Life History of the Ambrosia Beetle Xylosandrus germanus ...

Life History of the Ambrosia Beetle Xylosandrus germanus
(Coleoptera: Scolytidae) 1
B. C. WEBER 2 AND J. E. McPHERSON 3
Ann. Entomol. Soc. Am. 76: 455-462 (1983)
ABSTRACT The field life history of Xylosandrus germanus (Blandford) was studied in North Carolina,
Ohio, Tennessee, and Illinois, particularly as associated with black walnut (Juglans nigra L.),
and the number of instars was determined. The beetle was also reared in the laboratory on artificial
diet. Galleries are excavated by the females and are composed of entrance tunnels, brood chambers
where eggs are laid and larval development begins, and one to three branch tunnels where development
continues. The species is bivoltine and overwinters as adults, primarily females, in galleries of its host
plants. The number of progeny per gallery averaged 16.5; the sex ratio was 10 females to 1 male.
Flight activity began in late March to early April and continued until late August and early September.
Average seasonal flying heights of adult females ranged from 1.2 to 1.6 m. On artificial diets in the
laboratory, X. germanus laid an average of 1 egg per day and an average of 18.0 eggs per female.
Development time from egg to adult was 24.9 days at 24°C. X. germanus has three instars. Behavior
during mating, and brood production and rearing, is discussed.
Xylosandrus germanus (Blandford) (=Xyleborus germanus),
a sexually dimorphic ambrosia beetle, occurs
in Japan, Korea, the Kuril Islands, Vietnam, China,
Taiwan, central Europe, and the United States (Nobuchi
1981). It is found on a wide range of plants and attacks
apparently healthy plants and those dying or recently
dead (Weber 1982). The beetle appears to be increasing
in economic importance in the United States (Weber
1982).
The beetle's life cycle is not well understood. It has
two to three generations per year in the United States
and one to two in Europe and Japan (Weber 1982).
Adults overwinter in the host plants (e.g., Hoffman 1941).
Each female lays 2 to 54 eggs in the United States (Hoffmann
1941) and produces 12 to 20 adult progeny in
Germany (Heidenreich 1960). It has three (Kaneko 1965)
or four (Ueno 1960) instars in Japan.
Hoffmann (1941) described the galleries of X. germanus
in elm (Ulmus sp.) and red maple (Acer rubrum
L.), and Heidenreich (1960) described them in oak
(Quercus sp.).
X. germanus has been reared in the laboratory from
egg to adult on bolts of red pine (Pinus resinosa Aitkin)
(Buchanan 1941) and on the roots of tea plants (Thea
sp.) (Kaneko 1965, Kaneko et al. 1965a,b); larvae have
been reared to adults on agar plates containing fungi,
including Ceratocystis spp., Pestalozzia sp., and an unidentified
species (Buchanan 1941).
Black walnut, Juglans nigra L., is a commercially
valuable tree that increasingly is being grown in plantations
in the United States. X. germanus attacks it, and,
therefore, may become an economic pest. Because of
the limited information about this beetle's biology in the
United States, we studied its life history (particularly as
associated with black walnut) including gallery patterns,
life cycle, predators, parasites and other associates, and
'In partial fulfillment of the requirements for the Ph.D. (B. C. Weber).
Received for publication 6 August 1982; accepted 1 January 1983.
2 North Central For. Exp. Stn., Forestry Sciences Laboratory, Southern
Illinois University, Carbondale, IL 62901.
3 Dept. of Zoology, Southern Illinois University, Carbondale, IL 62901.
455
flight activity, and its reproduction and development under
laboratory conditions.
Areas of Study
Two primary areas of study were used. The first, in
North Carolina, maintained from August 1976 through
October 1979, was a 2.0-ha plot within a 4.9-ha black
walnut plantation planted in 1973. It is about 335 m
above sea level on sandy loam soil in McDowell County
about 65 km east of Asheville at latitude 35°41'30" N
and longitude 81°53'30" W.
Within this study area 1,520 trees were spaced at 3.66
x 3.66 m. The plantation was hand-weeded the first
two years (1973-1974) and disked every year thereafter
through 1979. No chemicals were applied. Lateral-branch
pruning was done in late winter 1974 and again in 1975.
In summer 1976 and winter 1978-1979, basal sprouts
and dead trees were cut and removed from the plantation.
The second study area, in Illinois, maintained from
September 1978 through October 1980, was a 2.8-ha
black walnut plantation planted by the USDA Forest
Service in 1973. It is about 153 m above sea level on
clay loam soil in the Shawnee National Forest in Alexander
County (T14S, R2W, Sees. 20 and 21), at latitude
37°17'30" N and longitude 89°20'30" W.
Within this plantation 2,700 trees were spaced at 1.83
x 3.66 m. Chemical weed-control treatments (simazine
+ atrazine) were applied the first three years (1973—
1975).
Other walnut plantations were also used at irregular
intervals. These included a plantation with one- to eightyear-old
trees near Rarden, Scioto County, Ohio, a twoyear-old
plantation near Nashville, Davidson County,
Tennessee, and a four-year-old plantation in the Shawnee
National Forest in Jackson County, Illinois. Nonwalnut
areas used included a small plantation (age unknown)
containing sweetgum (Liquidambar styraciflua
L.) and tulip trees (Liriodendron tulipifera L.) in the
North Carolina State tree nursery near Clayton (Johnston
County), and a nonplantation, forested, partially cutover
area containing oaks (Quercus spp.) and other hard-

456 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 76, no. 3
woods in the Shawnee National Forest in Alexander
County, Illinois.
Field Studies
Materials and Methods
Gallery patterns. Galleries of X. germanus in black
walnut collected from the McDowell County, North
Carolina, and Alexander County, Illinois, plantations
were sectioned with a band saw. Longitudinal and crosssections
were made. The cut surfaces were sanded lightly
before being photographed.
Life cycle. Most seasonal life cycle data came from
black walnut trees. However, because of difficulties in
finding enough heavily infested walnut trees, other species
were included.
Each infested tree (usually less than 7 cm in diameter)
was cut at the ground-line with a camp saw; the top and
branches were removed because beetles attack chiefly
the lower main trunk. The trunk was then sectioned into
1.5 m logs, which were sectioned in the laboratory into
2 cm lengths; and individual galleries were exposed with
a wood chisel. For each galley, date of collection and
number of beetles at each developmental stage were recorded.
From 1976 through 1979, black walnut trees (4 to 7
cm in diameter) from the North Carolina (McDowell
County) plantation, which had been attacked annually
by X. germanus since 1976, were collected at irregular
intervals from June to October. In 1980, four-year-old
walnut trees (2 to 3 cm in diameter), attacked by X.
germanus in late April, were cut weekly from the plantation
in Illinois (Jackson County) from 15 May until
28 June when no more beetle-attacked trees could be
found. In 1979, walnut trees about four years old (2 to
4 cm in diameter), attacked by X. germanus in May,
were taken in June from the plantation in Ohio (Scioto
County). In 1980, two-year-old walnut trees (1 to 3 cm
in diameter), attacked by X. germanus in April, were
cut in May and September from the plantation in Tennessee
(Davidson County).
In May 1979, sweetgum and tulip trees (5 to 10 cm
in diameter, age unknown), were taken from the North
Carolina State tree nursery (Johnston County) after a
recent infestation by X. germanus. From 2 July until 15
September 1980, pieces of an infested oak stump in the
partially cutover area in Alexander County, Dlinois, were
collected weekly to complete the seasonal development
information.
Progeny data are expressed as x ± SE.
Number of instars. The number of instars was determined
by measuring head-capsule widths of specimens
removed from dissected galleries in black walnut,
sweetgum, and tulip trees.
Predators, parasites, and other associates. All stages
of X. germanus were checked for parasites, the galleries
were checked for predators or evidence of predation,
and inhabitants other thanX. germanus were noted. Adults
were examined for phoretic mites.
Flight activity. To determine the seasonal flight activity
of X. germanus, window traps (McPherson and
Weber 1980) were used. In the North Carolina study
area in 1977 and 1978, 28 window traps were placed
throughout the area using a random numbers table; four
traps were suspended at each of the following heights:
1, 2, 3, 4, 5, 6, and 7 m. In 1979, 12 of the 28 traps
were randomly positioned only at the 1-m height because
in 1977 and 1978 most activity was at 1 m. The
remaining 16 traps were placed in the Illinois plantation
in 1979 and 1980, using a random numbers table; four
traps were suspended at each of the following heights:
1, 2, 3, and 4 m.
Collections from the North Carolina traps were made
weekly from 1 April to 14 October 1977, 31 March to
13 October 1978, and 23 March to 13 October 1979.
Collections from the Illinois traps were made weekly
from 30 March to 13 October 1979, and 28 March to
17 October 1980.
Flying height and seasonal flight activity data are expressed
as x ± SE.
Laboratory Studies
Beetles were reared to adult in the laboratory in 15cm-long
pyrex test tubes containing about 12 ml of artificial
diet. Because X. germanus had not previously
been reared on artificial media, other ambrosia beetle
diets were used (Norris and Baker 1968 (1), Saunders
and Knoke 1967 (2), and French and Roeper 1972 (3)).
Diets were prepared about two days before use and autoclaved
at 120 pounds pressure for 15 min. A mature
field-collected female was placed in each tube with sterile
forceps. The female had been surface-sterilized with
1% sodium hypochlorite for 1 min and then rinsed in
sterile distilled water for 1 min. About 100, 50, and 200
beetles were maintained on diets 1, 2, and 3, respectively.
Twenty sterilized beetles were also maintained on autoclaved
12- to 14-cm-long sections of black walnut, up
to 1 cm in diameter, in separate tubes.
All tests tubes were stoppered with cotton or plastic
caps and kept in the dark at 24 ± 0.6°C, except during
observations. Behavior of the female parent, and the
behavior and number of progeny, were observed and
recorded every two days.
Lengths of stadia are expressed as x ± SE.
Field Studies
Results and Discussion
Gallery patterns. Gallery patterns of X. germanus in
black walnut, tulip, sweetgum, and oak trees were similar
to those reported in elm (Hoffmann 1941), red oak
(Heidenreich 1960), and beech (Groschke 1953). The
gallery entrance was about 1 mm in diameter. At the
start of gallery formation, wood particles pushed out by
the female often extended from the entrance hole. From
the entrance hole a horizontal entrance tunnel extended
2 to 3 mm and then widened to form a brood chamber
(Figs. 1 and 2), which was elongate (7 to 12 mm) and
excavated vertically with the grain of the wood. Here
eggs were laid and larval development began. Extending
radially from the brood chamber were one to three branch

May 1983 WEBER AND MCPHERSON: LIFE HISTORY OF X. germanus 457
FIG. 1. Longitudinal section of black walnut showing entrance
hole and brood chamber of X. germanus gallery.
V_Branch Tunnels /
FIG. 2. Cross section of black walnut showing entrance
hole, brood chamber, and two branch tunnels of X. germanus
gallery.
tunnels (Fig. 2), where larval development continued.
These tunnels extended up to 25 mm into the wood. In
small-diameter wood (i.e., under 4 cm), these tunnels
sometimes extended directly into the pith, which was
hollowed out by the female to form another brood chamber
(as in Fig. 1). In other instances, the tunnels circumvented
the pith. Pupation occurred in both the brood
chamber and branch tunnels.
During active gallery formation and progeny development,
the brood chamber and tunnels were lined with
a white ambrosia fungus, Ambrosiella hartigii Batra.
Old galleries, however, were stained black and the ambrosia
fungus had disappeared.
The diameter of wood attacked ranged from 0.9 cm
(young black walnut trees) up to about 50 cm (recently
cut oak and beech stumps).
Life cycle. Data from black walnut, sweetgum, and
tulip trees from North Carolina, Tennessee, and Ohio,
were combined because we had too few data from any
one species for a complete life cycle. These data suggest
two generations per year (Fig. 3A).
The general life cycle of X. germanus in Illinois (black
walnut and oak) (Fig. 3B) is similar to that in Fig. 3A.
Two generations are apparent.
By the first of August, many of the galleries in North
Carolina and Illinois were either empty or contained a
dead female ( = parent?) in the gallery entryway. Dead
females in entryways have also been observed by Kaneko
(1965).
Adults overwinter, often clustering in galleries at the
bases of trees. For example, in North Carolina, one
gallery opened in mid-October contained 112 females
and 42 males. Because this was more beetles than could
be produced by one female (see below), beetles probably
moved in from nearby galleries. Clustering of many
overwintering adults in galleries was also observed in
Germany (Gauss 1960). The abundance of males in the
North Carolina gallery was unusual when compared to
10 additional overwintering sites in North Carolina and
Illinois in which nearly all beetles were females (116
females, 7 males).
Each of 24 females in wood from Illinois produced
an average of 16.5 ± 2.42 (range = 1-53) progeny and
the sex ratio was 10 females: 1 male. This range resembles
those found by others: 20-50 progeny per female in
Japan (Kaneko 1965, Ueno 1960) and 2-54 in the contiguous
United States (Hoffman 1941). Sex ratios of 9:1
and 10:1 (Kaneko 1965, Kaneko and Takagi 1965, respectively),
of reared insects are like ours from fieldcollected
material.
Number ofinstars. Head-capsule measurements of 153
larvae showed three peaks of which the first two (0.26
mm, 0.32 mm) were relatively close to each other (Fig.
4). Whether the first two peaks represented two separate
instars or variation in one instar was not readily apparent.
The third peak (0.46 mm) appeared to represent a
distinct instar, which varied from 0.43 to 0.51 mm.
Lekander (1968), working with larval scolytids, found
that the average ratio between average head capsule widths
in two consecutive instars was 1.32 (range: 1.17-1.48).
To apply his method to X. germanus, we first defined
the ranges of the peaks (Fig. 4); because the ranges of
peaks one and two were continuous and because the
lowest point between the two peaks was 0.29 to 0.31
mm, we selected 0.30 mm as the division between the
two ranges. The average for each of the ranges was:
instar I, 0.25 ± 0.003 mm; instar II, 0.33 ± 0.003 mm;
and instar III, 0.47 ± 0.003 mm. The ratios of the
average head capsule widths of instar II to instar I and
instar III to instar II were 1.32 and 1.42, respectively,
both within the range determined by Lekander (1968).
The ratio of the average head capsule width of individ-

458 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 76, no. 3
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FIG. 3. Life cycles of field-collected X. germanus from (A) black walnut, tulip, and sweetgum trees from 1977 to 1979 in
North Carolina, Ohio, and Tennessee and (B) black walnut and oak trees in 1980 in Illinois. Vertical bars represent the presence
of that stage in the sample, dashed lines mean that individuals were not collected but were probably present, and a question mark
indicates that the exact time of beginning or ending of a developmental stage was not known.
uals between 0.21 and 0.39 mm (instar I plus instar II)
(Fig. 4) to that of individuals between 0.43 and 0.51
mm (instar ID) was 1.66, outside Lekander's range.
Therefore, X. germanus apparently has three instars.
Scolytids with the same number of instars have last
to first instar average head capsule ratios that fall within
a relatively narrow range (Lekander 1968). Therefore,
it is possible to estimate number of instars from these
ratios. For X. germanus, the ratio of last to first instar
average head capsule width (instar III to instar I) was
1.88, which is within the range of scolytids with three
instars (Lekander 1968). This provides additional evidence
that X. germanus has three instars.
Predators, parasites, and other associates. No parasites
of X. germanus were found. Gauss (1960), however,
found a wasp, Tetrastichus sp. (Hymenoptera:
Eulophidae), in a gallery in Germany, although its relationship
to X. germanus was not known. Another Te-
trastichus sp. is known to parasitize Xylosandrus
compactus and X. morigerus (Blandford) in Java (Browne
1961,Entwistle 1972).
Unlike some other bark beetles (e.g., Moser 1975,
Moser and Roton 1971), X. germanus does not carry
phoretic mites.
About 60 mites (Schwiebia sp.) were collected in empty
or old galleries and in a gallery in which a live X. germanus
adult female was found without progeny. The
mites appeared to be feeding on the fungal lining of the
galleries. Their relationship to X. germanus was not determined,
nor was that of a mite, Histiogaster hylocoeti
Koch, Gauss (1960) found in galleries of X. germanus
in Germany.
Four nematodes were discovered on slide mounts of
the ambrosia fungus lining of a gallery of X. germanus.
They were not identified. Their small size suggests that
they are fungus-feeders rather than predators. In another
1
1

May 1983 WEBER AND MCPHERSON: LIFE HISTORY OFX. germanus 459
vidual
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No.
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24-
20-
16-
12-
8-
4-
•t
.17 .21 .25 .29 .33 .37 .41 .45 .49 .53
Head Capsule Width (mm)
FIG. 4. Frequency distribution of head capsule widths of
X. germanus larvae, n = 153.
gallery, a nematode nearly as large as an adult female
X. germanus (inadvertently lost), was found on a dead
female in an old gallery, suggesting it was a scavenger.
Although we never found nematodes in or on live X.
germanus, Roeper (pers. comm.) found nematodes in
mycangia of the ambrosia beetle Xyleborus qffinis
Eichhoff. Nematodes have not previously been reported
in association with xyleborine ambrosia beetles. But some
nematodes prey on eggs of other bark beetles, and other
species sterilize their hosts by infecting the gonads
(Massey 1974).
Predation we observed was a single attack of an immature
hemipteran (probably a reduviid) on an adult X.
germanus that had recently emerged from an oak log.
Flight activity. In North Carolina, the numbers of
adult female X. germanus caught in window traps in
1977 (n = 31) and 1978 (n = 45) were low, when the
traps were positioned at seven heights. Average flying
heights were 1.6 ± 0.036 m in 1977 and 1.2 ± 0.011
m in 1978 (Fig. 5). In 1979, when all traps (12) were
located only at the 1-m height, 276 beetles were caught.
In Illinois, the number of X. germanus caught were 214
in 1979 and 196 in 1980. Average flying heights were
1.6 ± 0.004 m in 1979 and 1.4 ± 0.004 m in 1980
(Fig. 5).
These results indicate that X. germanus flies low to
the ground; the low flying height also corresponds closely
with attack patterns on host trees (Weber 1982).
Seasonal flight activities in North Carolina and in Illinois
were similar (Fig. 6), although flight began about
mid-April in Illinois, about two weeks later than in North
Carolina. Of the beetles trapped, most had been collected
in North Carolina (about 80%) and Illinois (about
77%) by mid-June.
The early May peak of flight activity in North Carolina
(Fig. 6) was followed by the presence of eggs in
galleries in late May and probably as early as early-mid-
May (Fig. 3A). The late April and early May peaks,
and the early June peaks, in Illinois (Fig. 6) were followed
by the appearance of eggs in galleries in mid-
May and late June, respectively (Fig. 3B). These results
show (1) that overwintered females have a flight period
before establishing new galleries, (2) that their female
offspring also have a flight period before they, too, reproduce,
and (3) that X. germanus activity patterns are
similar across the central part of the beetle's range in
the contiguous United States.
Laboratory Studies
Of the three diets used, the beetles successfully reproduced
(offspring reached adulthood) only on Norris
and Baker's (1968) and French and Roeper's (1972)
diets. On these diets females became trapped in condensed
water on the diet surface. The problem of water
droplets was alleviated by letting the diet dry for two
days after being autoclaved.
Of the 100 and 200 female adults introduced onto the
Norris and Baker and French and Roeper diets, respectively,
only about 30% per diet successfully reproduced.
Of the remainder, 50% never attempted gallery excavation
and died after 2 or 3 weeks, 10% excavated short
tunnels but did not lay eggs, and 10% tunneled and laid
either sterile eggs or eggs which yielded larvae that died.
The females that successfully reproduced began excavating
galleries one to three h after being introduced
onto the diets, usually at the interface of glass and diet.
Each female tunneled the length of her body and then
began excavating a brood chamber area. Egg laying did
not begin until the chamber had been formed and A.
hartigii, the white ambrosia fungus, was growing on the
chamber wall, about five days after introduction. Eggs
were laid on the walls of the chamber and on the inner
surface of the glass and usually were clumped. Branch
tunnels, where larvae completed development and pupated,
were begun by the female soon after the first eggs
were laid; these tunnels radiated out from the main
chamber. We found no differences between tunnels in
the two diets. Gallery patterns in the diets were similar
to those found in trees.
Because the Norris and Baker diet dehydrated rapidly,
we confined our observations to beetles on the French
and Roeper diet. The beetles were reared through two
generations (about 100 F, females were used to start the
second generation) and there were no obvious differ-

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Vol. 76, no. 3
FIG. 5. Height distributions of X. germanus adults caught in window traps in 1977 and 1978 in North Carolina and in 1979
and 1980 in Illinois in black walnut plantations. 1977, n = 31; 1978, n = 45; 1979, n = 214; 1980, n = 196.
ences between the generations. Therefore, data from these
generations were combined.
The oviposition period for 10 females averaged 25.9
± 3.53 days, with each female laying 1.0 ± 0.11 eggs
per day; total fecundity per each of 10 females averaged
18.0 ± 1.97 eggs. The incubation period averaged 6.0
days (Table 1).
Larvae fed on the A. hartigii fungus that lined the
main chamber and, eventually, the branch tunnels. Within
about three weeks, the diet began to dehydrate and pull
away from the glass, thus exposing additional diet surface
already covered with fungus. Some larvae (exact
numbers not determined) left the main chamber, perhaps
because of crowding, and fed on the newly available
fungal mycelia. The female parent provided little apparent
care to the eggs, larvae, and pupae. Pupae were
found both in the main chamber and between glass and
diet.
Developmental times for larvae and pupae averaged
11.9 and 7.0 days, respectively; total development time
from egg to adult averaged 24.9 days (Table 1). That
relatively few individuals were used for the determination
of developmental times (Table 1), compared to the
number of females that successfully reproduced, was
due to impossibility of observing each individual within
its gallery in a test tube over several days. Although the
main chamber was usually constructed next to the glass,
eggs were often pushed around by later instars and could
not be positively identified from day to day. Larvae,
after hatching, often moved from the main chamber into
branch tunnels to feed and were no longer visible to us.
Developmental times for X. germanus immatures in
elm from New Jersey were 4 days for eggs and 7.34
days for pupae (Hoffmann 1941). Developmental times
for Japanese specimens reared on tea roots in the laboratory
at 23° to 24°C were 2 to 3 days for eggs, 8 to 9
days for larvae, and 5 to 6 days for pupae; overall developmental
time was 15 to 18 days from egg to adult
(Kaneko 1965, Kaneko et al. 1965a). Differences between
these developmental times and ours may be due
to the use of artificial diet versus natural host material.
The sex ratio of progeny reared on French and Roeper's
diet was five females:one male, which differs
markedly from those of beetles reared on tea roots by
Kaneko (1965) (9:1) and Kaneko and Takagi (1965)
(10:1). As noted earlier, we found a ratio of 10:1 under
field conditions. Xyleborine ambrosia beetles have skewed
sex ratios (Browne 1961, Norris 1975) and females are
believed able to control the sex of their offspring (e.g.,
Nobuchi 1972).

May 1983 WEBER AND MCPHERSON: LIFE HISTORY OF X. germanus 461
40
North Carolina
1977-1979
— Apr May Jun Jul Aug Sep
Apr May Jun Jul Aug Sep
FIG. 6. Combined seasonal flight activities of X. germanus
adults caught in window traps from 1977 to 1979 in North
Carolina and from 1979 to 1980 in Illinois in black walnut
plantations. North Carolina, n = 352; Illinois, n — 410.
Mating attempts were observed twice in the culture
tubes. On both occasions, a male followed a recently
emerged female as she wandered around the main chamber
area. With no noticed preliminary courtship, he repeatedly
attempted to mount her but success could not
be verified. Copulation probably occurs more often in
the main chamber area, where there is more room, rather
than in the branch tunnels.
Cannibalism was evident in three galleries in artificial
diets, although which developmental stage was responsible
for the cannibalism could not be determined. In
one gallery, the mandibles of a larva were found (sex
unknown). In the remaining two galleries, chewed wing
covers and partially eaten bodies were found, and males
were usually the victims. Cannibalism has been observed
in other ambrosia beetles, especially associated
with "grooming" (Kalshoven 1962). Hopkins (1898)
reported cannibalism of X. xylographus Say pupae by
larvae, and Leist (1902) observed the mutilation of teneral
adults by other Xyloterus lineatus (Olivier) (= Trypodendron
lineatum (Olivier)) adults.
Table 1. Duration (in days) of immature stages of A", germanus
Stage Number observed x ± SE Range
Egg
Larvae
Pupa
23
11
3
At the end of the second generation, about 30 new
female adults reared on French and Roeper's diet were
transferred to fresh diets but did not reproduce. These
beetles usually made a tunnel in the diet equal to the
length of their bodies and then became inactive. Although
light and temperature were the same as for previous
generations, this group of beetles apparently went
into diapause. All attempts to break diapause were unsuccessful
(e.g., chilling for at least 60 days and exposing
the culture tubes to light). Norris (personal
communication) reported similar failures in attempting
to break diapause in various species of Xyleborus. These
results suggest that X. germanus has an obligatory diapause
in North America, although it has no diapause in
Germany (Schneider 1975).
Attempts to rear 20 X. germanus females on sections
of black walnut in test tubes failed. Females made no
galleries in the wood but occasionally laid eggs on the
bottoms of the test tubes between glass and wood. Eggs
and larvae usually died within two weeks.
Acknowledgment
We appreciate the helpful comments and valuable suggestions
on earlier drafts of this manuscript from: J. A. Beatty,
R. A. Brandon, and W. G. Dyer, Department of Zoology,
SIUC; A. Pappelis, Department of Botany, SIUC; and D. T.
Funk, Forestry Sciences Laboratory, Northeastern Forest Experiment
Station, Durham, N.H.
We thank the following for their help in this research: D. L.
Brenneman, W. Boyette, and the staff of Morganton Forestry
Center, N.C. Department of Natural Resources and Community
Development, Morganton, for the use of their experimental
black walnut plantation and for their help in maintaining collecting
traps; W. Hoffard and E. Cordell, Southeastern Area,
State and Private Forestry, Asheville, N.C, for their assistance
in the field work; R. C. Schlesinger and F. D. McBride, Forestry
Sciences Laboratory, North Central Forest Experiment
Station, Carbondale, for assistance with the design and maintenance
of the collecting traps; S. L. Wood, Brigham Young
University, Provo, Utah, for his help in determining the number
of larval instars; R. A. Roeper, Alma College, Alma, Mich.,
for identifying the ambrosia fungus and for providing information
about nematode associates in ambrosia beetles; E. W.
Baker, Insect Identification and Beneficial Insect Introduction
Institute, USDA, Beltsville, Md., for identifying the mites; D.
M. Norris, University of Wisconsin, Madison, for providing
information about the breaking of diapause in ambrosia beetles;
and D. M. Baines and D. E. Alley, Forestry Sciences Laboratory,
Carbondale, and personnel of the Young Adult Conservation
Corps (YACC), for providing assistance with both
the field and laboratory work.
6.0 ± 0.16
11.9 ± 0.53
7.0 ± 0.57
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5-8
9-15
6-8
Cumulative mean
age (days)
6.0
17.9
24.9

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