- Larval mortality of leafcutting bee, Megachile pacifica (Panzer)
(= rotundata (F.)), is high, usually exceeding 50%. Parasites and
predators are not a limiting factor in the survival of the progeny.
The disease chalk brood has been an important mortality factor since
1974. Inadequate food supply, pollen and nectar, may affect the survival
of the progeny. Genetic differences within and between populations
and the saponin content of alfalfa leaves do not affect brood production
or survival. Domicile design and protection may also have a
great influence on survival.
Temperatures exceeding 50°C occur in cells when nesting material
receives direct sunlight or when nesting material is housed in domiciles
with poor ventilation and insulation. There are temperature
differences between cells in a series, between nest positions within a
domicile, and between types of nesting materials. There is no direct
relationship between ambient and cell temperatures because the latter
are influenced by exposure, nesting media, placement, and domicile
High internal bee body temperatures may occur during most bee
activities, excluding resting. The effects of high temperatures on the
adult bee or the eggs she contains are not known.
Although larval development can proceed at temperatures below 21°C,
survival is reduced. Adult activity, like metabolic larval development, can be conditioned to unusually high or low temperatures; thus, there
is no absolute temperature threshold for development or activity in
Eggs and young larvae reared at a constant 30°C had over 85%
survival, in most years. An ambient temperature of 45°C applied for
one to three hours resulted in a higher mortality of eggs and early
instars than the control temperature of 30°C; at 50°C, mortality was
complete. Exceptions were obtained for either situation. Half hour
exposure at 50°C ambient temperature was also detrimental to immatures.
An ambient temperature of 40°C in general does not affect survival of
immatures. In-cell temperatures were at least 5°C lower than ambient
temperatures in incubators during treatments.
Repeating heat treatments on two or more days was not as severe as
the duration of treatments. Larvae showed heat tolerance when exposed
to two to three hours at 45°C but not to one hour, but some exceptions
occurred. The mechanism for heat tolerance is not well understood, and
may be related to a conditioning of individuals to high temperatures.
A seasonal effect on survival was obtained and appears not to be
related to the age of the laying females, nor to the generations, but
rather to the thermal history to which the immatures were exposed.
Heat susceptibility of eggs and early instars seemed to be similar.
Fourth and fifth instars were the most heat tolerant of all larval
Exposure of young larvae to low temperatures before they were
exposed to high temperatures did not increase mortality. However, sublethal
high temperatures were generally less harmful to the immatures
that were conditioned but this acclimation of the larvae did not occur
in every test. Upper threshold temperature limits cannot be precisely
defined, nevertheless, cell temperatures over 40°C result in egg and
Brief exposure to 45°C was the upper limit that developing pupae
could tolerate; 50°C was lethal. Pupae were most heat sensitive between
three and six days before emergence. When exposed to high temperatures,
pupae and emerging adults were able to arrest development.
Pupae and emerging adults can be conditioned to tolerate short exposures to lethal temperatures up to seven days before emergence.
Low temperatures did not affect the survival of pupae.
Development of pupae and emerging adults could be arrested for up to
a week at 15.6°C without harmful effect. Development and emergence
proceed at 21 °C, but pupae need at least 2.5 hours per day of temperatures
above 21°C to survive when not in an arrested state. Pupae not
exposed to temperature above 29°C during 24 hours, emerged normally.
Incubation at 29.5°C for less than 10 hours per day delayed the emergence.
Pupae maintained at 15.6 °C for 22 hours per day for 8 days
or for 20 hours per day for 16 days emerged after a delay longer than
the period of cold. Cooling the emerging bees after high temperature
treatment appeared to be more detrimental than cooling before exposures
to high temperatures. The detrimental effect of extreme temperatures
was shown on the survival of eggs, young larvae, and pupae, but
possible chronic effects on later stadia were not studied.