Guest essay by Eric Worrall

Apparently painting bee boxes black, to “simulate a warmer, future climate”, results in higher mortality and greater stress.

Climate change linked to potential population decline in bees

Study finds that warmer temperatures push bees to their physiological limits, may drive local extinction

NORTHWESTERN UNIVERSITY
PUBLIC RELEASE: 28-JUN-2018

Study finds that warmer temperatures push bees to their physiological limits, may drive local extinction

EVANSTON, Ill. — A new study from Northwestern University and the Chicago Botanic Garden has found that climate change may drive local extinction of mason bees in Arizona and other naturally warm climates.

In a two-year, in situ field experiment that altered the temperature of the bees’ nests to simulate a warmer, future climate, 35 percent of bees died in the first year and 70 percent died in the second year. This is compared to a 1-2 percent mortality rate in the control group.

“The projected temperatures appear to be pushing this species up against its physiological limits,” said Northwestern’s Paul CaraDonna, who led the research. “This is evidence that we might see local extinction in the warmer parts of this species’ range, which is pretty sobering.”

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To study how climate change affects mason bees, CaraDonna’s team set up three types of nesting environments in Arizona’s Santa Catalina Mountains, where the bees thrive. The team manipulated the temperatures of the nests by painting them to simulate past, present and future climates. The team painted a third of the nests black to absorb more radiant heat, thus simulating a future climate predicted for the years 2040 to 2099. By painting another third with a white, reflective, cooling treatment, the team sent that third of the nests back in time to a climate similar to that of the 1950s. As a control, the team painted the final third nests with a transparent paint, leaving their natural wood color for a control group. The experiment included 90 nests total, each housing anywhere from 2 to 15 bees.

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Read more: https://eurekalert.org/pub_releases/2018-06/nu-ccl062618.php

The abstract of the study;

Experimental warming in the field delays phenology and reduces body mass, fat content and survival: Implications for the persistence of a pollinator under climate change

Paul J. CaraDonna James L. Cunningham Amy M. Iler
First published: 27 June 2018

1. Climate change is rapidly altering thermal environments across the globe. The effects of increased temperatures in already warm environments may be particularly strong because organisms are likely to be near their thermal safety margins, with limited tolerance to additional heat stress.
2. We conduct an in situ field experiment over 2 years to investigate the direct effects of temperature change on an early‐season solitary bee in a warm, arid region of the Southwestern USA. Our field experiment manipulates the thermal environment of Osmia ribifloris (Megachilidae) from larval development through adult emergence, simulating both previous cooler (c. 1950; nest boxes painted white) and future warmer (2040–2099; nest boxes painted black) climate conditions. In each year, we measure adult emergence phenology, linear body size, body mass, fat content and survival.
3. Bees in the warming treatment exhibit delayed emergence phenology and a substantial increase in phenological variance. Increases in temperature also lead to reductions in body mass and fat content. Whereas bees in the cooling and control treatments experience negligible amounts of mortality, bees in the warming treatment experience 30%–75% mortality.
4. Our findings indicate that temperature changes that have occurred since c. 1950 have likely had relatively weak and non‐negative effects, but predicted warmer temperatures create a high stress thermal environment for O. ribifloris. Later and more variable emergence dates under warming likely compromise phenological synchrony with floral resources and the ability of individuals to find mates. The consequences of phenological asynchrony, combined with reductions in body mass and fat content, will likely impose fitness reductions for surviving bees. Combined with high rates of mortality, our results suggest that O. ribifloris may face local extinction in the warmer parts of its range within the century under climate change.
5. Temperature increases in already warm ecosystems can have substantial consequences for key components of life history, physiology and survival. Our study provides an important example of how the responses of ectothermic insects to temperature increases in already warm environments may be insufficient to mitigate the negative consequences of future climate change.

Read more: https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2435.13151

Reading the full text, the scientists gathered nests and put them in their painted boxes.

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Our experiment has four stages: collection of newly completed nests from unmanipulated nest boxes and transfer to experimental nest boxes; exposure to experimental temperature treatments in the field from April until emergence in the following year; monitoring emergence phenology; and collection of emergent bees and postprocessing in the laboratory to measure adult body size and fat content.

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The warming treatment consisted of nest boxes painted with flat black acrylic latex‐based paint (Figure 2); because a black surface absorbs more radiant energy, this treatment warms internal nest temperatures. The control treatment consisted of identical nest boxes painted with a transparent acrylic paint (Figure 2); these resemble the unmanipulated nest boxes placed in the wild nearby. The cooling treatment differed between the 2 years of the experiment, although treatment effects were similar (see Section 3).

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Aside from the absurd suggestion that a slight range shift is a serious enough problem to warrant the term “local extinction”, there is another curiosity which caught my attention.

The team seem to have logged daily maximum temperature inside the boxes, but I didn’t see any attention to daily minimum temperature. Painting the boxes black would have caused higher maximum temperatures from absorption of sunlight in the daytime, but the black painted boxes would also have emitted heat faster at night.

So it seems possible that much of the damage to the bees in the black boxes was caused by colder night time temperatures, rather than warmer daytime temperatures. The Tucson, Arizona region where the experiment was conducted experiences some cold nights most of the year, with an average of 16 days per year below freezing in Tucson, likely even more cold nights in the nearby Santa Catalina Mountains where the experiment was conducted.