Modeling Insect Development in Earth’s Changing Climate
Objectives
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Does the temperature of the environment affect the rate at which development
occurs in a significant way?
If so, is the rate affected in a manner that fits well with what we generally know about
enzyme kinetics and metabolic rates?
Introduction
Global Warming resulting in the potential for range expansion in agricultural and forest pests
has been an area of serious concern in the last few decades for scientists, natural resource
managers, and farmers alike [3,4,5,6]. As we learn more about the effects of climate on
ecosystems and the influence of the human population on both climate and ecosystems, we
have engaged in complex conversations regarding the future ramifications of such influences.
Importantly, scientists have been investigating the implications of global warming on adaptation
and evolution from a developmental biology perspective and generating models to predict
changing ecosystems in relation to climate change [6].
To study this phenomenon in a developmental biology context, scientists examine organismal
phenology, the timing of developmental milestones, as an important determinant of fitness in
biological populations [1, 2]. For poikilotherms (heterotherms), phenology is largely dependent
on environmental temperatures. Phenology shifts have been identified and linked to warming
temperatures in several species including insects [3] and plants [4]. In some cases this has
resulted in range expansion both toward the poles [5] and into higher elevations [6]. Quantifying
the relationship between phenology and temperature is crucial for predicting the impacts of
climate change on insect populations.
The recent northward and upward (in elevation) expansion of mountain pine beetles in western
North America is an important example of a problem in which scientists have linked changing
phenology to climate change. Mountain Pine Beetles have received increasing attention in both
the scientific literature and the popular press over the last decade due to their impact on
lodgepole, ponderosa, and white bark pine stands in western North America [7]. The destruction
of large areas of forested land affects many areas of life from social (outdoor activities) to
industrial (lumber). A recent Seattle Times article reported on the expansion of Mountain Pine
Beetle populations in the Pacific Northwest [7] (see attached); other recent expansions in the
west have been linked by scientists to increasing temperatures due to global climate change
[5,6] . To further exacerbate the issue, an article published in the American Naturalist presents
data showing that an increase in temperature allows the beetles to not only increase their
altitude range but also to move from a univoltine to bivoltine reproduction cycle (once a year to
twice a year) [8].
An example of a tropical beetle whose range may change due to warming climates is the bean
beetle, Callosobruchus maculatus, which has long been considered an agricultural pest in
tropical regions globally. Fertilized eggs from this beetle species are laid on the surface of seeds
(beans) of legumes. The first instar larva hatches and burrows into the bean where it feeds as it
progresses through four larval instars prior to pupation. Following pupation, the adult beetle
emerges from the bean. Bean beetles are a model organism quickly growing in popularity in the
laboratory setting and we will use them in the experiment(s) for this exercise.
In your experiment you will examine factors that affect the developmental timing of bean beetle
embryos. Since the first instar larva forms within the egg prior to penetrating the seed coat, a
convenient visual marker signaling that embryonic development is nearly complete is the
formation of the pigmented larval head capsule (Figure 1). Because this is easily observable
using a dissecting microscope, we will use the timing of this particular marker to study the
impacts of environmental factors on development time.
Figure 1. Images of bean beetle eggs on black-eyed peas. On left: immediately following oviposition (no head
capsule). Right: after head capsule formation/darkening.
Mathematical models of temperature-dependent phenology are used to predict the
developmental timing of poikilotherms under varying temperature conditions in the field [1,6,9].
These models are fit to median development rate data collected in laboratory experiments at
constant temperatures. Although most of these models are empirically derived, Sharpe and
DeMichele [12] proposed a widely used mechanistic model that is based on enzyme kinetics.
Thus, development rates of organisms are assumed to be determined by the rates of underlying
enzyme-controlled reactions that are temperature-dependent. Although measuring the rates of
the underlying reactions is beyond the scope of this lab, it is important to keep this mechanism
in mind when thinking about the phenology of developing organisms.
Materials
The following materials will be available in class:
a.
Environmental chambers or incubators (with temperature control and, if available, humidity
control)
b.
Adult bean beetles reared on a variety of substrates (black-eyed peas, mung beans,
adzuki beans)
c.
Beans (black-eyed peas, mung beans, adzuki beans)
d.
Plastic Petri dishes
e.
Dissecting microscope
Since this experiment focuses on embryonic development, it is useful to fix the beans (bearing
embryos) to glass slides for easy observation under the microscope (see Figure 2). For this, the
following materials will be provided:
a.
b.
c.
h.
Glass slides
Modeling clay
Lids from a culture dish (e.g., 96-well) to hold the slides with beans on them
Fine tip black marker for labeling beans.
Figure 2. Beans with embryos mounted on slides using modeling clay. Beans are oriented so that embryos
face up for easy observation using dissecting microscopes. Slides are placed into the lid of a 96-well plate for
easy management.
Experimental Design
Answer the following questions before coming to class:
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Why would you expect temperature to have an impact on the developmental timing of
poikilotherms?
How you expect warming temperatures to affect developmental timing? Cooling
temperatures?
What are other factors that might affect developmental timing?
How so?
Design an experiment that will allow you to test your hypotheses by investigating the
impacts of temperature and at least one other factor on the developmental timing of
bean beetle embryos (i.e., the timing of pigmented head capsule formation).
What data will you need to collect and how will it be analyzed?
Literature Cited
1. Bentz, B.J., J.A. Logan, and G.D. Amman, Temperature-Dependent Development of the
Mountain Pine-Beetle (Coleoptera, Scolytidae) and Simulation of Its Phenology. Canadian
Entomologist, 1991. 123(5): p. 1083-1094.
2. Memmott, J., et al., Global warming and the disruption of plant-pollinator interactions.
Ecology Letters, 2007. 10(8): p. 710-717.
3. Parmesan, C. and G. Yohe, A globally coherent fingerprint of climate change impacts
across natural systems. Nature, 2003. 421(6918): p. 37-42.
4. Visser, M.E., Holleman, L.J.M., Warmer springs disrupt the synchrony of oak and winter
moth phenology. Proc. R. Soc. Lond., 2001. 268: p. 289-294.
5. Carroll, A., Taylor, S., Régnière, J., Safranyik, L., Effects of climate change on range
expansion by the mountain pine beetle in British Columbia, in Mountain Pine Beetle
Symposium: Challenges and Solutions. Information Report BC-X-399., T. Shore, Brooks, J.,
Stone, J., Editor. 2003, Natural Resources Canada, Canadian Forest Service, Pacific Forestry
Centre, Victoria, BC.: Klowna, British Columbia. p. 223–232.
6. Logan, J.A., Powell, James A., Ghost forest, global warming, and the mountain pine beetle
(Coleoptera: Scolytidae) American Entomologist 2001. 47(3): p. 160-173.
7. Climate change, beetle may doom rugged pine, The Seattle Times, Nov. 5, 2011,
http://seattletimes.nwsource.com/html/localnews/2016699269_barkbeetle06m.html
8. Mitton, Jeffry B. and Ferrenberg, Scott M., Mountain Pine Beetle Develops an
Unprecedented Summer Generation in Response to Climate Warming, American Naturalist
2012, 179(5), pp. E163-E171.
9. Sharpe, P., DeMichele, D., Reaction kinetics of poikilotherm development. J. Theor. Biol.,
1977. 64: p. 649-670.
This study was written by A. Putzke and B. Yurk, 2012 (www.beanbeetles.org).