Effects of temperature and other abiotic and biotic factors on development and survival of the immature stages of the alfalfa leafcutting bee, Megachile pacifica (Panzer) (=rotundata (F.)) (Hymenoptera:Megachilidae) Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/r207tr74b

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  • 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 structure. 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 this bee. 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 stages. 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 larval mortality. 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.
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