinctly as showing considerable difference in the extent of winter mortality.
First, an area represented by three stations in southern New Hampshire and Maine where winter mortality averaged 35.5 per cent; second, two localities in Massachusetts in inland river valleys where winter mortality averaged 22.6 per cent; third, four localities in eastern Massachusetts where mor- tality averaged 3.8 per cent; and fourth, five stations on Cape Cod where winter mortality averaged only .88 per cent. Observations in the spring of 1924
In the fall of 1923 a series of experiments similar to those described for the previous year were placed in the field in the -
infested area of New England. As in the former instance the experiments were placed in the field in the fall after activity of the larvae had ceased and they were recovered in the spring before activity commenced so that the loss through larval mi- gration was expected to be limited to a few individuals. Each experiment consisted of four stakes to each of which were at- tached five infested corn stalks. The stations, 50 in number, were chosen in an attempt to represent the whole infested area in as fair a manner as possible. Using Arlington, Mass., as a center, these stations were run out in lines as far as the infestation was known to exist to the northeast, the north, the north by northwest, the northwest, the west, the southwest, the south and the southeast. On these stations 47 were recovered in good ,
condition, 16 of which exhibited considerable feeding by birds. The average winter mortality of these 16 stations which were all located in Massachusetts was 1.7 per cent of the total number of larvae recovered. The extent of winter mortality is shown in table 5. In this table the stations are grouped in three main divisions similar to those into which the area seemed to be naturally divided the previous spring.




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Psyche
[December
TABLE No. 5.
Maine and New
I Eastern Massachusetts
Ham]
Per cent oi
winter
mortality
Number of Per cent oi
stations winter
mortality
-----
Number oi
stations
1
3
2
2
1
1
1
1
Average mortality 9.3 %
Average mortality omit-
ting the two highest
5.4%
-
Cape Cod
0%
less than
1 to2%
2to3%
3 to4%
4to5%
5 to6%
6to7%
7 to 8%
Average
Per cent of
winter
mortality
0%
less than 1%
1 to 2%
2 to 3%
Number of
stations
Average mortality 1.3%
Although the mortality was not nearly as extensive as it had been during the preceding winter, the same general conditions are noted; that is, a relatively higher rate of mortality at sta- tions in Maine and New Hampshire and an extremely low rate on Cape Cod.
Only two stations showed an important mor- tality-Concord, N. H. (39 per cent) and Bristol, N. H. (24 per . cent). Why these stations should have shown extensive winter mortality while other stations in the same region did not, is a point not clearly understood.
They yere in more exposed loca-
tions, however, and may not have benefited by snow protection to so great an extent as the other stations. The following table exhibits the
protecting influence of
snow as found in an experiment overwintering in Wells, Maine and examined April 28, 1923.




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19241
Winter Mortality of European Corn Borer
TABLE No. 6.
THE EFFECT OF SNOW PROTECTION ON THE EXTENT OF WINTER MORTALITY. Longfellow flint corn
1 Sweet Corn (24 stalks) 1 (29 stalks)
Middle section of stalks.. . 435 190 43% 223 Q/Q 4'1% ---------PA--
Bottom foot of stalks.. . . .I 68 1 13 19% 1 49 1 1.3 l26.5% Top foot of stalks.. . . . . . .
It is believed that the top of these stalks was exposed most of the winter, that the middle sections were exposed some of the time during the winter months and that the bottom foot was