- Biological interrelationships between the ambrosia beetle
Xyleborus dispar (F.) (Coleoptera:Scolytidae) with its symbiotic
fungus, Ambrosiella hartigii Batra (Fungi Imperfecti) were investigated
in western Oregon.
Postdiapause adults of X. dispar collected in March through
June with rotary nets, and excised from overwintered and newly
attacked host material, produced a single generation when the beetle
was reared in vitro with A. hartigii. Diapause beetles excised from
host materials in the fall failed to oviposit. This study is the first
record of rearing a temperate zone scolytid in vitro and on a fungus
in the genus Ambrosiella. Such in vitro rearing allowed controlled
studies of ecological, behavioral, developmental and physiological
aspects of this ectosymbiosis. Comparisons were made between
wild and in vitro populations.
Seasonal variations of the fungus within the female beetles
mesonotal mycangium, and the synchronization of ovariole development
Comparative concentrations of solube proteins and free amino
acids suggested that the fungus in the mycangia was built up from
free amino acids of the insects. At the period of emergence, flight
and attack of new hosts, the females were found to have a concentration
of soluble proteins more than double that found in the beetles
during the remainder of the year. Whereas, the free amino acids
were the lowest values recorded during this period (March-October).
Ovariole development and oviposition only occurred after the post-diapause female had fed on the ambrosial form of A. hartigii. These
beetles attacked a new host with empty intestinal tracts. The relationship
between the number of progeny and the volume of the galleries
Experiments were conducted in an attempt to terminate maturation
diapause in this univoltine species of ambrosia beetle using temperature,
Juvenile Hormone Analogs (JHA) and an olfactometer.
Termination of the maturation diapause was not achieved with
Qualitative and quantitative analyses of the major nitrogenous
excretory products were made on the various life stages of X. dispar. The main nitrogenous product found in excreta and hindguts of
beetles, larvae and pupae, was uric acid (range 7.6-14.8 μg uric
acid/beetle). No ninhydrin-positive compounds were located in
excreta of the beetles. The concentration of ammonia-nitrogen in
the various life stages averaged between 0.70-1.13 μg NH₃-N/beetle.
Total nitrogen determinations were made on sapwood samples
of Malus sylvestris (0.34 ± 0.005% N by dry weight), attacked wood,
"pre-brood" (0.31 ± 0.005% N by dry weight) and attacked wood-"post-brood" (0.17 + 0.02% N). Similar determinations of the artificial
medium (L-asparagine) indicated that a nitrogen requirement
of about 0.08 -0.1% N by dry weight was necessary before oviposition
Fixation of atmospheric nitrogen by individual X. dispar
beetles in vitro was not indicated using the acetylene ethylene reductase
method. In vivo situations may be different, but were not
Proteolytic enzyme activity was not found on examination of
dispause beetles, their excreta, larval and pupal excreta, and the
ambrosial and mycelial forms of A. hartigii.
Bioassays were used to study the interactions and effects of
the various life stages of X. dispar on the induction of the ambrosial
form of its symbiotic fungus, A. hartigii. Postdiapause adults
and pupae of X. dispar were able to cause a change from the mycelial
to the ambrosial form of A. hartigii in culture.
Larvae fed on the mycelia' form in vitro, but ambrosia is
required by larvae to develop and pupate. One of the main factors
inducing the ambrosial form of A. hartigii is probably a secretory
product of X. dispar. Nitrogenous compounds are considered necessary
for the cause of ambrosial induction, but not the primary factor
alone for the continued growth of the ambrosia. Morphogenic compounds
that caused ambrosial induction in vitro responded negatively
to the Azocoll procedure, indicating no proteolytic activity.
The symbiotic association of X. dispar and its sole food fungus,
A. hartigii is a reciprocal biochemical alliance based on carbon
and nitrogen metabolic integration. The beetles contribute their
free amino acids and nitrogenous excretions, and the fungus con -
tributes its cellulose-degrading enzymic ability to the association,
plus synthesizing proteins, sterols, vitamins and other growth factors.
The beetle-fungus symbiosis has been depicted in the form
of an abstract pattern.