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Black bears forage on army cutworm moth aggregations in the Jemez Mountains, New Mexico

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Abstract

Observamos osos negros (Ursus americanus) alimentándose de agregaciones de la mariposa (Euxoa auxiliaris) (Lepidoptera: Noctuidae) en felsenmeers (campos de rocas) subalpinos de las montañas Jemez de Nuevo México. Estas agregaciones sirven de alimento para el oso pardo (Ursus arctos horribilis) en el norte de las Montañas Rocosas de Wyoming y Montana. Sin embargo, no se había registrado el uso de estas agregaciones por los osos negros, ni se habían documentado estas agregaciones tan al sur en las Montañas Rocosas.
278 vol. 50, no. 2The Southwestern Naturalist
BLACK BEARS FORAGE ON ARMY CUTWORM MOTH AGGREGATIONS
IN THE JEMEZ MOUNTAINS, NEW MEXICO
J
ONATHAN
D. C
OOP
,* C
HARLES
D. H
IBNER
,A
ARON
J. M
ILLER
,
AND
G
REGORY
H. C
LARK
Department of Botany, University of Wisconsin Madison, Madison, WI 53706 (JDC)
Santa Fe Soil Survey, 1939 Warner Circle, Santa Fe, NM 87501 (CDH, AJM)
Charleston Soil Survey Office, 683 Castle Drive, Charleston, IL 61920 (GHC)
*Correspondent: jdcoop@wisc.edu
A
BSTRACT
We observed black bears (Ursus americanus) foraging on aggregations of army cut-
worm moths (Euxoa auxiliaris) (Lepidoptera: Noctuidae) in subalpine felsenmeers (block fields)
in the Jemez Mountains of New Mexico. Moth aggregations serve as food for grizzly bears (Ursus
arctos horribilis) in the northern Rocky Mountains of Wyoming and Montana. However, black bears
have not been reported to use these aggregations, nor have such aggregations been documented
to occur this far south in the Rocky Mountains.
R
ESUMEN
Observamos osos negros (Ursus americanus) alimenta´ndose de agregaciones de la
mariposa (Euxoa auxiliaris) (Lepidoptera: Noctuidae) en felsenmeers (campos de rocas) subalpi-
nos de las montan˜as Jemez de Nuevo Me´xico. Estas agregaciones sir ven de alimento para el oso
pardo (Ursus arctos horribilis) en el norte de las Montan˜as Rocosas de Wyoming y Montana. Sin
embargo, no se habı´a registrado el uso de estas agregaciones por los osos negros, ni se habı´an
documentado estas agregaciones tan al sur en las Montan˜as Rocosas.
Aggregations of army cutworm moths (Eux-
oa auxiliaris) (Lepidoptera: Noctuidae) are ex-
cavated from high-elevation talus slopes and
consumed by grizzly bears (Ursus arctos horribi-
lis) in the northern Rocky Mountains of the
USA (Chapman et al., 1955; Sumner and
Craighead, 1973; Servheen, 1983; Klaver et al.,
1986; Mattson et al., 1991; O’Brien and Lind-
zey, 1994; White et al., 1998a). Army cutworm
moths migrate in the spring from the Great
Plains to the alpine tundra of the mountains
and then return to the prairies in the fall
(Pruess, 1967; Burton et al., 1980; Kendall et
al., 1983). Moths spend the early summer for-
aging on nectar-rich alpine forbs at night (Ken-
dall et al., 1983) and roosting in dense aggre-
gations in the interstitia of talus slopes during
the day (O’Brien and Lindzey, 1994; White et
al., 1998b). Later in the summer, the moths en-
ter a period of dormancy whereby nocturnal
foraging ceases (Kendall et al., 1983; O’Brien
and Lindzey, 1994; White et al., 1998b) before
their return to the plains. Throughout the
summer months, moths are eaten by grizzly
bears, common ravens (Covus corax), Clark’s
nutcrackers (Nucifraga columbiana), and rosy
finches (Leucosticte tephrocostis) (O’Brien and
Lindzey, 1994; White et al., 1998a). However,
to our knowledge, black bears (Ursus american-
us) have not been documented to forage on
these moth aggregations, nor has it been well
known how far south in the Rocky Mountains
such moth aggregations extend. Here we de-
scribe 3 observations of black bears foraging
on aggregations of army cutworm moths in the
Jemez Mountains of New Mexico.
On the afternoon of 19 August 2003, while
3 of us were working on the western slopes of
Redondo Mountain in the Valles Caldera Na-
tional Preserve, we saw a single adult black
bear within a large (ca. 10 ha) felsenmeer, at
an elevation of 3,250 m. Felsenmeers (German
for ‘‘sea of rocks,’’ also known as block fields)
are periglacial landforms composed of angular
blocks of frost-shattered bedrock. Because the
bear was upwind and involved in a noisy activ-
ity, it remained unaware of our presence and
we were able to observe it from within ca. 60
m. The bear was digging slowly among the
large rocks, flipping them over and aside in
what seemed to be a deliberate manner. As it
excavated, the bear prodded its snout in and
among the rocks, as though searching for or
consuming some food buried within the rock
June 2005 279Notes
F
IG
.1 A black bear (Ursus americanus) foraging on aggregations of army cutworm moths in a subalpine
(3,250 m) felsenmeer on Redondo Mountain, New Mexico.
field. The bear continued with this activity for
ca. 15 minutes after it was first noticed (Fig. 1).
Large groups of ravens were active in the area,
but were not foraging on the rocks at this time.
Over the next week, we observed 2 more ep-
isodes of activity on the felsenmeers of Redon-
do Mountain from a vantage point on a ridge
ca. 1 km distant that allowed us to survey a
much greater area. We spotted 2 bears on the
afternoon of 20 August. These bears were us-
ing the same felsenmeer as the single bear had
been using the day before, but were ca. 200 m
farther upslope. On the afternoon of 22 Au-
gust, we spotted 3 bears in this felsenmeer,
while a lone bear appeared to be foraging in
a separate felsenmeer ca. 1.5 km north.
Though the distance from which these obser-
vations were made precluded a more precise
determination of the activity of the bears, their
behavior was consistent with the foraging we
observed previously. Bears stayed within the
felsenmeers for the entirety of each
.
10-mi-
nute period of observation and were either sta-
tionary or moving slowly across the rock fields.
Large flocks of ravens were present in the vi-
cinity of the bears, but we were not able to
determine the nature of their activity on either
of these days.
On the morning of 23 August, one of us vis-
ited these felsenmeers. At both sites where we
had seen bears, army cutworm moths were ev-
ident in great numbers just beneath the sur-
face of the rocky debris. Frequently, scores of
moths could be found simply by displacing just
a few large rocks. The presence of dead moths
in various stages of degradation suggested that
moths had been using these sites for a long
time. Large, recent excavations of rocks were
numerous in both areas, and abundant bear
scats seemed to be comprised mostly or entire-
ly of the chitinous exoskeletons of the moths.
No bears were seen during this visit, but nu-
merous ravens, Clark’s nutcrackers, Audubon’s
warblers (Dendroica coronata), and a house wren
280 vol. 50, no. 2The Southwestern Naturalist
F
IG
.2 Dense aggregations of army cutworm moths (Euxoa auxiliaris) just beneath the surface of the
felsenmeer, Redondo Mountain, New Mexico.
(Troglodytes aedon), were foraging among the
rocks.
Contrasting these observations with pub-
lished studies of grizzly bear use of moth ag-
gregations in the Northern Rockies revealed
both similarities and differences. On Redondo
Mountain, moths were aggregating in felsen-
meers composed of blocky slabs of rock, rang-
ing in size from ca. 0.25 to 1.5 m in diameter,
derived from a densely welded volcanic tuff
the Tsirige member of the Bandelier Tuff
(Smith et al., 1970). These felsenmeers oc-
curred on steep (45 to 70%) slopes, with west-
ern to northwestern aspects ranging from 270
8
to 320
8
. Hence, in terms of rock size and slope
inclination, these sites were similar to moth ag-
gregation sites in talus slopes described by
O’Brien and Lindzey (1994) in Wyoming and
by White et al. (1998a) in Montana. Aspects
were somewhat more northerly than those re-
ported by these authors, though this distinc-
tion is relatively minor. Differences in the veg-
etation were much more pronounced. Moth
aggregations described by O’Brien and Lind-
zey (1994) and White et al. (1998a) occurred
in close proximity to alpine plant communi-
ties, where nectar-rich, animal-pollinated forbs
were generally quite abundant. Army cutworm
moths are thought to forage in alpine vegeta-
tion during the summer (Kendall et al., 1983;
White et al., 1998b) and consequently to in-
crease in body mass and lipid content (White
et al., 1998b) before fall migration to the Great
Plains. In contrast, there are no alpine plant
assemblages in the Jemez Mountains. Subal-
pine vegetation at the elevations at which the
moths were aggregating (between 3,150 and
3,250 m) on Redondo Mountain was com-
prised of coniferous forests and grasslands,
with seemingly lower abundances of nectar
sources. Moths aggregating on Redondo
Mountain might be foraging widely in these
June 2005 281Notes
plant communities to meet their energetic re-
quirements, or they might increase their use of
other nectar resources during migration.
Lastly, our observations raise questions re-
garding the extent of aggregations of army cut-
worm moths and their use by U. americanus
south of the range of U. arctos. Both might be
widespread but underreported, or might, in
fact, be uncommon occurrences. During the
spring of 2003, migrating populations army
cutworm moths were noted to be exceptionally
high throughout the southern Rocky Moun-
tain region (W. Cranshaw, pers. comm.). Per-
haps black bears only take advantage of this
resource during infrequent years of high moth
abundance.
We thank the Valles Caldera Trust staff and board
members for providing access and support for our
work on the preserve, and we commend their efforts
to foster a greater scientific understanding of this
remarkable area. Funding for JDC provided by an
NSF Doctoral Dissertation Improvement Grant
(DEB #0309347) and a Sigma Xi Grant-in-Aid-of-Re-
search. We also would like to thank J. T. Hogg and
R. Vinkey, who suggested publication of these obser-
vations. C. Jones and an anonymous reviewer provid-
ed helpful suggestions on the manuscript. H. Nalini
Morzaria-Luna assisted with translation of the ab-
stract.
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Submitted 20 January 2004. Accepted 24 August 2004.
Associate Editor was Cheri A. Jones.
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