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Abstract Insects exposed to low temperature stress can experience chill injury, but incorporating fluctuating thermoprofiles increases survival and blocks the development of sub-lethal effects. The specific parameters required for a protective thermoprofile are poorly understood, because most studies test a limited range of thermoprofiles. For example, thermoprofiles with a wave profile may perform better than a square profile, but these two profiles are rarely compared. In this study, two developmental stages of the alfalfa leafcutting bee, Megachile rotundata, eye-pigmented pupae, and emergence-ready adults, were exposed to one of eight thermoprofiles for up to 8 weeks. All the thermoprofiles had a base of 6°C and a peak temperature of either 12°C or 18°C. The duration at peak temperature varied depending on the shape of the thermoprofile, either square or wave form. Two other treatments acted as controls, a constant 6°C and a fluctuating thermal regime (FTR) with a base temperature of 6°C that was interrupted daily by a single, 1-h pulse at 20°C. Compared with constant 6°C, all the test thermoprofiles significantly improved survival. Compared with the FTR control, the thermoprofiles with a peak temperature of 18°C outperformed the 12°C profiles. Bees in the eye-pigmented stage exposed to the 18°C profiles separated into two groups based on the shape of the profile, with higher survival in the square profiles compared with the wave profiles. Bees in the emergence-ready stage exposed to 18°C profiles all had significantly higher survival than bees in the FTR controls. Counter to expectations, the least ecologically relevant thermoprofiles (square) had the highest survival rates and blocked the development of sub-lethal effects (delayed emergence).
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Generalized Linear Mixed Model Approach to Time-to-Event Data with Censored Observations
The time-to-event response is commonly thought of as survival analysis, and typically concerns statistical modeling of expected life span. In the example presented here, alfalfa leafcutting bees, Megachile rotundata, were randomly exposed to one of eight experimental thermoprofiles or two control thermoprofiles, for one to eight weeks. The incorporation of these fluctuating thermoprofiles in the management of the bees increases survival and blocks the development of sub-lethal effects, such as delayed emergence. The data collected here investigates the question of whether any experimental thermoprofile provides better overall survival, with a reduction and delay of sub-lethal effects. The study design incorporates typical aspects of agricultural research; random blocking effects. All M. rotundata prepupae brood cells were randomly placed in individual wells of 24-well culture plates. Plates were randomly assigned to thermoprofile and exposure duration, with three plate replicates per thermoprofile x exposure time. Bees were observed for emergence for 40 days. All bees that were not yet emerged prior to fixed end of study were considered to be censored observations. We fit a generalized linear mixed model (GLMM), using the SAS® GLIMMIX Procedure to the censored data and obtained time-to-emergence function estimates. As opposed to a typical survival analysis approach, such as Kaplan-Meier curve, in the GLMM we were able to include the random model effects from the study design. This is an important inclusion in the model, such that correct standard error and test statistics are generated for mixed models with non-Gaussian data.
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- Award ID(s):
- 1826834
- PAR ID:
- 10163553
- Date Published:
- Journal Name:
- Southeast SAS Users Group Conference
- Page Range / eLocation ID:
- 160
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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ABSTRACT Variable spring temperatures may expose developing insects to sublethal conditions, resulting in long-term consequences. The alfalfa leafcutting bee, Megachile rotundata, overwinters as a prepupa inside a brood cell, resuming development in spring. During these immobile stages of development, bees must tolerate unfavorable temperatures. In this study, we tested how exposure to low temperature stress during development affects subsequent reproduction and characteristics of the F1 generation. Developing male and female M. rotundata were exposed to either constant (6°C) or fluctuating (1 h day−1 at 20°C) low temperature stress for 1 week, during the pupal stage, to mimic a spring cold snap. Treated adults were marked and released into field cages, and reproductive output was compared with that of untreated control bees. Exposure to low temperatures during the pupal stage had mixed effects on reproduction and offspring characteristics. Females treated with fluctuating low temperatures were more likely to nest compared with control bees or those exposed to constant low temperature stress. Sublethal effects may have contributed to low nesting rates of bees exposed to constant low temperatures. Females from that group that were able to nest had fewer, larger offspring with high viability, suggesting a trade-off. Interestingly, offspring of bees exposed to fluctuating low temperatures were more likely to enter diapause, indicating that thermal history of parents, even during development, is an important factor in diapause determination.more » « less
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